A study of water exchanges between the Bunnefjord and the Vestfjord, inner Oslofjord

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A study of water exchanges between the Bunnefjord and the Vestfjord, inner

Oslofjord

Using observational data and high-resolution model simulations

Pipatthra Saesin

Master’s Thesis, Autumn 2021

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Abstract

This study is aimed to investigate water exchanges in the inner Oslofjord, between the Bunnefjord and the Vestfjord. A 46-year historical dataset of salinity and temperature shows that the surface- mixed layer is about 20 m deep and has high seasonal variations. Summer heating at the surface diffuses to the lower layer which results in relatively high temperature at depth in winter. The deepwater below the Bunnefjord-Vestfjord sill depth, which of 55 m, is homogenized in both temperature and salinity. Two significant events related to deepwater exchanges were observed from the salinity time series. First, the stagnant periods of deepwater, the periods of progressively decrease of salinity due to the vertical eddy diffusivities. Vertical eddy diffusivities were estimated in the periods of stagnation using the Budget method. The results confirm earlier claims that the Vestfjord has higher diffusivities than the Bunnefjord. This is a result of breaking internal waves that are generated at the Drøbak sill. Second, actual deepwater renewal events, periods of rapidly increase of salinity, require availability of dense water outside the basin in order to replace old basin water. In addition, northerly winds are needed to push the surface water out of the basins and induce the deepwater influx. New observations were conducted by the F/F Trygve Braarud research vessel during 7 August-11 December 2020 in order to study details of water exchanges in the inner fjord. The FjordOs numerical ocean model was compared to the observations. In return, the model was used to help interpret the observations. Both model and observations agree on the time-average inflows at a station, a station between Nesoddtangen and Bygdøy, and time-average outflows at another station near the Nesoddtangen. Local river discharge and tides play minor roles on the exchange flows. Winds and pressure (SSH) gradient are important factors influencing the water exchanges. The circulations in the surface layer are mainly influenced by the winds, whereas the pressure gradients relate to the exchanges of intermediary water. The model reveals recirculation cells in the Bunnefjord and the Vestfjord, particularly in the surface layer. This may suggest that most of the water exchanges between the Vestfjord and the Bunnefjord take place in intermediate and bottom layers.

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Acknowledgements

I would like to express my special thanks to my supervisor Pål Erik Isachsen for his supervision of this thesis. Furthermore, I would like to thank my co-supervisor Andre Staalstrøm and Nils Melsom Kristensen for inspiring discussions and help with the observations, data, and model. Moreover, I am thankful to the crew onboard R/V Trygve Braarud, who enabled the collecting of data. I am really thankful to Lars Petter Røed for giving useful input.

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List of Figures

1.1 Circulation and physical processes in fjords from Stigebrandt, 2012 . . . 2 1.2 Topograhpy of Oslofjord and the inner of Oslofjord is in the black frame. . . 5 2.1 Topography of the inner of Oslofjord with the locations representing the Bunnefjord, Ep1,

the Vestfjord, Dk1, and out of the Drøbak sill, Im2. . . 10 2.2 Climatology mean of potential temperatures and salinity at station Ep1 in the Bunnefjord

and station Dk1 in the Vestfjord during 1973-2018. . . 12 2.3 Potential temperature and salinity at station Ep1 in the Bunnefjord and station Dk1 in

the Vestfjord during 2000-2008. Black dots represent the data points. . . 13 2.4 Time series of potential temperature at 8, 20, 40 60, 80 m depth at station Ep1 in the

Bunnefjord and station Dk1 in the Vestfjord. . . 14 2.5 Time series of salinity at 8, 20, 40 60, 80 m depth at station Ep1 in the Bunnefjord and

station Dk1 in the Vestfjord. . . 15 2.6 Time series of salinity below 60 m at station Ep1, Dk1, and Im2 during 1973-2018. White

shaded areas define the deepwater renewal events, and stagnant periods are in the grey shaded areas. Numbers in the plots are the highest salinity in each renewal period. . . 16 2.7 Sketch of exchange in a slab of water from Stigebrandt, 2012 . . . 17 2.8 Time series of salinity and the north-south wind component (daily mean in blue, weekly

mean in orange, and monthly mean in green) during two example of deep water renewals in the Bunnefjord, during 2005-2007 and 2010-2011. . . 19 2.9 Climatology mean of wind components, north-south winds (V-winds) in blue, and east-west

(U-winds) winds in orange at the Blindern station during 1973-2018. The thick black lines represent the period of renewals with year of occurrences . . . 20 3.1 Locations of ADCPs deployment at R2 and R3, and the stations of CTD measurement,

in the Bunnefjord, Cp2, in the Vestfjord, Bn1, and at the sill, R2. A black line across the Bygdøy and the Nesoddtangen is a transect for studying vertical exchanges from the model in the section 3.9 . . . 24 3.2 Deployment of ADCPs at the sill. . . 25

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viii LIST OF FIGURES 3.3 Temperature (blue), salinity (red) and density (black) profiles at station Bn1 (top), R2

(middle) and Cp2 (bottom) on 7 August (ADCP deployment), 29 September (student cruise), 27 October (battery change), and 11 December (recovery) in 2020. Solid lines represent observations and the dotted lines are the model simulations. . . 26 3.4 Time-averaged velocity at R2 and R3, 7 August - 11 December 2020, from observations

(left) and hindcast simulations (right). The vectors show the direction of the flow at difference depths. The horizontal arrows are east-west flows. . . 28 3.5 Time-averaged velocity during 7 August - 11 December 2020. The color vectors are the

average from the observations in three layers, surface in blue, mid-depth in green, and bottom in yellow, and the variance ellipses at R2 and R3. The average velocity from simulations are plotted as black vectors. The dash lines are the line transects at the sill. . 28 3.6 The 1 hour-averaged velocity components, u-eastward and v-northward, at R2 and R3

during the observation period, 7 August – 11 December 2020. The observations are on the left and hindcast simulations are on the right(right column) . . . 29 3.7 Velocity rose plots at R2 and R3 from observations and hindcast simulations . . . 30 3.8 The 36-hour filtered u-eastward component at R2 and R3 in surface and mid-depth layers.

Solid lines are observations and dash lines are hindcast simulations. . . 31 3.9 Locations of four rivers that may influence on the flows between the Bunnefjord and the

Vestfjord. . . 32 3.10 Time series of river discharge in the Gjersjøelva (blue), the Alna (yellow), the Akerselva

(green), and the Frognerelva (red). The sum of river discharge from four rivers and the zonal velocity (u-eastward) at the surface layer at R2 from observation and hindcast simulation. The black crosses are peaks of discharge and peaks of outward flows. . . 33 3.11 Time series of daily climatological river discharge (2013-2017) in the Gjersjøelva (blue),

the Alna (yellow), the Akerselva (green), and the Frognerelva (red) from the FjordOs model. 33 3.12 The scatter plots of total currents (light blue dots) and tides (pink dots) with the variance

ellipses at R2 and R3 from observations and hindcast simulations. The variance of observed currents are in blue ellipses, and variance of estimated tides are in red ellipses. The red and blue lines are the first and second axis of principal component for total currents and tides, respectively. The time-average velocities are plotted in black vectors. . . 34 3.13 Observed surface air pressure, wind components at Blindern station, and the observed

current at R2 and R3. The simulated surface air pressure from AROME-MetCoOP model.

