Highlights

General Circulation Features

Northern Hemisphere

During May, zonal circulation dominated the mid-high latitudes in the mean 500hPa geopotential height field. The polar vortex was weaker than normal. In the height anomaly field, negative height anomaly of –4~–8 dam dominated the northeastern Atlantic. Meanwhile, the northern North Pacific, northwest to Baikal and the regions from the central North Atlantic to Europe were covered by positive anomaly of 4~8 dam. The Northwest Pacific subtropical high was slightly stronger than normal. Its area was larger than normal and its position shifted southward.

Southern Hemisphere

During May, the mean 500hPa geopotential height field exhibited a 3-wave pattern in the mid-high latitudes. Negative height anomalies below –4 dam were over the central South Atlantic, the central South Pacific and the central South India Ocean in the regions south to 40°S, with center value below –12 dam. The Antarctic and the region south to South America were covered by positive height anomalies of 4~12 dam. The rest regions of the mid-high latitudes were near normal.

Climate for China

In May, monthly mean surface air temperatures were 1~2℃ below normal in the central and the western Tibet, parts of Xinjiang and the northwest Northeast of China. The temperatures in most of China are near or above normal. Temperatures were 1~2℃ above normal in the central and southern Northeast China and most of South China. In this month, rainfall totals of 30~200% more than normal were received in such regions as the central and western Inner Mongolia, most of Xinjiang and the northern Hunan. The monthly precipitation totals were near or below normal in rest regions of China. Rainfall totals of 30~80% less than normal were received in most of Northeast China, most of the regions between the Huaihe River and the Yangtze River.

Tropical Features

Sea Surface Temperatures (SSTs)

During May, negative SST anomalies across the eastern and central equatorial Pacific obviously enhanced and extend westward. Negative SST anomalies below –0.5℃ controlled the region east to 150°W, with the center value below –3.0℃. The SST indices of NINO 4, NINO 3 and NINO 1+2 decreased to 0.2℃, –0.9℃and –1.7℃. NINO 4, NINO 3 and NINO 1+2 indices dropped 0.3℃, 0.6℃and 0.6℃ from last month, respectively. In addition, positive SST anomalies about 0.5℃ maintained in most of the equatorial Indian Ocean.

Circulation

Wind Field

In May, at the lower troposphere (850hPa), the region of westerly anomalies expanded over the eastern equatorial Pacific and the tropical western Pacific. Zonal wind was near normal over the equatorial the India Ocean. At the upper troposphere (200hPa), westerly anomaly covered most of the equatorial Pacific. Strong westerly anomaly with center value above 12 m/s controlled the eastern equatorial Pacific where was dominated by weak easterly anomaly last month. Zonal wind was near normal in the most of equatorial India Ocean.

Southern Oscillation

The Southern Oscillation Index (SOI) was –0.5 in May.

Convection over the Tropics

During May, the anomalous outgoing longwave radiation (OLR) distribution displayed that convections enhanced over the western Pacific north to equator and the center was west to Philippines. Suppressed convection was over the eastern equatorial Pacific. In the lower troposphere (850hPa) divergent wind field over the tropical Pacific, the convergent center was around 130°E，10°N, while the divergent center was around 95°W，5°S. Correspondingly, the divergent center was over 145°E，5°N, while the convergent center was around 100°W，15°S at 200hPa.