What Causes Dryness in Earth’s Subtropics?

Dry regions, where evaporation and evapotranspiration exceed the annual mean precipitation, cover about 40% of Earth’s land surface and affect the livelihood of nearly two billion people, primarily in developing countries (Safriel and Adeel, 2005). Dry regions are primarily found in the subtropics, within a wide latitudinal band ranging from ~10 to 40˚ latitude in each hemisphere. At these latitudes great deserts are found over continents (for example, the Sahara and the Namibian deserts over Northern and Southern Africa ,respectively). Dryness extends over the subtropical oceans where the large excess of evaporation over precipitation rates feeds the global atmospheric water cycle (Peixoto and Oort, 1992a). As this subtropical water vapor is advected into the deep tropics and midlatitudes, it acts as the main source for precipitation in the Intertropical Convergence Zone (ITCZ) and in synoptic storms (synoptic storms are New Haven’s primary source of precipitation).The extent and aridity level of these dry regions vary greatly with seasons, sometimes in opposite ways when comparing regions at the same latitude. In particular, the most extreme variations in hydrology are found over continents during summer when some regions experience the largest precipitation rates globally while others receive virtually none at all. For instance, while Dhaka in Bangladesh receives about to 1250mm during northern summer (June to September), Muscat receives less than 1mm at the same latitude.To a large extent, these large differences in summer precipitation determine the locations of the most productive ecosystems (the Amazonian basin in S. America) and those of parched deserts (the Namibian desert in South Africa).

What controls the extent or the aridity level of these dry regions in present-day climate remains unclear, and a number of mechanisms have been proposed to explain precipitation distribution in the subtropics and its seasonal variations. Quantifying the relative importance of these mechanisms in present-day climate has been challenging due to the inherent complexity of the climate system. Even less is known about how sensitive these factors are to climate change.

The most basic explanation for subtropical aridity is the Hadley circulation, a meridional overturning circulation organized in a cell-like structure with a narrow latitudinal band of ascent located in the equatorial regions and wide latitudinal bands of weak descent in the subtropics in each hemisphere (Peixoto and Oort, 1992b). This dynamic pattern is associated with a narrow zonal band of strong precipitation in the deep tropics (the ITCZ) and drier conditions over most of the subtropics. In the summer hemisphere, the ITCZ can reach poleward 10˚ latitude while the Hadley circulation expands poleward (Nguyen, 2013), leading to large precipitation rates at the equatorward margins of the subtropics, but enhanced dryness at its poleward margin. In the winter hemisphere, dry conditions prevail in the deep tropics while synoptic storms moisten the poleward margin of the subtropics. This zonally-symmetric pattern can qualitatively explain a wide array of seasonal changes on Earth. In particular, it is consistent with monsoon precipitation occurring in summer (for example, the African monsoon over the Sahel), or with poleward margins of deserts becoming drier in summer and wetter in winter (for instance, over North Africa). Yet the Hadley circulation does not explain large contrasts in precipitation during summer between regions experiencing strong precipitation (monsoon regions) and those experiencing nearly none (deserts) at the same latitude. Instead, other mechanisms relying on persistent surface inhomogeneities are at play.

Subtropical continents often show a stark hydrological contrast during summer months between intense dryness along their western margins and humid conditions along their eastern margins (Yang et al., 1992). A number of mechanisms have been proposed to explain this zonal hydrological asymmetry. In general, prevailing directions of near-surface winds during the summer season and their interactions with stationary low-pressure over the continents explains why hydrology is organized primarily along an east-west contrast. During the summer season, continental interiors tend to be warm and dry due to the limited role of evaporation in cooling the land (Xue and Shukla, 1993; Yang and Lau, 1998). In contrast, cool and moist conditions prevail over the oceans (Chou et al., 2001; Chou and Neelin, 2003; Privé and Plumb, 2007b). This land-sea contrast induces a large cyclonic circulation over the continental interior, deflecting midlatitude westerlies and tropical easterlies toward the continental interior (Xie and Saiki, 1999). Hence, prevailing moist and warm southeasterlies from the equatorial ocean set favorable conditions for precipitation over the southeastern part of subtropical continents, while cool and dry northwesterlies prevail on the northwestern parts of subtropical continents — inhibiting precipitation (Privé and Plumb, 2007b). Precipitation distribution over continents are sensitive to only a few climate parameters controlling the land-sea contrast, such as ocean heat transport or moisture content of the land (Chou and Neelin, 2003; Privé and Plumb, 2007a,b).

Secondary circulations can also arise from preexisting circulation patterns due to topography (Smith, 1979; Hoskins and Karoly, 1981) and other factors, which then act to further enhance or dampen the hydrological imbalance. Similarly, localized precipitation induced by a local surface forcing can produce secondary flow and lead to enhanced dryness to the west (Hoskins and Rodwell, 1995). The relevance of these mechanisms to the presence of dry regions on Earth today, as well as their sensitivity to global climate change, remains to be quantified.

Observations suggest that Earth’s largest dry regions have experienced a widening and drying during the past 30 years (Hu and Fu, 2007; Johanson and Fu, 2009) consistent with GCM simulations of 21st century climate (Lu et al., 2007). Yet, large discrepancies in GCM outputs are evident when comparing hydrologic changes with global warming scenarios in the subtropics, particularly near the margins separating dry and wet regions (Seager and Coauthors, 2007). Uncertainties can be partly ascribed to differences in cloud and land surface representations between GCMs, which can enhance or dampen hydrologic changes with climate change. Ultimately, better predictions for subtropical precipitation changes for the 21st century will require a quantitative assessment of the various dynamical mechanisms impacting hydrology and their sensitivities to climate change.

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