Monthly Highlights

1. Climate in China

In August 2011, the whole country monthly mean surface air temperature was 21.3℃, with 1.0℃ above normal (20.3℃) being the 4^th highest since 1951. Temperatures were near and above normal in most China, with temperatures 1~2℃ above normal in mid-northern Northeast China，most Xinjiang, most Gansu, most Inner Mongolia, northwestern Qinghai, the boundary regions between Sichuan, Chongqin,Yunnan and Guizhou, with 2~4℃ above normal in parts of above regions. Meanwhile, temperatures were near or below normal in southern North China， Huanghuai, Jianghan and Jianghuai, with 1~2℃ below normal in parts of above regions.  

For the month, the average precipitation over China was 85.7 mm, with 16.5% less than normal (102.6 mm) being the 5^th least since 1951. Precipitations were observed at least 30% more than normal in eastern Jianghuai and Huanghuai, Jianghan, northern and western Xinjiang, western Inner Mongolia, northwestern and parts of northern Qinghai, with 100% ~ 200% more than normal in parts of above locations. Meanwhile, precipitations were 30% ~ 80% less than normal in most Northeast China, southern Jiangnan, most South China, middle and eastern Southwest China, middle Xinjiang, middle and eastern Inner Mongolia, and eastern Tibet, with 80% less than normal in parts of above regions.

In August, daily maximum temperature extremes, consecutive hot day extremes, daily rainfall amount extremes and consecutive rainy day extremes were mainly observed in China. Daily maximum temperature extremes occurred in 142 stations in Northwest China, eastern Southwest China, South of the Yangtze River and South China, of which 19 stations having temperatures breaking records in Sichuan, Chongqing, Yunan and Guizhou. Consecutive hot day extremes occurred in 158 stations in China, with 11 stations reaching or breaking records in Chongqing, Yunnan and Sichuan. During the month, daily rainfall amount extremes occurred in 33 stations in China, with amounts breaking records at 7 stations, such as Gucheng(228.6mm), Ningde (274.0mm), Jinhua(140.0mm), Zhuanghe (152.0mm), Gulang(43.8mm), Menghai (156.5mm) and Shenzha(39.0mm). Meanwhile, consecutive rainy day extremes were observed at 38 stations in mid-eastern Northwest China, southeastern Hebei and Jiangsu, with 8 stations of them breaking records.

2. World Climate

During August 2011, monthly mean air temperatures were observed at least 2℃ above normal in most South Europe, northern East Europe, mid-eastern North Asia, northern East Asia, most West Asia, central Africa, central and southern Australia, northeastern North America, mid-northern South America, with temperatures at least 4℃ above normal in locations of northeastern Siberia, parts of southwestern Russia and southeastern West Asia. Meanwhile, monthly mean surface air temperatures were observed 1~ 4℃ below normal in southern East Europe, mid-northern Russia, most Far East Russia, southwestern West Africa, northwestern Canada, southern South America and northern Australia.

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

3. General Circulation Features

Northern Hemisphere: During August 2011, the field of mean 500hPa geopotential height exhibited a 4-wave pattern over the mid-high latitudes. In the field of height anomaly, positive anomalies above 4 dam were located in most of the Arctic region, western East Siberia, the region from northeastern North America to the northern North Atlantic Ocean and northeastern Europe, respectively, with central values above 12 dam over the polar region. Meanwhile, negative anomalies below -4 dam covered Alaska, the region from the northeastern Atlantic Ocean to northwestern Europe and West Siberia. For the month, the area and intensity of West Pacific Subtropical High were a bit larger and stronger than normal, while the western ridge point and ridge line position were a bit more eastern and northern than normal, respectively.

Southern Hemisphere: During August 2011, the field of mean 500hPa geopotential height exhibited a 3-wave pattern over the mid-high latitudes. In the field of height anomaly, positive anomalies above 4 dam covered the southern South Pacific Ocean south to Australia, the Southwest Indian Ocean, and the region from Antarctic continent to the southern tip of South America, with central values above 20 dam over the Antarctic Peninsula. Meanwhile, negative anomalies below -4 dam mainly covered the Southeast Indian Ocean, the South Atlantic Ocean and the region from the southwestern tip of South America to the Southwest Pacific basin.

4. Tropical Features

Sea Surface Temperatures (SSTs): During August 2011, SSTs were below normal in most of the equatorial central and eastern Pacific with the minimum below -1.0℃. The negative SSTA below -0.5℃ expanded in the equatorial eastern Pacific. NINO 1+2, NINO 3, NINO 4, NINO 3.4 and NINO Z indices were -0.2℃, -0.4℃, -0.2℃, -0.5℃ and -0.3℃, with drops of 0.5℃, 0.4℃, 0.1℃, 0.4℃ and 0.3℃ from last month, respectively. Meanwhile, SSTs were about 0.5℃ above normal in the equatorial Indian Ocean.

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

Warm Pools: During August 2011, the area of the Indian Ocean warm pool was slightly larger than normal, and that of the western Pacific warm pool was near normal. Their intensities were both above normal.

Wind Field: In August 2011, at lower troposphere (850hPa), easterly anomalies dominated over the equatorial Pacific to the west of 160°W, and westerly anomalies were observed over the equatorial Pacific to the east of 160°W. At upper troposphere (200hPa), westerly anomalies covered most of the equatorial Pacific with anomalous westerlies above 4 m/s.

Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 0.4, with a drop of 0.5 from last month.

Convections over the Tropics: During August, the anomalous outgoing long-wave radiation (OLR) showed that enhanced convection was still over the equatorial western Pacific near the Indonesia Islands with reduced area. Meanwhile, suppressed convection was slightly developed over the western and central Pacific.

In the divergent wind field at lower troposphere (850hPa), the convergent and divergent centers were around 140°E, 15°N and 80°W, 22.5°S, respectively. At upper troposphere (200hPa), the corresponding divergent and convergent centers were near 140°E, 10°N and 90°W, 22.5°S, respectively.

5. ENSO Monitoring and Outlook

Since late July 2011, the temperatures in the equatorial east-central Pacific ocean have decreased, and the atmospheric circulations above it reflect aspects of La Niña at all times, and at present, the negative sea surface temperature anomalies cover the equatorial east-central Pacific. Based on the results of the ENSO model forecasts, ENSO-neutral (-0.5℃<Nino z index <0.5℃) is expected to last through the Northern Hemisphere fall 2011, with ENSO-neutral or La Niña conditions (Nino z index ≤-0.5℃) equally likely thereafter.

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