Extratropical Highlights –May 2016

 

1. Northern Hemisphere

The 500-hPa circulation during May featured above-average heights over the Gulf of Alaska, Canada, the central North Atlantic, and northern Europe. (Fig. E9). The circulation also featured below-average heights across southeastern Europe and central Russia. At 200-hPa in the subtropics, the circulation featured anticyclonic streamfunction anomalies over the extreme eastern Pacific in both hemispheres, along with cyclonic anomalies over the western Pacific in both hemispheres (Fig. T22). These conditions partially reflected a lingering atmospheric response to the dissipating El Niño (Figs. T18, T25).

The main land-surface temperature signals during May included above-average temperatures across Alaska, western Canada, and northern Europe (Fig. E1). The main precipitation signals included above-average totals in the southeastern U.S. and in Texas, and also across central/ southern Europe, eastern China and eastern Mongolia (Fig. E3).

 

a. North Pacific/ North America

The 500-hPa circulation during May featured above-average heights over the Gulf of Alaska and Canada, and below-average heights over the southwestern U.S. (Fig. E9). This overall circulation reflected a pronounced split-flow pattern over the western United States, with a broad trough extending across the southern U.S.

Overall, surface temperatures remained well above average in Alaska and western Canada, with departures over most of Alaska exceeding the 90^th percentile of occurrences (Fig. E1). Precipitation was above average across portions of the southern and southeastern U.S., with the most significant surpluses recorded in Texas and in the mid-Atlantic region (Figs. E3, E5).

According to the U.S. Drought Monitor, exceptional or extreme drought continued across central and southern California during May, while extreme or severe drought persisted in western Nevada. However, the spatial coverage of these drought areas continued to decrease during May. Also, moderate drought persisted in southeastern Oregon, and in the southern half of both New Mexico and Arizona. Moderate drought developed during the month in northern Alabama and Georgia, and in eastern Tennessee.

 

b. Europe

The 500-hPa circulation during May featured an amplified ridge over northern Europe and a broad tough across southern Europe (Fig. E9). This pattern was associated with above-average surface temperatures across northern Europe (Fig. E1). It was also associated with a above-average totals across northern Europe and below-average precipitation totals across central and southern Europe (Fig. E3). For southern Europe, area-averaged totals were the largest in the 1979-present record for May (Fig. E4).

 

2. Southern Hemisphere

The mean 500-hPa circulation during May featured above-average heights to the south of South America and Africa, and below-average heights to the south of Australia (Fig. E15). At 200-hPa in the subtropics, the circulation featured anticyclonic streamfunction anomalies over the extreme eastern Pacific in both hemispheres, and cyclonic anomalies over the western Pacific in both hemispheres (Fig. T22). These conditions partially reflected a lingering atmospheric response to the dissipating El Niño (Figs. T18, T25).

The main surface temperature signals during May included significantly above-average temperatures in eastern and northern Australia, and below-average temperatures in central South America (Fig. E1). The main precipitation signal during May was a continuation of below-average totals in eastern Australia (Fig. E3).