The meteorological setting at the time and place of the two cruises ASTRA-OMZ and M91, is presented in this section. The cruises took place along the west coast of South America, during October 2015 (ASTRA-OMZ) and during December 2012 (M91). More information about ASTRA-OMZ is given in Chapter 3, and the M91 cruise is described in more detail in Fuhlbrügge et al. (2016a) and Hepach et al. (2016).
The ASTRA-OMZ cruise in took place during the development of a very strong El Nino. The development of the 2015/16 El Niño was anticipated by researchers at the beginning of 2015 (Hu and Fedorov, 2017). The year before, in 2014, the same projection was made, but no El Niño developed. This time, however, researchers were right. An extreme El Niño event developed in 2015/16 (Wang and Hendon, 2017). According to Stramma et al. (2016), the 2015 El Niño was a clear EP El Niño in October 2015. However, the 2015/2016 event became dominated by the CP El Niño dynamics after October 2015, as reported by Paek et al. (2017). The El Niño started early in 2015, but the shift to El Niño water mass distribution in the equatorial East Pacific (EP) was surprisingly slow (Stramma et al., 2016). In October 2015 the El Niño signal was found to be strongest at the equator, along the ASTRA-OMZ cruise track (Stramma et al., 2016). Large positive temperature anomalies over the equatorial EP in October 2015, can be seen in Figure 2.7a, highlighting the ongoing El Niño event (CDB, October 2015). At the time of the M91 cruise in December 2012, there was an ENSO neutral state, as seen in Figure 2.7b (CDB, December 2012). The negative outgoing longwave radiation (OLR) anomaly along the Peruvian coast indicates enhancement of oceanic upwelling (Figure 2.7b).
A discrepancy in the convective situation over the tropical Pacific between ASTRA-OMZ and M91 is expected because of the different ENSO states. A measure of deep con-vection is the anomaly of OLR. Negative OLR anomalies indicates enhanced concon-vection, more cloud coverage, and higher and colder cloud tops which emits less infrared radia-tion into space. The opposite occurs for positive OLR anomalies. As expected, stronger convection in the tropical EP prevail during October 2015 than in December 2012 (Figure 2.8). There are no significant anomalies in the OLR in December 2012, which is typical for an ENSO neutral year, although there is some enhanced convection over the Andes Mountains.
The evaluation of the convective situation during El Niño 2015 and ENSO neutral 2012, is of interest since this thesis investigating the transport of VSLS to the strato-sphere. Hence, a time-longitude section of the anomalous equatorial OLR for 2015/16 and 2012/13 is shown in Figure 2.9. The shift of the convection centre from the West Pacific to the East and Central Pacific can be seen in Figure 2.9a.
Under El Niño conditions, more accentuated tropical convection has previously been found to drive a stronger and narrower Hadley Cell (HC) (Chang, 1995; Seager et al., 2003). The width of the Hadley Cell (HC) is limited by the subtropical jet. Hence, the 200 hPa winds are presented for October 2015 and December 2012 in Figure 2.10. The subtropical jets were situated farther north over the EP in October 2015 than in November 2012 (Figure 2.10). The position and stenght of the subtropical jets, from November to December in 2012 and 2015 appeared similar, although the southern subtropical jet was slightly enhanced for 2015 Center (CDB).
In Figure 2.11, radiosonde measurements of relative humidity (RH) from the two cruises ASTRA-OMZ and M91 are shown. It should be noted that two different types of radiosondes were used; ASTRA-OMZ used Graw, and M91 used Vaisala sondes. The av-erage CPT is located at about 17 km for both cruises (Alina Fiehn and Steffen Fuhlbrügge, personal communication, Nov 2017).
(A)
(B)
FIGURE2.7: Monthly sea surface temperature and temperature anomaly average for (A) ASTRA-OMZ , (B) M91 (CDB, Oct 2015 and Dec 2012).
A pronounced humid layer at around 1 km is found for both cruises (Figure 2.12), which is characteristic of the trade inversion layer. The trade inversion layer was higher for ASTRA-OMZ, especially when the cruise crossed the equator (Figure 2.12). In the free troposphere, less humidity was detected during ASTRA-OMZ than during the M91 cruise. This may indicate weaker vertical transport of humid air masses from the MABL
(A)
(B)
FIGURE2.8: Monthly outgoing longwave radiation and radiation anomaly average for (A) Oct 2015, ASTRA-OMZ , (B) Dec 2012, M91 (CDB, Oct 2015
and Dec 2012).
into the free troposphere during ASTRA-OMZ than during M91. It may also be due to
(A) (B)
FIGURE2.9: Anomalous outgoing longwave radiation averaged between 5N-5S for (A) 2015/16 and (B) 2012/13 (CDB, Mar 2016 and Mar 2013).
radiosonde differences (Kirstin Krüger, personal communication, Nov 2017). The mean height of the MABL was 307 m for M91 (Fuhlbrügge et al., 2016a) and 470 m for ASTRA-OMZ (Alina Fiehn, personal communication, Nov 2017).
