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Extratropical Highlights - May 2002

1. Northern Hemisphere
The Northern Hemisphere circulation during May featured above-average 500-hPa heights over Alaska, the southern United States, the central North Atlantic, and eastern Siberia, and below-average heights across Canada, the high latitudes of the North Atlantic, and western Russia (Figs. E10, E12). The circulation anomalies over the Pacific/ North American sector and the North Atlantic have persisted since mid-January, when an abrupt transition occurred to the negative phase of the Pacific/ North American (PNA) teleconnection pattern and to the positive phase of the North Atlantic Oscillation (NAO) (Fig. E8). The daily time series of these indices show that both patterns remained in these respective phases nearly every day through the end of April, with the PNA index remaining strongly negative until May 29 when a second abrupt transition to the positive phase occurred. Within this 4 month period the monthly PNA index averaged at least 1.5 standard deviations below normal from March-May (Table E1, Fig. E7). Over the North Atlantic the north-south dipole pattern of circulation anomalies that had prevailed from mid-January through the end of April was also very persistent during May, although the anomaly centers shifted slightly during May such that they no longer projected onto the positive phase of the NAO.

a. North Pacific and North America

The strong negative phase (-1.6) of the PNA teleconnection pattern during May was again accompanied by easterly wind anomalies at 200-hPa throughout the climatological exit region of the East Asian jet stream (Fig. T21, E11, Right). These anomalies reflected a weakening of that jet stream in the vicinity of the date line, but not a complete disappearance of the jet stream from this region as was observed in April (Fig. T21, Top). This westward retraction of the East Asian jet stream was accompanied by a corresponding westward shift of the mean upper-level ridge axis from the Rocky Mountain region of western North America to Alaska (Figs. E10, T22, Top), and by a westward expansion across Canada of the downstream upper-level trough axis normally located over eastern North America (Fig. E10). This circulation pattern again resulted in significantly below-average surface temperatures throughout Canada and the northern United States (Fig. E1). Below-average temperatures have prevailed across Canada since March.

The anomalous north-south dipole pattern of height anomalies across North America and the North Atlantic was again associated with an anomalous jet core extending from the upper midwestern United States eastward to Europe, with the confluent entrance region of this anomalous jet spanning the entire central U.S. (Fig. E11). Only one major upper-level trough moved across the western U.S. during the month, and this trough dissipated within the anomalous confluent jet entrance region (Fig. E13). As a result significantly below-average precipitation was again recorded over much of the western half of the country (Fig. E3), with the largest deficits observed in the Plains States and the Inter-Mountain area (Fig. E5). In the Southeast and the Gulf Coast region the combination of the enhanced upper-level ridge and suppressed cyclonic activity, also contributed to well below average rainfall during the month. Each of these regions has accumulated large precipitation deficits over the past several months, with below-average precipitation observed in the Inter-Mountain region since May 2001, in the Great Plains region since June 2001, and in the Gulf Coast region since November 2001.

b. North Atlantic and Europe

A strong dipole pattern of 500-hPa height anomalies again dominated the North Atlantic during May, with below-average heights at high latitudes and above-average heights in the middle latitudes (Fig. E10). This anomaly pattern was accompanied by a nearly continuous jet stream that extended from the north-central United States eastward to Portugal (Figs. E11, left and T22, top). This circulation contributed to above-average surface temperatures throughout northern and eastern Europe (Fig. E1), and to significantly below-average precipitation (below the 10th percentile) over northeastern Europe and northwestern Russia (Fig. E3).

Above-average surface temperatures also covered most of eastern Asia during May, in association with above-average 500-hPa heights throughout the region. These conditions are similar to, but not quite extensive as, the extreme warmth observed across most of Asia during February and March.

2. Southern Hemisphere

In the Southern Hemisphere the 500-hPa circulation during May featured an anomalous zonal wave-3 pattern with above-average heights over the high latitudes of the eastern South Pacific, and in the area south of both Australia and Africa, and below-average heights over the central South Pacific, the central Indian Ocean, and the high latitudes of the South Atlantic (Fig. E16).

Over Australia significantly below-average rainfall was observed over much of the continent during the month (Fig. E3). This marks the sixth consecutive month of below-normal rainfall in northeastern Australia (Fig. E4), which coincides with the onset of above-average rainfall over the central equatorial Pacific in December 2001. Significantly above-average surface temperatures also covered the western half of Australia during May (Fig. E1) in association with an anomalous upper-level ridge over the region (Fig. E16). During the past several months the excessive warmth across much of Australia has been associated with a much larger area of above-average SSTs spanning the eastern Indian Ocean and western equatorial Pacific (Fig. E1).

Over South America above-average temperatures were observed over southeastern Brazil during May (Fig. E1), while cooler and wetter than normal conditions covered southern South America. These conditions are similar to those observed in April in association with a persistent upstream trough at upper-levels over the eastern South Pacific (Fig. E16).

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