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

1. Northern Hemisphere

The 500-hPa circulation during May featured above average heights across the North Pacific from Japan to Alaska, over the southern United States, the eastern North Atlantic, and central Russia, and below-average heights over the central subtropical North Pacific, Canada, Europe, and northern China (Fig. E9). The circulation anomalies over the North Pacific and North America were especially persistent during the month (Fig. E11), and contributed to a near-record negative value (-3.5) of the Pacific/ North American (PNA) teleconnection index (Table E1, Fig. E7).

            Prominent temperature departures during May included warmer than average conditions across the southern U.S. and central Russia, and well below-average temperatures across Canada (Fig. E1). The prominent precipitation anomalies during May includeded above-average rainfall across the Midwest and Great Lakes regions of the United States, and below-average totals over the extreme southeastern U.S. and western North Atlantic (Figs. E3, E5).

a. Pacific/North America

The circulation over the Pacific/North American sector reflected a near-record amplitude (-3.5) for the negative phase of the PNA teleconnection pattern (Table E1, Fig. E7). Key aspects of this pattern included above-average 500-hPa heights over the Gulf of Alaska and southern U.S., and below-average heights over the central subtropical North Pacific and Canada (Fig. E9). Associated with this anomaly pattern the mean upper-level ridge and trough axes were located over Alaska and western Canada, approximately 30 west of their climatological positions over the Rocky Mountains and eastern Canada, respectively.

In the middle latitudes the negative PNA pattern was associated with a pronounced weakening and westward retraction of the East Asian jet in the vicinity of the date line, and with increased diffluence throughout the jet exit region (Figs. T21, T22). It was also associated with an anomalous jet core across the northern U.S. and southeastern Canada (Fig. E10).

            Over North America this anomalous jet stream is consistent with an enhanced north-south surface temperature gradient, as indicated by near-record cold surface temperatures (2-5 below average) across most of Canada and anomalously warm temperatures (1.0-1.5C above average) across the southern and central U.S. (Fig. E1). In Canada surface temperatures were generally in the lowest 10th percentile of occurrences. These unusually cold temperatures are related to the northwesterly flow of arctic air from Alaska and the Beaufort Sea into western Canada in the area downstream of the mean upper-level ridge axis (Figs. E8, E10).

            The anomalous circulation across North America also contributed to above-average rainfall across the northern United States. The largest surpluses were observed in the Midwest and Great Lakes regions where area-averaged totals reached the 90th percentile of occurrences (Fig. E3). These regions were situated downstream of the mean upper-level trough axis within the region of enhanced surface temperature contrast.

b. North Atlantic

North Atlantic SSTs have been considerably above average during the past year. During May they remained above the 90th percentile across the high latitudes of the North Atlantic (Fig. E1), which contributed to ongoing warmth across Iceland and parts of Great Britian. This anomalous warmth is likely associated with the ongoing warm phase of the Atlantic multi-decadal mode that began in approximately 1995.

2. Southern Hemisphere

In the Southern Hemisphere the 500-hPa circulation during May featured above-average heights over the central South Pacific, southern South America, and southern Africa, and below-average heights over southern Australia, at high latitudes over the eastern South Pacific and the central South Atlantic, and over the central Indian Ocean (Fig. E15).

Over Australia the circulation reflected a strengthening and equatorward shift of the jet stream (Fig. T21). Southeastern Australia experienced upper-level convergence (Fig. T23) and descending motion within the right entrance region of the jet, which resulted in below-average precipitation during the month (Fig. E3). An anomalous inland penetration of cold air at 850-hPa from the Great Australian Bight was also evident (Fig. T20), which resulted in anomalously cool surface temperatures across southeastern Australia (Fig. E1).

Much of central extratropical South America experienced significantly below-average surface temperatures during May, with temperatures in many regions east of the Andes Mountains falling within the lowest 10th percentile of occurrences. These cool temperatures appear to be related to the significant weakening of the westerly winds and associated downslope flow equatorward of the strong upper-level ridge (Figs. T20, T21, E15).          The below-normal temperatures in this region and over parts of Australia contributed to the lowest value of the Southern Hemisphere land-only temperature time series since 1992 (Fig. E2, bottom).


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