Highlights

General Circulation Features

Northern Hemisphere

During April, the mean 500hPa geopotential height field exhibited a 4-wave pattern in mid-high latitudes. In the anomaly field, negative anomalies dominated the region from eastern Europe to the Northwest Pacific, with center values below –8 dam. Positive anomalies above 4 dam were observed in Polar region, North Europe and northwestern North America, with center values above 16 dam. In this month, both the Northern Hemisphere subtropical high and the Northwest Pacific subtropical high were stronger than normal.

Southern Hemisphere

In the mean 500-hPa geopotential height field during April，zonal circulation dominated the mid-high latitudes. In the height anomaly field, below-normal heights were observed over the southeastern Pacific, the southeastern Atlantic and the east-central India Ocean. Above-normal heights were observed over most of Antarctica and the region around.

Climate for China

During April, the mean surface air temperatures were above average in much of China. The positive anomalies above 2℃ were observed in much of Northwest China and North China, Central China and western Tibet, with the maximum values above 4℃. In this month, rainfall totals were above 50% more than normal in Yunan, southern Tibet, southwestern Xinjiang, western Hainan and parts of North China. Rainfall totals in the other regions of China were near or below normal. The monthly precipitation totals were 50~90% less than normal in central and eastern Northwest China, the western regions between the Yellow River and the Huaihe River and middle of Northeast China.

Tropical Features

Sea Surface Temperatures (SSTs)

During April, the negative SST anomalies below -0.5℃ were observed over the equatorial Pacific east to 120°W. The negative anomalies near South America weakened. The positive anomalies above 0.5℃ maintained in the western equatorial Pacific. Sea surface temperatures were near normal in the central equatorial Pacific. Correspondingly, the SST indices of NINO 4, NINO 3 and NINO 1+2 regions were 0.3℃, 0℃ and -0.3℃, respectively. In addition, positive anomalies above 0.5℃ dominated much of the equatorial India Ocean.

Circulation

Wind Field

In April, the zonal wind anomalies at the lower troposphere (850hPa) over the equatorial Pacific changed little compared with that in March. The weak easterly anomalies covered the region around the dateline. Westerly anomalies about 2m/s remained in the rest regions of the equatorial Pacific. Easterly anomalies about 2~4m/s controlled the equatorial India Ocean. At the upper troposphere (200hPa), weak westerly anomalies were observed in the central and eastern equatorial Pacific and the equatorial India Ocean. Weak easterly anomalies were observed over the western equatorial Pacific.

Southern Oscillation

During April, the Southern Oscillation Index was -1.3.

Convection over the Tropics

During April, the anomalous outgoing longwave radiation (OLR) distribution exhibited that convections enhanced over the region around the dateline and were suppressed obviously over Indonesia. In the lower troposphere (850hPa) divergent wind field, the convergent center was over equator 155°E, and the divergent center was around 85°W, 20°S. Correspondingly, the divergent and the convergent centers at the upper troposphere (200hPa) were over equator 155°E and around 90°W, 25°S, respectively.