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Extratropical Highlights - February 2000

1. Northern Hemisphere

The extratropical circulation during February featured above-normal heights over the lower mid-latitudes of the central North Pacific, and across the United States and North Atlantic to south-central Europe (Figs. E9, E11). The circulation also featured below-normal heights across the high latitudes of both the North Pacific and North Atlantic Ocean basins. Over the North Atlantic, the anomalous circulation reflected a very strong positive phase of the North Atlantic Oscillation (Table E1, Figs. E6, E7). The NAO has been primarily in the positive phase since December 1998, and was a contributing factor to the anomalous warmth observed over large portions of North America during both the 1998/99 and 1999/2000 winter seasons.

Overall, global mean temperature anomalies averaged 1.2°C during February, which is the second largest value in the historical record dating back to 1950 (Fig. E2, top). This anomalous warmth was due almost entirely to conditions in the Northern Hemisphere, which also recorded its second largest mean anomaly (1.4°C) since 1950 (Fig. E2, middle). In contrast, Southern Hemisphere temperatures have been near-average for the past four months (Fig. E2, bottom).

In the subtropics, there was again considerable symmetry of the upper-level circulation anomalies during February in both hemispheres (Fig. T22, bottom). Notable aspects of this anomaly pattern included amplified troughs 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 was also prominent throughout 1999 in association with ongoing La Niņa conditions, and is a leading mode of atmospheric variability on both the interannual and interdecadal time scales.

a. North America

Exceptionally warm temperatures were observed across the United States, southern Alaska, Mexico, and central Canada, during February, with temperatures averaging 3°-5°C above average and exceeding the 90th percentile over much of the central and southwestern U.S. and Mexico (Fig. E1). The month also featured exceptionally dry conditions over the eastern seaboard of the U.S. (Figs. E3, E5) and a continuation of very dry conditions in the Gulf Coast states.

Rainfall has also been substantially below-average in the southwestern U.S. and northwestern Mexico since August (see box labeled Southwest in Fig. E5). This dryness reflects an early end to the regions’ monsoon rains in August, and a nearly complete absence of measurable rainfall since October. This region typically receives below-average rainfall during the cool season in association with La Niņa conditions.The Gulf Coast region also recorded significantly below-normal rainfall during November 1999 - February 2000 (Fig. E5), which is typical of mature La Niņa conditions. However, it is likely that the extreme rainfall deficits observed in this region during the period were also partly linked to the strong positive phase of the NAO (Table E1).

Other regions of the United States that have accumulated large precipitation deficits during the past several months include 1) the Midwest, which has recorded below-average precipitation since July 1999, 2) the Southeast, which has recorded below-average precipitation since November 1999, 3) the Great Lakes region, which has recorded below-average precipitation since March 1999, 4) the Northeast, which has recorded substantially below-average precipitation in many months since April, and 5) the Mid-Atlantic region, which has recorded substantially below-average precipitation since November 1999.

b. North Atlantic/ Europe

The atmospheric circulation over the North Atlantic again featured the strong positive phase of the North Atlantic Oscillation (NAO) (Figs. E6, E7, Table E1), which was associated with an enhanced jet stream (Fig. E10, right) and increased storminess (Fig. E13, right) across the high latitudes of the North Atlantic and into northern Europe/ Southern Scandinavia. The pattern was also associated with an anomalous anticyclonic circulation and reduced storminess across the east-central Atlantic and southern Europe. These conditions contributed to above-average temperatures across northern Europe, Scandinavia and large portions of central Russia (Fig. E1), to above-average precipitation over portions of northern Europe and Great Britain (Fig. E3), and to below-average rainfall across southern Europe, northwestern Africa and large portions of the Middle East.

2. Southern Hemisphere

In the Southern Hemisphere extratropics, the circulation during February featured a continuation of above-normal heights over large portions of the middle latitudes (Fig. E15). 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 also observed in the Northern Hemisphere (Fig. T22, bottom).

a. Australia

Anomalously wet (Fig. E3) conditions again covered northern Australia during February, with above-average rains also observed in the arid interior. In contrast, below-average rains were again reported in portions of southeastern Australia during the month. This region has recorded below-average rainfall in almost every month since April1999, with the most significant deficits observed since June. Overall, totals since April were generally more than 200 mm below average across nearly all of Victoria, which includes major cities such as Melbourne and Sydney. The state of Victoria has recorded significantly below-average rainfall since late 1996, with December 1996-present deficits exceeding 500 mm throughout the region. These long-term moisture deficits have led to severe drought over most of southeastern Australia.

b. Southern Africa

Monsoon-like rains during February were again above-average over central and southeastern Africa, with totals exceeding the 90th percentile throughout the region (Fig. E3, bottom) for the second consecutive month. Two-month totals rainfall totals reached 250-1365 mm in this region, and moisture surpluses ranged from 100-1000 mm. The increased rainfall during February occurred partly in association with Tropical Cyclone Eline, which produced extremely heavy downpours throughout southern Africa. Extreme rainfall amounts from this storm contributed to devastating floods in the low-land areas of Mozambique, which killed hundreds and left tens of thousands homeless.

The above-average rainfall during January and February was also associated with an anomalous large-scale circulation pattern that featured 1) enhanced low-level easterly winds across the central Indian Ocean in association with an amplification and poleward shift of the Mascarene High (Fig. T20, bottom), 2) an anomalous low-level circulation center over south-central Africa, with anomalous westerly winds across southern Africa between 10°-20°S, and 3) anomalous upper-level easterly winds in association with a poleward extension of the subtropical high (Fig. T22) and a corresponding poleward shift of the main upper-level westerly winds to south of the continent (Fig. T21).Overall, these conditions were associated with confluence extending southwestward along the Mozambique coast, and with anomalous large-scale convergence across southeastern Africa and the Mozambique Channel. Collectively, these conditions represented an enhanced monsoonal circulation over southeastern Africa. This anomaly pattern was linked to the large-scale pattern of anticyclonic streamfunction anomalies at upper levels previously noted in the lower and middle latitudes of both hemispheres (Fig. T21, bottom).

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