Monthly Highlights

Climate in China

Monthly mean temperature of China is -2.3°C during December 2007, which is 1.6°C above the 1971-2000 base period year mean (-3.9°C). It is the highest since 1950. Monthly mean surface air temperatures were above or near normal in most of China, with temperatures 2-4℃ above normal in central and northern Northeast China, northeastern and central Inner Mongolia, northern Shanxi, most Zhejiang, most Fujian, southern Guangdong, Hainan, eastern Yunnan, central and western Xizang, southern Qinghai. For the month, precipitations were observed 20-50% above normal in western and northeastern Northeast China, the eastern Huanghuai area and the Jianghuai area, the most Jiangnan area, northern South China, the Sichuan basin, most Inner Mongolia, Shanxi, northeastern Qinghai, with at least 200% above normal in parts of the above regions. Meanwhile, precipitations were 20-50% below normal in western and southern Xizang, western Qinghai, southern Xinjiang, Yunnan, Hainan, and southern Guangdong, with 50-80% below normal in parts of above regions.

Climate in the World

During December 2007, Temperatures were 1-2℃ above normal in most Europe, most Asia, eastern West Africa, southeastern and northwestern America, Alaska, eastern South America, and southeastern Australia，with temperatures at least 4℃ above normal in most middle and high latitude areas of Eurasia, and northern Alaska. Meanwhile, temperatures were about 1-2℃ below normal in southwestern Europe, southwestern Central Asia, the northern Central Siberian Plateau, parts of central and northern North America, shore of western South America, parts of western and eastern Australia.

For the month, precipitations were observed 50-100% more than normal in northwestern and northeastern North Asia, eastern Central Asia, eastern East Asia, part of southern Africa, most America, northwestern South America and part of central South America, eastern and southwestern Australia, with precipitations at least 200% more than normal in locations. Meanwhile, precipitations were at least 50% less than normal in central and southwestern Europe, most South Asia, southwestern East Asia, the northern and southeastern Central Siberian Plateau, northwestern Africa, southern America and eastern Alaska, parts of eastern and southern South America, shore of southwestern Australia, with no precipitation in parts of above regions.

General Circulation Features

Northern Hemisphere: During December 2007, the mean 500hPa geopotential height field exhibited a 3-wave pattern at the mid-high latitudes, with the troughs over eastern Europe, the Japan Sea, and the northwestern North Atlantic Ocean. In the 500hPa height anomaly field, positive anomalies above 4 dam covered most Europe, the eastern Baykal, the Arctic Ocean in north of the Chukchi Sea, the central and eastern North Pacific, regions from southeastern North America to the central and western North Atlantic Ocean, with central values above 12 dam over northeastern Europe. Meanwhile, negative anomalies below -4 dam covered the Balkhash, the northern Bering Sea, and northwestern North America, the Greenland Island and neighborhood sea areas.

Southern Hemisphere: For the month, zonal circulation prevailed at most of the mid-high latitudes. In the 500hPa height anomaly field, positive anomalies above 4 dam covered part of the southern South Atlantic Ocean, the southeastern South Pacific, the southwestern South Pacific and the southern South India Ocean. Meanwhile, negative anomalies below -4 dam covered Antarctica.

Tropical features

Sea Surface Temperatures (SSTs): During December 2007, the sea surface temperature anomalies （SSTAs） below -0.5℃ lasted in the central and eastern equatorial Pacific. Compared with last month, SSTAs ascended slightly in the equatorial Pacific east of 120ºW. Indices of NINO Z, NINO 4,NINO 3, NINO 1+2 were -1.3℃, -0.9℃, -1.5℃ and -2.0℃, with increases of 0.2℃, 0.1℃, 0.3℃ and 0.1℃ from last month, respectively. And NINO 3.4 index was -1.5℃ as same as that in last month. Meanwhile, SSTs in most of the equatorial Indian Ocean were near normal.

Subsurface Temperatures: Compared with last month, anomalously cold subsurface water in the central-eastern equatorial Pacific strengthened slightly. Meanwhile, the intensity of warm subsurface water in the western equatorial Pacific also enhanced slightly.

Warm Pools: During December 2007, the western Pacific warm pool was near normal in area but strengthened in intensity, and shifted westward slightly compared with normal. Meanwhile, the Indian Ocean warm pool was near normal in area but strengthened slightly in intensity.

Wind Field：For the month, at lower troposphere (850hPa), weak westerly anomalies were over the equatorial Pacific east of 120ºW, and the easterly anomalies covered the central and western equatorial Pacific, with central values above 8m/s. Meanwhile, wind field was near normal over the most equatorial India Ocean. At upper troposphere (200hPa), westerly anomalies dominated the central tropical Pacific.

Southern Oscillation: For the month, Southern Oscillation Index (SOI) was 1.5, with a rise of 0.7 from last month.

Convections over the Tropics: During December 2007, the anomalous outgoing long wave radiation (OLR) showed that convections were enhanced over the eastern tropical Indian Ocean, while convections were suppressed over the central and western equatorial Pacific. For the tropical Pacific, in the divergent wind field at lower troposphere (850hPa), a convergent center and a divergent center located at 155ºE，5ºS, and 110ºW, 5ºS, respectively, while the corresponding divergent and convergent centers at upper troposphere (200hPa) were over 175ºW，15ºS, and 90ºW，15ºS, respectively.

ENSO Monitoring and Prediction

(1) For the month, anomalously cold surface water lasted in the central and eastern equatorial Pacific, the NINO Z index was -1.3℃, and the cumulate value of NINO Z achieved -5.6℃ for recent 5 month from August to December, showing a La Nina events has came into being.

(2) Anomalously cold subsurface water maintained in the central and eastern equatorial Pacific with the intensity enhanced slightly, while the intensity of warm subsurface water in the western equatorial Pacific also enhanced slightly.

(3) Southern Oscillation Index (SOI) was 1.5 during this month, with a rise of 0.7 from last month.

(4) At lower troposphere (850hPa), easterly anomalies covered the central and western equatorial Pacific.

(5) Convections were suppressed over the central and western equatorial Pacific.

(6) Most ENSO models predicted La Nina conditions are likely to persist in the following 3 months but its intensity slightly weaken.

In a word, a La Nina event has came into being in the central and eastern equatorial Pacific, and will persist through following 1-2 months.