Monthly Highlights

1. Climate in China

In September 2011, the whole country monthly mean surface air temperature was 16.4℃, with 0.4℃ above normal (16.0℃). Temperature were about 1～2℃ above normal in most Xinjiang, the Tibet, most Qinghai, western Sichuan and most Yunnan, with 2～4℃ above normal in locations. Meanwhile, temperatures were about 1～2℃ below normal in middle and southern North China, northern Huanghuai, northern Jianghan, southeastern Northwest China, parts of western and northeastern Inner Mongolia, with 2～4℃ below normal in locations.

For the month, the average precipitation over China was 65.0 mm which was near normal (65.3 mm). Precipitations were observed at least 30% more than normal in eastern Northwest China, locations of western Inner Mongolia, northeastern Heilongjiang, middle and southern North China, northern Huanghuai, northeastern Sichuan, western and southern Qinghai, locations of western Xinjiang and the western Tibet, southern Yunnan and Hainan, with 100%-200% more than normal in eastern Northwest China, southern North China, north Huanghuai. Meanwhile, precipitations were 30% ~ 80% less than normal in most Xinjiang and Inner Mongolia, western Gansu, north Qinghai, most Northeast China, Jianghuai, northern Jiangnan, Guizhou, southeastern coastal regions and locations of the eastern Tibet, with at least 80% less than normal in parts of above regions.

In September, daily maximum temperature extremes, consecutive hot day extremes, daily rainfall amount extremes，consecutive rainfall amount extremes and consecutive rainy day extremes were mainly observed in China. Daily maximum temperature extremes occurred in 21 stations, which were mainly located in regions between Sichuan，Chongqing and Yunnan. Meanwhile, Consecutive hot day extremes occurred in 43 stations, mainly distributing in parts of eastern Southwest China and coast of South China. During the month, daily rainfall amount extremes occurred in 22 stations, with rainfall amounts of 3 stations, Putian (313.8mm), Tiantai (182.5mm)and Longling (167.7mm), breaking record. Consecutive rainfall amount extremes were observed in 29 stations，which were mainly located in northern Huanghuai, northern Jianghan, southern Shaanxi and northeastern Sichuan, with rainfall amounts of Ziyang (496.0mm) breaking record. Additionally, consecutive rainy day extremes occurred in 71 stations，with 11 stations reaching or breaking record, such as Lushan (25 days), Ziyang (17 days), Danjiangkou (17 days) and Chengkou(17 days).

2. World Climate

During September 2011, monthly mean temperatures were 1-4℃ above normal in most Europe, West Siberia and northwestern Central Siberia, northern Central Asia, southwestern East Asia, most West Asia, parts of northern, eastern and southern Africa, most North America and parts of South America, with temperatures at least 4℃ above normal in locations. Monthly mean surface air temperatures were observed 1~ 2℃ below normal in East Siberia, regions from southeastern Central Siberia to northeastern East Asia, northwestern South Asia, parts of central and southern North America, parts of northern Australia, with temperatures 2~ 4℃ below normal in locations.

For the month, precipitations were at least 50% more than normal in regions from northeastern Europe to western of West Siberia, the Far East, central and eastern East Asia, northwestern South Asia, parts of northern West Asia, parts of western and eastern Africa, most eastern and locations of western North America, locations of South America, with precipitations at least 100% above normal in locations. Meanwhile, precipitations were at least 50% less than normal in central and southern Europe, most Siberia, northern East Asia, central and northern Central Asia, parts of northern, eastern and southern Africa, most western North America, most central-eastern and parts of southern South America, most Australia, with no precipitation in locations.

3. General Circulation Features

Northern Hemisphere: During September 2011, in the field of 500hPa geopotential height anomaly, positive anomalies above 4 dam covered eastern Europe, West Siberia and northwestern North Pacific, with maximum above 12 dam. Meanwhile, negative anomalies below -4 dam covered northeastern East Asia, northeastern North Pacific and northern North Atlantic Ocean, with central values below -16 dam. For the month, the area and intensity of Northwest Pacific Subtropical High were larger and stronger than normal, while its western ridge point and ridge line were a bit more eastern and northern than normal, respectively.

Southern Hemisphere: During September 2011, in the field of 500hPa geopotential height anomaly, positive anomalies above 4 dam covered most of the polar region, with central values above 12 dam over the Ross Sea. Meanwhile, negative anomalies mainly covered most of the parallel between 60oS and 70oS, with central values below -4 dam over the southern South Pacific Ocean and the southwestern South Indian Ocean.

4. Tropical Features

Sea Surface Temperatures (SSTs): During September 2011, SSTs were at least 0.5℃ below normal  in almost all the central and eastern equatorial Pacific, and the minimum SSTa below -1.0℃. NINO 1+2, NINO 3, NINO 4, NINO 3.4 and NINO Z indices were -0.7℃, -0.6℃, -0.4℃, -0.7℃ and -0.6℃, with drops of 0.5℃, 0.2℃ 0.2℃, 02.℃ and 0.3℃ from last month, respectively. Meanwhile, SSTs were some 0.5℃ above normal in the equatorial Indian Ocean.

Subsurface Temperatures: During September 2011, anomalously cold subsurface water controlled most of the central and eastern equatorial Pacific. Negative anomalies in the east-central Pacific strengthened, expanded and uplift evidently, with central values below -4℃.

Warm Pools: During September 2011, the area of the Indian Ocean warm pool was slightly larger than normal, and that of the western Pacific warm pool was smaller than normal. The intensities of the Indian Ocean warm pool was above normal, and that of the western Pacific warm pool was slightly below normal.

Wind Field: In September 2011, at lower troposphere (850hPa), easterly anomalies dominated over the central and western equatorial Pacific to the west of 160°W, with centre value about 6m/s, and zonal wind anomalies were near normal over the central and eastern equatorial Pacific to the east of 160°W. At upper troposphere (200hPa), westerly anomalies covered most of the equatorial Pacific with values above 4 m/s.

Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 1.2, with a rise of 0.8 from last month.

Convections over the Tropics: During September, the anomalous outgoing long-wave radiation (OLR) showed that enhanced convection was over most of the Northwest equatorial Pacific and the South China Sea, with area much larger than that in August.

In the divergent wind field at lower troposphere (850hPa) over the tropical Pacific, the convergent and divergent centers were around 135°E, 15°N and 120°W, 15°S, respectively. At upper troposphere (200hPa), the corresponding divergent and convergent centers were near 135°E, 10°N and 125°W, 10°S, respectively.

Comparing with last month, negative SST anomalies in the east-central equatorial Pacific oceans enhanced obviously, showing an occurrence of La Niña conditions.

5. ENSO Outlook

Current monitoring results show the cold SSTs over the equator central-eastern Pacific persist to enhance in recent several months, and most ENSO models forecast a new La Niña event in the next 3 months. Statistical analyses also indicate most La Niña events strengthened during late autumn and early winter. So a new La Niña event is occurring and is expected to strengthen and continue through the boreal winter 2011-2012.

We will closely monitor the development of ENSO conditions and update our ENSO wrap-up in time.