Monthly Highlights

Climate in China

During December 2008, monthly mean surface air temperatures were near or above normal in most China, with temperatures 2-4℃ above normal in central Northeast China, southern Northwest China and parts of western Southwest China.

For the month, precipitations were observed 50-200% more than normal in northern North China, most Inner Mongolia, central Northeast China, eastern Southwest China and locations in southern South China, with at least 400% more than normal in locations. Meanwhile, precipitations were observed 30-80% less than normal in most remaining areas.

Climate in the World

During December 2008, temperatures were at least 1℃ above normal in central and northern Europe, most China, eastern and northwestern Siberia, southern West Africa and South Africa, southern and eastern North America, most Alaska, southern South America and parts of northeastern Australia, with temperatures 4-9℃ above normal in northern Europe, northwestern and eastern Siberia. Meanwhile, temperatures were 1-6℃ below normal in western and southern Europe, central Siberia, western North Africa, southern Australia and coastal areas of western Australia, northern North America and parts of northern South America (Fig.1).

For the month, precipitations were at least 50-100% more than normal in parts of southern Europe, most North Europe, southern Central Asia, northern and southeastern West Siberia, East Siberia, southeastern Mongolia, Northwest Africa, central and southern Australia, most America, with precipitations at least 200% more than normal in locations. Meanwhile, precipitations were observed at least 50% less than normal in parts of eastern Europe, most Central Asia, most China, most West Africa, eastern North Africa and South Africa, northeastern Australia and parts of western Australia, most Alaska, southern North America, northwestern and southern South America, with no precipitations in locations (Fig.2).

General Circulation Features

Northern Hemisphere: During December 2008, in the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered the northern North Atlantic Ocean, northern Europe and the northern Pacific Ocean, with central values above 20 dam over the northern Pacific Ocean. Meanwhile, negative anomalies below -4 dam covered the western Mediterranean Sea, central Siberia, parts of the southern North Pacific Ocean and northern North America (Fig.4). For the month, the area and intensity of the subtropical high was near normal over the Northwest Pacific, it was located in south of normal position and extended further east than normal.

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

Tropical Features

Sea Surface Temperatures (SSTs): During December, positive SST anomalies dominated the western equatorial Pacific and negative SST anomalies covered most of the central and eastern equatorial Pacific, with central values below -1.0℃ (Fig.5). Furthermore, comparing with the status of last month, SST anomalies further declined in the central and eastern equatorial Pacific. In December, indices of NINO 3, NINO 4, NINO 3.4 and NINO Z were -0.5℃, -0.6℃, -0.7℃ and -0.5℃, with drops of 0.3℃, 0.3℃, 0.5℃ and 0.2℃ from last month, respectively, while NINO 1+2 index was -0.4℃ the same as that in last month.

Subsurface Temperatures: During December，in terms of subsurface ocean, the cold water in the eastern equatorial Pacific strengthened with maximum below -5.0℃, while warm water in the western equatorial Pacific also enhanced (Fig.6).

Warm Pools: During December, 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 all above normal.

Wind Field：For the month, at lower troposphere (850hPa), easterly anomalies prevailed over the central equatorial Pacific, with central values reaching 6m/s (Fig.7). At upper troposphere (200hPa), strong westerly anomalies covered most of the central and western equatorial Pacific.

Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 1.3, with a drop of 0.3 from last month.

Convections over the Tropics: During December, the anomalous outgoing longwave radiation (OLR) showed that convection was enhanced over maritime continent, and suppressed over the central equatorial Pacific (Fig.8). In the lower troposphere (850hPa) divergent wind field, a convergent center was near 155ºE, 7.5ºS, and a divergent center was near 110ºW, 5ºS. At upper troposphere (200hPa), the corresponding divergent center was near 155ºE, 20ºS, and the convergent center was near 100ºW, 10ºS.

ENSO Monitoring and Prediction

During December, mean SSTa over the central and eastern Pacific dropped to -0.5ºC and SOI was 1.3, which reflect La Niña conditions in tropical Pacific. Analyses showed that current La Niña condition is not likely to maintain over 5 months to become a new La Niña event, the main reasons are as following:

1. No La Nina event starts in winter since 1951.

2. Cold phases of ENSO cycles generally reach peaks in winter, and current pattern of the subsurface equatorial Pacific is similar to that of a mature La Nina event, which means current La Nina conditions almost reaching the peak.

3. Easterly zonal wind anomalies at 850 hPa weakened recently and westerly zonal wind anomalies are likely to expand eastward over the western and central equatorial Pacific.

4. Predictions of dynamical and statistical models show a weak cold-phase process during the following several months.

In conclusion, current La Nina conditions are likely to continue about 3 months but unlikely to develop into a La Nina event.