Monthly Highlights
Climate in China
During September 2008, monthly mean surface air temperatures were at least 1℃ below normal in southwestern Xizang and parts of northern Heilongjiang, but almost above normal in other regions of China. Temperatures were observed at least 1℃ above normal in mid-eastern Northwest China, mid-eastern and parts of western Inner Mongolia, eastern and southern Northeast China, most Southwest China, Jianghuai, Jiangnan and South China, with 2-4℃ above normal in mid-eastern Jiangnan.
For the month, precipitations were observed 30-100% more than normal in western Xinjiang, regions from eastern Xinjiang to North China across central and eastern Northwest China, mid-western Inner Mongolia, northern Northeast China, southwestern Xizang, northwestern Sichuan basin and mid-western South China, with over 200% more than normal in locations. Meanwhile, precipitations were observed 30-80% less than normal in central-southern Xinjiang, northwestern and southeastern Xizang, northwestern and eastern Inner Mongolia, eastern and southern Northeast China, most Southwest China, northern Jiangnan, Jianghan, Jianghuai, southern Huanghuai and eastern South China, with no precipitations in parts of Xinjiang and Xizang.
During September 2008, temperatures were at least 1℃ above normal in northern Siberia, Far East Russia, most West Asia, most eastern China, eastern Canada, northeastern and western United States, Alaska, eastern South America, most North Africa and central Australia, with temperatures at least 2℃ above normal in locations. Meanwhile, temperatures were at least 1℃ below normal in southwestern and parts of northern Europe, regions from western Russia to Kazakhstan, eastern Siberia, regions from southern United States to northern Mexico, southern Brazil and South Africa, with temperatures at least 2℃ below normal in locations.
For the month, precipitations were at least 50% less than normal in North Europe, parts of mid-northern Russia, Mongolia, eastern and parts of southern West Asia, western North America, central and eastern South America, mid-northern and southeastern Africa, and western and southern Australia, with no precipitation in parts of above regions. Meanwhile, precipitations were observed at least 50% more than normal in eastern Russia, northern West Asia, middle Central Asia, regions from northern India to western Xizang of China, regions from mid-eastern Northwest China to North China, mid-eastern North America, parts of northwestern and mid-eastern Africa, and northern and eastern Australia, with precipitations at least 200% more than normal in locations.
Northern Hemisphere: During September 2008, in the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered regions from eastern North America to northern North Atlantic Ocean, and regions from North Europe to Far East Russia across polar regions, with central values above 8 dam. Meanwhile, negative anomalies below -4 dam covered Baffin Bay and Greenland Island. For the month, the subtropical high over the Northwest Pacific was larger and stronger than normal, and its position was more northward and westward than normal.
Southern Hemisphere: For the month, in the 500hPa geopotential height anomaly field, negative anomalies below -4 dam covered southeastern South Pacific and the seas to the south of Australia and Africa, but positive anomalies above 4 dam dominated other regions south of 30ºS. Negative height anomalies with minimum values below -12 dam covered southeastern South Pacific, while positive anomalies with maximum values above 20 and 16 dam respectively in southern South Indian Ocean and southeastern Antarctica.
Tropical Features
Sea Surface Temperatures (SSTs): During September, positive SST anomalies dominated the eastern and western equatorial Pacific, with central values above 1.0℃. Negative SST anomalies appeared in regions around the dateline with central values below -0.5℃. Furthermore, SST anomalies decreased significantly in the equatorial eastern Pacific. In September, indices of NINO 1+2, NINO 3, NINO 4, NINO 3.4 and NINO Z were 0.7℃, 0.3℃, -0.4℃, -0.2℃ and 0.0℃, with drops of 0.3℃, 0.4℃, 0.1℃, 0.3℃ and 0.3℃ from last month, respectively.
Subsurface Temperatures: During September，anomalously cold subsurface water strengthened significantly in the central equatorial Pacific, and extended eastward to the eastern equatorial Pacific. The anomalously warm water strengthened dramatically in the western equatorial Pacific at thermocline depth.
Warm Pools: During September, area of the western Pacific warm pool was near or just below normal while the Indian Ocean warm pool was above normal. Their intensities were above normal.
Wind Field：For the month, at lower troposphere (850hPa), weak westerly anomalies covered regions 120ºE-150ºE in the equatorial Pacific, while easterly anomalies prevailed over regions 150ºE-160ºW with central values below -4m/s. At upper troposphere (200hPa), easterly anomalies covered the most eastern equatorial Pacific.
Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 1.4, with a rise of 0.5 from last month.
Convections over the Tropics: During September, the anomalous outgoing longwave radiation (OLR) showed that convection was enhanced over Indonesia, and suppressed near the dateline in the equatorial Pacific. In the lower troposphere (850hPa) divergent wind field, a convergent center was over 125ºE, 15ºN, and divergent centers were observed over 115ºW, 20ºS and 95ºW, 25ºS respectively. At upper troposphere (200hPa), the corresponding divergent center was over 125ºE, 15ºN, and convergent centers were over 110ºW, 15ºS and 90ºW, 25ºS.
ENSO Monitoring and Prediction
(1) During September, positive SST anomalies dominated the eastern and western equatorial Pacific, while negative SST anomalies appeared in regions around the dateline. Furthermore, SST anomalies decreased significantly in the equatorial eastern Pacific.
(2) During September, anomalously cold subsurface water strengthened significantly in the central equatorial Pacific, and extended eastward to the eastern equatorial Pacific. The anomalously warm water strengthened dramatically in the western equatorial Pacific at thermocline depth.
(3) Southern Oscillation Index (SOI) was 1.4.
(4) For the month, at lower troposphere (850hPa), weak westerly anomalies covered regions 120ºE-150ºE in the equatorial Pacific, while easterly anomalies prevailed over regions 150ºE-160ºW.
(5) The convection was enhanced over Indonesia, and suppressed near the dateline in the equatorial Pacific.
(6) Predictions of most statistical and dynamic models indicated ENSO-neutral conditions before the end of 2008.
During September, the current status indicated that the tropical Pacific was still in ENSO-neutral conditions. Based on monitoring of recent atmospheric and oceanic conditions in the tropical Pacific and predictions of statistical and dynamic models, ENSO-neutral conditions are expected to continue into the end of 2008.