2.1.1 The Hadley Cell
The Hadley cell is named after George Hadley (1685-1768), who was an English meteorol-ogist (Wallace and Hobbs, 2006). When seeking for the origin of the trade winds, Hadley realized that they must be caused by the uneven distribution of solar insolation between the equator and the poles. He visualized one great thermally directly driven cell on each hemisphere. Heated air is convected over the equator and is transported towards the poles where it cools, sinks, and flows back towards equator (Holton, 2004, pp. 314–316).
Although Hadley’s idea of one major cell in each hemisphere did not prevail, his concepts of that differences in heating give rise to persistent large-scale atmospheric overturning circulation and of that zonal winds can be attributed deflection of meridional winds have (Aguado and Burt, 2010, pp. 201–202). A more realistic model is the three-cell model dividing the circulation of each hemisphere into three major transport cells, namely; the Hadley cell which circulates air between the tropics and sub-tropics, the mid-latitudinal Ferrel cell and the Polar cell (Aguado and Burt, 2010, p. 203).
The Hadley cell is a thermally direct cell, and it can be described as following: Con-vergence of warm and moist air near the equator, results in major convection, heavy precipitation, and release of latent heat. This zone of rising air, varies gradually over the seasons, and is called the Intertropical Convergence Zone (ITCZ). The rising air reaches the tropical tropopause layer (TTL), and is then forced poleward. The rotation of the earth leads to an eastward deflection of the winds, and the resulting subtropical jet streams. At about 30◦N/S, the winds start to subside. The now cool and dry air warms adiabatically, while sinking and traps the underlying moist and cold maritime air below. This results in an inversion layer called the trade wind inversion. The air then moves back towards the equator, absorbs moisture on its way, and completes the loop. Again because of the Coriolis effect, the winds are deflected westward, creating the southeasterly trade winds (Aguado and Burt, 2010, pp. 203–206). In Figure 2.1 a conceptual model of the Hadley cell is depicted.
FIGURE 2.1: Conceptual model of the Hadley Cells, together with the tropical tropopause layer (TTL) and the Inter-tropical convergence zone
(ITCZ). Adapted from Fiehn (2017, p. 6).
FIGURE2.2: Generalized Walker Circulation during ENSO Neutral condi-tions. NOAA Climate.gov drawing by Fiona Martin (Liberto, 2014)
2.1.2 The Walker Circulation
Due to the southeasterly trade winds over the equatorial Pacific, the surface water is pushed westward. This results in warm water piling up at the west Pacific, and a con-tinuous upwelling of cold water at the eastern boundary. The warm surface water in the west, heats the air above, leading to low surface pressure and a major convection center over the western Pacific. The rising air travels east over the ocean, resulting in high sur-face pressure over the eastern Pacific where dry air subsides. This east-west circulation of air over the equatorial Pacific ocean, is called the Walker Circulation (WC), and it is named after G. T. Walker who was the first to document the surface pressure pattern as-sociated with it (Holton, 2004, pp. 382–382). The WC is dependent on El Niño–Southern Oscillation (ENSO) conditions. The above description is valid for the ENSO neutral con-dition (see Figure 2.2).
FIGURE2.3: Conceptual model of a La Niña event, with the generalized Walker Circulation over a map of anomalous sea surface temperatures.
Blue indicates anomalous ocean cooling, and orange anomalous ocean warming. NOAA Climate.gov drawing by Fiona Martin (Liberto, 2014)
FIGURE2.4: Conceptual model of an El Niño event. NOAA Climate.gov drawing by Fiona Martin (Liberto, 2014)
2.1.3 The El Niño Southern Oscillation
ENSO describes an ocean atmospheric coupled oscillation, where EN stands for El Niño which is a recurrent pattern of positive temperature anomalies in the equatorial Pacific sea surface, and SO is the Southern Oscillation, an interannual fluctuation of atmospheric pressure between Darwin and Tahiti. ENSO is an irregularly interannual variation of the surface temperatures and winds over the Pacific Ocean, affecting the global climate (Wal-lace and Hobbs, 2006, pp. 431-438). ENSO has a warm phase and a cold phase, which are called El Niño and La Niña respectively, when the surface temperature anomaly is above 5◦ for several months in a row. During La Niña, the Walker Circulation is strengthened.
