Monthly Highlights

Climate in China

During September 2007, monthly mean surface air temperatures were above or near normal in most of China, with temperatures more than 1℃ above normal in most of Northeast China, most of Inner Mongolia, northern North China, eastern East China, northern Northwest China and the central and western Tibetan Plateau. For the month, precipitations were observed 30-100% above normal in southeastern Northeast China, most of Northwest China, western Inner Mongolia, central and southern North China, eastern parts of the regions between the Yellow River and the Yangtze River, most of Jiangnan and western Tibet, with more than 100% above normal in parts of the Shandong Peninsula, western Inner Mongolia and central and western Northwest China. Meanwhile, precipitations were 30-50% below normal in western Northeast China, central and eastern Inner Mongolia, northern Xinjiang, the northeastern Tibetan Plateau, the regions from the Sichuan Basin to the middle and upper reaches of the Huaihe River and eastern South China, with more than 50% below normal in locations of above regions.

Climate in the World

During September 2007, monthly mean surface temperatures were above or near normal in most of the globe. Temperatures were 1-2℃ above normal in most Asia, most Australia, eastern South America, central-eastern America, southern Canada, South Africa and northern Algeria, with more than 4℃ above normal in locations of above areas. Meanwhile, temperatures were about 1-2℃ below normal in central Europe, parts of southern China and western coasts of South America.

For the month, precipitations were observed 50-100% more than normal in central-eastern Europe, regions east to the Central Siberian Plateau, most of Northwest China, parts of South Asia, southern West Africa and parts of North America, with above 200% more than normal in locations. Meanwhile, precipitations were above 50% less than normal in West Asia, Central Asia, Northeast and Central China, northern Africa, most Australia and central and southern South America, with no precipitation in locations of above regions.

General Circulation Features

Northern Hemisphere: During September 2007, the mean 500hPa geopotential height field exhibited a weak 3-wave pattern at the mid-high latitudes, with the troughs over eastern North America, western Europe and the northern North Pacific. In the 500hPa height anomaly field, positive anomalies above 4 dam covered northern Asia, most Arctic, the northeastern North Pacific and the northern North Atlantic Ocean, with center above 12 dam over the northern North Atlantic Ocean and Arctic region. Meanwhile, negative anomalies below -4 dam covered northern North America, the polar region north to Europe and the Far East. For the month, the Northwest Pacific subtropical high was larger and stronger than normal, and withdrew eastward and its ridge line position was more northward than normal.

Southern Hemisphere: For the month, zonal circulation prevailed at most of the mid-high latitudes. In the 500hPa height anomaly field, positive anomalies above 4 dam covered Antarctica, the southwestern and southeastern South Pacific and the southwestern South Atlantic Ocean. Meanwhile, negative anomalies below -4 dam covered the eastern South Atlantic Ocean, most of the South Indian Ocean and the southern South Pacific, with center value below -12 dam over the South Pacific along 60^oS.

Sea Surface Temperatures (SSTs): During September 2007, the pattern of SSTa maintained “ cold in the east and warm in the west” in the equatorial Pacific. Negative SST anomalies enhanced and expanded westward to west of the dateline with minimum below -2.5℃. Positive SST anomalies above 0.5℃ remained in parts of the western equatorial Pacific. NINO Z, NINO 3.4, NINO 4, NINO 3 and NINO 1+2 were -1.0℃, -0.9℃, -0.4℃, -1.3℃ and -1.8℃, with drops of 0.4℃, 0.4℃, 0.5℃, 0.2℃ and 0.2℃ from last month, respectively. Meanwhile, SST anomalies were about 0.5℃ above normal in most of the equatorial Indian Ocean.

Subsurface Temperatures: Comparing with last month, anomalously cold subsurface water in the central and eastern equatorial Pacific expanded westward with -1.0℃ contour reaching west of the dateline, and its intensity enhanced, while warm subsurface water maintained in the western equatorial Pacific and its intensity enhanced slightly during September 2007.　

Warm Pools: During September 2007, both the western Pacific and Indian Ocean warm pools enlarged in areas and strengthened in intensity comparing with normal.

Wind Field：For the month, at the lower troposphere (850hPa), easterly anomalies covered the central and eastern equatorial Pacific and the center located at the central Pacific, while easterly anomalies prevailed over most of the equatorial Indian Ocean. Noticeably, westerly anomalies enhanced over Indonesia comparing with last month. At the upper troposphere (200hPa), westerly anomalies dominated the equatorial Pacific.

Southern Oscillation: For the month, Southern Oscillation Index (SOI) was 0.2, with a drop of 0.1 from last month.

Convections over the Tropics: During September 2007, the anomalous outgoing long wave radiation (OLR) showed that convections were enhanced over the western equatorial Pacific and suppressed over the central and eastern equatorial Pacific. For the tropical Pacific, in the divergent wind field of lower troposphere (850hPa), a main convergent center and a main divergent center located at 125ºE，15ºN, and 90ºW, 25ºS, respectively, while the corresponding divergent and convergent centers at the upper troposphere (200hPa) were over 120ºE，15ºN, and 95ºW，25ºS, respectively.

ENSO Monitoring and Prediction

(1) Comparing with last month, SST of the central and eastern Pacific decreased generally, and negative SST anomalies expanded westward to west of the dateline, and the index NINO Z dropped to -1.0℃, showing a enhancing La Nina conditions.

(2) Anomalously cold subsurface water of the east-central equatorial Pacific enhanced, while warm subsurface water maintained in the western equatorial Pacific.

(3) Southern Oscillation Index (SOI) was 0.2 during this month.

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

(5) Convections were suppressed over the central and eastern equatorial Pacific and enhanced over the western equatorial Pacific.

(6) Most NCC statistical models predicted La Nina conditions are likely to continue through early 2008.

In a word, the current oceanic and atmospheric conditions are favorable for La Nina development, and a weak-to-moderate La Nina conditions will likely persist through early 2008.