Indian Ocean Walker Cell in spring, which implies that their relationship could be affected by either multidecadal natural variability or anthropogenic forcing (Park et al. 2020).
2.4.4 The South American Monsoon
The South American Monsoon is part of the monsoon system of the Americas, encompassing most of the continent and featuring strong seasonal variability in a region lying between the Amazon and La Plata Basin (Silva Dias and Carvalho 2011). In the upper troposphere, the wet summer season is characterised by an anticyclonic circulation over Bolivia and a trough over the tropical and subtropical South Atlantic, near the coast of NE Brazil. There are several prominent low‐level features, including (i) surface high‐pressure systems and anticyclonic circulation over the subtropical Atlantic and Pacific Oceans; (ii) the Chaco thermal low centred over northern Argentina; (iii) the South Atlantic Convergence Zone (SACZ); and (iv) the South American Low‐Level Jet (SALLJ) east of the Andes, a low‐level NW flow east of the Andes that extends from the SW Amazon to southeast South America (Marengo et al. 2012).
Transient moisture flow from the Amazon is important for maintenance of the SACZ, and the location of the SACZ is influenced by the topography in central‐east Brazil, while it has a strong influence on the position and intensity of the Bolivian High. Climate variability associated with the SACZ and the SALLJ dipole‐like pattern has been observed on intra‐seasonal, inter‐seasonal, and decadal time scales. One phase of the dipole is characterised by an enhanced SACZ and suppressed convection to the south, whereas the other phase is characterised by a suppressed SACZ and increased convection in the subtropical plains. A strengthening of the SALLJ and associated transports of massive amounts of moisture from the Amazon Basin into the subtropics accompanies the latter phase.
The annual cycle of precipitation features distinct wet and dry seasons between the equator and 25°S. Rates of rainfall in the wet season are somewhat less than those in other monsoon systems but like those over the ocean. The annual cycle of rainfall in most pronounced in the southern Amazon, where some of the largest seasonal rainfall occurs, and extends to the southeast in the South Atlantic convergence zone, which is an extension of the wet season maximum at 10°S. Precipitation in this region is out of phase with that further south and is driven synoptically. The Madden‐Julian Oscillation (Section 2.6) increases the average daily precipitation by >30% and doubles the frequency of extreme events over central‐east South America, including the South Atlantic convergence zone (Grimm 2019).
ENSO plays an important role in the dynamics of the South American monsoon: during the warm phase of ENSO, precipitation decreases near the equator during the summer wet season and increases rainfall in SE South America. Large sub‐seasonal changes also occur within an ENSO cycle, but long‐term trends of precipitation are small in the Amazon Basin. However, downstream convergence of moisture advected from the Amazon Basin by a strong low‐level jet flowing southward along the eastern flank of the Andes results in some of the most frequent and large mesoscale systems on earth over the northern part of La Plata Basin, the second largest drainage basin in South America after the Amazon Basin. Those powerful systems contribute a larger proportion of total rainfall than any other region on earth.
The onset of the monsoon progresses towards the equator from an area in southern Brazil. Rainfall is at near minimum along the equator in December–February, while the wettest season occurs in April and May. At about 5°S, it begins to rain earlier than at 10°S, but rainfall stays below average until after the heavy rains begin farther south. Interannual variability in rainfall is related to variations in either the onset or ending of the wet season. More rainfall occurs when Atlantic SST anomalies are positive south of the equator, resulting in a southward displacement of the ITCZ. This may be a mechanism by which Atlantic SST influences monsoon rainfall, to the extent that these anomalies associated with a displaced ITCZ occur early and extend westward into the Amazon. Long‐term trends are not well understood due to lack of observation sites, but records of river discharge indicate alternating decadal trends. For example, the Amazon had low flows in the late 1960s, high flows in the mid‐1970s, and low flows again in the early 1980s. Such trends in rates of river discharge have significant impacts on marine processes.
2.5 Tropical Weather Systems
Earth averages about 85 revolving storms per year, with an average of 12 in the Atlantic, 17 in the NE Pacific, 26 in the NW Pacific, five in the north Indian, nine in the SW Indian, seven in the southeast Indian, and nine in the SW Pacific Ocean (Figure 2.8). These systems are known as hurricanes in the NE Pacific, North Atlantic and Caribbean, typhoons in the western Pacific, and cyclones in the Indian Ocean and SW Pacific.
Tropical cyclones form within regions of pre‐existing deep precipitating convection. Before cyclone formation, the convection is only loosely organised into mesoscale areas of enhanced cloudiness that are often called tropical disturbances. Only a small fraction of such disturbances become cyclones (Figure 2.8). Such storms initially form as a cloud mass to one side of the equator over the sea when temperatures are at or above 27 °C and occur far enough away from the equator for the Coriolis force to induce ‘twist’ on the storm; storms such as these do not develop on or within 5 °N or S of the equator.
Cyclogenesis is not well understood but considering that 80–90% of all tropical cyclones form within 20° of the equator (Webster 2020), the genesis of cyclones may be modulated by the family of equatorial and near‐equatorial waves that propagate zonally in this band (Sharkov 2012). The waves could cause cyclones to form by organising deep convection and/or by altering the flow in preferred areas. Mixed Rossby‐gravity waves, tropical depression‐type, or easterly waves, equatorial Rossby waves (large‐scale waves of low amplitude that move along the thermocline), and the Madden‐Julian Oscillation may play a role in cyclogenesis. These waves appear to enhance the local circulations by increasing the forced upward vertical motion, increasing the low‐level vorticity at the site of formation, and by modulating the vertical steer. Most convection occurs near the centre and this usually allows an ‘eye’ to develop due to convective subsidence; a ‘wall’ also develops around the centre due to dynamical forcing and development of a ring of cumulonimbus cloud as well as altostratus and nimbostratus clouds. Gale force winds develop due to falling pressure.
Even when conditions are favourable for cyclone formation, storms may not occur. For instance, hurricanes rarely spawn off the Brazilian coast despite the existence of favourable conditions because wind shear in the upper troposphere is too strong to permit storm development. Convective heating must be strong enough and upper‐level winds must be weak enough to permit development of a warm core.
These evolving storm systems deepen over the NW Pacific most commonly between March and December, between November and April in the south Indian Ocean, in the SW Pacific between December and April, and in the NE Pacific, North Atlantic, and Caribbean between June and November. Fewer such storms occur over the northern Indian Ocean, rarely during the SW monsoon and during the inter‐monsoon season, while no storms form in the SE Pacific where SSTs are too low. These storms all have one common characteristic: the need for a very warm water source. Such storms rapidly dissipate over land, so an immense supply of constant latent heat over the tropical ocean is required. Not surprisingly, rain bands are spiral in these revolving storms and are formed by cooler air and bring heavy rainfall and frequent thunderstorms. These rain bands are complex and can re‐energise over land near swamps and large lakes, or if they reach another parcel of warm water. For instance, cyclones spawned in the Gulf of Carpentaria west of Cape York peninsula in northern Australia have been observed to weaken considerably crossing the cape but then regenerate once back over the warm water of the Coral Sea. Similar regeneration can occur for storms moving westward from the South China Sea into the Bay of Bengal, and storms originating in the Bay of Bengal have been known to cross the