The simulated wind components at a collocated point to the R2 and R3 locations, and currents at R2 and R3 from hindcast simulations. . . 36 3.14 The development of low pressure system from the AROME-MetCoOp during the first low

pressure event on 24-26 September 2020. . . 36

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LIST OF FIGURES ix 3.15 Observed surface air pressure, wind components at Blindern station, and the observed

current at R2 and R3 during 20 September- 2 October 2020. The simulated surface air pressure from AROME-MetCoOP model. The simulated wind components at a collocated point to the R2 and R3 locations, and currents at R2 and R3 from hindcast simulations. . 37 3.16 Locations of SSH from the hindcast simulations in the Bunnefjord (BN), Vestfjord (VF),

Drøbak (DB), and Skagerak (SK). . . 38 3.17 Winds at R3, SSH difference and current components from hindcast simulations. The

BN-VF is the SSH at the Bunnefjord minus the Vestfjord, VF-BR is the SSH at the Vestfjord minus Breiangen, and VF-SK is the Vestfjord (VF) minus the Skagerrak (SK). 39 3.18 The velocity profiles at difference points along the headland. The dash line is the streamline.

The figure is implemented from Garrett, 2005. . . 41 3.19 A residual current that be created by flows past a headland (from Garrett, 2005). . . 41 3.20 Currents from hindcast simulations across the Bygdøy and the Nesoddtangen (transect is

showed in Figure 3.1 and 3.5) show inflows and outflows at the sill that may result in the recirculation cells during 21 - 23 August 2020. The arrows are streamlines while the color gives flow speed. . . 42 3.21 Winds from hindcast simulations during 21 - 23 August 2020. . . 43 3.22 Time-averaged currents across the sill from hindcast simulations show inflows and outflows

during 7 August-11 December 2020. . . 43 A.1 Time series of salinity and the north-south wind component (dailymean in blue, weekly

mean in orange, and monthly mean in green) in the Bunnefjord, during 1974-1976, 1976-1978 and 1983-1986. . . 52 A.2 Time series of salinity and the north-south wind component (dailymean in blue, weekly

mean in orange, and monthly mean in green) in the Bunnefjord, during 1989-1992, 1995-1997 and 2000-2002. . . 53 A.3 Time series of salinity and the north-south wind component (dailymean in blue, weekly

mean in orange, and monthly mean in green) in the Bunnefjord, in 2012-2013 and 2016-2018. 54

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List of Tables

2.1 Estimations of vertical diffusivity from salinity for each stagnant period. . . 18

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Chapter 1

Introduction

1.1 Fjords

Fjords are geological structures developed by glacial erosion which creates semi-enclosed coastal inlets that are partly filled with seawater. They were created once the glaciers melted and canyons/valleys were flooded by rising sea levels. The fjord’s inland end is called the head, where it gets freshwater from rivers, whereas the fjord’s seaward opening is referred to as the mouth. Fjords usually possess one or more submarine sills which limit water exchange between sub-basins (inlets). In particular, these sills act as barriers to deep water exchanges. Fjords are usually highly stratified due to low salinity in the surface layer and high salinity in the bottom layer.

Flow dynamics are critical to the ecosystem, water chemistry, and pollutant cycling in the fjord. Changes in nutrient, sediment, freshwater, or pollutant inputs in fjords can have significant consequences for ecology, such as plankton and related living communities, larval transport (e.g. Boyer et al., 2002; Davis et al., 2014), and water quality, such as hypoxia and acidification (e.g. O’Callaghan and Hamilton, 2007), and heavy metal accumulation (e.g. Zaborska and Włodarska-Kowalczuk, 2017).

Long-term stressors like climate change, for instant shifting of precipitation and runoff patterns, also have impacts on fjord dynamics (Straneo and Cenedese, 2015). Fjords are also beneficial to people close by because they provide leisure opportunities, transportation, and travel destinations.

The waters in fjords can generally divided into three water masses: estuarine/brackish water in the surface layer, intermediary water (coastal water) which lies between the brackish water and the sill depth, and basin water which is the densest water that is formed by the intrusion of dense water outside fjords (Stigebrandt, 2012). Figure 1.1 shows these three main water masses and physical processes involved in exchanges and mixing in fjords (Stigebrandt, 2012).

In the surface layer, freshwater and saline water of the intermediate layer combine to create brackish water. The salinity of surface brackish water near the head is relatively low due to freshwater supply from rivers, whereas the surface water at the mouth has higher salinity because of the seawater supply. This characteristic creates a density gradient between the head and the mouth, resulting in the

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2 CHAPTER 1. INTRODUCTION

Figure 1.1: Circulation and physical processes in fjords from Stigebrandt, 2012

outflow of low salinity toward the mouth, and inflow of higher salinity (sub-surface water) toward the head. Turbulent mixing is a crutial mechanism for mixing the surface fresh water and the saltier ocean water beneath. This is the actual mechanism making the surface water progressively saltier towards the mouth of a fjord. The above is the classical estuarine circulation that describes the flow in a basin where freshwater plays a role.

Wind is one of the important parameters affecting the dynamics of fjords (Pedersen, 1978).

Local winds cause turbulent mixing between the surface and intermediary waters (Stigebrandt, 2012) as well as forces the surface layer flow (Svendsen and Thompson, 1978). Farmer and Smith, 1978 and Svendsen, 1977 found that wind is more important to the surface circulation in fjords than the freshwater supply. However, freshwater can create strong stratification that traps traps the wind response to near-surface layer (Svendsen and Thompson, 1978).

Wind fields outside the fjords can also play a role in the exchange of water in fjords.

Offshore/inshore surface transports occur when the wind blows along the coastline, called Ekman transport. Offshore transport causes rising up of water beneath the surface as so-called upwelling events, whereas inshore transport is associated with downwelling. Upwelling and downwelling induce horizontal pressure gradients, both baroclinic and barotropic due to replacement of water mass which induces changes in density and sea level. Upwelling typically enhances pressure gradients along the fjord by increasing density at the fjord mouth thus enhancing exchange flows between the fjord and the ocean, while downwelling reduces the gradients, resulting in a reduction of the exchange flow

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1.1. FJORDS 3 and sometimes reversing the flow (Giddings and MacCready, 2017). This response relates to water exchange in the fjords, particularly in the intermediary layer (Aure and Stigebrandt, 1996, Giddings and MacCready, 2017, Pettersson, 1920, and Stigebrandt, 1990). A study of Stigebrandt, 1990 on the response of horizontal density gradients outside a fjord at the Norwegian west coast, the Ørsta fjord, found that water exchanges due to the pressure gradients are greater than exchanges driven by the estuarine circulation.

Tide generated in the ocean is also an important parameter for currents and mixing. Tides can affect both the exchange flows (the circulation) and the hydrography of the fjords. Tidal currents can push the water in and out of the fjords, and this water accelerates as it passes the sill. This leads to strong currents and mixing around the sill. Moreover, tides can induce vertical movements of pycnoclines over the topography (the sills) in stratified water, where the internal waves/tides are generated. Studies of internal wave energy in fjords suggest that more than half of breaking internal wave energy is dissipated near the sill, e.g., Arneborg and Liljebladh, 2001a, Arneborg and Liljebladh, 2001b and Klymak and Gregg, 2004. The turbulence generated induces mixing in water column. About 25% of the energy is contributed to turbulence and mixing in the basin water (Stigebrandt, 2012). An experiment in laboratory shows that most of the energy in the internal tides dissipate in the lower layer (Stigebrandt, 1976) and has the potential to alter deep basin water exchanges and renewals.

