Monthly Highlights

Climate in China

During January 2009, monthly mean surface air temperatures were 1-2℃ above normal in most Northeast China, parts of the Hetao area, the Qinghai-Tibet Plateau, most Xinjiang and western Yunan, with temperatures 2-4℃ above normal in northern Xinjiang and the most Qinghai-Tibet Plateau. Meanwhile, temperatures were observed 1-2℃ below normal in central-western Fujian, most Guangdong, Hainan and western Inner Mongolia. Temperatures were near normal in other regions of China.

For the month, precipitations were near or below normal in most other regions except for most Northeast China, northeastern Inner Mongolia, northern Xinjiang and locations of the south, western Sichuan and central Gansu where precipitations were observed 50-100% more than normal. Precipitations were observed 50-80% less than normal in most Northwest China, North China, the Huanghuai area, the northern Jianghuai area, the Jianghan area, the central-western Jiangnan area, most South China and western Yunnan, with at least 80% less than normal in most North China, the Huanghuai area, most Shanxi, eastern and western Xinjiang, southwestern Tibet and southern Guangdong.

Climate in the World

During January 2009, temperatures were at least 1℃ above normal in most Eurasia, central and western America, most Central America, most North Africa, southern South America and southern Australia, with temperatures at least 4℃ above normal in parts of Tibet, parts of western America and northern Russia and 8-10℃ above normal in northern West Siberia. Meanwhile, temperatures were 1-4℃ below normal in southwestern Europe, eastern East Siberia, southeastern China, the western Indochinese Peninsula, northeastern and northwestern North America, western West Africa, northern Argentina and northern Australia, with temperatures at least 4℃ below normal in locations .

For the month, precipitations were less than normal in most regions of the world. Precipitations were observed at least 30% below normal in central and northwestern Europe, most Asia, most America, eastern North Africa, southern West Africa, southeastern Australia and offshore in western Australia and southern South America, with no precipitations in locations. Meanwhile, precipitations were observed 30-100% more than normal in parts of southern and northwestern Europe, most Turkey, parts of southern Siberia, northeastern China, the southern Indochinese Peninsula, western Indonesia, parts of northwestern America, most Alaska, northwestern North Africa, northeastern and part southwestern Australia and parts of northern South America, with precipitations at least 200% more than normal in locations.

General Circulation Features

Northern Hemisphere: During January 2009, the mean 500hPa geopotential height field exhibited a 4-wave pattern at the mid-high latitudes. In the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered regions from the Baikal to northern India, regions from the northwestern North Pacific to east Siberia, parts of the central North Atlantic and regions from the northeastern North Pacific to western North America, with central values above 16 dam over the northeastern North Pacific. Meanwhile, negative anomalies below -4 dam covered regions from northeastern North America to Northwest Africa through northern North Atlantic. For the month, the area of the Northwest Pacific subtropical high (NWPSH) was larger and its intensity was near normal. It was located in north of normal position and its west end contracted further east than normal.

Southern Hemisphere: For the month, in the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered southern South Atlantic and parts of the southern South Pacific with central values above 12 dam. Meanwhile, negative anomalies below -4 dam covered the seas south of the Australia and parts of Antarctica.

Tropical Features

Sea Surface Temperatures (SSTs): During January, negative SST anomalies dominated most of the central and eastern equatorial Pacific, with central values below -1.5℃. Furthermore, comparing with the status of last month, SST anomalies further declined in the central equatorial Pacific but rose in the eastern equatorial Pacific. In January, indices of NINO 3, NINO 4, NINO 3.4 and NINO Z were -0.6℃, -0.8℃, -1.0℃ and -0.6℃, with drops of 0.1℃, 0.2℃, 0.3℃ and 0.1℃ from last month, respectively, while NINO 1+2 index was -0.1℃ with a rise of 0.3℃ from last month.

Subsurface Temperatures: During January，in terms of the subsurface equatorial Pacific, the pattern of “cold in the east and warm in the west” still remained. However, the position of cold water in the eastern equatorial Pacific rose, while warm water in the western equatorial Pacific expanded eastward obviously.

Warm Pools: During January, in terms of the western Pacific warm pool, its eastern boundary shrank slightly while both of its southern and northern boundaries expanded. Meanwhile the area of the Indian Ocean warm pool was above normal. The intensities of the two warm pools were both above normal.

Wind Field：For the month, at lower troposphere (850hPa), easterly anomalies prevailed over the central equatorial Pacific, with central values over 4m/s, at the same time westerly anomalies developed in the adjacent regions of 120ºE in the western equatorial Pacific. At upper troposphere (200hPa), westerly anomalies covered most of the central and eastern equatorial Pacific.

Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 0.9, with a drop of 0.4 from last month.

Convections over the Tropics: During January, the anomalous outgoing longwave radiation (OLR) showed that convection was enhanced over Philippines, and suppressed over the central equatorial Pacific. In the lower troposphere (850hPa) divergent wind field, a convergent center was near 150ºE,10ºS, and a divergent center was near 110ºW，5ºS. At upper troposphere (200hPa), the corresponding divergent center was near 150ºE,15ºS, and the convergent center was near 95ºW，5ºS.

ENSO Monitoring and outlook

During January, atmospheric and oceanic status of the tropical Pacific maintained La Nina conditions, with NINO Z index being 0.6℃. However, current monitoring in the beginning of February shows that cold conditions are weakening, and is unlikely to maintain over 5 months to become a new La Nina. The main reasons are as followings:

1. In the central and eastern equatorial Pacific, Oceanic heat content anomalies continued to be rising, while cold water starts to weaken remarkably during early February 2009.

2. Recently, westerly anomalies at 850hPa are developing in the western Pacific, which is in favor of the warm water in the western equatorial Pacific expanding eastward to weaken the cold water in the central and eastern equatorial Pacific.

3. Dynamical and statistical models Predicted a weak cold-phase process through the following several months.

In conclusion, current La Nina conditions are likely to continue about 1-2 months again and to gradually come back to the ENSO-neutral conditions. It is unlikely to develop into a new La Nina event.