Monthly Highlights

Climate in China

During December 2009, the surface air temperature averaged over China was near normal. Monthly mean surface air temperatures were above normal in most of Northwest and Southwest China, with temperatures at least 2-4℃ above normal in the central Qingzang Plateau and southern Xinjiang. Meanwhile, temperatures were 1-2℃ below normal in northern and central North China, most of Northeast China and Inner Mongolia and northern Xinjiang, with temperatures 2-4℃ below normal in most of Northeast China and eastern Inner Mongolia. Temperatures were near normal in most of central and eastern China.

The precipitation averaged over China was above normal. Precipitations were observed 30-200% more than normal in most of Northeast China, northeastern Inner Mongolia, the regions from the southern Huanghuai area to the mid-lower reaches of the Yangtze River, central and western parts of the Jiangnan area, eastern South China, northern Xinjiang, the regions from western and central Gansu to western Inner Mongolia and southeastern Tibet, with precipitations at least 200% more than normal in locations. Meanwhile precipitations were observed 30-80% less than normal in most of North China, central Inner Mongolia, western South China, most of southwestern China, southern Xinjiang and the southeastern part of Northwest China.

During late December 2009, 13 stations of Northeast China, Inner Mongolia and Hebei experienced severe low temperature events. The daily minimum temperature in Wudalianchi station of Heilongjiang province(-42.6℃) broke the historical record. Besides, extreme persistent rainy-day occurred in 105 stations of southern Huanghuai, Jianghuai, Jianghan, northwestern Jiangnan, eastern Southwest China, Xinjiang and eastern Inner Mongolia, with the persistent rainy days setting new records in 20 stations.

Climate in the World

During December 2009, monthly mean surface air temperatures were observed 2-6℃ below normal in regions from northeastern Europe to central Russia, Mongolia, northeastern China and central and western America, with over 10℃ below normal in locations. Meanwhile, temperatures were 2-4℃ above normal in northwestern America, eastern Canada, central and northern West Africa, regions from southeastern Europe to western West Asia, part of Central Asia, the Qingzang Plateau, the Far East Russia and parts of northern South America, with over 6℃ above normal in locations.

For the month, precipitations were observed at least 50% less than normal in northern Europe, central Russia, regions from Central Asia to South Asia, most of the Indo-China Peninsula, most of northwestern North America, northwestern South America, northern and parts of western Australia, most of West Africa and northern North Africa, with no precipitation in parts of above regions. Meanwhile, precipitations were at least 50% more than normal in southern Europe, regions from northern China to the Baikal Lake and regions east to it, the Far East Russia, central and southeastern North America, central South America and eastern Australia, with precipitations at least 200% more than normal in locations.

General Circulation Features

Northern Hemisphere: During December 2009, the 500hPa geopotential height anomaly field illustrated a canonical Arctic Oscillation (AO) pattern with negative anomalies over middle latitudes while positive anomalies over high latitudes. The abnormal negative centers were over the region from the northern North Atlantic Ocean to western Europe and the northern North Pacific, with central values below -12 dam. Meanwhile, the abnormal positive center value over the Greenland exceeded 20 dam. Furthermore, the Northwest Pacific subtropical high was larger and stronger and its location was more southward and westward than normal.

Southern Hemisphere: For the month, zonal circulation prevailed in the 500hPa geopotential height. In the anomaly field, positive anomalies above 4 dam covered the southern South Atlantic Ocean, the southeastern South Pacific, southeastern Australia and its vicinity and the southeastern South Indian Ocean, with central values over 12 dam over the southern South Atlantic. Meanwhile, negative anomalies below -4 dam were observed over much of the regions south to Australia.

Tropical Features

Sea Surface Temperatures (SSTs): During December, SSTs were more than 0.5℃ above normal in the central-eastern equatorial Pacific with central value above 2.5℃. Indices of NINO 1+2, NINO 3, NINO 4, NINO 3.4 and NINO Z were 0.3℃, 1.6℃, 1.4℃, 1.8℃and 1.4℃, with a rise of 0.3℃ for NINO 3, 0.1℃ for NINO 3.4 and NINO Z, with drops of 0.2℃ and 0.1℃ for NINO 1+2 and NINO 4 from last month, respectively.

Subsurface Temperatures: During December, anomalously warm subsurface water prevailed over the central-eastern equatorial Pacific and further shifted eastward in comparison to last month. Meanwhile, anomalously cold subsurface water was observed in the western equatorial Pacific west of 160ºE with central value below -2℃.

Warm Pools: During December, both the area and intensity of the western Pacific and Indian Ocean warm pool were above normal.

Wind Field：For the month, at lower troposphere (850hPa), westerly anomalies covered the equatorial Pacific east of 165ºW, the rest regions were close to normal, while easterly anomalies prevailed over the western equatorial Pacific in late December. At upper troposphere (200hPa), westerly anomalies prevailed over the equatorial Pacific west of 160ºE, while easterly anomalies covered the rest equatorial Pacific.

Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was -0.5, with a rise of 0.1 from last month.

Convections over the Tropics: During December, the anomalous Outgoing Longwave Radiation (OLR) showed that enhanced convection was present over the equatorial Pacific near the Date Line, suppressed convection was over Indonesia, the Philippines and the vicinity, and convection was near normal over the rest equatorial Pacific. In the lower troposphere (850hPa) divergent wind field, a convergent center in the tropical Pacific was near 165ºE, 5ºS, and a divergent center was near the equator 105ºW. At the upper troposphere (200hPa), the corresponding divergent and convergent centers were near 170ºE, 7.5ºS; 90ºW, 5ºS, respectively.

ENSO Monitoring and Outlook

Monitoring showed that during December in the central-eastern equatorial Pacific, SSTs were more than 0.5℃ above normal with central value above 2.5℃ and anomalously warm subsurface water further extended eastward. Index of NINO Z was 1.4℃. Meanwhile in the western equatorial Pacific, anomalously cold subsurface water extended in the month and easterly anomalies prevailed over the lower troposphere (850hPa) in late December. Most statistical and dynamic climate models predicted that El Nino was near or on its peak and would gradually weaken after peaking.

Aforementioned analysis indicates that current El Nino episode is during its peak phase. El Nino event is supposed to remain and gradually weaken in the future 2-3 months. Therefore, we will still closely monitor the developments of ENSO conditions and update our ENSO wrap-up in time.