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Extratropical Highlights - March 2001

1. Northern Hemisphere
The extratropical circulation during March featured above-normal heights over the eastern North Pacific, the polar region, and along an axis extending eastward from northern Africa to south-central Russia (Figs. E10, E12). The circulation also featured a large area of below-average heights extending northeastward from the eastern United States to northwestern Russia, along with a pronounced cyclonic circulation anomaly over the central subtropical North Pacific (Fig. T22, bottom). Over the North Pacific, these anomalies were accompanied by a pronounced westward retraction of the East Asian jet to well west of the date line (Fig. T21, top) and a highly amplified split-flow pattern over the eastern half of the North Pacific. These conditions are strongly linked to the ongoing La Niņa episode. Over eastern North America and the North Atlantic the circulation anomalies during March were associated with a strong negative phase of the North Atlantic Oscillation (NAO) (Table E1, Figs. E7, E8).

The subtropical circulation again reflected considerable symmetry of the upper-level circulation anomalies in both hemispheres (Fig. T22, bottom). Notable aspects of this anomaly pattern included amplified troughs (cyclonic circulation anomalies) over the mid-Pacific of both hemispheres, and anticyclonic circulation anomalies extending eastward in the Northern Hemisphere from the Atlantic Ocean to Asia and in the Southern Hemisphere from the Atlantic Ocean eastward to Australia. This overall pattern has prevailed during much of the 1998-2001 Pacific cold episode, and is a leading mode of atmospheric variability on both the interannual and interdecadel time scales.

a. North America

The circulation over North America reflected above-average heights over the western United States and large portions of Canada, and below-average heights over the eastern half of the United States (Fig. E10). In the west these conditions were accompanied by an enhanced westerly flow of mild air into western Canada, while in the east they were associated with the negative phase of the NAO. Over the eastern U.S. these circulation anomalies were accompanied by an amplification of the mean trough, and amplified confluence within the entrance region of the North Atlantic jet stream (Fig. E10).

The pattern of temperature anomalies over North America closely matched the upper-level circulation anomalies, as indicated by above-average temperatures across the western U.S. and Canada and below-average temperatures across the eastern U.S. (Fig. E1). In Canada, temperatures in many areas averaged 2°-3°C above normal during the month, while in the U.S. temperatures generally averaged 2°C below average across the southeast and lower Midwest.

The prominent precipitation anomalies during the month included below-average rainfall in the Midwest and above-average rainfall along the Gulf Coast and portions of the eastern seaboard (Fig. E3). In the Gulf coast region these above-average rains were associated with the strong negative phase of the NAO. Nonetheless, substantial rainfall deficits remain throughout the region, in association with the ongoing La Niņa episode.

b. North Atlantic/Europe

The North Atlantic Oscillation is the most prominent, recurring circulation pattern to impact temperature and precipitation across Europe. During March the strong negative phase of the NAO was accompanied by a substantial strengthening of the North Atlantic jet stream (Fig. E11), as well as a southward shift of the mean jet core to northern Spain. This flow pattern resulted in increased marine flow into Europe, along with increased storminess and enhanced rainfall across central Europe. Above-average temperatures also prevailed across Europe and the eastern Mediterranean during the month (Fig. E1), with the most significant anomalies exceeding the 90th percentile from southern Italy eastward to the Middle East.

c. Stratosphere

Strong winter stratospheric warming over the Northern Hemisphere polar region reached peak levels at the end of January and early February (Fig. S4). In March, cold air once again became reestablished over the polar region in the middle and upper stratosphere (see 10 and 2 hPa in Fig. S4). However, in the lower stratosphere, positive anomalies (Fig. S3) and positive height anomalies (Fig. S1) remained dominant over the north polar region. Associated with the lower stratosphere positive height and temperature anomalies, there were also strong positive total ozone anomalies which prevailed over the north polar region (Fig. S6). Over middle latitudes of the Northern hemisphere, negative anomalies of up to 10 percent in total ozone predominated. However there were no extremely low values of total ozone over the north polar region during March 2001, such as were evident in winter and early spring of 1999-2000. The very low values in the region of depleted ozone during the winter of 2000-2001 disappeared by mid February, in association with the stratospheric warming in January and February and its related temperature and circulation changes.

2. Southern Hemisphere

In the Southern Hemisphere extratropics, the circulation during March featured a continuation of above-normal heights over large portions of the middle latitudes (Fig. E16). Over the central and eastern South Pacific, the above-normal heights were located immediately poleward of the La Niņa-related subtropical cyclonic circulation anomalies, which is similar to the anomaly pattern observed in the Northern Hemisphere (Fig. T22, bottom).

Well above-average temperatures (Fig. E1) and enhanced rainfall (Fig. E3) were observed over much of northern Argentina, Uruguay, and extreme southern Brazil during March. Anomalously warm temperatures were also observed over southeastern Africa and portions of southern Australia, while considerably below-average temperatures prevailed over most of northwestern Australia.

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