Extratropical Highlights - February 1999

1. Northern Hemisphere

a. North Pacific

The circulation over the North Pacific during February featured below-normal heights at high latitudes and above-normal heights in the middle latitudes (Fig. E9). In the subtropics, an anomalous anticyclonic (cyclonic) circulation covered the western (eastern) half of the North Pacific (Fig. T22, bottom). Overall, these conditions reflected a northward shift of the East Asian jet stream to approximately 40°N (Fig. T21 ), well north of its climatological mean position near 32.5°N. They also reflected a retraction of the mean subtropical ridge to well west of the date line and a strengthening of the mid-Pacific trough in the region east of the date line. Accompanying this structure was an extremely well developed diffluent flow and jet exit region across the eastern subtropical North Pacific. The conditions in the middle latitudes and subtropics reflected increased zonal variability of the height, wind and temperature fields, and are linked to the increased east-west gradient in deep tropical heating associated with ongoing strong La Niña conditions. Similar circulation features were also evident over the subtropical South Pacific.

The overall northward shift of the wintertime jet stream across the North Pacific contributed to a continuation of well above-normal precipitation in the Pacific Northwest of the United States (Figs. E3, E5), and to a continuation of suppressed rainfall across the southwestern and southern states. During February, near-record high precipitation totals were observed in the Pacific Northwest (Fig. E5), while well below-normal rainfall was recorded in the Gulf Coast states. Below-normal precipitation was also observed in the Great Plains and Southeast regions during the month (Fig. E5).

Extremely warm surface temperatures covered North America during February (Fig. E1), with temperatures 4-6°C above normal extending from northeastern Mexico northward to central Canada. This warmth was linked to broad zonal flow across the United States and southwesterly flow across western and central Canada, in association with a pronounced weakening of the Hudson Bay Low and the establishment of a large-amplitude trough over the Gulf of Alaska (Fig. E9).

b. North Atlantic and Europe

The circulation during February featured above-normal heights across the middle latitudes of the North Atlantic (Fig. E9 ) and below-normal heights across Scandinavia and the eastern half of Europe. This circulation reflected the continuation of a strong positive phase of the North Atlantic Oscillation (NAO) (Table E1, Figs. E6, E7), which has prevailed for the past three months. It also reflected a strong negative phase (-1.7) of the Scandinavia teleconnection index. These conditions were accompanied by an enhanced (20 m s^-1 stronger than normal) jet stream across the high latitudes of the eastern North Atlantic (Fig. E10), and suppressed upper-level westerlies (15 m s^-1 weaker than normal) across the central North Atlantic.

Accompanying this circulation, significantly below-normal precipitation covered the eastern North Atlantic in the vicinity of the mean upper-level ridge, and well above-normal precipitation was observed across eastern Europe and portions of Scandinavia in the vicinity of the mean upper-level trough. In the Alps, a series of major snowstorms during the month caused deadly avalanches and cut off entire towns from outside communications. The persistent of the mean large-scale trough over this region contributed to this enhanced snowfall by favoring recurring cyclogenesis in essentially the same geographic region.

Farther east, the combination of a large-scale trough across Europe and a ridge over central Russia contributed to broad southwesterly flow and abnormally warm surface temperatures across central Russia and central Siberia.

The Southern Hemisphere extratropical circulation during February featured above-normal heights across the middle latitudes of the South Pacific and below-normal heights across the high latitudes of the eastern South Pacific (Fig. E15). At lower latitudes, the circulation featured a well-defined subtropical 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 Niña conditions

Another recurring feature of Pacific cold episodes is enhanced rainfall over southern Africa during that region's rainy season, which typically lasts from October - April. During February, area-average totals in this region reached the 90th percentile (Fig. E4), with most of the above-normal rainfall occurring in the climatologically wet regions of eastern South Africa, southern Mozambique and southern Tanzania (Fig. E3). Rainfall has averaged well above-normal across eastern South Africa since November 1998, following a delayed onset to the rainy season in October (Fig. E4).