The surface on or near the equator (dependent on solar zenith angle) receives the maximum intensity of solar radiation and therefore heats up the air above it and the air rises to higher altitude, leaving a low pressure zone near the surface. The relative high pressure in the tropics north and south of the minimum SZA drives the air southward and northward respectively to equalise the pressure. The converging air experiences Coriolis force as it travels in the meridional direction and deflects towards the west at the surface pressure level in both northern and southern hemi- spheres. Hence, the air from the north and south in turn forms northeasterly and
upward. This process circulates in the tropics and forms Hadley cells (see Fig.2.1) on both sides of the equator.
Figure 2.1.: Diagram of the Hadley cells and the corresponding wind direction, Im- age from NASA Earth Observatory
The area where the trade winds converge is called the Intertropical Convergence Zone (ITCZ). This is where the moist air rises and the water vapour within the air parcel condenses as the air cools during the uplift, usually resulting in cloud formation and subsequent precipitation. Over land, the position of the ITCZ changes according to solar zenith angle of the hemispheres due to variations of the latitude where surface receives maximum solar radiation. However, the seasonal positions of the ITCZ are not perfectly symmetric due to surface type and the orography
variations. This seasonal perturbation in the position of the ITCZ determines wet and dry seasons for many tropical regions, caused by the air moisture, clouds and precipitation associated with the convergence.
Asymmetry and seasonal variation of the ITCZ
Although the ITCZ is heavily influenced by the seasonal variation of the SZA over land, the ocean counterpart remains in the northern hemisphere all year round.
Fig. 2.2(a) and 2.2(b) show the rainfall maps for January and June 2006 respec- tively. The data used in the plots are from the Nasa’s TRMM mission. The ITCZ is one of the main sources of precipitation in the tropics due to the convective activ- ities and cloud associated with the convergence. Therefore, precipitation is a good proxy for the location of the ITCZ over the season. In Fig. 2.2, it can be observed that the rainfall band over land generally shifts towards the north of the equator in June and south in January. This is due to the shift in the maximum ISR associated with the minimum SZA during the months. However, the rainfall band over the ocean presents a different picture, the ITCZ remains north of the equator during both boreal winter and summer.
This has been investigated by Philander et al. (1996), where the asymmetries in the seasonal ITCZ are particularly prominenet in the eastern Pacific and Atlantic Oceans. It was suggested in the paper that this is the effect of: 1) the interactions between the ocean and atmosphere which turns symmetry into asymmetry, 2) the geometries of the continents that determines the effectiveness of the atmosphere and ocean interaction.
The first mechanism involves physical instabilities and feedback mechanisms which magnifies any small asymmetric perturbations into a sustainable asymmetry that we observe in the atmosphere. One such mechanism proposed by [Pike, 1970], where the cold upwelling of the ocean at the equator prevents the tropical convergence from centering directly above the cold water, as it suppresses the uplift of the air. [Charney, 1971] have further propose the asymmetry of the ITCZ is dependent at a feedback mechanism between the atmosphere and ocean caused by the moist deep convection and the convergence of air and moisture feeding the convection. The convection itself creates a low pressure on the surface and thus maintains the wind
(a) Total rainfall map over January 2006
(b) Total rainfall map over June 2006
Figure 2.2.: Total rainfall maps over months January and June 2006 [NASA, Earth Observatory website] (TRMM rainfall data) grey indicate no data present.
also prompts the downwelling of ocean water beneath the wind convergence and consequently, upwelling cold water in the south. This creates a warmer pool of water directly underneath the ITCZ relative to the colder sea surface in the other hemisphere and further maintains this asymmetry. This process, however, does not
impose any preferences on the ITCZ over the northern or southern hemisphere, it merely provides a feedback mechanism to maintain the asymmetry itself. To explain the origin of the asymmetry we consider the arguments related to the geometries of the continents.
The second mechanism considers the asymmetries of the continents in the south- ern and northern hemisphere and how the asymmetry of the seasonal movement of the ITCZ arises from those. In the African continent and Atlantic Ocean, the ITCZ has demonstrated a good example of the asymmetry, where the land ITCZ shifts according to the season, the ocean ITCZ, however, remains north of the equator most of the year.
In this case, the horizontal temperature gradient between the Gulf of Guinea and the land in the north is large. Therefore, creating a surface pressure difference between the two regions, and consequently, the surface air moves northward to balance the pressure. The meridional winds are cross equatorial [Philander et al., 1996], thus the wind in itself is already creating an asymmetry about the equator. This is shown in fig. 2.3 c where the meridional wind remains northward over the Atlantic ocean, north of the equator, up to around 5◦N. The cross equatorial meridional wind itself causes the asymmetry in two ways: first of which is forming a convergence north of the equator, thus promoting convective activities; Secondly, the wind causes Ekman transport which induces upwelling of cold water near the coast of southern Africa and the warm surface water is transported to the north of the equator, whereas, in the south, the coastal region is replaced by cold water. This will intensify the winds and amplify the asymmetries about the equator.
This asymmetry forms a convergence north of the equator and is observable via the annual averaged rainfall in fig. 2.3a where most of the precipitation over the Atlantic ocean is concentrated in the northern hemisphere.
In fig. 2.3a and 2.2 , a northern ITCZ bias appears over the tropical Pacific just as in the Atlantic. However, the asymmetries of the Pacific appear to be more complicated as there aren’t any major equatorial geometry asymmetry equivalent to that of the Atlantic. [Philander et al., 1996] have used an atmospheric GCM to investigate this and they have found the local coastal geometry, specifically the inclination of the coast to meridians on the west coast of Americas can contribute the asymmetry. The inclination causes the trade wind to be parallel to the coastline
Figure 2.3.: Time-averaged distributions of (a) rainfall estimated by Spencer (1993) using measurements recorded by the Microwave Sounding Unit on NOAA polar orbiting satellites; the contour interval is 5cm/month (b) Sea surface temperatures from Reynolds and Smith (1994); The contour interval is 1◦C (c) Meridional wind components at a height of 10 m from (Halpern et al. 1993) [figure taken from [Philander et al., 1996]]
in the southern hemisphere and perpendicular in the north. Therefore, the southern coast is expected to colder due to the Ekman transport which induces the upwelling of cold water in the south whilst driving the warm surface water towards the north. This asymmetry in temperature across the equator causes meridional wind and the asymmetry is intensified and maintained via the same mechanism mentioned above.