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Extratropical Highlights - December 1999

1. Northern Hemisphere

The extratropical circulation during December featured a large area of above-normal heights spanning the eastern Pacific, Canada, and the central Atlantic, as well as southern Russia and the Middle East (Figs. E9, E11). These above-normal heights contributed to extremely warm surface temperatures across Canada and the United States (Fig. E1). They also contributed to exceptionally dry conditions over both the western and southeastern United States, and across the eastern Mediterranean Sea and Middle East (Fig. E3). Elsewhere, extremely warm and dry conditions have prevailed across much of the Middle East for the past several months, which has led to drought conditions in crop-growing regions of Iraq, Iran and Syria.

Over Europe, the combination of above-normal heights across the central Atlantic and below-normal heights over Scandinavia was associated with an extremely strong jet stream (Fig. E10) that directed a series of exceptionally severe winter storms into central and northern Europe in late December (Fig. A2.3). These storms produced widespread damage from hurricane-force winds, and also caused many deaths and injuries.

In the subtropics, there was again considerable symmetry of the upper-level circulation anomalies over the Pacific sector of both hemispheres (Fig. T22), as well as considerable along-stream variation in these circulation features. Notable aspects of this low-latitude circulation pattern included enhanced ridges over the western Pacific and pronounced troughs over the mid-Pacific of both hemispheres, features that were directly linked to the La Niņa-related pattern of tropical convection.

These anomalous conditions at low-latitudes contributed to strong along-stream variations in the Northern Hemisphere East Asian jet, and to a confinement of that jet core to well west of the date line (Fig. T21). They also contributed to an extremely strong diffluent flow in the exit region of the East Asian jet, along with an amplification of the mean thermally-indirect transverse ageostrophic circulation throughout the jet exit region. These dynamical changes in the wintertime East Asian jet represent a fundamental aspect of the extratropical atmospheric response to mature cold episode conditions.

a. North America

The December mean circulation over North America was dominated by an amplified ridge over the eastern North Pacific and western North America, and by an anomalously weak upper-level trough over central and eastern Canada (Fig. E9). This circulation contributed to above-normal temperatures across North America (Fig. E1), with departures averaging more than 4°C above normal over most of central Canada for the second consecutive month. In portions of central Canada, monthly mean temperatures during December exceeded 6°C above normal, which is in the upper 10th percentile of the 1961-1990 distribution.

Accompanying this anomalous warmth, the storm track over the eastern Pacific and western North America was shifted northward to the northern flank of the amplified ridge, which resulted in increased storminess across the northern Gulf of Alaska and west-central Canada (Fig. E13), and in reduced precipitation in the United States both in the Pacific Northwest and the Southwest (Fig. E3). The southwestern U.S. has recorded significantly below-normal precipitation since August (Fig. E5), which is consistent with the expected La Niņa-related decrease in rainfall and storminess in that region.

Anomalously dry conditions were also observed over large portions of the remainder of the U.S. during December, including the Inter-Mountain region, the Great Plains, the Midwest, the Gulf Coast, the mid-Atlantic, and the Northeast. For the midwestern region as a whole, rainfall totals have been below normal throughout the July-December 1999 period (Fig. E5), with totals being substantially below normal during August, September, and November.

Much of the eastern U.S. has received substantially reduced precipitation since November (Fig. E5). During this two-month period, storminess has been reduced across the entire region (Fig. E13, right), in association with the persistence of a strong positive phase of the North Atlantic Oscillation (NAO) (Table E1, Figs. E6, E7). The Gulf Coast region also recorded significantly below-normal rainfall during the November-December period (Fig. E5), which is typical of mature La Niņa conditions. However, it is likely that the extreme rainfall deficits observed during November and December in this region were also partly linked to the strong positive phase of the NAO.

b. North Atlantic/ Europe

The strong positive phase of the NAO contributed to anomalous precipitation patterns from eastern North America to eastern Europe (Figs. E1, E3). It also contributed to a series of major winter storms across central and northern Europe late in the month. In northern Europe, precipitation totals averaged in the upper 90th percentile for the month as a whole (Fig. E4), with the largest percentiles observed from western France to southeastern Finland (Fig. E3, bottom).

Elsewhere, large portions of western Europe and northern Africa experienced near-normal temperatures and precipitation during December (Fig. E1), which is in stark contrast to the extremely warm and dry conditions that have dominated these regions in the past several months. However, extremely dry conditions persisted farther downstream in the region south of the Caspian Sea during the month. This overall persistence of extremely warm and dry conditions had been previously linked to an anomalous anticyclonic circulation at upper levels in association with the much larger-scale pattern of above-normal heights over large portions of the middle latitudes. During December this anomalous upper-level ridge was replaced by a large-amplitude trough that extended southward from Scandinavia to southern Europe. However, positive height anomalies remained intact in the vicinity of the Caspian Sea (Fig. E9) where they supported a continuation of drought conditions.

2. Southern Hemisphere

In the Southern Hemisphere extratropics, the circulation during December featured a continuation of above-normal heights over large portions of the middle latitudes (Fig. E15), with the largest positive anomalies again observed in the region poleward of Australia, in the area east of New Zealand, and over the central South Atlantic. 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). This strong inter-hemispheric symmetry of circulation features over the central and eastern Pacific is typical of mature La Niņa episodes. At high latitudes, another prominent feature of the Southern Hemisphere circulation was negative height anomalies throughout Antarctica, in association with an amplified polar vortex.

a. Australia

Anomalously cold (Fig. E1) and wet (Fig. E3) conditions covered most of Australia during December. Anomalously cold conditions were also observed across the continent during November, while extremely wet conditions were observed across northern Australia. This abundant rainfall across northern Australia during November and December is typical of mature La Niņa conditions, and reflected a very good start to the region’s rainy season which typically lasts from November to April.

b. Southern Africa

The rainy season in southern Africa typically extends from October to April, with many regions recording more than 75% of their mean annual rainfall during this period. The heaviest rains are generally observed over southeastern Africa, while considerably less rainfall is observed farther west in areas such as the Kalahari Desert of south-central Africa. Rainfall throughout southern Africa exhibits a strong relationship to the ENSO cycle, with above-normal rains generally observed during La Niņa episodes and below-normal rainfall observed during El Niņo episodes.

Overall, below-normal rainfall during October indicated a slow start to the 1999-2000 rainy season across southeastern Africa (Fig. E4). However, above-normal rains were observed across the region during November and December (Fig. E5, bottom), with seasonal totals averaging above normal by year’s end. Also during December, significantly above normal rainfall was observed across the desert regions of southern Africa, with portions of both the Kalahari desert recording totals exceeding the 90th percentile (Fig. E5, bottom).

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