Highlights

General Circulation Features

Northern Hemisphere

During April, the mean 500hPa geopotential height field exhibited a 4–wave pattern in the mid–high latitudes. Troughs over eastern Asia and eastern North America were deeper than normal. In the height anomaly field, negative anomalies dominated northeastern Asia, the west coast of North America and the regions from northeastern North America to the northern North Atlantic, with minimum value below –8 dam over northeastern Asia. Positive anomalies covered Norwegian Sea and the northern Pacific, with maximum value above 12 dam. The Northwest Pacific subtropical high was obviously stronger than normal and its area was obviously larger than normal. Meanwhile, its position slightly shifted southward.

Southern Hemisphere

During April, the mean 500hPa geopotential height field exhibited a 3–wave pattern in the mid–high latitudes. Troughs over the southern Indian Ocean, southern Pacific and southern Atlantic were correspondent with negative anomalies, with minimum anomaly below –12 dam over the southern Pacific. The ridge to the south of Australia was correspondent with positive anomalies, with center anomaly above 12 dam.

Climate for China

During April, monthly mean surface air temperatures were near or above normal in most of China, with 2~3℃ above normal in most of Northeast China and southeastern Southwest China. But temperatures were 1~3℃ below normal in northern Xinjiang. In this month, precipitation totals were less than 30% below normal in most of Northeast China, Southwest China and southern South China. In the rest regions of China, monthly precipitation totals were near or above normal, with more than 100% above normal in parts of Xinjiang, Tibet, Qinghai, Inner Mongolia, Hebei and Shandong provinces.

Tropical Features

Sea Surface Temperatures (SSTs)

Positive SST anomalies across the central equatorial Pacific decreased to 0.5℃ during April. Meanwhile, negative SST anomalies dominated regions to the east of 135°W equatorial Pacific, with the minimum below –2.5℃ in the offshore of South America. The SST indices of NINO 4, NINO 3 and NINO 1+2 were 0.5℃, –0.3℃ and –1.1℃, respectively. In addition, positive SST anomalies above 0.5℃ still dominated the most of equatorial Indian Ocean.

Circulation

Wind Field

During this month, at the troposphere, zonal wind was near normal over the equatorial Pacific and equatorial Indian Ocean. At the lower troposphere (850hPa), westerly anomalies over the eastern and central equatorial Pacific obviously weakened. Meanwhile, easterly anomalies over the east part of western Pacific slightly weakened and westerly anomalies over the west part of western Pacific strengthened. At the upper troposphere (200hPa), weak easterly anomalies dominated the eastern equatorial Pacific and weak westerly anomalies dominated the west part of western equatorial Pacific.

Southern Oscillation

The Southern Oscillation Index (SOI) was –0.1.

Convection over the Tropics

During April, the anomalous outgoing longwave radiation (OLR) distribution displayed that enhanced convection center around the dateline during last month obviously shifted northward and suppressed convection remained over the eastern and central equatorial Pacific. In the lower troposphere (850hPa) divergent wind field over the tropical Pacific, the convergent center was around 150°E，5°N, while the divergent center was around 80°W，20°S. Correspondingly, at 200hPa, the divergent and the convergent center were around 160°E，0° and 90°W，20°S, respectively.