Extratropical Highlights –April 2015

 

1. Northern Hemisphere

The mean 500-hPa circulation during April featured above-average heights across the central North Pacific and Europe, and below-average heights over the high latitudes of the North Pacific, eastern Canada, the middle and high latitudes of the North Atlantic, and western Russia (Fig. E9). 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 have links to El Niño (Fig. T25).

The main land-surface temperature signals during April included above-average temperatures across the eastern half of the U.S., Alaska, Scandinavia, western Europe, and central Siberia (Fig. E1). The main precipitation signals included above-average totals in the southeastern and eastern U.S., and below-average totals in the northwestern U.S., and southern Europe (Fig. E3).

 

a. North Pacific/ North America

Over the North Pacific, the mean 500-hPa circulation during April featured above-average heights in the middle latitudes and below-average heights at high latitudes (Fig. E9). Over the central subtropical Pacific, anti-cyclonic streamfunction anomalies at 200-hPa reflected El Niño (Fig. T22).

Over the U.S., a broad trough was present across the west, with much of the southeastern U.S. situated in an area of anomalous southwesterly flow downstream of the trough axis (Fig. E14). This circulation contributed to above-average surface temperatures across the eastern half of the U.S., with monthly departures generally above the 70^th percentile of occurrences (Fig. E1). Similar levels of anomalous warmth were observed across most of Alaska.

The southeastern quadrant of the U.S., in addition to above-average surface temperatures, recorded above-average precipitation during April. Most areas recorded departures in the upper 70^th percentile of occurrences, while the western Gulf Coast region experienced totals in the upper 90^th percentile of occurrences (Fig. E3). Area-averaged monthly rainfall totals in the Gulf Coast region were the largest in the 1979-2015 record, while those in the Southeast and Ohio Valley regions were in the upper 90^th percentile of occurrences (Fig. E5). In contrast, the Pacific Northwest region of the U.S. recorded below-average precipitation for the second month, and southern California recorded a continuation of below-average precipitation for the 4^th straight month.

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

 

b. Eurasia

The 500-hPa circulation featured above-average heights across Europe, and below-average heights across the North Atlantic, Greenland, and western Russia (Fig. E9). This pattern was associated with well below-average precipitation in southern Europe (Fig. E3), where area-averaged totals were the lowest in the 1979-2015 record (Fig. E4). It was also associated with above-average surface temperatures in central Siberia in the region downstream of the mean trough axis. This area has recorded warmer than average conditions for the past three months.

 

2. Southern Hemisphere

The mean 500-hPa circulation during April featured an anomalous zonal wave-3 pattern, with above-average heights located generally south of the three continents, and below-average heights over southeastern Australia and Antarctica (Fig. E15).

At 200-hPa, anti-cyclonic streamfunction anomalies over the central subtropical Pacific reflected El Niño (Fig. T22). Anti-cyclonic streamfunction anomalies were also observed over western Australia and in the area west of South Africa (Fig. T22).

In Australia, the amplified ridge was associated with an anomalous flow of air from the Great Australian Bight into the central part of the continent (Fig. T20). This flow resulted in below average surface temperatures across the region (Fig. E1).

The South African rainy season lasts from October to April. Rainfall during April was above average for the region as a whole (Fig. E4), with the main surpluses recorded in the northeast (Fig. E3). Overall, the 2015 South African rainy season featured above average rainfall in December and April, and below average totals during October- November and January- March (Fig. E4).