Climate Diagnostics Bulletin - Extratropical Highlights

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Extratropical Highlights - February 2005

1. Northern Hemisphere

The 500-hPa circulation pattern during February featured above-average heights over the central North Pacific, the high latitudes of the North Atlantic , and throughout the polar region, and below-average heights over the southwestern United States , and across the central North Atlantic eastward to the Caspian Sea (Fig. E9). The main surface temperature departures during the month included above-average temperatures over North America and the central/ high latitudes of the North Atlantic , and below-average temperatures across the southern half of Europe and much of central/ eastern Asia (Fig. E1). The main precipitation anomalies reflected well above-average totals over the southwestern United States and the Caspian Sea region, and significantly below-average precipitation from Greenland eastward to central Russia (Figs. E3, E5, E6).

a. North America

The 500-hPa circulation pattern during February featured above-average heights over much of North America . A pronounced split-flow configuration was also evident over western North America , with one branch of the westerlies entering the continent over northern British Columbia and the other entering over the Baja Peninsula . These conditions were associated with a southward shift of the main jet steam and storm track across the western U.S., which resulted in significantly below-average precipitation in the Pacific Northwest and above-average precipitation in the Southwest, Southern California, and the southern Inter-Mountain regions of the United States (Figs. E3, E5). Significantly above-average precipitation during October 2004-February 2005 has led to a dramatic improvement in long-term drought conditions in these latter three regions.

b. North Atlantic and Europe

A high-amplitude omega-block dominated the circulation over the North Atlantic during February (Fig. E9). Individual components of this blocking pattern included a large-amplitude ridge over the central North Atlantic , and flanking upper-level troughs over both the western North Atlantic and southern Europe . This pattern reflected an ongoing strong negative phase (-1.5) of the East Atlantic teleconnection pattern, which has prevailed in five of the last six months (Table E1, Fig. E7).

During February, the primary storm activity in Europe was associated with the southern branch of a pronounced split-flow pattern (Fig. E10). As a result, southeastern Europe , northern Africa , and portions of the Middle East , recorded above-average precipitation during the month, while the high latitudes of the North Atlantic , Scandinavia , and western/ central Russia recorded significantly below-average precipitation (Fig. E3).

The North Atlantic also saw a continuation of exceptionally warm SSTs at both high latitudes and in the subtropics (Fig. T18). This warmth reflects the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science, 2001).

2. Southern Hemisphere

The 500-hPa circulation pattern during February featured above-average heights over the western and central South Pacific, southern South America , and the central Indian Ocean , and below-average heights over the high latitudes of the eastern South Pacific, in the areas just west of and just east of central South America , and over southeastern Australia (Fig. E15). In South America , the main surface temperature and precipitation anomalies were associated with the persistent upper-level trough-ridge-trough pattern centered over the continent. This circulation contributed to exceptionally warm surface temperatures across southern South America , with departures in many areas exceeding the 70^th percentile of occurrences (Fig. E1). Well below-average precipitation was also observed over much of central and southern South America , in association with the persistent upper-level ridge (Fig. E3).

In Australia , anomalously warm and dry conditions in the north were consistent with a near record negative value of the SOI during February (Table T1). Anomalously cool and wet conditions in the southeast were consistent with an enhanced upper-level tough and below-average surface pressure.

In southern Africa the rainy season normally lasts from October to April. During February area-mean precipitation totals were well below-average, and generally in the lowest 20^th percentile of occurrences (Fig. E4). The largest rainfall deficits were observed in Mozambique , where totals in many areas were below the 10^th percentile of occurrences. These deficits reflected an equatorward contraction of the main region of tropical convection (Fig. T25), which occurred in association with an anomalous upper-level cyclonic circulation across southern Africa (Fig. T22). Exceptionally warm conditions were again observed in South Africa during February, along with a continuation of near-record sea-surface temperatures (ranging from 1°-2°C above average) surrounding the country. These anomalously warm SSTs began developing during June-August 2004, and have been at near-record levels since November 2004.


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