Monthly Highlights

Climate in China

During February 2008, the monthly mean surface air temperatures were observed 1-4℃ above normal in central and northern Northeast China, northeastern Inner Mongolia and northern Xinjiang. Meanwhile, temperatures were below or near normal in other regions of China. Temperatures were 2-4℃ below normal in most of northwestern China, western North China, eastern Southwest China, southern Jiangnan and South China, with over 4℃ below normal in parts of above regions (Fig.1). For the month, precipitations were observed above normal in southwestern Xinjiang, eastern Qinghai, most Sichuan and Chongqing, southern Yunnan and most Guangxi. Meanwhile, precipitations were observed below or near normal in other regions of China, with over 80% less than average in northern Jilin, southern Liaoning, southeastern Inner Mongolia and eastern Hebei.

Climate in the World

During the month, temperatures were 1-2℃ above normal in most of Eurasia north to 45°N, southern and parts of northwestern North America, southern South America and western West Africa, with temperatures 4-8℃ above normal in most of Eurasia north to 55°N. Meanwhile, temperatures were over 1℃ below normal in most of the regions from West Asia to China, Far East Russia, central and northern North America, Alaska, western and southeastern Australia, with 4-6℃ below normal in parts of above regions (Fig.2).

For the month, precipitations were observed 50-100% more than average in parts of northern Europe, central and western Russia, southern South Asia, Indo-China Peninsula, northeastern America, northwestern Canada, western South America, western and the east coasts of Australia, with precipitations at least 200% above normal in locations. Meanwhile, precipitations were at least 50% less than average in southern Europe, West Asia, northern South Asia, most of northern China, Mongolia, Far East Russia, southern North America, most of Alaska, central Australia, most northwestern Africa and parts of East Africa, with no precipitations in parts of above regions (Fig.3).

General Circulation Features

Northern Hemisphere: During February 2008, the mean 500hPa geopotential height field exhibited a 3-wave pattern at the mid-high latitudes. In the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered the regions from the northern North Atlantic to West Asia, the northeastern North Pacific and the polar region, with central values above 12 dam over southern Europe. Meanwhile, negative anomalies below -4 dam covered northern North America, Northeast Asia and parts of the central-northern North Pacific, with central values below -8 dam over Northeast Asia (Fig.5). For the month, the subtropical high over the West Pacific was larger and its position was more eastward and northward than normal.

Southern Hemisphere: For the month, zonal circulation prevailed at most of the mid-high latitudes. In the 500hPa geopotential height anomaly field, negative anomalies below -4 dam covered most of the regions south to 60°S, the southeastern South Pacific, the southwestern South Indian Ocean and the regions southeast to Australia, with central values below -12 dam over the polar region. Meanwhile, positive anomalies above 4 dam covered the southwestern South Pacific, southern South America and the southeastern South Indian Ocean.

Tropical features

Sea Surface Temperatures (SSTs): During February 2008, sea surface temperature anomalies (SSTAs) below -0.5℃ remained in most of the central and eastern equatorial Pacific. Compared with last month, cold situation persisted in the central and eastern equatorial Pacific west of 110 ºW, and weakened remarkably in regions east of 110ºW. In February, indices of NINO 1+2, NINO 3, and NINO Z were 0.3℃, -1.3℃ and -1.3℃, with rises of 0.9℃, 0.2℃ and 0.1℃ from last month, respectively. NINO 3.4 index was -1.9℃, with a drop of 0.1℃ from last month, and NINO 4 index was –1.6℃ the same as that in last month. Meanwhile, SSTs in most of the equatorial Indian Ocean were near normal.

Subsurface Temperatures: Compared with last month, the anomalously cold subsurface water remained in the central and eastern equatorial Pacific, and its center shifted westward slightly. Meanwhile, the anomalously warm subsurface water still sustained in the western equatorial Pacific and its range expanded eastward obviously.

Warm Pools: During February 2008, the area of the western Pacific warm pool was smaller slightly but the intensity was stronger than normal, with its eastern boundary shrank strictly and northern boundary expanded obviously. Meanwhile, the Indian Ocean warm pool was near normal both in area and intensity.

Wind Field：For the month, zonal wind anomalies showed the dipole pattern in both meridional and vertical structures. At lower troposphere (850hPa), easterly and westerly anomalies were over regions west and east of 135 ºW, with maxima above 6m/s and 4m/s, respectively. Meanwhile, at upper troposphere (200hPa), westerly and easterly anomalies dominated regions west and east of 125 ºW, with maxima above 16m/s and 8m/s, respectively. Zonal winds over the equatorial Indian Ocean were near normal.

Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 1.9, with a rise of 0.6 from last month.

Convections over the Tropics: During February 2008, the anomalous outgoing long wave radiation (OLR) showed that convections were enhanced over the western equatorial Pacific and the eastern equatorial Indian Ocean, while convections were suppressed over the central equatorial Pacific. For the tropical Pacific, in the divergent wind field at lower troposphere (850hPa), a convergent center was observed over 155ºE，10ºS, and a divergent center was located at 115ºW, 5ºS. Correspondingly, at the upper troposphere (200hPa), the divergent center was observed over 155ºE，20ºS and the convergent center was over equator near 110ºW.

ENSO Monitoring and Prediction

(1) During February 2008, anomalously cold surface water continued in the central and eastern equatorial Pacific, but its intensity decreased remarkably in regions east of 110 ºW.

(2) For the month, the pattern “cold in the east and warm in the west” in the subsurface equatorial Pacific still maintained, while warm subsurface water in the western Pacific expanded eastward obviously.

(3) Southern Oscillation Index (SOI) was 1.9 during this month, with a rise of 0.6 from last month.

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

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

(6) Most ENSO models predicted that SSTA below -0.5℃ over the central and eastern equatorial Pacific are likely to persist through the following 3 months but cold water intensity will weaken continuously .

In a word, La Nina conditions continued in February 2008, and it will probably undergo a continuous weakening during the coming 3 months.