Monthly Highlights

1．Climate in China

During August 2009, the monthly mean surface air temperatures were near or above normal in most of China, with 1-2℃ above normal in parts of central eastern Inner Mongolia, southern South China, southern and western Tibetan Plateau, Tarim Pendi, and at least 2℃ above normal in some areas of central eastern Inner Mongolia, except for temperatures 1-2℃ below normal in parts of Ordos region.

Precipitation exhibited 30-100% more than normal in the partial regions between the upper and middle reaches of the Yangtze River and the Yellow River, the lower reaches of the Yangtze River and its southern areas, and northwestern Northeastern China. Meanwhile, precipitation with 30-80% less than normal was present in some areas of southern Tibetan Plateau, most of northwestern China, central eastern Inner Mongolia, central southern Northeast China, and most of South China.

In August, daily maximum temperature extremes occurred in eastern South China and Liaoning province. The maximum temperatures prompted new records in Xingning station (39.0 ℃) of Guangdong, Emei station (41.7℃) of Sichuan. Daily precipitation extremes occurred in the central parts of Huanghuai, coast of Fujian and Zhejiang, southwest Chongqing. New records of daily maximum precipitation were reported in following 5 stations, i.e. Zherong (408.8mm) of Fujian, Feixian (298.3mm) of Shandong, Tongliang (228.6mm) of Chongqing, Keping (74.0mm) of Xinjiang, Molidawa (118.8mm) of Inner Mongolia. Both daily maximum temperature and precipitation extremes were observed in Changle station of Fujian and Xingwen station of Sichuan.

2．World Climate

During August 2009, the monthly mean surface air temperatures were primarily above normal in most of the world. The temperatures were at least 1-2℃ above normal in parts of western Europe, western Siberia, northern North China, southern West Asia, eastern and western seacoasts of North America, and most Africa, South America, and Australia, with at least 4℃ above normal in two small areas of southwestern West Asia and central Australia. Moreover, temperatures were 1-2℃ below normal in parts of central North America, southeastern Europe and around the boundary between Europe and Asia, with even more than 2℃ below normal in some of the above areas.

For the month, precipitation was at least 50% more than normal in parts of central West Asia, the northwest to Caspian Sea, around Mongolia, northwestern and southern Africa, central western and northwestern North America, central South America, with precipitation at least 200% more than normal in some of the above-mentioned areas. Meanwhile, precipitation was 50% less than normal in parts of southeastern Europe, central and northern Asia, southeastern and northwestern Africa, western and northeastern North America, central and northern South America, and most Australia, with no precipitation in some of the above-mentioned areas.

3．General Circulation Features

Northern Hemisphere: During August 2009, the mean 500hPa geopotential height field mainly showed a 4-wave pattern at the high-mid latitudes. In the anomaly field, positive anomalies above 4 dam covered parts of central southern West Asia, western Europe, northwestern Russia and western Bering Sea. In addition, there was a center of positive anomalies above 8 dam which was located in the western Arctic Ocean. In the meantime, there was a center of negative anomalies below -4 dam around Caspian Sea. Also, negative anomalies dominated parts of the high-mid latitude North Atlantic Ocean and the eastern Arctic Ocean with central values below -8 dam, respectively. Furthermore, the subtropical high over the Northwest Pacific covered a larger area and more intense along with stretching more westward than its normal conditions. But its mean north-south position was nearly normal.

Southern Hemisphere: During August 2009, the mean 500hPa geopotential height field exhibited a 3-wave pattern at the high-mid latitudes. In the anomaly field, the south to 60°S and Antarctica were dominated by intense positive anomalies with central values above 20 dam. The positive region extended northward to around 35°S at 10°-80°E. In addition, the southeastern subtropical Pacific, the central eastern Australia and the southeastern Pacific to Australia were covered by above 4 dam positive anomalies. Negative anomalies covered the middle latitudes of southeastern Pacific and South Atlantic Ocean with center values below -4 dam, respectively. Another negative center below -8 dam was located over the southern ocean to Australia.

4. Tropical Features

Sea Surface Temperatures (SSTs): During August, equatorial SSTs were 0.5℃ above average across most of the Pacific Ocean, and at least 1.0℃ above average near the Date Line and in parts of the eastern Pacific, with maximum positive departure above 1.5℃. In August, indices of NINO 1+2, NINO 3, NINO 4, NINO 3.4 and NINO Z were 0.9℃, 1.0℃, 0.8℃, 0.8℃ and 0.9℃, with a rise of 0.2℃ and 0.1℃ from last month for the indices of NINO 4 and NINO Z, respectively.

Subsurface Temperatures: During August, the anomalously warm subsurface water prevailed over most of the equatorial Pacific. Positive equatorial sub-surface temperature anomalies weakened in the eastern Pacific, while strengthened in the central Pacific with the largest anomalies near 125m depth.

Warm Pools: During August, both the area and intensity of the western Pacific and Indian Ocean warm pool were above normal.

Wind Field：For the month, at lower troposphere (850hPa), westerly wind anomalies have covered large portions of the equatorial Pacific, except the regions near the Date Line. At upper troposphere (200hPa), except for westerly wind anomalies to the east of 110ºW, easterly anomalies covered most of the equatorial Pacific, with maximum negative departure of -8m/s in the central and eastern Pacific.

Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was -0.3, with a drop of 0.4 from last month.

Convections over the Tropics: During August, the anomalous outgoing longwave radiation (OLR) showed that the convection was near normal in most of the equatorial Pacific. In the lower troposphere (850hPa) divergent wind field, a convergent center was near 115ºW, 12.5ºN and a divergent center was near 110ºE, 5ºS in the tropical Pacific. At upper troposphere (200hPa), the corresponding divergent and convergent center was near 115ºW, 10ºN and 80ºE, 10ºN.

5. ENSO Monitoring and Outlook

During August 2009, the monitoring results showed that El Nino features remain slowly enhanced over the tropical Pacific, with warmer sea surface over the central and eastern equatorial Pacific, 0.9℃ of Nino Z index, anomalously warm subsurface water across most of the equatorial Pacific, low-level westerly wind anomalies and upper-level easterly wind anomalies corresponding to weakening Walker circulation. However, we should pay more attentions to the present equatorial conditions such as the lasting warmth in the western Pacific, the suppressed convection over the central and eastern Pacific and the slow drop of SOI. Most statistical and dynamic climate models predict that equatorial SST anomalies will keep above 0.5℃ in the central and eastern Pacific till winter 2009-2010.

Based on the aforementioned analysis, El Nino conditions are supposed to remain in 3 months and El Nino event is expected to form in the central and eastern equatorial Pacific. We will continue to monitor the further development of the El Nino conditions and to update the information on ENSO monitoring, diagnosis, prediction and services in time.