The Tropospheric Seasonally Varying Mean Climate over the Western Hemisphere (1979-1995)

The seasonally varying mean circulation features are intimately linked to horizontal gradients in temperature, which arise from differential heating as a function of both latitude and the underlying surface characteristics. The tropics are heated strongly throughout the year, while middle and high latitude regions experience considerable variation in heating from summer to winter. As a result, the mean meridional temperature gradient and, consequently, the strength of the tropospheric zonal wind (westerlies) vary considerably from summer to winter (compare Figs. 1 and 2). The strongest meridional temperature gradients and strongest westerlies are observed in the middle latitudes of the winter hemisphere. The Southern Hemisphere winter jet stream (maximum in the westerlies in July, Fig. 2) is closer to the equator than the corresponding Northern Hemisphere jet stream (January, Fig. 1). The greatest anticyclonic shear for either hemisphere is found on the equatorward flank of the SH winter jet stream, due to the greater intensity and more equatorward position of the SH jet stream and to the presence of equatorial easterlies throughout the troposphere at that time of the year. Strong sinking motion is observed on the equatorward flank of the winter jet streams, with the strongest direct (Hadley) circulation occurring during the southern winter (Fig. 2).

Zonal asymmetries in the atmospheric circulation in the western hemisphere arise primarily due to the difference in thermal capacity between land and water. Continental areas are often warmer during the summer and cooler in the winter when compared to neighboring oceanic regions. This results in a seasonally varying zonal temperature gradient in the vicinity of the east and west coasts of continents, and a seasonal variation in the intensity of the mean meridional wind (Figs. 3 and 4). The troposphere over the southwestern United States (hereafter US) is heated strongly during the boreal summer season (June-September), which contributes to a pattern of zonal temperature gradients (Fig. 3, top) that favors southerly (northerly) upper-tropospheric winds over the West Coast (eastern US) (Fig. 3, bottom). This pattern is reversed during the winter (November-February). Thus, the mean upper-tropospheric circulation over the Southwest US is monsoon-like being anticyclonic during the summer and cyclonic during the winter. This reversal in the circulation between summer and winter and its close association to the annual cycle in heating over the continent have been discussed previously (e.g., Kousky and Srivatsangam 1983).

Similarly, the southern Amazon Basin and Bolivian altiplano are heated strongly during the austral warm season (October-March) resulting in an enhanced tropospheric zonal temperature gradient and enhanced upper-tropospheric meridional flow in the vicinity of both coasts of South America (Fig. 4). The strengths of the zonal temperature gradient and meridional flow near the west and east coasts of South America are measures of the intensity of the Bolivian anticyclone, which dominates the upper-tropospheric flow over the region during the austral summer. Unlike the pattern over the US, the flow over South America does not show a reversal in circulation from summer to winter. Instead, the winter flow in the upper troposphere is nearly zonal (near zero meridional component, Fig. 4, bottom), which is probably due to the fact that most of the South American land mass is located in the tropics and subtropics, and not in the middle and high latitudes as is the case for North America.

In both hemispheres the summer monsoon-like upper-tropospheric continental anticyclones are accompanied by oceanic troughs (e.g., Figs. 33 and 35). These troughs, located on the equatorward flanks of the subtropical surface highs, are cold core, and feature sinking vertical motion and a lack of deep convective clouds.