Highlights

General Circulation Features

Northern Hemisphere

During December, the mean 500hPa geopotential height field exhibited a 4-wave pattern at mid-high latitudes. The polar vortex was weaker than normal. Correspondingly, negative anomalies of –4~–8 dam covered the northeastern North Pacific and the regions between coast of northern Eurasia and the Far East. Meanwhile, positive anomalies above 4 dam controlled the central North Pacific and the regions from central North America to eastern Baikal via central Europe, with center values above 16 dam over the northern North Atlantic. In this month, Northwest Pacific subtropical high was also stronger than normal and extended westward remarkably.

Southern Hemisphere

In the mean 500hPa geopotential height field during December, zonal circulation dominated the mid-high latitudes. In the height anomaly field, negative anomalies below –4 dam were covered the southern South America，central-southern South Pacific, southern Africa，southeastern South Indian ocean. Positive height anomalies above 8 dam dominated the most of Antarctica.

Climate for China

During December, the mean surface air temperatures were near normal in most of China, with 2~6℃ above normal in parts of Northeast China and Qingzang Plateau, and 1~3℃ below normal in northern Northwest China, parts of Jiangxi and Fujian. In this month, rainfall totals were more than 50% below normal in most of Northwest China and North China, southern Northeast China and most of regions from the Yangtze River to South China, and 50%~100% above normal in the regions between Yellow River and Huaihe River, most of Southwest China, northern Northeast China.

Tropical Features

Sea Surface Temperatures(SSTs)

During December, about 0.5℃ positive SST anomalies remained in most of the equatorial Pacific. Its center values above 1.0℃ around dateline were weaker than last month. Correspondingly, the SST indices of NINO 4, NINO 3 and NINO 1+2 regions were 0.7℃, 0.5℃ and 0.1℃, respectively. In addition, positive anomalies around 0.5°C controlled most of India Ocean.

Circulation

Wind Field

In this month, at the lower troposphere (850hPa), westerly anomalies dominated the eastern equatorial Pacific, with center values above 4 m/s. Easterly anomalies of 2 m/s mainly covered the central-western equatorial Pacific. At the upper troposphere (200hPa), strong easterly anomalies controlled most of the eastern equatorial Pacific, with center values above 12 m/s. In other regions of the equatorial Pacific and the equatorial India Ocean, anomalies of zonal wind were near normal.

Southern Oscillation

During December, the Southern Oscillation Index was 1.0, which rose sharply from -0.4 in November.

Convection over the Tropics

During December, the anomalous outgoing longwave radiation (OLR) distribution exhibited that enhanced convection mainly located Indonesia and regions north to equator nearby the dateline, suppressed convection located regions south to equator nearby the dateline. In the lower troposphere (850hPa) divergent wind field, the convergent and divergent centers were over 160°E 5°S and around 100°W, 12.5°S, respectively. Correspondingly, at the upper troposphere (200hPa), the divergent centers were nearby 120°E, 5°S and 160°E, 8°S, and the convergent center was around 110°W, 10°S.