Monthly Highlights

Climate in China

During May 2007, monthly mean surface air temperature for China was observed the highest for May in record. Temperatures were 1-2℃ below normal in central Yunnan, while temperatures were near or above normal in the remainder of China, with temperatures 2-4℃ above normal in the middle and lower reaches of the Yangtz River, Huanghuai, eastern and south-western North China, central and eastern Northeast China, central Tibet and south-western Xinjiang. For the month, precipitations were observed mainly below normal over China, with 30-200% above normal in eastern Northeast China, eastern North China, central and southern Southwestern China, western Tibet, northern and eastern Xinjiang, and were more than 30% below normal in the remainder of China.

Climate in the World

During May 2007, monthly mean surface air temperatures were observed above normal in most of the world. Temperatures were more than 2℃ above normal in the Far East of Russia, most of China, most of Mongolia, south-eastern Europe, central and eastern America, with temperatures more than 4℃ above normal in the Far East of Russia. Meanwhile, temperatures were more than 2℃ below normal in the northern West Siberian Plains, north-western Alaska, the areas between the Indochina Peninsula and Yunnan China.

For the month, precipitations were observed 50-200% more than normal in central and western Russia, western Europe, central America and south-eastern Brazil. Meanwhile, precipitations were 50-100% less than normal in parts of China, parts of South Asia, most of West Asia, West Africa, most of South Africa, most of Australia, the Bering Strait and the regions around it, western and eastern America, southern South America.

General Circulation Features

Northern Hemisphere: During May 2007, the mean 500hPa geopotential height field exhibited a weak 5-wave pattern at the mid-high latitudes, with the troughs over West Europe, the West Siberia Plain, East Asia, the oceans south to Alsaka and north-eastern North America. In the 500hPa height anomaly field, positive anomalies above 8 dam covered the Bering Sea and the Great Lakes in North America. Meanwhile, negative anomalies below -8 dam covered north-eastern Canada and the oceans east to Iceland. Heights were near normal in other regions. For the month, the Northwest Pacific subtropical high was obviously larger and stronger than normal, and extended farther west, and its ridge line position was near normal.

Southern Hemisphere: For the month, zonal circulation prevailed at the mid-high latitudes. In the 500hPa height anomaly field, positive anomalies above 4 dam covered most of Antarctica and the 40°S latitude band of South Pacific, with central value above 12 dam. Meanwhile, negative anomalies below -8 dam covered the oceans south to Australia and the Bellingshausen Sea. Heights were near normal in other regions.

Tropical Features

Sea Surface Temperatures (SSTs): During May 2007, SST still exhibited a pattern of “cold in the east and warm in the west” in the equatorial Pacific. Positive SST anomalies above 0.5℃ west of the Date Line in the western equatorial Pacific enlarged, while negative SST anomalies in the equatorial eastern Pacific strengthened and extended westward, with central values below -2.0℃ in the Pacific offshore of South America. SST anomaly indices of NINO Z, NINO 3.4, NINO 4, NINO 3 and NINO 1+2 were -0.4℃, -0.2℃, 0.2℃, -0.7℃ and -1.5℃, with drops of 0.3℃, 0.3℃, 0.1℃, 0.4℃ and 0.3℃ from last month, respectively. Meanwhile, SST anomalies were 0.5-1.0℃ above normal in most of the equatorial Indian Ocean.

Subsurface Temperatures: During May 2007, comparing with last month, the area of anomalously warm subsurface waters enlarged in the western equatorial Pacific, while anomalously cold subsurface waters remained in the eastern equatorial Pacific.

Warm Pools: During May 2007, areas of the western Pacific and Indian Ocean warm pools were above normal and their intensities were both stronger than normal.

Wind Field：For the month, at the lower troposphere (850hPa), weak westerly anomalies covered the eastern Pacific, while weak easterly anomalies covered most of the central and western equatorial Pacific. Easterly anomalies covered the equatorial Indian Ocean, with central values below –6 m·s^-1. At the upper troposphere (200hPa), easterly anomalies below -4 m·s^-1 prevailed over part of the eastern Pacific.

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

Convection over the Tropics: During May 2007, the anomalous outgoing longwave radiation (OLR) showed that convections were near normal over most of the equatorial Pacific. In the lower troposphere (850hPa) divergent wind field, a convergent center was observed over 150ºE, 10ºN, and a divergent center was located in 100ºW, 10ºS. Correspondingly, at the upper troposphere (200hPa), the divergent center was found over 150ºE, 5ºN, and the convergent center was over 120ºW, 20ºS.

ENSO Monitoring and Prediction

(1) SST still exhibited a pattern of “cold in the east and warm in the west” in the equatorial Pacific during May, while SST anomaly indices of NINO Z, NINO 3.4, NINO 4, NINO 3 and NINO 1+2 all dropped from last month, and NINO Z was –0.4℃.

(2) The area of anomalously warm subsurface waters enlarged in the western equatorial Pacific, while anomalously cold subsurface waters remained in the eastern equatorial Pacific.

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

(4) At the lower troposphere (850hPa), weak westerly anomalies covered most of the equatorial eastern Pacific, while weak easterly anomalies were observed over most of the central and western equatorial Pacific.

(5) Convections were near normal over the equatorial Pacific.

(6) Most of models predicted that the SST anomalies in the equatorial eastern Pacific would drop and come into La Nina status in the following 1-3 months.

In conclusion, the anomalously SSTs in the equatorial Pacific appeared to be the normal-cold conditions in the mid-eastern Pacific, and it is expected to maintain in the following 1-2 months.