Monthly Highlights

Climate in China

During April 2010, monthly mean surface air temperatures were 1-4℃ above normal in most Tibet, southwestern Qinghai and central Yunnan. Meanwhile, temperatures were observed 1-4℃ below normal in most central and eastern China, eastern and northern Northwest China, with temperatures at least 4℃ below normal in locations.

For the month, precipitations were observed 30-100% more than normal in most Northeast China, northwestern North China, northeastern and northwestern Northwest China, the southern Huanghuai area, the Jianghuai area, the southern Jiangnan area, northern South China, southeastern Tibet, central and western Yunnan, eastern and central Inner Mongolia, with precipitations at least 100% more than normal in locations. Meanwhile, precipitations were observed 30-80% less than normal in southeastern North China, northwestern Heilongjiang, central Tibet, central eastern Qinghai, southwestern Xinjiang and central Guizhou, with precipitations at least 80% less than normal in locations.

In April 2010, daily maximum temperature extremes occurred in 16 stations of Yunnan, Guizhou, Sichuan, Guangxi, Hainan. The maximum temperature broke new records in 4 stations , Songtao station(42℃) of Guizhou, zhenyuan station（37℃）of Yunnan, Leye station（34.4℃）of Guangxi, Emeisan tation（24.2℃）of Sichuan. During April, the number of consecutive precipitation days in 6 stations of Guangxi, Guizhou, Guangdong, Fujian exceeded the 95th percentile thresholds in which Danzhai station (25day) of Guizhou broke historical records.

Climate in the World

During April 2010, temperatures were 1-4℃ above normal in most Europe, northern Siberia, northwestern West Asia, southeastern Central Asia, most South Asia, Southwest China, the western Indo-China Peninsula, mid-eastern and northwestern North America, West Africa, North Africa, central and western Australia, central and northern South America, with temperatures at least 4℃ above normal in locations. Meanwhile, temperatures were 1-4℃ below normal in partly eastern and southern Siberia, most East Asia and Southwestern North America, with temperatures 4-6℃ below normal in locations.

For the month, precipitations were observed at least 50% more than normal in western Central Asian, northern East Asia, the northwestern Indo-China Peninsula, eastern South Asia, northeastern and northwestern Siberia, most western North America, southwestern West Africa, locations of North Africa, costal area of southeastern Brazil and partly northwest Brazil, and central Australia, with precipitations at least 100% more than normal in locations. Meanwhile, precipitations were at least 50% less than normal in West Europe, partly east Europe, western West Asia, most central and western South Asia, southern Siberia, northern Africa, western and eastern Australia, southern South America, with no precipitation in parts of above regions.

General Circulation Features

Northern Hemisphere: During April 2010, the mean 500hPa geopotential height field exhibited a 4-wave pattern at the mid-high latitudes. In the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered regions from northern North America to the polar regions, regions from North Africa to West Europe, regions from East Africa to the Central Indian Ocean, regions from the Bay of Bangle and South Asia to West Siberia and the polar regions and location of the northeastern North Pacific, with central values above 12 dam in the polar regions. Meanwhile, negative anomalies below -4 dam covered northeastern East Asia, the northwestern and partly northeastern North Pacific and the northern North Atlantic Ocean, with central values below -8 dam in the northwestern North Pacific. For the month, the Northwest Pacific subtropical high (NWPSH) was larger and stronger. It extended abnormally westward.

Southern Hemisphere: During April 2010, zonal circulation prevailed in the mean 500hPa geopotential height field. In the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered the southeastern and partly southwestern South Pacific, parts of ocean south of Australia, southern Africa, Madagascar, the partly southern South Indian Ocean. Meanwhile, negative anomalies below -4 dam covered the southwestern South Indian Ocean, the southeastern South Pacific, locations of the South Atlantic Ocean.

Tropical Features

Sea Surface Temperatures (SSTs): During April 2010, SSTs were more than 0.5℃ above normal in the most equatorial central and eastern Pacific. Positive anomaly centers were located near the Date Line and in the equatorial eastern Pacific, with the maximum above 1.0℃. NINO 1+2 index was 0.6℃, with an increase of 0.8℃ from last month. NINO 3 index was 0.6℃, the same as one last month. Indices of NINO 4, NINO 3.4 and NINO Z were 0.8℃, 0.7℃ and 0.7℃, with drops of 0.3℃, 0.4℃ and 0.1℃ from last month, respectively.

Subsurface Temperatures: During April 2010, anomalously warm subsurface water decreased evidently in the equatorial central and eastern Pacific from last month. Meanwhile, anomalously cold subsurface water enhanced greatly in the equatorial western Pacific and shifted eastward. The anomalously negative center moved to 150ºW, with the minimum below -3℃.

Warm Pools: During April 2010, both the area and intensity of the western Pacific and the Indian Ocean warm pool were above normal.

Wind Field：For the month, at lower troposphere (850hPa), westerly anomalies covered the equatorial Pacific east of the Date Line, and easterly anomalies prevailed over the Pacific west of it. At upper troposphere (200hPa), easterly anomalies covered the equatorial Pacific east of the Date Line, with westerly anomalies to the west of it.

Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 1.4, with an increase of 2.1 from last month.

Convections over the Tropics: During April 2010, the anomalous Outgoing Longwave Radiation (OLR) showed that enhanced convection was observed over the equatorial central and eastern Pacific east of the Date Line, and convection also developed over the equatorial western Pacific. In the lower troposphere (850hPa) divergent wind field, a convergent center was at 135ºE, 5ºS over the tropical Pacific, and a divergent center was near 90ºW, 25^oS. At the upper troposphere (200hPa), the corresponding divergent and convergent centers were near 140ºE, 5ºS and 105ºW, 25ºS, respectively.

ENSO Monitoring and Outlook

Monitoring showed that during April 2010, SST anomalies were still at least 0.5℃ in most of the equatorial central and eastern Pacific. But compared with last month, anomalously warm subsurface water decreased evidently in the equatorial central and eastern Pacific, while anomalously cold water enhanced under the equatorial western Pacific and shifted eastward greatly. Positive SOI indicated that tropical atmosphere was turning to the normal conditions. Most statistical and dynamic climate models predicted that a transition to ENSO-neutral conditions was expected by summer 2010.

Aforementioned analysis indicates that warm conditions over the equatorial central and eastern Pacific will decrease continuously in the future 1-2 months, and the El Nino event is possibly to end in May 2010. Therefore, we will still closely monitor developments of ENSO conditions and update our ENSO conditions in time.