Skip Navigation Links 
NOAA logo - Click to go to the NOAA home page National Weather Service   NWS logo - Click to go to the NWS home page
Climate Prediction Center

HOME > Expert Assessments > Climate Diagnostics Bulletin


Extratropical Highlights - April 2001

1. Northern Hemisphere
The Northern Hemisphere circulation was dominated by above-normal 500-hPa heights in the middle latitudes during April (Fig. E10), and by anticyclonic circulation anomalies at 200 hPa across most of the subtropics (Fig. T22, bottom). The exception was the eastern subtropical Pacific, where cyclonic circulation anomalies were again evident in the subtropics in response to the ongoing La Niņa-related pattern of tropical convection. Similar anomaly patterns were also evident in the Southern Hemisphere middle and low latitudes (Figs. E16, T22, bottom). This strong inter-hemispheric symmetry of the upper level circulation features is the leading mode of atmospheric variability on the interannual time scale, and is indicative of continued strong forcing from the Tropics.

a. United States

The prominent temperature and precipitation anomalies during April occurred over the United States), where the eastern half of the country experienced significantly above-average temperatures (Fig. E1) and below-average rainfall (Fig. E3). This warmth was associated with the development of the strong positive phase of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7) during the second half of the month (Fig. E8). During this period a large region of above-normal heights developed across the eastern half of the country, and the mean Hudson Bay trough that normally extends southward to cover the eastern United States disappeared. This circulation pattern resulted in an enhanced transport of mild air from the Gulf of Mexico to the central and east-central U.S. (Fig. T20). Upper-level convergence (Fig. T23) and large-scale sinking motion of this warm air downstream of the anomalous ridge axis then contributed to a nearly complete lack of rainfall over parts of the Gulf Coast and mid-Atlantic states, and to exceptionally dry conditions in the northeast and Ohio Valley regions (Fig. E5).

b. Europe

Western Europe was influenced during April by an upper-level ridge over the east-central North Atlantic and an upper-level trough over northern Europe (Fig. E10). This circulation was accompanied by enhanced jet stream winds (Fig. E11) which contributed to increased storminess and above-average precipitation across northern Europe (Fig. E3). It also resulted in large-scale sinking motion and below-average rainfall over the western Mediterranean Sea.

2. Southern Hemisphere

The Southern Hemisphere circulation was also dominated by above-normal 500-hPa heights in the middle latitudes (Fig. E16), and by anticyclonic circulation anomalies at 200 hPa across most of the subtropics (Fig. T22, bottom). Abnormally dry conditions were observed over central and eastern Brazil, eastern Africa and Madagascar, and over many sections of Australia (Fig. E3). The dry conditions in Brazil have persisted since the beginning of 2001, with rainfall deficits of more than 400 mm in some areas.

One of the more unique aspects of the Southern Hemisphere conditions during April was significantly above-average rainfall across western Africa in the area south of the equator (Fig. E3). This rainfall occurred across the normally very arid regions of western South Africa, Botswana, and Namibia, and produced totals in many areas exceeding the 90th percentile. Above-average rains also covered central and southeastern South Africa, which is the third consecutive month of enhanced rainfall during that region’s rainy season (Fig. E4).

NOAA/ National Weather Service
NOAA Center for Weather and Climate Prediction
Climate Prediction Center
5830 University Research Court
College Park, Maryland 20740
Page Author: Climate Prediction Center Internet Team
Page last modified: August 24, 2007
Information Quality
Privacy Policy
Freedom of Information Act (FOIA)
About Us
Career Opportunities