The residence time of the deep basin water may depend on density variability of the coastal water, the rate of diapycnal mixing of the basin water (eddy diffusivity), and the time to fill the basin with new deepwater. New basin water outside the fjord flows into the basin along the bottom, climbs up above the sill depth due to tides or winds, entraining old basin water. As a result, the new deep water will contain a mix of new dense coastal water and old basin water (Stigebrandt, 2012). The renewal of deep water occurs when the coastal water is denser than the old basin water. Here, eddy diffusivity plays an important role in reducing the density of the old water and allowing the new dense water to flow underneath. This renewal event is a relatively rapid process, typically taking only a few weeks, and tends to take place at the same time of the year (Gade, 1973). A model developed by Erlandsson (Erlandsson, 2006) considered water exchange which is driven by horizontal density differences between the fjord and the offshore area above the sill level. Their model demonstrates that the basin water can be stagnant for several years due to combination of weak mixing rate and a too light offshore density field. This model distinguished two types of basin water systems. ‘Mixing systems’ have a short time of water renewal, and the residence time depends on the rate of diapycnal mixing in the basin water and on the distribution of the density variability in the coastal water. If mixing in the basin water is weak, long-term variation in the offshore density field, e.g. set by large-scale atmospheric forcing, may determine the residence time. The other system is the ‘transport systems’, where the new basin water takes time to fill up the basin and the residence time is determined by the baroclinic transport capacity of the mouth. However, this model assumes that new basin water does not mix with the old basin water, which is an unrealistic system. After deepwater renewals, stagnant periods typically occur. During these periods, the basin water are residing below the sill depth for a year or longer with

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4 CHAPTER 1. INTRODUCTION less exchanges and mixing. Here, turbulent diffusion could be the only mechanism that influences the water properties. Water salinity/density will progressively decrease at rate set by the diffusivity. This less saline basin water due to the diffusivity is one of the requirements of deepwater renewals in order to allow the replacement of the new denser water at the bottom.

1.2 Oslofjord

Oslofjord is an approximately 100 km long estuary in south-eastern Norway, extending from the city of Oslo to the Færder lighthouse in Skagerrak (figure 1.2). The fjord is divided into five regions: the outer Oslofjord from Færder to Horten-Moss, the central basin at Breiangen, the Drøbak sound, the inner Oslofjord, and the Drammenfjord which is connected to the western end of Breiangen.

The Drøbak sound is divided in two by the island Håøya. The sill depths at the Drøbak Sound is 20 m, approximately, with 19.5 m in the western passage and 18.5 m in the eastern passage (Gade, 1970). The inner fjord widens out to form the Vestfjord in the north of the Drøbak Sound. The Vestfjord is connected to the Bunnefjord situated east of the Vestfjord. Both inner Oslofjord basins have approximately maximum depths of 160 m with approximately 55 m of sill depth that connects these two basin.

The inner fjord is the most densely populated area in Norway and provides a source of sea foods and recreational values. There are numerous activities around and inside the fjord, i.e., transportation, summer beaches, harbors, leisure boats, and industrial area. Water quality and pollution have been main concerns in the inner Oslofjord since the 1930s, particularly in the Bunnefjord. The Bunnefjord is well known as an intermittent anoxic basin due to limitation of water exchange coupled with a high organic matter supply or sewage originating from river discharge, household and industrial drainage around the fjord. Sporadic strong overflow of the rivers disturbs the cycle of nutrients and leads to excess nutrient inputs (eutrophication) in surface and intermediate waters (Pinturier-Geiss et al., 2002 and Dale et al., 1999). This phenomenon can result in blooms of phytoplankton responsible for fish death. The blooms are usually followed by the depletion of oxygen in the surface and intermediate water (i.e. Ruud, 1968 and Pinturier-Geiss et al., 2002) and formation of H₂S in the basin water (Magnusson et al., 1995). Understanding of circulation and water exchanges in the inner Oslofjord is therefore crucial for water management and planning.

Gade, 1970 suggested five mechanisms that drive the surface circulation in the Oslofjord: tides, non-tidal water level variations, the estuarine circulation, density currents (including internal waves), and wind drift. Only the tides and associated internal waves are permanent processes, while the rest of the mechanisms arise intermittently and with varying intensities. Because those processes are independent, their intermediate impacts on currents can be superimposed. A study of water exchanges, particularly in the inner Oslofjord by Gade, 1968 suggested that only three main processes control the water exchanges between the Bunnefjord and the Vestfjord. These processes are the estuarine circulation, tidal currents and large-scale winds.

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1.2. OSLOFJORD 5

Figure 1.2: Topograhpy of Oslofjord and the inner of Oslofjord is in the black frame.

The major sources of freshwater to the Oslofjord are Glomma and Drammenselva with a mean discharge of 729m³/s and 317m³/s, respectively (Milliman and Farnsworth, 2011). Many small local rivers provide additional freshwater to the inner Oslofjord making up a maximum discharge in spring and fall of about 50m³/s. But the minimum discharge in summer barely exceeds surface evaporation.

Previous studies suggested that the estuarine circulation is mostly restricted to the upper 20 meters.

However, exchange of the surface water appears to be controlled primarily by other mechanisms than the estuarine circulation (Gade, 1968 and Staalstrøm, Aas et al., 2012). Importantly, Gade, 1968 found that the brackish water entering the inner Oslofjord originates from the outer Oslofjord, mainly

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6 CHAPTER 1. INTRODUCTION from the Drammenselva discharge. This could mean that the estuarine circulation in the classic sense (outflows of the surface water) probably does not exist in the inner Oslofjord.

Gade, 1968 suggested that surface drift in the inner Oslofjord is linked to sustained winds.

Winds are primarily from the north in winter, while they are more variable with a preference for south-to-southwesterly in summer. The availability of continuous northerly winds and the presence of not too much brackish water in the fjord give optimal conditions for deepwater exchange in the inner Oslofjord. These two conditions are typical of the winter and early spring months, just before the seasonal snow melts (Gade, 1968). Study of deepwater renewal in the inner Oslofjord (Gade, 1970) suggested a hypothesis that the northerly winds drive the surface waters out of the fjord and is replaced by denser deep water. Observations in 1962-1965 showed that deepwater flows into the Vestfjord every winter and is associated with the northerly winds. But in the Bunnefjord, the deep water influxes were observed only once, in 1963. The processes required at least three-week continuous northerly winds.

However, the wind-forced deepwater influxes will not last to complete the process. In the case of the Bunnefjord, the coupling of both wind-induced deepwater inflow and decreasing of basin water density are necessary.

In Oslofjord, the tides fluctuate by about 24 cm (32 cm spring and 18 cm at neap tide). The dominant tidal component M2 is about 15.0 cm (Stigebrandt, 1976 and Staalstrøm, Aas et al., 2012).

Contributions from the harmonic overtides M S4, M4 and 2SM2 due to shallow water effects are observed during spring tides and also present in the internal tides (Staalstrøm, Aas et al., 2012).

