Monthly Highlights
1. Climate in China
During April 2013, the surface air temperature averaged over China was 11.0°C, which was equal to the normal. Monthly mean surface air temperatures were generally below normal in Northeast China, eastern Inner Mongolia, most of North China, northern Huanghuai, southern Jiangnan and South China, with 2-4°C below normal in most of Northeast China, eastern Inner Mongolia, eastern North China and northern Huanghuai and 4°C below normal in locations. Meanwhile, positive anomalies were around 1-2°C in most of Northwest China and most of Southwest China, with 2°C above normal in locations .
For the month, the mean precipitation of China was 43.2 mm, which was 3.4% less than normal (44.7mm). Percentages of precipitation anomalies were observed 20-50% above normal in eastern Xinjiang, eastern Inner Mongolia, southern Qinghai, mid-eastern Tibet, central Southwest China and southern South China, with precipitations 50% more than normal in locations. Meanwhile, precipitations were 50-80% below normal in northern Northeast China, mid-western Inner Mongolia, most of Northwest China, most of North China, most of Huanghuai, eastern Jianghuai, most of Jiangnan, southern Southwest China and northwestern Tibet, with 80% below normal in locations.
In April, Daily Temperature Drop (DTD) and Consecutive Temperature Drop (CTD) were observed in China. DTD extremes occurred in 34 stations, which were located in Shaanxi, Shanxi, Sichuan, Hunan, Yunnan and other provinces (districts), with widespread DTDs of 8~15°C, with Lintong (12.0°C) and Huozhou (12.0°C) breaking record. During the month, CTD extremes were observed in 41 stations，distributed in Shanxi, Henan, Hubei and other provinces. CTDs ranged from 10°C to 20°C, with Xinan (17.6°C), Mengjin (20.9°C) , Gongyi (17.3°C) , Yongji (19.1°C), Xiangfan (17.0°C) and Lufeng (12.6°C) setting new records.
2. Climate in the World
During April 2013, mean temperatures were 1-4°C above normal in southeastern Europe, northwestern and central Asia, Russian Far East, parts of northwestern and eastern Africa, parts of southwestern North America and southern South America, southwestern Australia, with temperatures 4-6°C above normal in parts of northwestern and eastern Russia and at least 6°C above normal in locations. Meanwhile, mean temperatures were 1-4°C below normal in mid-eastern Asia, parts of southern Central Asia, parts of northern South Asia and northwestern West Asia, most of North America and central South America, with temperatures 4-8°C below normal in regions from northwestern to northern North America and at least 8°C below normal in locations.
For the month, precipitations were 50%-200% above normal in northeastern Europe, parts of western and central Siberia, most Central Asia, northwestern and central South Asia, eastern West Asia, mid-western and southern Africa, parts of mid-eastern and northwestern North America, and central and northern South America, with precipitations at least 200% above normal in parts. Meanwhile, precipitations were at least 50% below normal in parts of northern and western East Asia, northwestern Southeast Asia, parts of northwestern and northeastern Africa, parts of southwestern North America, locations of western South America, most of Australia, with no precipitation in parts.
3. General Circulation Features
Northern Hemisphere: During April 2013, 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 40 gpm covered over regions from the southern Europe to the western East Asia, and the northern and eastern North Pacific, with central values above 200 gpm over the northern North Pacific. Meanwhile, negative anomalies below -40 gpm covered regions over the Sea of Okhotsk, the middle part of North Pacific, the northern North America, and the northeastern North Atlantic Ocean, with central values below -120 gpm in the middle part of North Pacific. For the month, the extent of western North Pacific subtropical high was near normal, and its intensity was slightly weaker with location more eastward and slightly southward compared with its normal situation.
Southern Hemisphere: For the month, the field of mean 500hPa geopotential height exhibited a three-wave pattern over mid-high latitudes. In the field of height anomaly, positive anomalies above 40 gpm covered regions over the southern South Atlantic Ocean and the southwestern South Pacific, with central values above 120 gpm in the southern South Atlantic Ocean. Meanwhile, negative anomalies below -40 gpm covered over the Antarctic Peninsula and sea areas to the north, the middle part of South Atlantic Ocean, the southeastern South Pacific, regions from the southeastern South Indian Ocean to the southwestern South Pacific, and the southwestern South Indian Ocean, with central values below -80 gpm over the Antarctic Peninsula and sea areas to the north.
4. Tropical Features
Sea Surface Temperatures (SSTs): During April 2013, sea surface temperatures (SST) were near normal across most of the eastern and central equatorial Pacific, except that negative SST anomalies were in the eastern equatorial Pacific nearby South America with central value below -2.0°C. Niño 1+2, Niño 3, Niño 4, Niño 3.4 and Niño Z indices were -0.9°C, -0.2°C, 0.0°C, -0.1°C and -0.2°C,respectively. Comparing with last month, Niño1+2, Niño3, NiñoZ decreased while Niño4 and Niño3.4 increased slightly. During April, SSTs were near or slightly above normal in the equatorial India Ocean, and positive anomalies weakened in the tropical southern India Ocean.
Subsurface Temperatures: During April 2013, anomalously warm subsurface water persisted in the western equatorial Pacific with the central value above 3°C. Meanwhile, anomalously cold subsurface water remained in the central equatorial Pacific and weakened in east of the eastern equatorial Pacific.
Warm Pools: During April 2013, the area of the Indian Ocean warm pool were below normal and the intensity were above normal. The area and intensity of the western Pacific warm pool were both above normal.
Wind Field: During April 2013, at lower troposphere (850hPa), the weak easterly wind anomalies about 2-4 m/s covered the western equatorial Pacific, while weak westerly wind anomalies about 2 m/s were observed over the eastern equatorial Pacific. At upper troposphere (200hPa), westerly wind anomalies controlled the eastern and central equatorial Pacific, and the zonal winds were near normal in the western equatorial Pacific.
Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 0.3, with a drop 0.9 from last month.
Convections over the Tropics: During April 2013, the anomalous outgoing long wave radiation (OLR) showed that convections enhanced in the western equatorial Pacific north to Indonesia while convections were suppressed in the equatorial Pacific around date line. At lower troposphere (850hPa) divergent wind field, a convergent center was around equator 140ºE, and a divergent center was near 107.5ºW，17.5ºS. At upper troposphere (200hPa), the corresponding divergent center was near 145ºE，5ºS, and a convergent center was around 105ºW，20ºS.
5. ENSO Monitoring and Outlook
On the base of the monitoring and diagnosis of the atmospheric and oceanic behaviors, as well as the results of climate system and statistical models, ENSO-neutral conditions are predicted in boreal summer 2013.
Therefore, we will closely monitor the development of ENSO conditions and update our ENSO wrap-up in time.