Monthly Highlights

1. Climate in China

During December 2005, monthly mean surface air temperature for China was the lowest one for the same period for the past 20 years. While tmperatures were 1-2℃ above normal in the eastern Tibet Plateau and western Yunnan, they were found more than 1℃ below normal in most of China, with 2-4℃ below normal from central and southern Northeast China westward to eastern and northern Xinjiang, and more than 4℃ below normal in parts of above regions. For the month, precipitations were near or below normal in most of China. Precipitations were more than 50% above normal in central and eastern Xinjiang, central and western Northeast China, most of Inner Mongolia, southern Southwest China and the western Tibet Plateau, with 100~200% more than average in parts of above regions. Meanwhile, precipitations were 30~80% less than normal in central and eastern China, most of Tibet Plateau and most of South China, with more than 80% below normal in parts of above regions.

2. Climate in the World

During December 2005, monthly mean surface air temperatures were generally 2-4℃ below normal in southwestern Europe, from the southern Central Siberia Plateau southward to northern China and Japan, western Australia, northwestern and southeastern America and southeastern Argentina, with 4-6℃ below normal in the southern Central Siberia Plateau. Meanwhile, monthly mean surface air temperatures were 2-4℃ above normal in West Asia and western Central Asia, offshore of northern Russia, eastern Tibet Plateau, eastern Australia, southwestern and northern American, and southern West Africa, with 4-6℃ above normal in offshore of northern Russia.

During December 2005, precipitations were observed more than 30% above normal in the region from eastern Europe to Ukraine, from the Aral Sea to Northwest China, from the lake Baikal to Northeast China, Southwest China, central Australia, western and northern America, with more than 100% above normal in parts of above regions. Meanwhile precipitations were observed more than 30% below normal in central and western Europe, northern West Africa，regions from South Asia to east China, from Kazakhstan to the Far East of Russia, eastern and western Australia, South Africa, central and southern America, with no precipitations in South Asia.

3. General Circulation Features

Northern Hemisphere: During December 2005, the mean 500hPa geopotential height field exhibited a 4-wave pattern at the mid-high latitudes, and the polar vortex was near normal. In the 500hPa height anomaly field, the high latitudes was mainly dominated by positive anomalies, with above 8 dam positive anomalies over Siberia, northern North America and northern North Atlantic, and the central value above 20 dam over Siberia. Meanwhile, negative anomalies below -4 dam were observed over northern North Pacific, Eastern America and South Europe, with the central value below -12 dam over regions around Japan. During the month, the Northwest Pacific subtropical high was both larger and stronger than normal.

Southern Hemisphere: During December 2005, the mean 500hPa geopotential height field exhibited a 4-wave pattern at the mid-high latitudes. In the 500hPa height anomaly field, above 8 dam anomalies dominated most of polar region, with the central value above 12 dam. Below -4 dam anomalies were observed over the southern South Pacific, the southeastern and southwestern South Indian Ocean and the southwestern South Atlantic Ocean, with the central values below -16 dam over the first three regions.

4. Tropical Features

Sea surface temperatures (SSTs): During December 2005, SST anomalies decreases obviously in the central and eastern equatorial Pacific, with negative SST anomalies below -0.5℃ in the east to 150ºW and the central value below -1.5℃ near 120ºW. In the meantime, positive SST anomalies about 0.5℃ were observed in the western equatorial Pacific, and near normal SST over the equatorial Indian Ocean. The index of NINO 1＋2 was -0.6℃, with arise of 0.4℃ compared with last month and these of NINO 3, NINO 4 and NINO Z were -0.9℃, 0.1℃ and -0.5℃ all with drops of 0.2℃ compared with last month, and the index of NINO 3.4 was -0.6℃, with a drop of 0.4 compared with last month.

Subsurface Temperatures: For the month, abnormal subsurface cold water in the eastern equatorial Pacific shrank eastward slightly, strengthened and extended to the surface compared with last month. Meanwhile, the western equatorial Pacific warm water also strengthened slightly.

Warm Pools: For the month, the western Pacific and Indian Ocean warm pool was larger in area and stronger in intensity than normal.

Wind Field：For the month, at the lower troposphere (850hPa), 4~12m/s easterly anomalies were over the central and western equatorial Pacific, while westerly anomalies were found over the equatorial Indian Ocean and the eastern Pacific, with central values above 12m/s over the equatorial Indian Ocean. At the upper troposphere (200hPa), weak easterly anomalies mainly covered the equatorial Pacific and Indian Ocean.

Southern Oscillation: During December 2005, Southern Oscillation Index (SOI) was 0.2, with arise of 0.5 from last month.

Convection over the Tropics: For the month, the anomalous outgoing long wave radiation (OLR) showed that convections were enhanced over the region from the western tropical Pacific to the eastern Indian Ocean and suppressed over the date line. In the lower troposphere (850hPa) divergent wind field, a convergent center was over nearby 140°E, 5ºS, and a divergent center was around 110ºW, 7ºS. Correspondingly, at the upper troposphere (200hPa), a divergent center was found around equator at 115°E, and a convergent center was observed over nearby 110ºW, 15ºS.

5. ENSO Monitoring and Forecasting

① During December 2005, SST anomalies dropped obviously in the central and eastern equatorial Pacific. With indices of NINO1+2, NINO3, NINO3.4, NINO Z all being ≤ -0.5℃.

②The SOI was 0.2 in December 2005, with a rise of 0.5 from last month.

③During December 2005, abnormal subsurface cold water in the eastern equatorial Pacific strengthened obviously. Meanwhile, abnormal subsurface warm water also strengthened slightly in the western equatorial Pacific.

④During December 2005, at the lower troposphere, easterly anomalies were over most of the central and western equatorial Pacific, while westerly anomalies covered the eastern equatorial Pacific.

⑤ Most of ENSO models showed that negative SST anomalies will persist in the eastern equatorial Pacific in the following 1~3 months

In conclusion, tropical oceanic and atmospheric conditions reflected obvious characteristics of cold phase, and these conditions were predicted to persist during the following 1~3 months. The air – sea development in the tropical Pacific will continue to be carefully monitored.