1. Climate in China
During April 2012, the surface air temperature averaged over China was 11.7 ℃ in April 2012, which was 1.0 ℃ warmer than normal and 0.3℃ warmer than last year (11.4 ℃). Monthly mean surface air temperatures were 1~2℃ warmer than normal in Most China, and with 2～4℃ warmer than normal in northern Xinjiang, central North China, eastern Jianghuai and Jiangnan. While monthly mean surface air temperatures were 0~1℃ below and near normal in middle and east Tibet, southeastern Qinghai, western Sichuan, eastern North China.
In April, average precipitation over China was 45.6 mm, which was 5.8% more than normal (21.3mm) and 114% more than last year (21.3mm). Precipitations were observed 30-200% more than normal in western Inner Mongolia, most Qinghai, central and eastern Tibet, central and southern Northeast China, most North China, most huanghuai and South China, and with Precipitations at least 200% more than normal in locations. Meanwhile precipitations were observed 50%-80% less than normal in most Xinjiang, central and eastern Inner Mongolia, northern Northeast China, western Jianghan and Jianghuai, most Southwest China, and with precipitations at least 80% less than normal in locations.
During April, the extreme events of Daily Temperature Drop (DTD), Consecutive Temperature Drop (CTD), High Temperature (HT), Daily Precipitation amount (DPA) were observed in China. DTD extremes occurred in 29 stations, which were located in Inner-Mongolia, Heilongjiang, Jilin, Liaoning，Hebei, Sichuan and Fujian etc., with widespread DTDs of 12-19℃ and new records in 9 stations, such as Xinbayouqi(16.1℃),Kezuozhongqi(14.2℃), Kailu (15.9℃), Wenniuteqi (18.4℃), Naimanqi(15.7℃), Ningcheng(16.3℃), Zhangjiakou(13.9℃), Zhangzhou(14.1℃). CTD extremes occurred in 3 stations, which were located in Manzhouli(21.5℃), Xinbayouqi(18.8℃) and Dunhuang(19.5℃). HT extremes occurred in 10 stations, which were located in Guangxi, Guizhou and Yunnan, and new records in 2 stations, such as Cehen(39.9℃) and Wangmo（40.4℃）. DPA extremes occurred in 2 stations, which were located in Jingan(192mm) and Kaiping(234.6mm).
2. Climate in the World
During April 2012, temperature in most of the world was warmer than normal, except in western Europe, northeastern East Asia, northeastern North America, western and southern South America, western and southern Africa and northern Australia, where temperatures were below normal. Temperatures were 1-4℃ above normal in central and western Eurasia continent, central and western North America, with temperatures more than 4℃ above normal in central Russia and part of Central Asia.
For the month, the distributions of precipitation were inhomogeneous. The precipitations were observed at least 50% above normal in northern North America, northern South America, northern and eastern part of Africa, western Europe, northwestern and central-eastern Russia, northern South Asia, eastern East Asia, southeastern China and northern Australia. Meanwhile, precipitations were observed 50% less than normal in southern part of North America and South America, northern Africa, southern Europe, western West Asia, Central East Asia and central-southern Australia.
3. General Circulation Features
Northern Hemisphere: During April2012, the field of mean 500hPa geopotential height exhibited a four-wave pattern over mid-high latitudes. In the field of height anomaly, positive anomalies above 4 dam covered regions from the northern North Atlantic Ocean, western Siberia, northwestern North Pacific as well as the Arctic polar area. Meanwhile, negative anomalies below -4 dam covered West Europe, northeastern North Pacific and northeastern North America, with central values below -8 dam over western West Europe. During the month, the area of the northwestern Pacific subtropical high was smaller than normal. Its intensity was weaker, west end of its ridge shifted further east.
Southern Hemisphere: For the month, zonal circulation dominated over mid-high latitudes of the mean 500hPa geopotential height. In the field of height anomaly, positive anomalies above 4 dam covered the regions from the southeastern and southwestern South Pacific, southern South Indian Ocean, and also the southeastern South Atlantic Ocean, with central values above 8 dam over the southeastern South Atlantic Ocean. Meanwhile, negative anomalies below -4 dam covered the regions from the South Pole to the southern South Atlantic Ocean, southeastern South Indian Ocean, and also the southwestern South Atlantic Ocean.
4. Tropical Features
Sea Surface Temperatures (SSTs): In April 2012, positive SSTAs have persisted in the eastern Pacific, and SSTAs were near normal in the rest equatorial Pacific. Niño 1+2, Niño3, Niño4, Niño3.4 and Niño Z indices were 1.3℃, 0.1℃, -0.4℃, -0.4℃ and 0℃, respectively, all with rising tendency, especially for Niño 1+2 with rises of 1.1℃ from last month. Meanwhile, SSTAs were close to normal in the equatorial Indian Ocean.
Subsurface Temperatures: During April, anomaly cold subsurface water in the central equatorial Pacific weakened markedly, while the anomaly warm subsurface water in eastern equatorial Pacific developed.
Warm Pools: During April, the areas of the Indian Ocean warm pool and the western Pacific warm pool were near normal. The warm pool intensity was near normal in the Indian Ocean and above normal in the western Pacific.
Wind Field: During April, at low troposphere (850hPa), weak easterly anomalies dominated over the central equatorial Pacific, while westerly anomalies have maintained in the eastern equatorial Pacific. At upper troposphere (200hPa), westerly anomalies dominated over most of the equatorial Pacific.
Southern Oscillation: During April, the Southern Oscillation Index (SOI) was -0.1, and the tropical atmosphere showed response to the ENSO-neutral conditions.
Convections over the Tropics: During April, the anomalous outgoing long-wave radiation (OLR) showed that suppressed convection was over sea regions of Indonesia.
5. ENSO Outlook
The zonal wind anomalies over the western equatorial Pacific at 850-hPa level have changed to the easterly anomaly since middle April despite the westerly anomalies prevailing from late March to early April. The impacts of the strongly East Asian winter monsoon in 2011/2012 on ENSO development in following seasons are significant, in which the relationship had been proved by many studies in the past several years. The impacts include two aspects of meanings, which exist that the strong winter monsoon is going to favor the La Nina conditions transition to ENSO-neutral conditions; on the other hand, it also benefits El Nino conditions showing up during the second half year depending on the westerly anomaly enhanced continuously in the western equatorial Pacific.
Based on models’ prediction and trend diagnoses of the oceanic and atmospheric variations, the ENSO-neutral conditions are expected to persist through the coming summer, and the occurrence or not of El Nino conditions depends on how is the development of westerly anomaly in the western equatorial Pacific in following several months.