Monthly Highlights

1. Climate in China

During April 2006, monthly mean surface air temperatures were near or above normal in most of China, with temperatures 2℃ above normal in most of the Jiangnan and parts of eastern Northwest China. Exceptions were temperatures around 1~3℃ below normal in Northeast China and eastern Inner Mongolia. For the month, precipitations were observed 30~200% more than average in the Jianghuai, southern Southwest China, eastern Northeast China and southern Northwest China, with precipitations more than 300~400% in parts of above regions. Meanwhile, precipitations in most of China were 30~80% less than average, with no precipitation in parts of it.

2. Climate in the world

During April 2006, monthly mean surface air temperatures were 2~5℃ below normal in area from northern Asia to Alaska, northeastern East Asia and most parts of Australia, with temperatures 5~7℃ below normal in the north coasts of mid-west Siberian. Meanwhile temperatures were near or above normal in the other regions over the world ,with temperatures 1~6℃ above normal in most of East Asia, Central Asia, western South Asia, most of Europe, Northwest Africa and most of North America.

During the month, precipitations were observed over 50% less than average in most of China, northern Siberian, Central Asia, West Asia, the western South Asia, eastern and western Europe, West Africa, the southeastern and western coasts of Australia, most of both southern and northern North America，northeastern and extreme southern South America, with no precipitations in parts of above regions. Meanwhile, precipitations were 50% more than average in northeastern East Asia, the Jianghuai and western Southwest China, eastern India, central Europe, most of the West Siberia Plain, the northern and southern coasts of Australia, most of western and northern America, central and northern Brazil.

3. General Circulation Features

Northern Hemisphere: During April 2006, the mean 500hPa geopotential height field exhibited a 4-wave pattern at the mid-high latitudes. Areas of the polar vortexes were near normal, and there was an abnormal strong ridge developed around Novoya Zemlya. Correspondingly, in the 500hPa height anomaly field, positive anomalies above 4 dam were observed over Novoya Zemlya and its neighbouring Arctic area, North Pacific, regions from North America eastward to the Mediterranean Sea, with positive anomalies above 22 dam around Novoya Zemlya. Heights in the other regions were found near or below normal. Negative anomalies were found below -12 dam over the northern North Atlantic Ocean. For the month, the area of Northwest Pacific subtropical high was a little larger than normal. Its ridge-line was located in north of the normal position, and its strength and west end of ridge were near normal.

Southern Hemisphere: For the month, zonal circulation prevailed over mid-high latitudes in the mean 500hPa geopotential height field. Negative anomalies below -4 dam were observed over southeastern South Pacific and the oceans of south of both the Australia and Africa with central values lower than -8 dam. Meanwhile, heights were near normal over the rest of the mid-high latitudes, with central values higher than 24 dam over southwestern South Pacific.

4. Tropical Features

Sea Surface Temperatures (SSTs): During April 2006, negative SST anomalies weakened in most of the eastern equatorial Pacific but enhanced markedly in the extreme eastern equatorial Pacific with central values below -3℃. SSTs were found near normal in most of the equatorial Pacific and in the equatorial Indian Ocean. The index of NINO 1+2 was -1.4℃ with drop of 1.7℃ from last month. The indices of NINO 3 and NINO 4 were -0.2℃ and 0℃ respectively, with rise of 0.4℃ and 0.3℃ respectively from last month. The indices of NINO Z and NINO3.4 were -0.2℃, -0.1℃, respectively, with both being weaken.

Subsurface Temperatures: During April 2006, compared with last month, the area of the abnormal subsurface cold water in the central and eastern equatorial Pacific shrank eastward apparently with its centers being shifted eastward. Meanwhile, the abnormal subsurface warm water extended eastward and its central values decreased slightly in the western equatorial Pacific.

Warm Pools: During April 2006, the areas of the western Pacific and Indian Ocean warm pool were larger than normal and their intensities were stronger than normal.

Wind Field：During April 2006, at the lower troposphere (850hPa), easterly anomalies lower than -4m/s covered the central and western equatorial Pacific Ocean, while westerly anomalies controlled the eastern equatorial Indian Ocean. At the upper troposphere (200hPa), westerly anomalies dominated most of the equatorial Pacific, while zonal winds over the rest equatorial Pacific were near normal.

Southern Oscillation: For the month, Southern Oscillation Index (SOI) was 1.0, with drop of 0.3 from last month.

Convection over the Tropics: During April 2006, the anomalous outgoing longwave radiation (OLR) showed that convections enhanced in the western tropical Pacific but suppressed over the dateline and its east of the equatorial Pacific. In the lower troposphere (850hPa) divergent wind field, a convergent center was over 150ºE, 5ºS, and a divergent center was over around 90ºW, 10ºS. Correspondingly, at the upper troposphere (200hPa), the divergent center was over 145ºE, 10ºS, and the convergent center was over 100ºW, 10ºS.

5. ENSO Monitoring and Forecasting

① During April 2006, negative SST anomalies weakened in most of the eastern equatorial Pacific but enhanced markedly along the coasts of South America. The indices of NINO Z and NINO3.4 were -0.2℃, -0.1℃,respectively, with both being a little weaker than last month.

② During April 2006, the area of the subsurface cold water in the mid-eastern equatorial Pacific shrank eastward apparently compared with last month.

③ For the month, Southern Oscillation Index (SOI) was 1.0, with drop of 0.3 from last month.

④ At the lower troposphere, easterly anomalies covered most of the central and western equatorial Pacific Ocean.

⑤The anomalous outgoing longwave radiation (OLR) showed that convections enhanced in the western tropical Pacific but suppressed over east of the dateline in the equatorial Pacific.

⑥ Most of ENSO models showed that weak negative SST anomalies could persist in the eastern equatorial Pacific in the following 1~3 months.

In conclusion, tropical oceanc conditions appeared to be in the near neutral conditions, and these conditions were estimated to persist in the following 1~2 months.