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

Throughout the year, mean zonal flow dominates the southern mid-latitudes (Figs. 33-36), while at subtropical latitudes there is marked contrast between the zonal asymmetries observed during the austral summer (Fig. 33) and the near zonal flow found during winter (Fig. 35). During summer, dry conditions, upper-level convergence and mid-tropospheric sinking motion are observed over the eastern portions of the tropical and subtropical Pacific and Atlantic Oceans (Figs. 17 and 33). Two northwest-southeast bands of rising motion and heavy precipitation are observed; one in the central South Pacific (South Pacific Convergence Zone - SPCZ) and the other over South America and the neighboring South Atlantic (South Atlantic Convergence Zone - SACZ). Both of these bands are southeastward extensions of heavy precipitation from regions of intense tropical convection (over the western Pacific, and the Amazon Basin).

During the southern winter (Fig. 19), the precipitation pattern displays a zonally symmetric appearance, with heavy precipitation in the intertropical convergence zones (ITCZs), located between 5 and 10 N over the Pacific and Atlantic Oceans, and in the vicinity of the Southern Hemisphere storm tracks (20-40 S). The precipitation bands in the extratropics of the Southern Hemisphere display relative maxima near 30 S, 140 W and 30 S, 45 W. During the austral spring (Fig. 20), precipitation increases over the Amazon Basin and over the Pacific in the region of the SPCZ, and bands of maximum precipitation develop linking the tropical convection and precipitation activity in the lower extratropics in these regions. These bands continue into the austral summer and spring seasons.

The subtropical highs over the South Pacific and South Atlantic Oceans display very little seasonality in either central pressure or position. However, there is a marked seasonal cycle in sea level pressure (SLP) over central South America, with the lowest pressures occurring during DJF and the highest pressures occurring during JJA (Figs. 17, 19, 49 and 51). This pattern is consistent with the seasonal variation in the low-level meridional flow, which features strong northerly winds from the Amazon Basin southward to northern Argentina during the austral summer (DJF) and much weaker northerly flow during winter.

There is substantial seasonal variation in precipitable water (PW) in the region from the southern Amazon Basin south to northern Argentina. Over the latter region, summer values of PW are near 30 mm, while winter values are about 15 mm. This seasonality in PW is consistent with the seasonality in precipitation, moisture advection and temperature, which are all greatest over northern Argentina during summer.