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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
Extratropical Highlights - April 2006

1. Northern Hemisphere

The 500-hPa circulation pattern during April featured positive height anomalies across the central North Pacific, the high latitudes of the North Atlantic , and the polar region, and negative anomalies over the eastern Pacific, across Greenland to Scandinavia , and over north- central Russia (Fig. E9). In the subtropics, cyclonic 200-hPa streamfunction anomalies over the central North Pacific and anticyclonic anomalies across the western North Pacific and southeastern Asia reflected the ongoing weak Pacific cold episode (La Niņa) (Figs. T22, T25). These conditions were associated with a pronounced westward retraction of the East Asian jet core, and a westward shift of the associated jet exit region to well west of Hawaii . Farther downstream a pronounced westward shift of the mean upper-level ridge and trough axes was also evident.

The main surface temperature departures during April reflected warmer than normal conditions across North America , Europe , and central Asia , and cooler than normal conditions across Alaska and eastern Siberia (Fig. E1). The main precipitation departures included above-average totals in the northwestern U.S. and portions of south-central Canada , and below-average totals in the Gulf Coast region (Figs. E3, E5, E6).

a. Pacific/ North America

The circulation across the North Pacific strongly reflected the ongoing Pacific cold episode. In particular, the westward retraction and confinement of deep tropical convection to the western Pacific resulted in the combination of an amplified subtropical ridge over the western Pacific and an amplified trough over the central and eastern subtropical Pacific (Fig. T22). Accompanying this pattern, the East Asian jet stream was retracted well west of the date line, and the associated jet exit region was shifted to well west of Hawaii (Fig. T21). Also in the extratropics a corresponding westward retraction of the mean downstream ridge and trough positions at 500-hPa was also evident, with the mean ridge shifted from western North America to the central North Pacific, and the mean trough shifted from Hudson Bay to the eastern Pacific (Fig. E9). This pattern reflected a complete disappearance of the mean Hudson Bay trough.

The main surface temperature and precipitation departures reflected this anomalous circulation, with above-average temperatures across North America (Fig. E1), and above-average precipitation near Hawaii and over much of the northwestern U.S. (Fig. E3). In contrast, the Gulf Coast region recorded below-average precipitation during the month. The Gulf Coast has recorded below-average precipitation in every month since September 2005. During both March and April 2006, area-averaged totals in this region were below the 10th percentile of occurrences.

b. North Atlantic / Eurasia

The 500-hPa circulation pattern during April featured above-average heights across southern Europe extending eastward to the Caspian Sea , and below-average heights over central Russia (Fig. E9). This pattern was associated with an enhanced westerly flow into southern Europe , and an anomalous northerly flow into eastern Siberia . These conditions contributed to above-average temperatures from southern Europe to central Asia , and to below-average temperatures across eastern Siberia and portions of Mongolia .


2. Southern Hemisphere

In the subtropics, the mean 200-hPa circulation pattern during April was consistent with ongoing weak La Niņa conditions, as indicated by an enhanced ridge over Australia and an amplified trough over the central South Pacific (Fig. T22). This pattern was associated with a large area of positive extratropical height anomalies at 500-hPa across the central South Pacific (Fig. E15) similar to that observed in the Northern Hemisphere. The 500-hPa circulation pattern also featured positive height anomalies over the central South Atlantic and negative anomalies south of Africa , Australia and South America .

In southern Africa the rainy season normally lasts from October to April. Area-averaged rainfall was above normal during April, marking six consecutive months of above-average rains (Fig. E4). An enhanced South African rainy season is consistent with a La Niņa episode.



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