Even more warm surface water piles up in the west, leading to enhanced convection over the West Pacific, stronger subsidence of air over the East Pacific, intensified upwelling in the coastal East Pacific, and large Darwin-Tahiti pressure differences (Figure 2.3). While during an El Niño event (Figure 2.4), the Pacific WC is weakened, or even reversed, and the Darwin-Tahiti pressure difference is small. The convection center over the West Pa-cific propagates eastward, leading to less subsidence of air over the East PaPa-cific, or even
FIGURE 2.5: Schematic of tropical deep convection, with the convective boundary layer (CBL), and the tropical tropopause level (TTL). The level of the cold point tropopause (CPT) and the level of zero radiative heat-ing (LZRH) is shown. Pink lines indicate typical tracer routes, red arrows mass redistribution, and a typical temperature profile is shown in green
(Carpenter et al., 2014a, p. 1.34).
convection i.e. precipitation over the Atacama desert (Holton, 2004, pp. 384–385). Cur-rently, many classification methods are used for identifying the type of an El Niño event.
One such method is to analyze the type according to where the surface ocean tempera-ture anomalies are situated (Pascolini-Campbell et al., 2015). There are two types of El Niño; the Eastern Pacific El Niño and the Central Pacific El Niño.
2.1.4 Troposphere-to-Stratosphere Transport in the Tropics
Over the past decades, it has become clear that the transition from the troposphere to the stratosphere is gradually rather than abrupt. In the tropics, a transitional regime between the troposphere and stratosphere extends over several kilometers. This transi-tional regime in the tropics has been named the Tropical Tropopause Layer (TTL). The TTL spans the distance between the connectively dominated overturning circulation of the Hadley cell, to the slow upwelling region of the lower stratospheric Brewer-Dobson circulation (Fueglistaler et al., 2009). Air enters primarily the stratosphere in the Trop-ics. Thus the TTL acts as a gateway for atmospheric tracers to the stratosphere (Holton et al., 1995), such as for the very short-lived halogenated substances (VSLS). The base of the TTL is defined as the height of the temperature lapse rate minimum. The cold point tropopause (CPT) is used to define the top, and , the level of zero radiative heat-ing (LZRH) is in the middle (Carpenter et al., 2014a) (Figure 2.5).
Transport and chemical processes in the TTL are important for the VSLS source gas injection (SGI) and product gas injection (PGI) (Carpenter et al., 2014a). Significant trans-port to the stratosphere of particularly short-lived substances, with atmospheric lifetimes of several days or less, is unlikely unless emitted close to deep convection (Hossaini et al., 2012). Very deep overshooting convection may transport air masses directly through the TTL, allowing also the shortest-lived VSLS to reach the stratosphere, although these
events are relatively rare (Carpenter et al., 2014a). See Figure 2.5 for an illustration of deep convection and overshooting convection, together with the TTL.
Trajectory calculations from several studies found that troposphere-to-stratosphere transport (TST) trajectories mostly entered the TTL over the West Pacific (Bonazzola and Haynes, 2004; Fueglistaler and Haynes, 2005; Fueglistaler et al., 2004; Hatsushika and Yamazaki, 2003). The tropical West Pacific is under neutral ENSO conditions, a region with major vertical transport of air, due to the rising branch of the WC. The largest mod-ifications of the TST occur due to major ENSO events (Fueglistaler et al., 2004). Krüger et al. (2008) found that the TTL becomes colder and drier during La Niña over the western Pacific, and warmer and less dry during El Niño.