Internal waves in the Oslofjord were first documented by Gade, 1967. They are generated at the Drøbak sill and propagate into the inner fjord (Stigebrandt, 1976 and Staalstrøm, Aas et al., 2012).

These waves break when they face the sloping bottom at the head of Vestfjord, and lose their energy, approximately, 40−75% within 10 km by the sill (Staalstrøm, Aas et al., 2012 and Staalstrøm, Aas et al., 2012). The breaking internal waves dissipate energy to the water by turbulence. Observations suggest that turbulent diffusivities in Vestfjord are higher than in Bunnefjord. Gade, 1970 estimated vertical diffusivity during stagnant periods in 1963-1965 between 90 and 125 m depth and found that diffusivity in the Bunnefjord was 1.0±0.5cm²/sand 7.6±4.1cm²/sin the Vestfjord. An estimation of vertical diffusivity in 2003-2009 from Staalstrøm, Aas et al., 2012 is 1.8±0.7cm²/sin the Bunnefjord and 4.9±0.5cm²/s in the Vestfjord.

There are few studies focusing on the inner Oslofjord. Most of them were focused on tidal mixing by the breaking internal waves (e.g. Staalstrøm, Aas et al., 2012, Staalstrøm and Røed, 2016, and Stigebrandt, 1979) and were handled in such short-period time series (about 3-7 years). A study of deepwater renewals by Gade, 1970 covered only 4-year observations which includes only few renewals.

It also used a very old dataset (almost 60 years ago). Hence, it is worth revisiting the process of deepwater renewal using updated and longer datasets. This could be an initiative to repeat and compare to the previous results. In addition, the long time series could be a very useful data to study long-term exchanges and influences of climate change. There are much rarer studies on other forcing,

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1.3. THIS THESIS 7 particularly on respect to the details of exchanges between Vestfjord and Bunnefjord. This could support the environmental concerns, especially in the Bunnefjord.

1.3 This thesis

This thesis is dedicated to investigate water exchanges between the Bunnefjord and the Vestfjord, the inner Oslofjord, using both observational data and a high-resolution model. The first part of the thesis will work with a 46-year historical dataset. It will start with a general description of temperature and salinity in both basins. Estimations of vertical diffusivity will be repeated as had been done in Staalstrøm, Aas et al., 2012 and Gade, 1970 but with the longer and updated time series that cover several stagnant periods. Moreover, related conditions that may induce deepwater renewals will be investigated. The second will focus on the details of water exchanges around the sill. A 4-month field campaign was conducted on 7 August - 11 December 2020. This is the first time that currents have been observed at the sill between the Bunnefjord and the Vestfjord. Related drivers, including river discharge, tides, winds, and sea level gradient forcing will be discussed. In addition, a high-resolution model for the Oslofjord, called "FjordOS", which has been developed by the Norwegian Meteorological Institute (MET), will be used in the study. The model was set up with emergency response in mind;

for instance, oil spills or search and rescue, and has been evaluated with observational data for the whole region of Oslofjord. This is a chance to validate the model performance, particularly in the inner Oslofjord, using the new field observations. In return, the model will be used to help interpret the observations with respect to the spatial structure of the flow field. As will be seen, the model can also be used to study non-linear processes that would be hard to predict using the observations alone. The non-linear circulation are often neglected in previous studies. But such flow features are needed in order to gain a complete understanding of water exchanges and circulations in the inner Oslofjord.

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Chapter 2

General hydrography and deepwater

exchanges from a 46-year historical dataset

This first part of thesis is an investigation of temperature and salinity in the inner Oslofjord using a 46-year historical dataset provided by the Norwegian Institute for Water Research (NIVA).

Climatological-mean seasonal cycles of both temperatures and salinity are described together with their variability. The estimation of vertical diffusivity in the deepwater takes place in order to understand the exchanges of basin water. The estimations had been done before, but with much shorter datasets.

With a near-complete 46-year dataset, previous estimates can be verified and also evaluated against year-to-year variability. The chapter, and indeed the whole thesis, has a special focus on the Bunnefjord, where the water quality is mainly concernd. The deepwater renewal events in the Bunnefjord and related conditions will be investigated, including availability of dense water outside the basin and the influence of winds, as hypothesized by Gade, 1968 and Gade, 1970.

2.1 General hydrography in the inner Oslofjord

Historical hydrographical data used in this thesis are from three datasets. The Norwegian Institute for Water Research (NIVA) provides the hydrographic data from 1933-2004, and via the

‘Aquamonitor’ program from 1990-2014. Observations from monitoring programs of Norconsult, stored in Miljødirektoratets dataset, are available from 2015-2018. Two selected stations are representing hydrography in the Bunnefjord, Ep1, and one in the Vestfjord, Dk1 (see Figure 2.1). Due to incompleteness of the data, the study period is limited to 1973-2018. These dataset provide various parameters related to water quality such as dissolved oxygen, nutrients concentration, and Chlorophyll.

But only temperatures and salinity are investigated in this thesis.

Climatological seasonal cycles of temperatures and salinity were constructed from the 46-year historical data and are shown in Figure 2.2. Unsurprising, surface temperature is at a minimum in winter and last to the middle of spring (April). The surface temperature then starts to warm and reaches a maximum in summer. In winter, surface waters lose heat to the cold air while summer heat

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CHAPTER 2. GENERAL HYDROGRAPHY AND DEEPWATER EXCHANGES FROM A 46-YEAR HISTORICAL DATASET

Figure 2.1: Topography of the inner of Oslofjord with the locations representing the Bunnefjord, Ep1, the Vestfjord, Dk1, and out of the Drøbak sill, Im2.

diffuses to the lower water layers. This results in the low temperature in the surface layer and high temperature at depth during the wintertime. The seasonal changes are clearly observed in the surface layer of about 25 m. The temperature at Ep1, particularly below the sill depth, approximately at 55 m, seems to be nearly constant in all seasons. At Dk1, the high temperature in wintertime is observed at 25 m down to bottom (90 m). This may be a result of higher eddy diffusion of heat in the Vestfjord than in the Bunnefjord (Gade, 1970). Estimation of mean vertical thermal eddy diffusion from an observation campaign in 1962-1965 (Gade, 1970) were 0.18cm²/sand 0.32cm²/s in the Bunnefjord and the Vestfjord, respectively. The higher rate of heat diffusivity means that the heat can penetrate faster and further down to the deeper layer.

Seasonal cycles of salinity in the Bunnefjord and the Vestfjord are also seen in the surface layer (0-25 m). Low salinity in the surface layer occurs in summer when a relatively large amount of freshwater discharge feeds into the basins. High salinity is observed during winter. This could be a result of ice formation at the surface, as salt is then released to the underlying water. It seems that strong stratification appears in summer when the vertical salinity gradient is strong. A halocline at the sill depth, about 55 m, is clearly seen at Ep1. The salinity at Dk1 tends to be lower than at Ep1. This

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2.1. GENERAL HYDROGRAPHY IN THE INNER OSLOFJORD 11 could be a result of higher eddy diffusivity of salt in the Vestfjord than in the Bunnefjord (Gade, 1970) that induces more mixing between surface and lower waters. In colder seasons when the stratification is not so strong, the eddy diffusivity is more effective on vertical mixing. It can be seen in Figure 2.2, the more homogenized in salinity below 25 m, approximately, in autumn and winter.

