Monthly Highlights

Climate in China

During May 2008, the monthly mean surface air temperatures were observed 1-2℃ below normal in eastern Heilongjiang, northeastern Jilin, central and eastern Inner Mongolia. Meanwhile, temperatures were at least 1℃ above normal in the most Huanghuai area, the Jianghuai area, the Jianghan area, the northern Jiangnan area, most northwestern China, eastern Sichuan and Chongqing, with temperatures 2-4℃ above normal in southern Henan, central Hubei, northern Hunan and Xinjiang. Temperatures were near normal in other regions of China. For the month, precipitations were observed 30-200% more than normal in northeastern China, northeastern Inner Mongolia, most Shandong, most southwestern China, western Qinghai and central Xinjiang, with precipitations 200% more than normal in locations. Meanwhile, precipitations were observed 30-80% less than normal in eastern northwest China, central and western Inner Mongolia, northern and western Xinjiang, most Jiangxi, eastern Guangdong, southwestern Guangxi and Hainan, with at least 80% less than normal in parts of northwestern China.

Climate in the World

During May 2008, temperatures were 1-2℃ above normal in most China, most Central Asia, northern Siberia, central and western Europe, eastern Canada, the regions from southern America to northern Mexico, most Alaska, southwestern Australia, Northwest Africa, southern West Africa and South Africa, with temperatures at least 4℃ above normal in locations. Meanwhile, temperatures were 1-2℃ below normal in the regions from northeastern China to eastern Mongolia, the regions from Yunnan of China to the most Indo-China peninsula, eastern Europe, most America, eastern Australia and southern South America.

For the month, precipitations were observed 50-100% more than normal in southwestern Europe, western Russia, the regions from northeastern China to southeastern Serbia, the regions from southwestern China to the most Indo-China peninsula, northern India, central America, southern West Africa, most South Africa, northern and southern South America, with precipitations at least 200% more than normal in locations. Meanwhile, precipitations were at least 50% less than normal in northern and southeastern Europe, most Russia, most northwestern China, Central Asia, most India, western and southeastern America, Alaska, Australia and northern Argentina, with no precipitations in parts of above regions.

General Circulation Features

Northern Hemisphere: During May 2008, the mean 500hPa geopotential height field exhibited a 5-wave pattern at the mid-high latitudes. In the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered the Far East of Russia, the northeastern North Pacific and the regions from northwestern Europe to northern Canada through Greenland, with central values above 16 dam over northern Canada. Meanwhile, negative anomalies below -4 dam covered the northern North Pacific and northeastern North America (Fig.4). For the month, the subtropical high over the West Pacific was near normal in area and intensity, and its western part shrank eastward compared with its normal position.

Southern Hemisphere: For the month, zonal circulation prevailed at the mid-high latitudes. In the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered the regions from the southern South Pacific to most Antarctica. Meanwhile, negative anomalies below -4 dam covered the southwestern South Pacific, the southern Indian Ocean and the regions from the southeastern South Pacific to the southern South Atlantic through oceans south of South America, with central values below -8 dam over oceans south of South America.

Tropical Features

Sea Surface Temperatures (SSTs): La Nina event ended in May 2008. Negative SST anomalies remained in the central equatorial Pacific between 165ºE-140ºW, with central values below -1.0℃ while positive SST anomalies almost dominated the rest of the equatorial Pacific. In May, indices of NINO 3, NINO 4, NINO 3.4 and NINO Z were 0.0℃, -0.8℃, –0.6℃ and -0.3℃, with rises of 0.2℃, 0.2℃, 0.3℃ and 0.2℃ from last month, respectively, while index of NINO 1+2 was 0.0℃,with a drop of 0.6℃ from last month. Meanwhile, SSTs in most of the equatorial Indian Ocean were near normal.

Subsurface Temperatures: During May, the anomalously cold subsurface water in the central equatorial Pacific weakened in intensity and shrank in area obviously, with central values below -1℃, while the anomalously warm subsurface water in the eastern equatorial Pacific strengthened in intensity and expanded westward distinctly.

Warm Pools: During May 2008, the area of the western Pacific warm pool was smaller than normal, with its eastern boundary shrinking strictly and its northern boundary expanding obviously, and its intensity was near normal. Meanwhile, the area of the Indian Ocean warm pool was smaller slightly and its intensity was weaker than normal.

Wind Field：For the month, at lower troposphere (850hPa), easterly anomalies covered the central equatorial Pacific between 150ºE-155ºW, with the maxima about 4m/s, while westerly anomalies prevailed over the eastern equatorial Pacific, with the maxima about 4m/s. Meanwhile, at upper troposphere (200hPa), zonal wind over the equatorial Pacific was near normal.

Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was -0.3, with a drop of 0.9 from last month.

Convections over the Tropics: During May 2008, the anomalous outgoing long wave radiation (OLR) showed that the convection pattern of “enhanced in the west and suppressed in the east” remained but weakened over the western and central equatorial Pacific. For the tropical Pacific, in the divergent wind field at lower troposphere (850hPa), a divergent center and a convergent center were observed over 115ºW，15ºS and 130ºE, 15º N, respectively. Correspondingly, at upper troposphere (200hPa), the convergent center was over 115ºW, 20ºS, and the divergent center was over 130ºE, 15º N.

ENSO Monitoring and Prediction

(1) During May 2008, anomalously cold surface water weakened continuously in the central equatorial Pacific, and NINO Z index was -0.3℃.

(2) For the month, the anomalously cold subsurface water in the central equatorial Pacific weakened in intensity and shrank in area obviously, while the anomalously warm subsurface water in the eastern equatorial Pacific enhanced.

(3) Southern Oscillation Index (SOI) was -0.3.

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

(5) The convection pattern of “enhanced in the west and suppressed in the east” remained but weakened over the western and central equatorial Pacific.

(6) Most ENSO models predicted that neutral conditions in the central and eastern equatorial Pacific are considered the most likely outcome for the middle part of the year.

In conclusion, the La Niña event beginning in August 2007 ended in May 2008. The La Nina lasted 9 months from August 2007 to April 2008, and peaked at November with NINO Z index being -1.5℃. And the accumulation of NINO Z monthly index during the La Nina event was -9.5℃, which ranked a weak La Nina event. In following summer, neutral conditions are expected in the central and eastern equatorial Pacific.