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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
Extratropical Highlights - February 2004

1. Northern Hemisphere

The 500-hPa circulation during February 2004 featured above-average heights across Canada, the North Atlantic, and the polar region, and below-average heights over the high latitudes of the North Pacific, the southwestern U.S., the central North Atlantic, and Scandinavia (Fig. E10). In the subtropics the circulation featured an anomalous anticyclonic circulation across nearly the entire hemisphere (Fig. T22 bottom), with the largest anomalies evident along the equatorward flank of the East Asian jet stream (Fig. T21). A corresponding anomalous anticyclonic circulation was also evident in the Southern Hemisphere across Australia and the South Pacific.

In the Northern Hemisphere this anomaly pattern was associated with an enhanced East Asian jet stream extending well east of normal, and with an eastward shift in the associated jet exit region to just upstream of California (Fig. T21). It was also associated with a southward shift of the North Atlantic jet stream, and with a near absence of upper-level westerlies over the eastern North Atlantic and Europe.

Prominent temperature departures during February included significantly warmer than average conditions across Alaska and Canada, southern Russia, and China, and a continuation of warmer than average SSTs over large portions of the North Atlantic (Fig. E1). Prominent precipitation anomalies during the month included above-average totals in California, the Inter-Mountain and Gulf Coast regions of the U.S., and eastern Europe, and below-average totals over the mid-western and northeastern U.S. (Figs. E3, E5, E6).

a. Pacific/North America

The mean upper-level circulation during February featured persistent north-south-dipoles of height anomalies over both the eastern North Pacific and western North America, with a change in sign of the anomalies occurring between the two regions (Figs. E10, E12). This circulation was associated with an eastward extension of the East Asian jet stream, and with a pronounced split-flow configuration over western North America. The northern branch of this split flow pattern brought mild, marine air into Canada and contributed to above-average surface temperatures across the country (Fig. E1). The southern branch of the jet stream was associated with a pronounced southward shift of the storm track, and brought increased storminess and above-average precipitation to the southern tier of the U.S. (Fig. E3), while also leaving a swath of below-average totals from Missouri northeastward to Maine (Fig. E6).

The Inter-Mountain region of the western U. S. has received above-average precipitation in three of the last four months (Fig. E5), and also recorded above-average totals during the 2002-03 winter. This region had previously experienced large precipitation deficits during the prolonged 1998-2001 Pacific cold episode. For the Gulf Coast and Southeast regions of the U.S., February marks the first month of above-average precipitation since August 2003.

b. North Atlantic/Eurasia

Over the North Atlantic positive 500-hPa height anomalies at high latitudes reflected a continued absence of the mean Icelandic Low. These anomalies, combined with below-average heights in the middle latitudes, were associated with enhanced jet stream winds (Fig. E11) and increased storminess (Fig. E14) over the western North Atlantic. They were also associated with a pronounced split flow pattern across the eastern North Atlantic and western Europe, with the northern branch of the flow extending across Iceland and central Scandinavia, and the southern branch extending across the Mediterranean Sea primarily during the second half of the month (Fig. A2.1). This circulation, combined with the exceptionally warm SSTs at high latitudes, contributed to a continuation of above-average surface temperatures over Great Britain (Fig. E1).

The main precipitation anomalies during February were observed in eastern Europe, where totals in some areas exceeded the 90th percentile. Much of this precipitation occurred during the second half of the month when a well-developed southern branch of the jet stream brought increased storminess to the region (Fig. A2.1).

c. Central Russia and China

Much of central Russia experienced well above-average surface temperatures during February in response to broad southwesterly flow between the mean upper-level trough and ridge axes (Fig. T22 top). Central China also experienced above-average surface temperatures in response to broad upper-level southwesterly flow along the equatorward flank of the enhanced East Asian jet entrance region (Fig. T21).

2. Southern Hemisphere

In the Southern Hemisphere the 500-hPa circulation during February featured above-average heights over Antarctica, southern South America, and the subtropics from Australia to the central South Pacific, and below-average heights in the middle latitudes from the central Indian Ocean to the eastern South Pacific, and across the subtropical western South Atlantic. (Figs. E16, T22). This circulation was associated with enhanced tropical convection across northern Australia (Fig. T25), and a poleward shift of the South Pacific jet stream to approximately 45S (Fig. T21). Southeastern Australia experienced below-average precipitation in response to its location along the anticyclonic flank of the jet core (Fig. E3).

Over South America anomalously warm conditions were observed in the extreme south beneath the persistent upper-level ridge axis. Eastern Brazil experienced wetter-than-average conditions, while northeastern Argentina, Uruguay, and southern Brazil were drier than average for the second consecutive month (Figs. E3, E4). This dipole pattern is consistent with the anomalous mid- and upper-tropospheric cyclonic circulation observed over the western South Atlantic (Figs. T21, E16).

In southern Africa the rainy season normally lasts from October to March. Area-averaged rainfall was near-normal during February (Fig. E4, with portions of northeastern South Africa receiving above-average totals (Fig. E3) and portions of Mozambique and Tanzania receiving below-average rains. Overall, the 2003-2004 rainy season has been below-average, with significantly below-average totals during November and December and above-average totals in January.

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