Monthly Highlights

 

1. Climate in China

In April 2011, the monthly mean surface air temperature was 11.3℃, with 1.0℃ above normal (10.3℃) being the 7th highest since 1961. And monthly mean temperature was the 3rd lowest since 1961 in Hainan. Temperatures were 1~2℃ below normal in some parts of Hainan, while they were 2~4℃ above normal in northern and eastern Xinjiang, most Gansu, northwestern Qinghai, southern Ningxia, southwestern Shaanxi and some parts of Inner Mongolia.

In the month, average precipitation over China was 21.4 mm, with 51.2% less than normal (43.81 mm) being the least in the past 50 years. While precipitations were observed close to normal or above normal in northern and eastern North China, western and southern Southwest China, eastern Heilongjiang, southern Qinghai and parts of Xinjiang, they were more than 30% below normal in most of the rest of China, with over 80% less than normal in southern Xinjiang, northwestern Gansu, locations of Inner Mongolia, northern Anhui, southern Jiangxi and eastern Guangdong.

In April, daily temperature drop extremes occurred in 75 stations, which were mainly located in Shandong, Henan, Jiangsu and Hebei, with 20 stations setting new records. Consecutive days of no precipitation extremes occurred in 21 stations, which were mainly located in Liaoning, Jiling，Shandong, Inner Mongolia and Heilongjiang, with 6 stations setting new records.

 

2. World Climate

During April 2011, monthly mean temperatures were 1-4℃ above normal in most Europe, parts of northern and eastern Africa, southern North America, locations of eastern South America, with temperatures 4-8℃ above normal in most mid-northern Russia, western Europe. Monthly mean surface air temperatures were observed 1-2℃ below normal in regions from southeastern Europe to southwestern Russia, most East Asia, most northern North America and mid-northern Australia, with temperatures at least 4℃ below normal in locations.

  For the month, precipitations were at least 50% more than normal in regions from southeastern Europe to northern West Asia, parts of northern Russia, southern South Asia, the Indo-China Peninsula, parts of northern, eastern and southern Africa, eastern North America, parts of mid-northern South America, northern Australia, with precipitations at least 200% above normal in locations. Meanwhile, precipitations were at least 50% less than normal in locations of southeastern Russia, parts of northern and southeastern East Asia, regions from southwestern Central Asia to northwestern South Asia, parts of West Africa and Central Africa, southern and parts of northwestern North America, locations of South America, southern Australia, with no precipitation in locations.

 

3. General Circulation Features

Northern Hemisphere: During April 2011, the field of mean 500hPa geopotential height exhibited a 3-wave pattern over the mid-high latitudes, and the center of polar vortex covered North America. In the height anomaly field, positive anomalies above 4 dam covered regions from the northeastern North Atlantic Ocean to western Europe, regions from Central Asia to most of Russia through Mongolia, and the northeastern North Pacific, with central values above 12 dam over western Europe. Meanwhile, negative anomalies below -4 dam covered central and northern North America, the northwestern North Pacific and parts of eastern Europe, with central values below -20 dam. During the month, the area of Northwest Pacific Subtropical High and its western ridge point were near normal, with its intensity weaker than normal and its ridge line a bit southerly.

Southern Hemisphere: During April 2011, in the field of 500hPa geopotential height anomaly, positive anomalies above 4 dam covered most of the polar regions, the southern South Atlantic Ocean and the southwestern South Indian Ocean, with central values above 12 dam. Meanwhile, negative anomalies below -4 dam mainly covered oceans south to South Africa and oceans south to the southmost of South America, with central values below -8 dam.

4. Tropical Features

 ace water enhanced slightly andSea Surface Temperatures (SSTs): During April 2011, negative SST anomalies weakened rapidly. Although negative SST anomalies of -0.5~ -1.0℃ remained in the central equatorial Pacific, positive SST anomalies around 0.5～1.0℃ appeared in the eastern equatorial Pacific offshore South America. NINO 1+2, NINO 3, NINO 4, NINO 3.4 and NINO Z indices were 0.2℃, -0.2℃, -0.6℃, -0.7℃ and -0.3℃, and rises of 0.5℃、0.5℃、0.1℃、0.2℃ and 0.4℃ for others from last month, respectively.

Subsurface Temperatures: Oceanic Kelvin waves propagated from the western equatorial Pacific eastward along thermocline, resulting the weakening of the negative subsurface temperature anomalies in the central and eastern equatorial Pacific. Anomalously warm subsurface water strengthened significantly in the central-eastern equatorial Pacific and its center uplifted and reached east of 120oW during April 2011.

Warm Pools: During April 2011, the area of the Indian Ocean warm pool was near normal, while its intensity was slightly above normal. The area of the western Pacific warm pool was below normal, but its intensity was above normal.

Wind Field: For the month, at lower troposphere (850hPa), westerly anomalies prevailed to the west of 130ºE in equatorial western Pacific and the east of 150ºW in equatorial eastern Pacific, while easterly anomalies were observed over the east of 140ºE in equatorial western Pacific and equatorial central Pacific. At upper troposphere (200hPa), westerly anomalies covered the  equatorial central-eastern Pacific Ocean, while easterly anomalies covered the equatorial western Pacific。

Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 2.2, with a rise of 0.1 from last month.

Convections over the Tropics: During April, the anomalous outgoing long wave radiation (OLR) showed that enhanced convection was observed over the equatorial western Pacific Ocean, while suppressed convection was observed over the equatorial central-western Pacific Ocean. At lower troposphere (850hPa) divergent wind field, a convergent center was around 140ºE, 5ºS, and a divergent center was near 115ºW, 5ºN. At upper troposphere (200hPa), the corresponding divergent center was near 145ºE, 10ºS, and a convergent center was around 90ºW, 20ºN.

5. ENSO Monitoring and Outlook

Since January 2011, oceanic Kelvin waves propagated from the equatorial western Pacific Ocean eastward along thermocline, leading to the weakening of the negative subsurface temperature anomalies in the equatorial central and eastern Pacific Ocean. In April 2011, SST anomalies in the equatorial central-eastern Pacific Ocean was -0.3℃, and this condition will continue. The low-level westerly anomalies developed and extended eastward over the equatorial western Pacific Ocean. These tropical atmospheric and oceanic features indicate that the La Niña event ended.

Most statistical and dynamic models predicted that the SST anomalies over the equatorial central-eastern Pacific Ocean would become near zero or slightly positive in the next two or three months.

Based on current observations, analyses and model forecasts, a transition to near or above normal conditions is expected during late spring-summer 2011.