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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
 
Extratropical Highlights - May 2005
 

1. Northern Hemisphere

      

The 500-hPa circulation pattern during May featured above-average heights over northeastern Canada, across the Mediterranean Sea, and over western Russia, and below-average heights across the central North Pacific, the eastern United States and Scandinavia (Fig. E9). These circulation anomalies were associated with a strong negative phase (-1.3) of the North Atlantic Oscillation (NAO) and a strong positive phase (+1.7) of the Pacific/ North American teleconnection pattern (PNA) (Table E1, Fig. E7). Another prominent aspect of the circulation pattern was cyclonic streamfunction anomalies at 200-hPa extending from the eastern subtropical North Atlantic to central Asia in the Northern Hemisphere, and from the eastern subtropical South Atlantic to the central Indian Ocean in the Southern Hemisphere (Fig. T22). The pronounced inter-hemispheric symmetry to this pattern suggests a link to the suppressed convection seen across the central and eastern equatorial Atlantic and equatorial western Africa (Fig. T25).

The main surface temperature departures during May are above-average temperatures across Alaska, southern Europe, western Russia, and both the high latitudes and subtropical latitudes of the North Atlantic (Fig. E1). The main precipitation anomalies reflected above-average totals in the northwestern U.S., northern Europe, and central China, and below-average totals in the eastern U.S. and southern Europe (Figs. E3, E5, E6).

 

a. Pacific/ North America

The main 500-hPa circulation anomalies affecting the Pacific/ North American sector during May included above-average heights across Alaska and eastern Canada, and over the southwestern U.S., and below-average heights across the central North Pacific and the northeastern United States (Fig. E9). In the middle latitudes, this anomaly pattern reflected a strong positive phase (+1.7) of the Pacific/ North American teleconnection pattern (PNA) (Table E1, Fig. E7). This pattern was associated with an enhanced East Asian jet stream across the central and eastern North Pacific, and with an eastward extension of the jet exit region to the eastern North Pacific (Figs. E10, T21).

This circulation resulted in above-average precipitation throughout the northwestern U.S., with monthly totals in the Pacific Northwest exceeding the 90th percentile of occurrences (Fig. E3). Area-averaged monthly totals in the Inter-Mountain region of the western U.S. reached the 80th percentile of occurrences, making this the 7th consecutive month of above-average precipitation in that region (Fig. E4). Southern California also experienced a continuation of above-average precipitation during May. This region has also recorded large precipitation surpluses since October 2004. In contrast, monthly precipitation totals were generally below average over the eastern United States in the region upstream of the mean upper-level trough axis.

Both Alaska and northeastern Canada experienced well above-average temperatures during May, with departures in both regions averaging 2-3C above normal and exceeding the 90th percentile of occurrences (Fig. E1). In contrast, exceptionally cool temperatures were observed across the eastern U.S. in association with anomalous northwesterly flow upstream of the mean upper-level trough axis.

 

b. North Atlantic, Eurasia

The 500-hPa circulation pattern during May featured a strong negative phase (-1.3) of the NAO, with above-average heights over the high latitudes of the western and central North Atlantic, and below-average heights extending across the eastern U.S. and western North Atlantic (Fig. E9). Farther downstream, persistent negative height anomalies were centered over Scandinavia and persistent positive height anomalies were observed across southern Europe and most of western Russia. These conditions contributed to above-average precipitation in northern Europe and below-average precipitation in southern Europe. They also contributed to exceptionally warm temperatures throughout western Russia, with monthly values averaging +2-3C above average and exceeding the 90th percentile of occurrences.

Exceptionally warm SSTs were again observed throughout the high latitudes and subtropical latitudes of the North Atlantic, with departures in both regions exceeding the 90th percentile (Figs. T18, E1). This anomaly pattern 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 mean 500-hPa circulation pattern during May featured above-average heights from Australia eastward to the central South Pacific, and over the central South Atlantic, and below-average heights over the low latitudes of the South Pacific, southern South America, and the central Indian Ocean (Fig. E15). The pattern of positive height anomalies was very persistent during the month (Fig. E17), and was associated with large temperature and rainfall departures across both Australia and southeastern South America.

In Australia anomalously warm and wet conditions were observed west of the mean ridge axis and anomalously cool and dry conditions were observed east of the ridge axis (Figs. E1, E3). In southeastern South America, anomalously warm and wet conditions were observed in the area upstream of the anomalous upper-level ridge, with monthly temperature and precipitation departures both exceeding the 90th percentile of occurrences in this region. Southern South America also recorded above-average precipitation during May, in association with enhanced storminess immediately downstream of the mean upper-level trough axis.

 


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