(A)
(B)
FIGURE2.10: Monthly wind speed and wind speed anomaly average for (A) Oct 2015, ASTRA-OMZ , (B) Dec 2012, M91 (CDB, Oct 2015 and Dec
2012).
FIGURE 2.11: Relative humidity in the troposphere and lower strato-sphere, for the M91 cruise (left) and ASTRA-OMZ (right), measured with Vaisala and Graw radiosondes respectively. CPT = cold point tropopause, LRT = lapse-rate tropopause (Alina Fiehn and Steffen Fuhlbrügge,
per-sonal communication, Nov 2017).
FIGURE 2.12: Relative humidity in the lower troposphere, for the M91 cruise (left) and ASTRA-OMZ (right) (Alina Fiehn and Steffen Fuhlbrügge,
personal communication, Nov 2017).
Chapter 3
Data and Methods
3.1 The ASTRA-OMZ Cruise
Data from the two cruises ASTRA-OMZ and M91 is used in this thesis. The ASTRA-OMZ cruise was conducted on the R/VSONNE(SO243; 5 to 22 October 2015) from Guayaquil in Ecuador to Antofagasta in Chile, and the M91 cruise was carried on R/VMETEOR(1 to 26 December 2012) starting and ending in Lima, Peru. Descriptions of the meteorological observations, and the oceanic and atmospheric halocarbons, for ASTRA-OMZ, are given in the subsequent sections. The M91 cruise have been described earlier by Fuhlbrügge et al. (2016a) and Hepach et al. (2016).
3.1.1 Meteorological Observations
Meteorological observations of sea surface temperature (SST), surface air temperature (SAT), wind speed, wind direction, relative humidity, and air pressure were done every second. The wind were measured at about 30 m height above sea level. The data were averaged to 10 min intervals. GRAW DFM-09 radiosondes were regularly launched every six hours, with a total of 64 launches (Alina Fiehn, personal communication, Nov. 2017 and Marandino, 2016).
3.1.2 Surface Ocean and Atmospheric Halocarbon Measurements
Both surface oceanic and atmospheric halocarbon measurements were collected every 3 hours. Water samples were taken at a depth of about 5 m from a continuously working water pump in the hydrographic shaft of the ship. The water samples were then analyzed for halogenated trace gases with a gas chromatographer attached to a mass spectrome-ter (GC/MS) onboard the ship (Alina Fiehn, personal communication, Nov. 2017 and Marandino, 2016). The precision of the analysis is of 10% (1α). More detailed description of the measurements is given by Hepach et al. (2014).
The atmospheric air samples were taken at about 10 m height above sea level at the bow of the ship, using a jib of 4 meters. The samples were collected in stainless steel canisters, pressurized to 2 atm, and later analyzed at the Rosenstiel School for Marine and Atmospheric Sciences, University of Miami (Alina Fiehn, personal communication, Nov.
2017 and Marandino, 2016). Details about the atmospheric very short-lived halogenated substances (VSLS) samplings can be read in Fuhlbrügge et al. (2013).
3.1.3 Halocarbon Emissions
For calculating sea-to-air VSLS emission, the level of halocarbon saturation in the ocean surface layer is taken and converted to a flux by multiplying with an average transfer ve-locity (kw) (Moore et al., 1995a). The level of saturation is given as the difference between
the actual water concentration (cw) and the water concentration at which the concen-tration is at equilibrium with the above air. The theoretical equilibrium concenconcen-tration is given as catmH , wherecatm is the atmospheric concentration of the halocarbon, and His Henry’s law constant. Henry’s law constant is defined as the concentration in air devided by the equilibrium water concentration (Moore et al., 1995a).
F= kw·(cw−catm
H ) (3.1)
The transfer velocity coefficient by Nightingale et al. (2000) was used for the cruise VSLS emissions experiment.kwvaries with sea level pressure, sea surface temperature, sea sur-face salinity, and the wind speed at 10 m height. Air pressure and sea sursur-face temperature are taken from the ERA-Interim monthly means, and the sea surface salinity is taken from the World Ocean Atlas 2005. The 10 m wind speeds are parameterized from the observed wind speeds during ASTRA-OMZ and M91, using a logarithmic wind profile:
u10=u(z) κ
√CD κ
√CD+log(10z) (3.2)
whereκ = 0.41 is the von Kármán constant, CD is the neutral drag coefficient (Garratt, 1977), andz is the height of the observed wind speed u. For more information on the method of calculating the halocarbon emission flux see Hepach et al. (2014).