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HOME > Expert Assessments > Climate Diagnostics Bulletin

Extratropical Highlights - April 1999

1. Northern Hemisphere

a. North Pacific

The circulation over the North Pacific during April featured below-normal heights at high latitudes and above-normal heights in the middle latitudes (Fig. E9). In the subtropics, it also featured anticyclonic (cyclonic) circulation anomalies across southern Asia (the central North Pacific) (Fig. T22 , bottom). Each of these features was also present during February and March. Overall, these conditions reflect a pronounced westward retraction of the East Asian jet stream toward Asia, along with a slight northward shift of the jet (Fig. T21). They also reflect a confinement of the mean low-latitude ridge to the Indonesia sector, and a strengthening of the mid-Pacific trough in the region east of the date line. Similar features were also evident in the large-scale atmospheric circulation over the South Pacific during the period. In the Northern Hemisphere, these circulation features were again accompanied by an extremely well-developed diffluent flow and jet exit region over the central subtropical North Pacific. These conditions are consistent with ongoing cold-episode conditions and the pattern of La Niņa-related tropical heating and convection.

At higher latitudes a persistent upper-level trough across the North Pacific during the past three months (Fig. E9 ) has completely suppressed the high-latitude blocking activity which is typical of La Niņa conditions, and therefore has contributed to above-normal temperatures and to a reduced frequency of cold-air outbreaks across Canada and portions of the U.S. (Fig. E1). Accompanying these conditions, an increased westerly flow of marine air has persisted during the past few months in western Canada and the northwestern United States, which has contributed to above-normal precipitation in these regions (Figs. E4, E5).

A large region of positive height anomalies at upper levels has also persisted across central and eastern Canada during the past three months, in association with a reduced strength of the climatological mean Hudson Bay Low (Fig. E9). This pattern has contributed to above-normal temperatures across eastern Canada during the period.

b. North Atlantic, Europe, Asia

The circulation during April featured above-normal heights across the high latitudes of the North Atlantic (Fig. E9 ) and below-normal heights over the western North Atlantic. Above-normal heights also covered the eastern North Atlantic and northern Africa during the month. This circulation contributed to above-normal precipitation across the high latitudes of the North Atlantic, and to below-normal rainfall across the Mediterranean Sea and the Middle East (Figs. E3, E4). Farther north and east, a large-amplitude ridge at upper levels covered western Russia and eastern Europe during the month. This feature contributed to above-normal temperatures throughout the region, with values averaging 1-3°C above normal in many locations (Fig. E1).

2. Southern Hemisphere

The Southern Hemisphere extratropical circulation during April again featured above-normal heights across the middle latitudes of the South Pacific (Fig. E15). At lower latitudes, the

circulation featured a well-defined low-latitude ridge over the eastern Indian Ocean and Australia and a large-amplitude trough east of the date line (Fig. T22). Overall, these conditions were associated with abnormally weak winds at jet stream level across the middle latitudes of the eastern South Pacific (Fig. T21), and a poleward shift of the main jet stream to higher latitudes. These conditions and their Northern Hemisphere counterparts are consistent with ongoing strong La Niņa conditions.

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