Monthly Highlights

Climate in China

During April 2009, the monthly whole-country mean surface air temperature of China was 12.2℃, with 1.9℃ above normal, which was the second maximum value since 1951. While temperatures were near normal in South China and eastern Southwest China, they were observed above normal in most of China. They were 2-4℃ above normal in most of Northwest China, most of Inner Mongolia, western Jilin, southwestern Heilongjiang, central Tibet and northern Shanxi, with at least 4℃ above normal in locations. For the month, precipitations were observed 30-200% more than normal in much of Northeast China, central-eastern Inner Mongolia, eastern North China, northern Huanghuai, most of Hanjiang, parts of Southwest China, Hainan and northern Xinjiang, with at least 200% more than normal in locations. Meanwhile, precipitations were observed 30-80% less than normal in most of Northwest China, western North China, southeastern Jiangnan, eastern South China, eastern Jianghuai, most of Tibet, northeastern Inner Mongolia and northwestern Heilongjiang, with at least 80% less than normal in locations.

Climate in the World

During April 2009, in the temperature anomaly field, it displayed a pattern of “+ - +” in Eurasia from west to east, with temperatures 1-2℃ below normal in western Russia and western Asia and 1-2℃ above normal in most of the other regions, with at least 4℃ above normal in locations. In the remainder of the world, temperatures were observed 1-2℃ above normal in eastern coast of America, most of Alaska, eastern West Africa, northwestern Australia, most of Venezuela and southern South America, with temperatures at least 4℃ above normal in locations. Meanwhile, temperatures were 1-2℃ below normal in mid-western America, southwestern Canada, northwestern North Africa and parts of southern Australia.

For the month, precipitations were observed 50-100% less than normal in most of Europe, southeastern Siberia, western China, India, parts of America, northern Mexico, northwestern Australia, central West Africa, parts of North Africa, northern Venezuela and southern South America, with no precipitation in parts of above regions. Meanwhile, precipitations were 50-100% more than normal in parts of southwestern Europe, regions around the Ural Mountains, most of Central Asia, northeastern China, most of the Indo-china Peninsula, mid-southern America, western Alaska, southern West Africa and northeastern Brazil, with precipitations at least 200% more than normal in locations.

General Circulation Features

Northern Hemisphere: During April 2009, in the mean 500hPa geopotential height field, longitudinal circulation was above normal over Eurasia. A pressure ridge covered eastern Europe, while a deep trough located over the Ural. In the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered regions from central Siberia to northeastern China and Mongolia, most of eastern Europe, the northern North Atlantic Ocean, the northern and parts of eastern North Pacific, with central values above 12 dam over eastern Europe. Meanwhile, negative anomalies below -4 dam covered regions from the Ural to the North Pole and the Greenland, with central values below -16 dam over the North Pole. For the month, the area of the Northwest Pacific subtropical high (NWPSH) was larger than and its intensity was above normal. The western end of the ridge contracted further eastward.

Southern Hemisphere: For the month, in the 500hPa geopotential height anomaly field, while negative anomalies below -4 dam covered parts of southern South Pacific, positive anomalies above 4 dam covered most of the middle-high latitude regions, with central values above 12 dam over the South Pole.

Tropical Features

Sea Surface Temperatures (SSTs): During April, the SSTs were near normal in most of the equatorial Pacific except for the SSTs were observed 0.5℃warmer than normal in the region of 120ºE-160ºE and 120ºW-90ºW. In April, indices of NINO 1+2, NINO 3, NINO 4, NINO 3.4 and NINO Z were 0.4℃, 0.0℃ , -0.1℃, -0.2℃ and 0.0℃, with a rise of 0.5℃, 0.7℃, 0.2℃, 0.3℃ and 0.5℃, respectively.

Subsurface Temperatures: During April (25days), the anomalously warm subsurface water shifted eastward into the region east of 120ºW and upward. The anomalously warm subsurface water dominated most of the equatorial Pacific except for part of the central and eastern equatorial Pacific.

Warm Pools: During April, 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), easterly anomalies prevailed over the central Pacific with central values over 4m/s, weak westerly anomalies covered the western equatorial Pacific. At upper troposphere (200hPa), weak westerly anomalies covered the western and central equatorial Pacific.

Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 1.2, with a rise of 1.0 from last month.

Convections over the Tropics: During April, the anomalous outgoing longwave radiation (OLR) in the equatorial Pacific showed that the convection was suppressed over the central-eastern equatorial Pacific, and enhanced over the Philippines and its vicinity. In the lower troposphere (850hPa) divergent wind field, a convergent center was near 135ºE, 5ºN, and a divergent center was near 105ºW, 10ºS. At upper troposphere (200hPa), the corresponding divergent center was near 125ºE, 5ºN, and the convergent center was near 100ºW, 10ºS.

ENSO Monitoring and outlook

Along with eastward propagation of the anomalously warm subsurface water into the eastern equatorial Pacific, the Nino indices have transited into a neutral status in April. While, some features of tropical atmosphere still reflected weak La Nina conditions, with being 1.2 of Southern Oscillation Index (SOI), suppressing of the convections over the central-eastern equatorial Pacific, and continuously prevailing of the easterly anomalies over the central equatorial Pacific, which all hinted a transitional phase of current tropical oceanic and atmospheric status were in. Thus, the SST anomalies in the central and eastern equatorial Pacific will persist neutral condition in May 2009. Furthermore, based on a majority of the model forecasts, the neutral-ENSO conditions in the central and eastern equatorial Pacific are expected in the coming 1-2 months.

Based on monitoring of recent atmospheric and oceanic conditions in the tropical Pacific and predictions of statistical and dynamic models, the SST anomalies in the central and eastern equatorial Pacific will persist neutral condition in May 2009. However, considering recent occurrence of rapidly adjustment in the tropical Pacific, the SSTs were expected 0.5℃ warmer than normal in the central and eastern equatorial Pacific in June 2009. And, we will closely monitor the ENSO status and issue our updating prediction on it.