Extratropical Highlights –March 2015

 

1. Northern Hemisphere

The mean 500-hPa circulation during March featured above-average heights in the middle latitudes and below-average heights at high latitudes (Fig. E9). Regional anomalies included above-average heights over the western U.S. and across northern Europe/ northwestern Russia, and below-average heights over the central North Pacific and the high latitudes of the North Atlantic. At 200-hPa, anti-cyclonic streamfunction anomalies in both hemispheres over the central subtropical Pacific reflected eastward extensions of the mean subtropical ridges (Fig. T22). These conditions have persisted for two months and appear to have links to enhanced convection over the central equatorial Pacific (Fig. T25).

The main land-surface temperature signals during March included above-average temperatures across western North America, Scandinavia, and most of Asia, and below-average temperatures in eastern Canada (Fig. E1). The main precipitation signals included above-average totals in Mexico, the south-central U.S., and southern Europe, and below-average totals in California, the northern U.S., and the central Plains states (Fig. E3).

 

a. North Pacific/ North America

The mean 500-hPa circulation during March featured above-average heights across the high latitudes of the North Pacific and the United States, and below-average heights over the central North Pacific and eastern Canada (Fig. E9). These conditions were associated with a continuation of exceptionally warm (Fig. E1) and dry conditions (Fig. E3) in the western U.S., with much of the region recording temperature departures in the upper 90^th percentile of occurrences and precipitation totals in the lowest 30^th percentile of occurrences. Elsewhere, precipitation was above average in the south-central U.S. and Mexico, and below average across the northern U.S. and the central Plains states.

The U.S. Drought Monitor indicated a continuation of exceptional drought in central/ southern California and western Nevada, with extreme or severe drought extending northward into central Oregon and southern Idaho. Also, areas of exceptional or extreme drought expanded in north-central Texas and large portions of western Oklahoma.

 

b. Eurasia

The 500-hPa circulation featured an extensive north-south dipole pattern of height anomalies across the North Atlantic and western Russia, with above-average heights in the middle latitudes and below-average heights across the high latitudes of the North Atlantic (Fig. E9). This pattern was associated with an extensive southwesterly flow of milder air into northern Europe for a third straight month (Fig. E10). This warmth contributed to a continuation of well above-average surface temperatures across most of Asia, with many areas recording monthly departures exceeding 3^oC (Fig. E1).

 

2. Southern Hemisphere

The mean 500-hPa circulation during March featured an anomalous zonal wave-3 pattern, with above-average heights at high latitudes over the central ocean basins, and below-average heights south of Australia and over the high latitudes of the eastern South Pacific (Fig. E15). At 200-hPa, anti-cyclonic streamfunction anomalies were observed over central Australia, while cyclonic streamfunction anomalies covered southern Africa (Fig. T22). Over Australia, above-average precipitation was observed in the west in the area upstream of the amplified ridge, while descending motion and suppressed precipitation persisted across the eastern half of Australia downstream of the ridge axis (Fig. E3).

The South African rainy season lasts from October to April. Rainfall during March was below average for the region as a whole (Fig. E4), with the main deficits recorded in the northeast (Fig. E3). Suppressed rainfall during the last two months appears to be linked to an anomalous upper-level trough centered over southern Africa (Fig. T22). To date, the South African rainy season was below average during October- November and January- March.  Rainfall was above average only in December (Fig. E4).