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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
Extratropical Highlights - September 2003

1. Northern Hemisphere

The 500-hPa circulation during September 2003 featured generally above-average heights in the extratropics, with the major areas of persistent positive anomalies observed over the western United States, eastern Canada, and much of Eurasia (Figs. E10, E12). Smaller areas of negative height anomalies were observed over the central United States, the central North Pacific, and the polar region.

Large areas of significantly warmer-than-average conditions during the month generally coincided with the persistent positive height anomalies (Fig. E1), while cooler-than-average temperatures were confined to the eastern half of the United States and eastern Alaska. Prominent precipitation anomalies during September included a continuation of above-average totals in the eastern United States, and below-average totals in Europe (Fig. E3).

a. North America

The mean circulation featured amplified upper-level ridges over western North America and eastern Canada (Fig. E10), and a trough over the central United States. The entire eastern U.S. was situated in the region of large-scale ascending motion downstream of the mean trough axis. This region experienced well above-average rainfall (Figs. E3, E6), with totals exceeding the 90th percentile in portions of the Mid-Atlantic States. This excess rainfall was also associated with Hurricane Isabele, which produced totals of 100 mm and more across portions of North Carolina, and Virginia. The Ohio Valley, Mid-Atlantic and Southeast regions have recorded above-average precipitation in almost every month since September 2002 (Fig. E5), and the Gulf Coast region has recorded above-normal rainfall for four consecutive months.

Well above-average surface temperatures were again observed in the western U.S. and central Canada during September, with the anomalous warmth also extending across eastern Canada and the North Atlantic (Fig. E1). In contrast temperatures were below-average in the eastern U.S. in the vicinity of the mean upper-level trough axis, with some areas in the deep South recording temperatures in the lowest 10th percentile of occurrences.

b. Europe

A strong upper-level ridge again prevailed across Europe during September (Fig. E10), resulting in continued warmer and drier than average conditions over most of the continent. Monthly mean temperatures exceeded the 70th percentile throughout Europe (Fig. E1) and rainfall totals were generally below the 30th percentile (Fig. E3). Area-averaged rainfall totals in both northern and southern Europe have been significantly below-average since March (Fig. E4).

However, slightly above-average totals were observed in extreme southern Europe during September, which marks the first time since March 2003 that this region has not experienced significant rainfall deficits. This increased rainfall was associated with storm activity within the southern branch of a fairly strong split flow pattern at upper levels (Figs. E10, T22).

c. West Africa

The West African monsoon season typically lasts from May-October, with the largest totals observed during June-September. The 2003 monsoon season featured slightly above-average rains across the African Sahel and African Sudan throughout this four-month period (Fig. E4), resulting in an above-average monsoon season. A characteristic feature of above-average monsoon seasons rains is the anomalous northward extent of the convective rainfall.

Considerable rainfall occurred as far as 20̊N during September, in association with a series of strong easterly disturbances moving along the equatorward flank of the African easterly jet (at 700-hPa), and with an overall northward shift of the associated wave energy and deep tropical convection. As is typical of active West African monsoon seasons these conditions were related to an overall northward shift of the mean African easterly jet throughout the season, and to above-average cyclonic shear along the equatorward flank of this jet stream overspreading the heart of the monsoon region (Fig. T20, bottom).

2. Southern Hemisphere

a. Circulation

In the Southern Hemisphere the 500-hPa circulation during September featured an anomalous zonal wave-3 pattern, with above-average heights over the central ocean basins, and below-average heights generally situated poleward of the three continents (Fig. E16). This anomaly pattern was associated with enhanced upper-level westerlies along the southern portions of the three continents (Fig. E17), and with a pronounced anomalous anticyclonic circulation across Australia. (Fig. T21).

Large portions of eastern Australia experienced a continuation of above-average temperatures during September (Fig. E1). The anomalous warmth also covered northern New Zealand, where monthly mean temperatures again averaged above the 70th percentile. In southeastern Australia the anomalous anticyclonic circulation was associated with significantly below-average rainfall, with monthly totals averaging below the 10th percentile

b. Stratospheric Ozone Hole

The stratospheric ozone hole over Antarctica (Fig. S6) reached record size during August and September (Fig. S8, top), covering close to 25 x 106 km2 by late August and reaching 27 x 106 km2 in September. This large ozone hole is related to record or near-record low temperatures at both 10-hPa and 2-hPa during June-September (Fig. S4), which resulted in an above-average amount of polar stratospheric cloud (PSC) cover (Fig. S8, bottom). The PSCís are comprised mostly of nitric acid and water, and form when stratospheric temperatures drop below -78oC. These clouds provide an ideal surface upon which inert chlorine compounds can react with sunlight to form active chlorine compounds such as chlorine peroxide. As sunlight reaches the polar region in August and September the resulting reactions destroy nearly all stratospheric ozone, often to altitudes of 20 km.

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