Figure 2.3 shows the temporal variations of temperature and salinity at Ep1 and Dk1 during 2000-2008 (the period with the best data availability). Extensive variations of both temperature and salinity are observed, particularly in surface and intermediate layers. In agreement with Figure 2.2, the time series show seasonal cycles of high temperature in surface layer during the summertime relates to low salinity. This could be a signal of input warm freshwater discharge in summer. Hence, freshwater discharge may be one of important factors of hydrographic variability in the inner Oslofjord. Variation of freshwater discharge and climate could affect the hydrography, especially in the surface layer.

Variations of temperature at 8 m, 20 m, 40 m, 60 m, and 80 m are shown in Figure 2.4.

Temperature at Ep1 at 8 m reveals the seasonal cycle, but the seasonal cycle is much weaker below 20 m . Variations tend to be even smaller at deeper depths. At Dk1, seasonal cycle of temperature can be observed at all depths, but below 60 m water tends to homogenize in temperature.

Time series of salinity at 8 m, 20 m, 40 m, 60 m, and 80 m are shown in Figure 2.5. Seasonal cycle of salinity at Ep1 is observed only at 8-40 m and has less variations below 60 m. There is an obviously drastic increase of salinity following by progressively decrease below 60 m in 2006 at both stations. The increased signals could indicate replacement of new denser water from the outer Oslofjord as part of deepwater renewal which will be investigated later in section 2.3. Salinity is expected to decrease during the stagnant period due to the eddy diffusivity. It is very clear signal of slowly decreased salinity in deepwater at Ep1 due to the slow diffusion rate. In contrast, salinity below 60 m at Dk1 shows more fluctuations which are likely reflect much more advection in the Vestfjord.

Estimations of vertical diffusivity in deepwater during several stagnant periods will be presented in section 2.2.

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12

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0

25 50 75 100 125

150 Ep1

Potential temperature [DegC]

0 3 6 9 12 15 18

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0

25 50 75 100 125

150 Ep1

Salinity [PSU]

15 18 21 24 27 30 33 36

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0

25 50 75 100 125

150 Dk1

Potential temperature [DegC]

0 3 6 9 12 15 18

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0

25 50 75 100 125

150 Dk1

Salinity [PSU]

15 18 21 24 27 30 33 36

Figure 2.2: Climatology mean of potential temperatures and salinity at station Ep1 in the Bunnefjord and station Dk1 in the Vestfjord during 1973-2018.

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2.1. GENERAL HYDROGRAPHY IN THE INNER OSLOFJORD 13

2000Jan Jan

2001 Jan

2002 Jan

2003 Jan

2004 Jan

2005 Jan

2006 Jan

2007 Jan

2008 0

20 40 60 80 100 120 140

Depth [m]

Temperature contour at Ep1

0 3 6 9 12 15 18

2000Jan Jan

2001 Jan

2002 Jan

2003 Jan

2004 Jan

2005 Jan

2006 Jan

2007 Jan

2008 0

20 40 60 80 100 120 140

Depth [m]

Salinity contour at Ep1

15 18 21 24 27 30 33 36

2000Jan 2001Jan 2002Jan 2003Jan 2004Jan 2005Jan 2006Jan 2007Jan 2008Jan 0

20 40 60 80 100 120 140

Depth [m]

Temperature contour at Dk1

0 3 6 9 12 15 18

2000Jan 2001Jan 2002Jan 2003Jan 2004Jan 2005Jan 2006Jan 2007Jan 2008Jan 0

20 40 60 80 100 120 140

Depth [m]

Salinity contour at Dk1

15 18 21 24 27 30 33 36

Figure 2.3: Potential temperature and salinity at station Ep1 in the Bunnefjord and station Dk1 in the Vestfjord during 2000-2008. Black dots represent the data points.

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14

2000Jan Jan

2001 Jan

2002 Jan

2003 Jan

2004 Jan

2005 Jan

2006 Jan

2007 Jan

2008 0

5 10 15 20

Potential temperature [degC]

8 m

Ep1

20 m40 m 60 m80 m

2000Jan Jan

2001 Jan

2002 Jan

2003 Jan

2004 Jan

2005 Jan

2006 Jan

2007 Jan

2008 4

6 8 10 12

40 m

Ep1

60 m80 m

2000Jan Jan

2001 Jan

2002 Jan

2003 Jan

2004 Jan

2005 Jan

2006 Jan

2007 Jan

2008 0

5 10 15 20

8 m

Dk1

20 m40 m 60 m80 m

2000Jan Jan

2001 Jan

2002 Jan

2003 Jan

2004 Jan

2005 Jan

2006 Jan

2007 Jan

2008 4

6 8 10 12

40 m

Dk1

60 m80 m

Figure 2.4: Time series of potential temperature at 8, 20, 40 60, 80 m depth at station Ep1 in the Bunnefjord and station Dk1 in the Vestfjord.

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2.1. GENERAL HYDROGRAPHY IN THE INNER OSLOFJORD 15

2000Jan Jan

2001 Jan

2002 Jan

2003 Jan

2004 Jan

2005 Jan

2006 Jan

2007 Jan

2008 15.0

17.5 20.0 22.5 25.0 27.5 30.0 32.5 35.0

Salinity [PSU]

8 m

Ep1

20 m40 m 60 m80 m

2000Jan Jan

2001 Jan

2002 Jan

2003 Jan

2004 Jan

2005 Jan

2006 Jan

2007 Jan

2008 30.5

31.0 31.5 32.0 32.5 33.0 33.5 34.0 34.5

Salinity [PSU]

40 m

Ep1

60 m80 m

2000Jan Jan

2001 Jan

2002 Jan

2003 Jan

2004 Jan

2005 Jan

2006 Jan

2007 Jan

2008 15.0

17.5 20.0 22.5 25.0 27.5 30.0 32.5 35.0

Salinity [PSU]

8 m

Dk1

20 m40 m 60 m80 m

2000Jan 2001Jan 2002Jan 2003Jan 2004Jan 2005Jan 2006Jan 2007Jan 2008Jan 30.5

31.0 31.5 32.0 32.5 33.0 33.5 34.0 34.5

Salinity [PSU]

40 m

Dk1

60 m80 m

Figure 2.5: Time series of salinity at 8, 20, 40 60, 80 m depth at station Ep1 in the Bunnefjord and station Dk1 in the Vestfjord.

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16

2.2 Vertical diffusivity in the Bunnefjord

Figure 2.6 shows time series of salinity during 1973-2018 at station Ep1 and Dk1. The data shows several periods of stagnant basin water, particularly in the Bunnefjord. The salinity below the 80 m undergo a slow decrease which is most likely an indication of vertical diffusion dominating over advection. We use 10 such periods to estimate vertical diffusivities, using the budget method proposed by Gargett (1984).

Figure 2.6: Time series of salinity below 60 m at station Ep1, Dk1, and Im2 during 1973-2018. White shaded areas define the deepwater renewal events, and stagnant periods are in the grey shaded areas.

Numbers in the plots are the highest salinity in each renewal period.

