Review Of Lower And Upperlevel Circulation In The Americas

In this section, we show the mean seasonal summer and wintertime ensembles of near-surface (925 hPa [hectoPascal]) and upper level (200 hPa) atmospheric circulation for December-February and June-August, drawing upon the studies of Hastenrath (1996) and Kousky and Ropelewski (1997).

The seasonally varying mean circulation features are intimately linked to the horizontal gradients in temperature. The tropics are heated strongly throughout the year, while middle and high latitudes experience considerable variation in heating from summer to winter due to variations in solar insolation. As a result, mean meridional temperature gradients, and consequently the strength of the low-level zonal winds, vary considerably from summer to winter (Figs. 1A and 1B and 2A and 2B). The strongest meridional temperature gradients and strongest zonal winds are generally observed in middle latitudes of the winter hemisphere, especially the Southern Hemisphere. The Southern Hemisphere winter jet stream (maximum in zonal winds, Fig. 2A) is closer to the equator than the corresponding Northern Hemisphere winter jet stream (Fig. 1A), reflecting the asymmetry in heating occurring between the hemispheres. Zonal asymmetries in 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 during the winter compared to neighboring oceanic regions.

The subtropical highs over the South Pacific and the South Atlantic display very little seasonality in either central pressure or position between winter and summer (Figs. 1B and 2B). However, there is a seasonal cycle in the sea level pressure (SLP) over central South America, with the lowest pressures occurring during December-February and the highest pressures occurring during June-August (Figs. 1B and 2B). This pattern is consistent with the seasonal variation in the low-level meridional flow, which features strong flow from the Amazon basin southward to northern Argentina during austral summer (December-January-February, DJF), and much weaker flow from the north but stronger flow from southern latitudes into lower latitudes during winter (June-July-August, JJA). An important feature of the upper level summertime circulation over South America is the presence of an anticyclonic circulation over the Bolivian Plateau that is absent during winter (Fig. 2A).

Seasonal variations in SLP are evident over the middle and high latitudes of the North Pacific and North Atlantic Oceans. There is a marked northwestward shift from winter to summer (Figs. 1B and 2B) in the central positions of the oceanic anticyclones, and their central pressure increases somewhat during summer in response to the relative coldness of the underlying surface. High pressure dominates continental North America during winter, and lower pressure dominates a b

FIGURE 1 Mean seasonal summer and wintertime ensembles for December-February, (A) Upper level circulation (200 hPa) over the Americas, and (B) near-surface circulation (925 hPa) over the Americas.

Jun-Aua Jun-Aua

Mean Circulation Summer
FIGURE 2 Mean seasonal summer and wintertime ensembles for June-August, (A) Upper level circulation (200 hPa) over the Americas, and (B) near-surface circulation (925 hPa) over the Americas.

it during summer; however, the seasonal variations in surface winds are not as apparent as over tropical South America.

Throughout the year a mean westerly (zonal) flow dominates much of the middle latitudes of both hemispheres (Figs. 1A and 2A). The zonal flow is strongest in each hemisphere's winter and weaker in the summer. However, the zonal flow is strongest in both hemispheres in the southern winter (Fig. 2A), and the weakening occurring in the southern summer reaches an intensity that still approximately matches the mean strength of the flow during the northern winter (Fig. 1A). The mean flow during the northern summer is seasonally the weakest on the planet. Comparison between hemispheres in Figs. 1A and 2A also reveals a greater wavelike structure, or meridionality, to the flow in the Northern Hemisphere. Northern winters are occasionally beset by periods in which the westerly flow decreases or becomes blocked, and meridional (north-south) flow begins to dominate the circulation. This flow is accompanied by an amplification of the wave ridges, notably the ridge visible at higher latitudes over western North America and the eastern Pacific (Fig. 1A). During periods of blocked westerlies, unusually cold weather persists over North America for 2-4 weeks. These long cold spells can have cold waves (26 days duration) embedded within them, although their severity may not always be nearly as great as those of some of the events described later in this chapter. The persistent cold meridional flow periods have created some extraordinarily cold winters, especially east of the Rocky Mountains, that are long remembered by many people. Some of these in recent memory include the winters of 1962-63, 1968-69, 1976-77, 197879, and 1981-82. The 1968-69 and 1978-79 winters did not, however, witness the occurrence of strong, well-known severe polar outbreaks. Blocking ridges can also occur in the Southern Hemisphere around and over the Andes, producing cold waves in South America, but the hemispheric climatological flow (Figs. 1A and 2A) gives little hint of their existence.

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