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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
Extratropical Highlights - March 2007

1. Northern Hemisphere   

The 500-hPa height pattern during March featured positive anomalies over the high latitudes of the central North Pacific Ocean , the United States , the eastern North Atlantic Ocean , and Scandinavia , and negative anomalies over Alaska and the western Mediterranean Sea (Figs. E9, E11). The 200-hPa streamfunction anomaly pattern showed a strong inter-hemispheric symmetry of anomalies across the subtropical Pacific Ocean , with anomalous ridges west of the date line, and anomalous troughs east of the date line (Fig. T22). This pattern is consistent with enhanced equatorial convection west of the date line and suppressed convection east of the date line (Fig. T25).

      The main surface temperature departures during March reflected warmer-than-average conditions across the United States , Europe , and portion of China (Fig. E1). For central and eastern China , this marks the third straight month with temperatures in the upper 10th percentile of occurrences (Fig. E1). The main precipitation anomalies during March included above-average totals in the Plains states and southwestern Canada , and below-average totals in the southeastern U.S. and Scandinavia (Fig. E3). 

a. Pacific/ North America

In the subtropics, the upper-level circulation during March strongly reflected the pattern of anomalous convection across the equatorial Pacific, with enhanced subtropical ridges flanking the region of enhanced convection and enhanced mid-Pacific troughs flanking the region of suppressed convection (Figs. T22, T25). These conditions are known to be associated with an enhanced split flow in the exit region of the East Asian jet stream (Fig. T21) and with high-latitude blocking over the central North Pacific, as was observed during March.

The 500-hPa circulation pattern featured persistent positive height anomalies across North America and persistent negative height anomalies over Alaska in the region immediately downstream of the persistent blocking ridge (Figs. E9, E11). This pattern was associated with well above-average temperatures across the United States , with departures of 3-4C observed in many regions (Fig. E1). In contrast, much of Alaska experienced well below-average temperatures in the region downstream of the blocking ridge, where departures in many regions were in the lowest 10th percentile of occurrences.

The circulation pattern over the eastern U.S. was also associated with a marked eastward shift of the mean Hudson Bay low to the western North Atlantic . The resulting anomalous zonal flow and reduced storm activity contributed to exceptionally dry conditions in portions of the Ohio Valley , Midwest , Gulf Coast , and mid-Atlantic regions, with totals in the latter three regions falling into the lowest 10th percentile of occurrences (Figs. E3, E5).

b. North Atlantic / Europe

The 500-hPa circulation pattern during March featured persistent positive height anomalies over the eastern North Atlantic and Scandinavia (Fig. E9). These conditions were associated with an anomalous flow of mild, marine air into the continent, which contributed to well above-average temperatures during the month. The most anomalous warmth was centered across northern Europe and Scandinavia , where temperatures at many locations were in the upper 90th percentile of occurrences. These regions also experienced well below-average precipitation, which coincided with the mean position of the upper-level ridge.

2. Southern Hemisphere

      The 500-hPa circulation pattern during March featured positive height anomalies in the polar region and generally negative height anomalies across the higher mid-latitudes (Fig. E15). In the subtropics, the upper-level circulation featured an anomalous ridge over Australia and the western South Pacific, and an enhanced mid-Pacific trough east of the date line (Fig. T22). This anomaly pattern is consistent with the anomalous equatorial convection across the Pacific basin (Fig. T25).

      In Australia , the persistent upper-level ridge contributed to extremely warm temperatures in the east. In southern Africa , the rainy season extends from October to April. For the region as a whole, precipitation was slightly below average during March, with the most significant dryness occurring in northern Mozambique (Figs. E3, E4). So far during the 2006-07 rainy season, area-averaged totals were below-normal in October, February, and March, near-normal in November and December, and above normal in January.



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