The budget method (Gargett, 1984) is based on conservation of mass and salt in order to explain the exchange in a slab of water which depends on diffusivity, vertical advection and horizontal flow through the slab (see Figure 2.7), as described by

∂S

∂t = 1 A

∂

∂z(Aκ∂S

∂z)−wA∂S

∂z +q(Sin−S)

(2.1) where S is salinity, A is area of the slab, q is horizontal flow, κis diffusivity coefficient, and w is

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2.2. VERTICAL DIFFUSIVITY IN THE BUNNEFJORD 17

Figure 2.7: Sketch of exchange in a slab of water from Stigebrandt, 2012

vertical velocity. The first term on the right hand side is vertical diffusion. The second term is vertical advection. We assume no net flow through the slab during stagnant periods,q(Sin−S) = 0, and also that vertical diffusion dominates over vertical advection. The remaining balance is then:

∂S

∂t = 1 A

∂

∂z(Aκ∂S

∂z)

(2.2)

Vertical integration of equation 2.2 from bottom depth H, to any given level z, gives the expression for vertical diffusivity:

κ_z= 1 A^∂S_∂z

Z z

H

∂S

∂tAdz (2.3)

In this thesis, historical salinity at both Ep1 and Dk1 were used to estimate diffusivities at different levels (at 80, 100, 125, and 150 m). For each stagnant period we calculate time derivative

∂S/∂tbetween two adjacent observations on every depths, then take averages. The vertical gradient

∂S/∂zwas computed between two adjacent depths on the same date, then averaged over the stagnant time. Values of vertical diffusivity at each stagnant period are presented in Table 2.1. The results show that mean depth-averaged vertical diffusivity over all such stagnant periods in the Bunnefjord, Ep1, is 1.74cm²/s. The lowest vertical diffusivity occurs during 2014-2016 at rate of 0.83cm²/s, while the highest value appears during 1992-1995 at 2.85cm²/s. At Dk1, the vertical diffusivity averaged over all periods is 7.26cm²/s, the highest diffusivity is 9.25cm²/s in 2011-2012 and the lowest is 6.14cm²/s in 1992-1995. Previous estimates by Gade, 1968 suggested that mean diffusivity in the Bunnefjord was 1.0cm²/s and 7.6cm²/s in the Vestfjord. A study by Staalstrøm, Aas et al., 2012 found that mean diffusivity was 1.8cm²/s in the Bunnefjord and 4.9cm²/s in the Vestfjord. Our mean values differ from their estimations. However, their estimates are within our range. An exception is Staalstrøm’s estimate in the Vestfjord which is lower than our range.

There are small differences in analysis method between our estimates and previous studies.

Both Gade, 1968 and Staalstrøm, Aas et al., 2012 estimated diffusivities between 90 m and 125 m.

Gade, 1970 suggested that the eddy diffusivity of salt tends to increase with depth. Hence, different depth interval between our and their studies could thus be responsible for differences in the estimates.

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18

CHAPTER 2. GENERAL HYDROGRAPHY AND DEEPWATER EXCHANGES FROM A 46-YEAR HISTORICAL DATASET Table 2.1: Estimations of vertical diffusivity from salinity for each stagnant period.

Period of stagnation K_z[cm²/s]

Ep1 Dk1

1975-1976 1.31 7.07

1978-1983 1.55 7.49

1986-1989 2.34 6.30

1992-1995 2.85 6.14

1997-2000 1.84 7.47

2002-2004 1.28 6.32

2007-2009 1.62 8.03

2011-2012 2.26 9.25

2014-2016 0.83 6.89

2017-2018 1.40 7.62

Mean 1.73 7.26

STD 0.60 0.94

The different data periods could also result in different estimates. Gade, 1968 estimated using data during 1963-1965, while Staalstrøm, Aas et al., 2012 used data during 2003-2009. However, our estimations suggest that there are variations of diffusivity in different stagnant periods. This could be explained by the different hydrographic conditions.

The stratification could diminish the diffusion rate in the basin water. Gargett, 1984 and Gargett and Holloway, 1984 suggested a relationship of diffusivity,κand the Buoyancy frequency,N as following

κ=a_oN^−q (2.4)

wherea_oandqare constants. The diffusivity is an inverse proportion toN. In the surface-mixed layer and in the homogeneous deepwater in which theN is low, the vertical eddy diffusivity tends to be higher than in the pycnocline layer. Temporal variations ofN at a certain depth could also result in the variability of vertical diffusivity. So, difference of hydrographic conditions during stagnant periods can alterN and result in different diffusion rates. This relationship has not been checked in this thesis but it could be done using this dataset for further studies.

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2.3. DEEPWATER RENEWALS IN THE BUNNEFJORD AND RELATED CONDITIONS 19

2.3 Deepwater renewals in the Bunnefjord and related conditions

For the actual deepwater renewals study, we focused on two aspects related to renewals: i) the availability of dense water outside the Bunnefjord and ii) the direction of winds. According to Gade, 1970, the renewals are expected to take place during periods of prolonged northerly winds that induce the outflows of surface water, then the deepwater influx could occur.

Figure 2.8: Time series of salinity and the north-south wind component (daily mean in blue, weekly mean in orange, and monthly mean in green) during two example of deep water renewals in the Bunnefjord, during 2005-2007 and 2010-2011.

The temporal resolution of historical salinity record is not good enough to study the exact time of the events. However, we still get some signals of deepwater renewals. Salinities of the deepwater at three locations, Ep1 in the Bunnefjord, Dk1 in the Vestfjord, and Im2 outside of the Drøbak sill (see Figure 2.1) were used to investigate the relationship between deep water renewal events and the

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20

CHAPTER 2. GENERAL HYDROGRAPHY AND DEEPWATER EXCHANGES FROM A 46-YEAR HISTORICAL DATASET availability of dense source waters outside the fjord. Unfortunately, the data at Im2 that was used as the indication of dense water in the outer Oslofjord is only available from 1973 to 2004. Time series of salinity of deepwater at the three stations are shown in Figure 2.6. The results show that several renewal events were observed as sudden increases of salinity (the white shaded area in the figure). As it turns out, the large salinity jump at Ep1 (Bunnefjord) are related to the increasing of salinity in the same period at the Vestfjord and outside the Drøbak sill. The strongest renewal was observed in 1996 when the largest increase of salinity were observed in the Bunnefjord corresponding with the highest salinity in the Vestfjord and outside the Drøbak sill. This supports the notion that deep water renewals in the Bunnefjord are intimately tied to the availability of dense waters in the outer fjord.

Figure 2.9: Climatology mean of wind components, north-south winds (V-winds) in blue, and east-west (U-winds) winds in orange at the Blindern station during 1973-2018. The thick black lines represent the period of renewals with year of occurrences

Historical wind data at Blindern station provided by the Norwegian Meteorological Institute (MET Norway) can be accessed from the Frost API (https://frost.met.no). In this study, we estimate wind components, i.e. east-west and north-south winds, from hourly wind speed and direction over the same period of the historical hydrological data from 1973-2018. The hourly wind data is very noisy, therefore, estimation of daily mean, weekly mean, and monthly mean of wind components were estimated in order to get smoother signals.

