北半球：7月，月平均500hPa高度场上，中高纬度环流呈5波型分布，槽区分别位于北太平洋东部、北美洲东北部、北大西洋东部至西北欧、中亚和东北亚上空上空

Monthly Highlights

Climate in China

During July 2007, monthly mean surface air temperatures were 1-2℃ above normal in most of the Jiangnan region, South China，eastern Inner Mongolia and eastern Xinjiang, with temperatures more than 2℃ above normal in parts of above regions. Meanwhile, temperatures were 1-2℃ below normal in western part of regions between the Yellow River and Huaihe River and northwestern Hubei. For the month, precipitations were observed 30-200% above normal in the Huaihe River and regions nearby, eastern Southwest China, western Inner Mongolia, central and northern Xinjiang and southern Tibet with precipitation above 200% in locations. Meanwhile, precipitations were 30-80% below normal in Jiangnan region, South China, northern Northeast China, northeastern Inner Mongolia, northern North China, the Hetao region, western Xinjiang and western Tibet. Precipitations were near normal in the remainder of China.

Climate in the World

During July 2007, monthly mean surface air temperatures were observed above normal in most of the Northern Hemisphere. Temperatures were more than 2℃ above normal in central and southern Europe, regions from Mongolia to northern West Siberia, the Bering Strait and regions nearby, western North America, southwestern Australia, with temperatures more than 4℃ above normal in some regions. Meanwhile, temperatures were about 2℃ below normal in parts of the Far East of Russia, southern U.S.A. and parts of southern South America .

For the month, precipitations were observed 50-200% more than normal in western and northern Europe, northern and western Russia, most of middle Asia, northwestern China, the Huaihe rivers, western coasts amd southern part of United States, southern Alaska with above 200% more than normal in locations. Meanwhile, precipitations were above 50% less than normal in parts of western Russia, most of southern Russia, parts of northern East Asia, southeastern coastal regions of China, most of South Africa, western and northern United States, southern South America and most of Australia with no precipitation in parts of above regions.

General Circulation Features

Northern Hemisphere: During July 2007, the mean 500hPa geopotential height field exhibited a weak 4-wave pattern at the mid-high latitudes, with the troughs over the eastern North Pacific, northeastern North America, most of Europe and northeastern Asia. In the 500hPa height anomaly field, negative anomalies below -4 dam covered regions from the eastern North Atlantic Ocean to western Europe, parts of the northeastern Pacific and north-central Russia, with center below -8 dam. Meanwhile, positive anomalies above 4 dam covered most of polar region，Bering Strait, regions from western Russia to Baikal, northwestern North America, with center above 8 dam. For the month, the Northwest Pacific subtropical high was larger and stronger than normal, and extended more westward and its ridge line position was more southward than normal.

Southern Hemisphere: For the month, meridional circulation was much obvious at most of the mid-high latitudes. In the 500hPa height anomaly field, strong positive anomalies covered most of Antarctica and the southern South Pacific between 70-110°W, with center over the South Pacific above 20 dam. Meanwhile, obviously negative anomalies covered the western South Indian Ocean and most of the South Atlantic Ocean with center value below -12 dam on the western South Atlantic Ocean.

Tropical Features

Sea Surface Temperatures (SSTs): During July 2007, SSTs still exhibited a pattern of “cold in the east and warm in the west” in the equatorial Pacific. Positive SST anomalies above 0.5℃ covered the western equatorial Pacific west of the Date Line, which shrank compared with last month. Negative SST anomalies below -0.5℃ in the eastern equatorial Pacific extended westward to the region around 140ºW. SST anomaly indices of NINO Z, NINO 3.4, NINO 4, NINO 3 and NINO 1+2 were -0.4℃, -0.3℃, 0.2℃, -0.8℃ and -1.5℃, with drops of 0.1℃, 0.4℃, 0.1℃, 0.2℃ and 0.1℃ from last month, respectively. Meanwhile, SST anomalies were 0.5-1.0℃ above normal in most of the equatorial Indian Ocean.

Subsurface Temperatures: During July 2007, comparing with last month, the warm subsurface waters strengthened in the western equatorial Pacific but shrank westward. Meanwhile, anomalously cold subsurface waters of the eastern equatorial Pacific extended westward to the Date Line.

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

Wind Field：For the month, at the lower troposphere (850hPa), weak easterly anomalies covered the equatorial Pacific near the Date Line, while zonal winds were near normal over other regions of the equatorial Pacific. Weak easterly anomalies covered the equatorial Indian Ocean. At the upper troposphere (200hPa), westerly anomalies covered most regions of the western equatorial Pacific west of the Date Line with central values above 4m/s, while easterly anomalies covered the mid-eastern equatorial Pacific between 120-150ºW, with central values below -8m/s. Westerly anomalies covered the equatorial Indian Ocean.

Southern Oscillation: For the month, Southern Oscillation Index (SOI) was -0.5, with a drop of 0.9 from last month.

Convection over the Tropics: During July 2007, the anomalous outgoing longwave radiation (OLR) showed that convections enhanced over the equatorial Pacific west of 155ºE and they were suppressed near the Date Line. In the lower troposphere (850hPa) divergent wind field, a convergent center was observed over 150ºE, 25ºN, and a divergent center was located in 75ºW, 25ºS. Correspondingly, at the upper troposphere (200hPa), the divergent center was found over 120ºE, 10ºN, and the convergent center was over 100ºW, 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 July, and the index NINO Z was -0.4℃.

(2) The anomalously warm subsurface waters strengthened in the western equatorial Pacific, while anomalously cold subsurface waters extended westward in the eastern equatorial Pacific.

(3) Southern Oscillation Index (SOI) was -0.5.

(4) At the lower troposphere (850hPa), wind field was near normal in most of the equatorial Pacific.

(5) Convections enhanced over the western equatorial Pacific and were suppressed near the Date Line..

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

The SSTs appeared to be the ENSO-neutral conditions in the mid-eastern Pacific, and they are expected to maintain in the following 1-2 months, but they could transfer to weak La Niña conditions before the end of the year.