Highlights

General Circulation Features

Northern Hemisphere

During September, the mean 500hPa geopotential height field exhibited a 4-wave pattern at mid-high latitudes. The polar vortex was weaker than normal. In the height anomaly field, negative anomalies below ? dam covered the regions from Greenland to Central Siberia. Positive anomalies above 4 dam controlled the regions between the eastern North America and central Europe and the northeastern Pacific, with center values above 12 dam over the eastern North America. In this month, Northern Hemisphere subtropical high was obviously stronger than normal. The Northwest Pacific subtropical high was stronger than normal and extended westward remarkably.

Southern Hemisphere

During September, the mean 500hPa geopotential height field exhibited a 3-wave pattern at mid-high latitudes. Positive anomalies dominated the Antarctica and the southwestern Atlantic, the south-central Pacific and the south-central India Ocean, with the center values above 16 dam over the Antarctica. Meanwhile, negative anomalies covered areas south to Australia and south to Africa and southwest to South America, with the minimum below ?2 dam over the region southeast to Australia.

Climate for China

During September, the mean surface air temperatures were near or above normal in most of China. Positive anomalies above 1℃ were  in most of northern China , northern Tibet and the middle and lower reaches of the Yangtze River, with the anomalies more than 2℃ above normal in the eastern region south to the Yangtze River. As to monthly precipitation, rainfall totals were 50%~200% more than normal in North China，the north part of region between the Yellow River and the Huaihe River, western and eastern Northwest China and central Tibet.

Meanwhile, rainfall totals were 50%~100% below normal in the eastern region south to the Yangtze River and the north part of Fujian Province. The precipitations in rest of China were near normal.

Tropical Features

Sea Surface Temperatures(SSTs)

In this month, sea surface temperatures were near normal in the central and eastern equatorial Pacific. Negative SST anomalies around -0.5℃ were observed in the offshore of South America. Positive SST anomalies above 0.5℃ remained in the eastern West Pacific. Correspondingly, the SST indices of NINO 4, NINO 3 and NINO 1+2 regions were 0.5℃, 0.1℃ and -0.4℃, respectively. In addition, positive anomalies around 0.5C remained in the equatorial India Ocean.

Circulation

Wind Field

During September, at lower troposphere (850hPa), weak westerly anomalies about 2m/s covered the central equatorial Pacific. Near-normal wind covered rest of the equatorial Pacific and the equatorial India Ocean. At the 200hPa, zonal wind anomalies were near normal.

Southern Oscillation

During September, the Southern Oscillation Index was -0.5.

Convection over the Tropics

During September, the anomalous outgoing longwave radiation (OLR) distribution displayed that tropical convection was stronger than normal over the equatorial western Pacific. In the lower troposphere (850hPa) divergent wind field, the convergent center was over the Philippines and the divergent center was around 105W, 10S. Correspondingly, the divergent and the convergent centers at the 200hPa were over the Philippines and around 95W, 25S, respectively.