Monthly Highlights
Climate in China
During April 2008, the monthly mean surface air temperatures were observed at least 1℃ above normal in Northeast China, Inner Mongolia, northern North China, eastern Northwest China, southern Southwest China, the Jiangnan area and central Tibet, with temperatures 2-4℃ above normal in most Northeast China and Inner Mongolia. Meanwhile, temperatures were near normal in other regions of China. For the month, precipitations were observed 30-200% more than normal in most North China, the Huanghuai area, the northern Jianghuai area, central Inner Mongolia, western and northeastern Northeast China, eastern Northwest China, northern Xinjiang and parts of Sichuan. Meanwhile, precipitations were observed 30-80% less than normal in most Northwest China, most Tibet, eastern Inner Mongolia, the mid-western Jiangnan area and the central Huanan area, with at least 80% less than normal in most Northwest China and northwestern Tibet.
During April 2008, temperatures were 1-2℃ above normal in most Europe, West Asia, northern East Asia, northeastern and southern North America, northern Alaska, and northern West Africa, with temperatures at least 4℃ above normal in locations. Meanwhile, temperatures were 1-2℃ below normal in northern Russia, central and western North America, southern Alaska and eastern Australia.
For the month, precipitations were observed 50-100% more than normal in mid-western Europe, northwestern Russia and mid-eastern Siberia, North China, northwestern South Asia, the southern Indo-China Peninsula, northern North America, central South America and parts of West Africa, with precipitations at least 200% more than normal in locations. Meanwhile, precipitations were at least 50% less than normal in parts of western Russia, West Asia, Central Asia, most northwestern China, western North America, parts of northern and southern South America, parts of northern Africa, South Africa and most Australia, with no precipitations in parts of above regions.
General Circulation Features
Northern Hemisphere: During April 2008, the mean 500hPa geopotential height field exhibited a 3-wave pattern at the mid-high latitudes. In the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered the regions from northeastern North America to northern West Asia through Greenland and the regions from northeastern East Asia to the northern North Pacific, with central values above 16 dam over the northern North Pacific. Meanwhile, negative anomalies below -4 dam covered northern Russia, the northeastern North Atlantic and northwestern North America. For the month, the subtropical high over the West Pacific was smaller and weaker than normal and its western part shrank eastward compared with its normal position.
Southern Hemisphere: For the month, zonal circulation prevailed at the mid-high latitudes. In the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered most Antarctica. Meanwhile, negative anomalies below -4 dam covered the mid-high latitudes of the southern South Pacific, the southwestern South Atlantic and the southern South Indian Ocean, with central values below -12 dam over the southwestern South Atlantic.
Tropical Features
Sea Surface Temperatures (SSTs): During April 2008, La Nina conditions remained but its intensity weakened continuously. Negative SST anomalies covered the equatorial Pacific between 155ºE-120ºW, with central values about -1.5℃, and positive SST anomalies dominated the equatorial Pacific east of 120 ºW. In April, indices of NINO 3, NINO 4, NINO 3.4, and NINO Z were -0.2℃, -1.0℃, –0.9℃, and -0.5℃, with rises of 0.3℃, 0.3℃, 0.2℃ and 0.2℃ from last month, respectively, while index of NINO 1+2 was 0.6℃,with a drop of 0.3℃ from last month. Meanwhile, SSTs in most of the equatorial Indian Ocean were near normal.
Subsurface Temperatures: During April, the anomalously cold subsurface water in the central and eastern equatorial Pacific weakened obviously, and the region enclosed with -1℃ contour split into two parts, while the anomalously warm subsurface water in the western equatorial Pacific strengthened and expanded eastward continuously, with central value 6℃ and the eastern boundary of 1℃contour arriving near 130 ºW.
Warm Pools: During April 2008, the area of the western Pacific warm pool was smaller but the intensity was stronger than normal, with its eastern boundary shrinking strictly and its northern boundary expanding obviously. Meanwhile, the area of the Indian Ocean warm pool was smaller and its intensity was weaker than normal.
Wind Field：For the month, at lower troposphere (850hPa), easterly anomalies prevailed over the equatorial Pacific between 135ºE-150ºW, with the maxima about 8m/s, and zonal winds were near normal over the rest of the equatorial Pacific. Meanwhile, at upper troposphere (200hPa), westerly anomalies dominated the central and eastern equatorial Pacific between 180º-130ºW, with center values about 8m/s.
Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 0.6, with a drop of 0.8 from last month.
Convections over the Tropics: During April 2008, the anomalous outgoing long wave radiation (OLR) showed that convections were suppressed over the equatorial Pacific between 160ºE-165ºW, while convections were enhanced from the northern Indian Ocean to the western equatorial Pacific. For the tropical Pacific, in the divergent wind field at lower troposphere (850hPa), a divergent center and a convergent center were observed over 115ºW，15ºS and 130ºE of the Equator, respectively. Correspondingly, at the upper troposphere (200hPa), the convergent center was over 115ºW, 15ºS, and the divergent center was over 135ºE of the Equator, respectively.
ENSO Monitoring and Prediction
(1) During April 2008, anomalously cold surface water weakened obviously in the central and eastern equatorial Pacific, and NINO Z index was -0.5℃ with a rise of 0.2℃ from last month.
(2) For the month, the anomalously cold subsurface water in the central and eastern equatorial Pacific weakened and its thickness became thinner.
(3) Southern Oscillation Index (SOI) was 0.6 during this month, with a drop of 0.8 from last month.
(4) At lower troposphere (850hPa), strongly easterly anomalies still covered the central equatorial Pacific.
(5) Convections were suppressed over the equatorial Pacific between 160ºE-165ºW.
(6) Most ENSO models predicted that current La Niña event is likely to end before the coming midsummer.
In conclusion, the present La Niña is expected to end during the next 3 months.