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

1. Northern Hemisphere

           The 500-hPa circulation pattern during March featured positive height anomalies across the central North Pacific and the polar region, and negative anomalies over the western U.S. , across the central North Atlantic , and across northern Eurasia (Figs. E9, E11). In the subtropics, cyclonic 200-hPa streamfunction anomalies across the eastern half of the North Pacific and anticyclonic anomalies across the western North Pacific and southeastern Asia reflected the ongoing 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 March reflected warmer than normal conditions across the eastern half of Canada and central Asia , and cooler than normal conditions across western North America , northern Europe , and western Russia (Fig. E1). The main precipitation departures included above-average totals in Hawaii , central California , and western Europe, and below-average totals across the southeastern and eastern U.S. (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 across the central and eastern subtropical Pacific (Fig. T22). Accompanying this pattern, the East Asian jet stream was retracted to well west of the date line, and the associated jet exit region was shifted westward to well west of Hawaii (Fig.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 high latitudes central and eastern North Pacific, and the mean trough shifted from Hudson Bay to western North America (Fig. E9). This pattern reflected a complete disappearance of the mean Hudson Bay trough.

The main surface temperature departures reflected this anomalous circulation, with below-average temperatures observed across western North America and above-average temperatures observed across the eastern half of Canada (Fig. E1). Precipitation was well above average in the vicinity of Hawaii (Fig. E3), which was situated just downstream of the mean upper-level subtropical trough axis and the area of largest cyclonic streamfunction anomalies. Monthly precipitation surpluses far exceeded 100 mm across the Hawaiian Islands , with the entire region recording totals above the 90th percentile of occurrences. In contrast, the southeastern and eastern U.S. recorded below-average precipitation in association with descending motion downstream of the mean ridge axis.

 

b. North Atlantic / Eurasia

      The 500-hPa circulation during March featured a persistent pattern of above-average heights over Greenland , and generally below-average heights across the central North Atlantic (Fig. E9). This pattern reflected a negative phase (-1.3) of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7). The area of negative height anomalies also extended eastward across northern Eurasia . This pattern was associated with a strong low-latitude westerly flow into central Europe , resulting in above-average precipitation in that region. It was also associated with anomalously northerly flow of arctic air into Scandinavia and western Russia , where temperatures in many regions were below the 10th percentile of occurrences. Over central Asia, a persistent upper-level ridge situated south of the area of negative height anomalies contributed to significantly above-average temperatures during the month, with many regions recording temperatures that were more than 3°-5°C above average, and above the 90th percentile of occurrences.

 

2. Southern Hemisphere

In the subtropics, the mean 200-hPa circulation during March was consistent with ongoing La Niņa conditions, as indicated by an enhanced ridge over Australia and a very strong trough across the central and eastern 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 both Africa and South America .

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

In Australia , rainfall was above average in the north in association with La Niņa, and below average in the south. Surface temperatures were generally above average across eastern Australia , with values in many areas exceeding the 70th percentile of occurrences.

 

 


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