Highlights

General Circulation Features

Northern Hemisphere

During July 2004, a strong anomaly ridge developed over Northeastern Europe in the 500hPa geopotential height field. Correspondingly, positive anomalies dominated the regions from Northeastern Europe to the polar, the Bering Sea and northeastern Canada, with maximums above 12 dam over the Novaya Zemlya. Negative anomalies below -4 dam were observed over the northeastern North Atlantic Ocean and regions from northern Canada to parts of the Arctic Ocean. During this month the Northwest Pacific subtropical high was stronger than normal.

Southern Hemisphere

For the month, a 3-wave pattern appeared in the mid-high latitudes at the mean 500hPa geopotential height field. In the height anomaly field, negative anomalies below -4 dam were observed over the southeastern South Pacific Ocean and the regions from the marine area south to Australia to Antarctica, with central value below -12 dam over the southeastern South Pacific. Positive anomalies of 4~12 dam were observed over the southern South Atlantic Ocean and the mid-southern South Pacific.

Climate for China

During July, the mean surface air temperatures were more than 1℃ above average in most of Northwest China, northeastern Inner Mongolia and the lower reaches of the Yangtze River, and 2~3℃ above average in the eastern Xinjiang. Temperatures were 1℃ below average in part of Guizhou, Sichuan and the Tibet. For the month, rainfall totals were 50~100% more than normal in northwestern Xinjiang, part of Tibet, southeastern Northeast China, the most of the regions between the Yellow River and the Huaihe River, and parts of Hunan, Guizhou and Guangxi. Meanwhile, rainfall totals were observed 50~90% below normal in most of Northwest China, northeastern Inner Mongolia and part of the lower reaches of the Yangtze River.

Tropical Features

Sea Surface Temperatures (SSTs)

During July, positive SST anomalies were 0.5℃ above normal in most of the central equatorial Pacific. Compared with last month, the area of positive anomalies was obviously larger and its central values enhanced. Negative SST anomalies were -0.5℃ below normal in much of the eastern equatorial Pacific. Correspondingly, the SST indices of NINO 4, NINO 3 and NINO 1+2 regions were 0.8℃, -0.2℃ and -1.1℃, respectively.

Circulation

Wind Field

For the month, at the lower troposphere (850hPa), westerly anomalies of 2~6 m/s covered the most of the equatorial Pacific. Compared with last month, westerly anomalies enhanced over the central and eastern equatorial Pacific. The zonal winds closed to normal in the equatorial India Ocean. At the upper troposphere (200hPa), westerly anomalies were found in most of the western equatorial Pacific and the equatorial Indian Ocean. Easterly anomalies covered the most of the central and eastern equatorial Pacific.

Southern Oscillation

During July, the Southern Oscillation Index was -0.7.

Convection over the Tropics

During July, the anomalous outgoing longwave radiation (OLR) distribution exhibited that convections closed to normal over the equatorial Pacific. In the lower troposphere (850hPa) divergent wind field, convergent center was not found over the tropical Pacific, and the divergent center was around 80°W, 25°S. Correspondingly, in the upper troposphere (200hPa), there was not divergent center over the tropical Pacific and the convergent center was around 85°W, 25°S.