Figure 2.8 illustrates the salinity below 80 m at Ep1 together with wind components during the periods of deepwater renewal in 2005-2007 and 2010-2011. The results show that northerly winds (negative north-south component) play a role in deepwater renewals in the Bunnefjord. This is consistent with Gade’s hypothesis. The relation of northerly winds and deepwater renewals in the Bunnefjord also agrees on the another eight renewal events (shown in Figure A.1, A.2, and A.3). The climatological seasonal cycle of winds and seasonal timing of observed renewals are shown in Figure 2.9.

The results show that renewals usually occur during January-May when climatology of north-south wind component tends to be negative (northerly winds). In addition, it is possible that the deepwater is replaced in late winter since the offshore waters are then cooled and dense. However, the data shows

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2.4. SUMMARY 21 a deepwater renewal in 1974 that occurred during May-July. This may be a cause of low temporal resolution of the data, particularly in this year.

2.4 Summary

This chapter has focused on investigation of hydrography and deep water exchanges between the Bunnefjord and the Vestfjord using historical data from 1973-2018. The study of deepwater exchanges covers both i) stagnant periods when the basin water is slowly made lighter by diffusion and ii) the renewal events.

High variabilities of temperature and salinity are observed, particularly in the surface layer which shows a strong seasonal signal. High temperature in the surface layer is observed in summer, and low temperature is observed in winter. The summer heat diffuses to the deeper water and takes time resulting in the relatively high temperature at depth in winter. So eddy diffusion plays role, particularly in the Vestfjord where the turbulent diffusivity is large due to the breaking of internal waves in the basin. The low salinity in summer and high salinity in winter can be related to seasonal fresh water discharge.

Vertical eddy diffusivity was estimated from salinity time series in the Bunnefjord and the Vestfjord. The mean vertical diffusivity over 10 periods in the Vestfjord was higher than in the Bunnefjord, likely due to stronger internal wave breaking in the Vestfjord. The results suggest that rate of diffusivity varies with time and depth. The vertical density gradient (stratification) could affect the rate of diffusion in the basin. Homogenized salinity in deepwater and surface mixed-layer are consistent with the higher diffusivity than in the pycnocline region.(Gade, 1970 and Gargett, 1984).

Study of deepwater renewal events suggests that dense waters outside the inner fjord are required for deepwater replacement. Our results also agree with the Gade’s theory of northerly winds inducing the renewals. Persistent northerly winds force the outflow of surface water, then the inflow of dense water is induced. The deepwater renewals tend to occur in January-May. In this period, cold and dense water is formed, and the occurrences of northerly winds are usually observed.

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Chapter 3

Exchanges at the Vestfjord-Bunnefjord sill

In this chapter, the short-term variability of water exchanges in the inner Oslofjord will be investigated using both observations and simulations of a high-resolution model. The observations were conducted during 7 August-11 December 2020. Both hydrographical data (including temperature and salinity) and current measurements were analyzed in order to study the exchanges between the Bunnefjord and the Vestfjord. Influences of river discharge, tides, atmospheric winds, and large-scale sea level gradients on the exchange flows at the sill were investigated. In addition, the simulations were compared to the observations in order to evaluate the model performance. Finally, the model was used to help interpret the observations.

3.1 Observations

Observations were collected from the F/F Trygve Braarud research vessel during 7 August – 11 December 2020. Two Acoustic Doppler Current Profilers (ADCPs) were deployed on the first field trip on 7 August 2020 in the sill region (station R2 and R3, see Figure 3.1). R3 is located near the Nesoddtangen, whereas R2 is at the middle between the Nesoddtangen and the Bygdøy peninsula.

Figure 3.2 illustrates the set-up of the ADCPs. Each ADCP was weighted to fix the location and depth. A Lightweight Release Transponder (LTR) contained rope inside and buoys were attached to the ADCPs that support the recovery process. The LTR releases the rope after it gets a signal, then the buoys float to the surface and are ready for the recovery. A high frequency ADCP, 1 MHz Aquadopp, was deployed at R3. This ADCP’s sensor was at 1.55 m above the seabed and has a blind zone about 0.4 m from the sensors. This ADCP measured average velocity every 0.5 m with 37 bins.

Therefore, it can measure the currents from the surface down to 20 m, approximately. A 400 kHz Aquadopp was deployed at R2. The sensor was at 1.90 m from the bottom and has a 1 m blind zone.

The bin size is 2.5 m and the instrument measured in 17 bins. The measured currents at R2 were hence available from 6 – 43 m, approximately. Both ADCPs were set up to collect data every 10 minutes.

During the observation period, ADCP batteries were changed on 27 October, therefore, the data is unavailable for a few hours on this day.

23

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24 CHAPTER 3. EXCHANGES AT THE VESTFJORD-BUNNEFJORD SILL Temperature and salinity were also measured using CTD at three standard stations, one in the Bunnefjord, Cp2, one in the Vestfjord, Bn1, and one at the sill, R2 (see Figure 3.1). The temperature and salinity were measured on the date of deployment (7 August), during student field work for the course GEO2320 on 28 September, when batteries were changed (27 October), and at recovery (11 December).

Figure 3.1: Locations of ADCPs deployment at R2 and R3, and the stations of CTD measurement, in the Bunnefjord, Cp2, in the Vestfjord, Bn1, and at the sill, R2. A black line across the Bygdøy and the Nesoddtangen is a transect for studying vertical exchanges from the model in the section 3.9

3.2 The high-resolution model, the FjordOS

A high-resolution model for the Oslofjord was developed by the Norwegian Meteorological Institute (MET), called “FjordOs”. The model is based on the Rutgers Regional Ocean Modeling System (ROMS) version 3.6. It applies curvilinear grid with 300x 900 grid points and the resolution varies throughout the domain. In the inner Oslofjord the horizontal resolution is about 50 m. An Arakawa C-grid is introduced to the horizontal model state variables. The vertical gird uses a stretched terrain-following coordinate (σ-coordinate), using 42 layers with increased resolution in the surface

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3.3. TEMPERATURE AND SALINITY VARIABILITY 25

Figure 3.2: Deployment of ADCPs at the sill.

layer and reduced resolution near the bottom.

The FjordOs model is forced by atmosphere and ocean models at its boundaries. Specifically, the atmospheric data is extracted form MET Norway’s operational NWP model (AROME-MetCoOp) and oceanic input is extracted form MET Norway’s operational ocean forecasting model NorKyst800 (Albretsen et al., 2011). In addition, the model uses daily climatology of river discharge during 2013-2017. The FjordOs model provides both operational runs and hindcast runs. However, in this thesis, only the hindcast simulations will be evaluated because it uses updated river inputs and bottom topography. The operational run also suffered from some missing dates during the observation period.

3.3 Temperature and salinity variability

The observational data (as well as calculated potential density) are shown in figure 3.3, together with the model fields from the same dates. Seasonal changes of temperature and salinity were observed, particularly at depth 0-10 m. The surface layer was relatively warm in summer, then decreased through the autumn and lowest in winter. In summer (7 August), at all stations, surface temperature above 10 m decreased slowly with depth. Here, surface wind may be a dominant mechanism for moderate mixing of the surface water. A thermocline appeared at about 10-25 m. Below the thermocline, temperature tended to be uniform. In early autumn (29 September) surface temperature was lower than in summer.

