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

1. Northern Hemisphere

The 500-hPa circulation pattern during October featured above-average heights across the high latitudes of the North Pacific and the North Atlantic, and below-average heights over the western United States , the central North Atlantic, and eastern Siberia (Figs. E9, E11). This pattern was associated with strong negative phases of both the North Atlantic Oscillation (NAO, -1.2) and the East Pacific teleconnection pattern (-1.6) (Table E1, Fig. E7).

The main temperature departures during October included warmer than normal conditions over Alaska , the southeastern U.S. , and Europe (Fig. E1). The main precipitation departures included above-average totals over the western and central United States and western Europe, and near record-low totals over eastern China (Figs. E3, E4).

a. Pacific/North America

The mean 500-hPa circulation pattern during October featured above-average heights over the eastern North Pacific and eastern Canada , and below-average heights over the western United States . These conditions were associated with above-average temperatures over Alaska , the southeastern U.S. , and eastern Canada (Fig. E1). They were also associated with increased storminess and above-average precipitation across the western and central U.S. (Fig. E3). Precipitation totals in the Pacific Northwest, Southern California, Inter-Mountain, and Great Plains regions were significantly above-average during the month, with area-average totals generally above the 80th percentile of occurrences.

b. North Atlantic/Europe

During October above-average heights at 500-hPa covered the high latitudes of the North Atlantic and below-average heights spanned the central North Atlantic . This anomaly pattern reflected a strong negative phase of the NAO (Table E1), and was associated with an anomalous southward shift of the mean North Atlantic jet stream toward southern Europe (Fig. T21). This circulation was associated with a continuation of well above-average surface temperatures across the high latitudes of the North Atlantic and Europe , and with increased storminess and above-average precipitation in western Europe.

c. Eastern China

Eastern China experienced significantly below-average precipitation during October, with area-average totals generally in the lowest 10th percentile of occurrences (Fig. E3). This dryness represents a continuation of below-average totals that have plagued the country for the past year (Fig. E4). During October the below-average precipitation was related to enhanced upper-level convergence (Fig. T23) occurring within an anomalous left jet exit region upstream of the mean trough axis (Figs. T21, T22).

2. Southern Hemisphere

The 500-hPa circulation pattern during October featured above-average heights over the central South Atlantic, southern Australia , and the polar region, and below-average heights over the high latitudes of the South Pacific (Fig. E15). At 200-hPa anticyclonic streamfunction anomalies prevailed across the South Pacific from southern Australia to South America (Fig. T22).

In Australia these conditions contributed to above-average surface temperatures throughout the continent (Fig. E1). They also produced significantly below-average precipitation in the southeast, with departures in some areas dropping below the 10th percentile of occurrences (Fig. E3).

In southern Africa the rainy season normally lasts from October to April. The beginning of the 2004-2005 rainy season began with slightly below-average precipitation in eastern South Africa , and slightly above-average totals in northern Mozambique and southern Tanzania .

The September-October 2004 Antarctic ozone hole was below the 1994-2003 mean, in response to a reduced polar stratospheric cloud (PSC) cover compared to recent years (Fig. S8). These conditions were related to above-average heights and temperatures over Antarctica in association with a below-average strength of the circumpolar vortex (Fig. E15).

 


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