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

1. Northern Hemisphere

The 500-hPa circulation during October 2003 featured above-average heights in the polar region, the high latitudes of both the North Pacific and North Atlantic, and the western United States, and below-average heights across the central North Pacific, eastern North America and Europe (Figs. E10, E12). Areas experiencing significantly warmer-than-normal temperatures during the month generally coincided with the persistent positive height anomalies, and included the western U.S., most of Alaska and Canada, and the North Atlantic (Fig. E1). The main region of below-average temperatures during October was Europe, which had previously experienced significantly warmer-than-normal conditions during March-September 2003. Prominent precipitation anomalies during October included above-average totals over southern and eastern Europe, and below-average totals over large portions of the U.S., eastern China and Japan (Fig. E3).

a. North America

The mean circulation featured an amplified upper-level ridge over western North America, and amplified troughs over the central North Pacific and eastern North America (Fig. E10). This circulation was associated with a pronounced eastward extension of the East Asian jet stream to the northwestern U.S. (Figs. E11, T21), and with an anomalous flow of relatively mild air into western North America which contributed to a continuation of above-average surface temperatures throughout the region (Fig. E1). The most significant temperature departures were observed in the inter-mountain region of the western U.S., where temperatures exceeded the 90th percentile and averaged more than +2oC above average during the month.

Above-average precipitation in the northwestern U.S. and southwestern Canada during October (Figs. E3, E6) is related to the upper-level divergence and increased storm activity found upstream of the mean ridge axis and along the cyclonic-shear side of the East Asian jet exit region. The northeastern states also experienced above-average precipitation during October, due mainly to increased storminess and large-scale ascending motion immediately downstream of the mean trough axis.

In contrast, much of the remainder of the U.S. was situated within and downstream of the mean upper-level ridge axis, and experienced near-normal to below-normal precipitation during the month. For the central and eastern states this represents a marked departure from the above-normal precipitation totals observed during the past five months, and in some areas for the past year (Fig. E5). For example, the Ohio Valley, Mid-Atlantic, and Southeast regions recorded precipitation surpluses in almost every month since September 2002, while the Gulf Coast and Great Lakes regions recorded above-normal rainfall during May-September, and June-September 2003, respectively.

b. Europe

A strong ridge-trough system spanned the North Atlantic and Europe during October (Fig. E10), resulting in below-average temperatures over most of Europe (Fig. E1) and significantly below-average precipitation over portions of Great Britain and Scandinavia (Fig. E3). This circulation was also associated with a pronounced split flow pattern over the eastern North Atlantic, with the strong southern branch contributing to above-average precipitation across southern and eastern Europe. Much of the remainder of Europe experienced near-normal precipitation during the month. These overall conditions are in marked contrast to the mean upper-level ridge and the associated exceptionally warm and dry conditions (Fig. E4) that had prevailed across Europe since the March.

2. Southern Hemisphere

a. Circulation

In the Southern Hemisphere the 500-hPa circulation during October 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). Above-average heights also covered most of the polar region during October, resulting in a slight weakening of the polar vortex in the region of Antarctica (Fig. E17).

In the middle latitudes this circulation was associated with enhanced jet stream winds across Australia (Fig. T21), with the northern part of the country situated in anomalous anticyclonic shear equatorward of the jet core and the southeastern part of the country situated in anomalous cyclonic shear poleward of the jet core. This anomalous cyclonic circulation extended down to 850-hPa, where it contributed to an anomalous inflow of cold air across southern Australia (Fig. T20). Surface temperatures across the region were below the 30th percentile of occurrences, with temperatures in the southeast dropping below the 10th percentile (Fig. E1).

The anomalous cyclonic circulation over southern South America contributed to generally warmer and wetter-than-average conditions across central and southern Argentina, with much of the region situated in broad southwesterly flow immediately downstream of the mean trough axis.

b. Stratospheric Ozone Hole

The polar stratospheric circulation during October (Fig. S1, top) reflected an overall shift of the Antarctic circumpolar vortex and the associated ozone hole (Fig. S6, bottom) toward eastern Antarctica. Mean 100-50-hPa temperatures within the polar vortex averaged more than 8oC below-normal during the month (Fig. S2).

The 2003 ozone hole reached record size from August through early October (Fig. S8, top), when it covered close to 25 x 106 km2. It then shrank considerably during the second half of October, and was down to 10 x 106 km2 by the end of the month.

The large 2003 ozone hole is related to significantly below-average stratospheric temperatures, which resulted in an increased polar stratospheric cloud (PSC) cover (Fig. S8, bottom). The decreased size of the ozone hole during October was associated with a significant drop in PSC coverage. 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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