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26 CHAPTER 3. EXCHANGES AT THE VESTFJORD-BUNNEFJORD SILL

Figure 3.3: Temperature (blue), salinity (red) and density (black) profiles at station Bn1 (top), R2 (middle) and Cp2 (bottom) on 7 August (ADCP deployment), 29 September (student cruise), 27 October (battery change), and 11 December (recovery) in 2020. Solid lines represent observations and the dotted lines are the model simulations.

Wind-driven and convective surface mixing result in almost constant temperature in the surface layer (0-10 m). The thermocline on 29 September was weaker than in summer at depths around 10-20 m. During the late autumn (27 October), temperature tended to be cooler at the surface resulting in increase of temperature with depth, a reverse thermocline, in the surface layer with the highest temperature at 10 m, approximately. Decrease of temperature with depth appeared in the mid-depth layer at approximately 10-20 m, with an almost constant temperature below 20 m. The thin layer of reverse thermocline also appeared in wintertime (11 December). The temperature below 10 m

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3.4. THE FLOW FIELD AT THE SILL 27 rarely changed in this season. However, the second highest temperature was observed approximately at 35 m in all stations. The reverse thermocline in late autumn and in winter is likely a response to cooling at the surface due to the low air temperature. The model shows the similar temperature distributions as from the observations. The depth of maximum temperature at 10 m is also presented in the model results, particularly in late autumn but not in winter. The simulations seem to lose detail of temperature profiles at depth, for instance, high temperatures at 10 and 40 m.

In fjords, density mainly depends on salinity rather than temperature, hence, the density profiles share similar patterns with salinity. Seasonal changes of salinity (and density) were clearly seen, particularly at depth 0-10 m. In summer river discharge was relatively high, resulting in low salinity at the surface. The surface-mixed layer was about 10 m at all stations in both summer and early autumn.

The halocline and, hence pycnocline, were located at the same depth as the thermocline (about 10-20 m). The salinity below the halocline slowly increased with depth, and was constant below 45 m. In late autumn, at Bn1, the surface salinity was comparable to that in summer, while the surface salinity at R2 and Cp2 were higher than in summer and early autumn. The halocline was located at the surface to about 10 m. Below the halocline, salinity slowly increased with depth. In winter, a thin layer of constant salinity appeared only few meters from the surface at station Bn1 and Cp2. The halocline was detected 2-10 m at all stations.

Simulated temperature and salinity profiles at all stations agree fairly well with the observations.

There are some differences in magnitude of temperature and salinity. The model generally underestimated temperature and overestimated in salinity. But the signals of surface-mixed layer in late summer and early autumn did not appear in the simulations.

From temperature, salinity, and density profiles, the water column can roughly be divided into three layers: a surface-mixed layer at 0-10 m where influence of winds and river discharge can reach, a mid-depth layer that contain the pycnocline 10-25 m, and a bottom layer below 25 m. This classification will be applied to identify water layers in order to investigate water exchanges in the fjord.

3.4 The flow field at the sill

The time-averaged velocity during the observation period at different depths (see Figure 3.4) showed outflows at R3 and inflows at R2 in entire water column. At R3, strongest outflows were observed at the surface, then decreased with depth and increased again. The outflows were weakest at depth around 10 m, near the boundary between the surface and mid-depth layers. At R2 there were two weak flows at about 10 m and 25 m. These were consistent with the water layers that were defined by the temperature and salinity profiles in section 3.3. The simulations were largely consistent with the observations, there were inflows and outflows at R2 and R3, respectively. But there were differences in the details. The minimum velocity at 10 m was not observed in the simulations. At R3 the minimum currents shifted up to 5 m, approximately, and at R2 it did not show up at all. But the

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28 CHAPTER 3. EXCHANGES AT THE VESTFJORD-BUNNEFJORD SILL minimum at 25 m depth at R2 can be seen in the model results. In fact, the model suggested a weak outflow around this depth.

Figure 3.4: Time-averaged velocity at R2 and R3, 7 August - 11 December 2020, from observations (left) and hindcast simulations (right). The vectors show the direction of the flow at difference depths.

The horizontal arrows are east-west flows.

Figure 3.5: Time-averaged velocity during 7 August - 11 December 2020. The color vectors are the average from the observations in three layers, surface in blue, mid-depth in green, and bottom in yellow, and the variance ellipses at R2 and R3. The average velocity from simulations are plotted as black vectors. The dash lines are the line transects at the sill.

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3.4. THE FLOW FIELD AT THE SILL 29 The time-averaged flow in the three different layers, surface, mid-depth, and bottom layers are plotted in Figure 3.5 together with variance ellipses. The variance ellipses are based on Principal Component Analysis (PCA). The PCA is a statistical procedure to explain covariance structure of data and identify the principal directions. The first and second principle components, PCA1 and PCA2, are major and minor axes of an ellipse. The size of the ellipse indicates the strength of the variability of the data and the orientation points out the principal direction.

Figure 3.6: The 1 hour-averaged velocity components, u-eastward and v-northward, at R2 and R3 during the observation period, 7 August – 11 December 2020. The observations are on the left and hindcast simulations are on the right(right column)

At R3 near the Nesoddtangen, variability of flows seemed to follow the coastline, as the variance ellipses aligned with the coast. R2 is located between the Nesoddtangen and Bygdøy peninsula so the influence of coasts on the flows may be less significant. The variance were greatest in the surface layer (large ellipses), then decrease with depth. The time-averaged flow fields from the model simulations are also shown in the Figure 3.5. The results confirmed that the model performed well as it corresponded to the observations. The model gave us a chance to compare the observations from R2 and R3 with surrounding currents. The model showed that the inflows at R2 and outflows at R3 may not necessarily reflect currents that reach deep into the respective basins. Instead, they may be a part of recirculations in the Bunnefjord. The recirculation cells appeared in the surface layer at both sides of the Nesoddtangen. Hence, the water in the Bunnefjord may not flow to the Vestfjord due to these recirculations. More detail of the recirculations will be discussed later in section 3.9. In contrast, the model gave clear indication that the bottom flow did not recirculate locally but tends to follow the bottom topography along the basin boundaries.

The 1-hour averaged velocity components from observations are shown in Figure 3.6. The results show high variability in both east-west (u) and north-south (v) velocity components, particularly above 10 m, approximately, at both stations. The result is consistent with the observed temperature and salinity which indicated a surface-mixed layer that is about 10 m. It is not as easy to differentiate the

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30 CHAPTER 3. EXCHANGES AT THE VESTFJORD-BUNNEFJORD SILL

Figure 3.7: Velocity rose plots at R2 and R3 from observations and hindcast simulations

mid-depth layer and bottom layer at R2. At R3, located near the coast at Nesoddtangen, relatively strong currents were observed below 10 m which may be an effect of topographic steering. There were significant signals at both stations during 24-25 September that had strong inflows (positive u-eastward component) in the entire water column followed by strong outflows at R3 in the surface layer. This phenomenon was related to atmospheric forcing which will be described in section 3.7. The model simulations and observations are well correlated. For example, the model simulated the strong flows in the surface layer. However, the simulations indicated strong inflows below 30 m at R2 which were less pronounced in the observations.

Variability of flow strength and directions at R2 and R3 in the three layers, surface, mid-depth, and bottom layers are illustrated as rose plots in Figure 3.7. Both observations and hindcast simulations

A study of water exchanges between the Bunnefjord and the Vestfjord, inner Oslofjord