Annual Highlights

Tropical oceanic-atmospheric condition

The La Niña event came to the end

The La Niña event which began in August 2007 came to end in May 2008 and lasted for 9 months. Cold-neutral conditions persisted since May.

Fig.1 Time -longitude section of SSTA in the equatorial Pacific (5ºN～5ºS) (Unit:℃ )

Fig.2 Variations of Niño Z , Niño 3.4 indices (Unit: ℃ ) and SOI

East Asia summer monsoon

The onset time of the South China Sea (SCS) was earlier than normal while the end time was later than normal with the intensity weaker than normal.

The onset of the South China Sea (SCS) summer monsoon was in the first pentad of May and earlier than normal. Subsequently, southwesterlies moved northward to regions south to the Yangtze River. During mid-June, southwesterlies extended northward to the Jianghuai area, and warm and humid air was prevalent south of the Huanghuai area. Southwesterlies moved northward to North China during mid-July. After mid-August, warm and moist air swiftly withdrew southward and moved back to the middle and lower reaches of the Yangtze River, withdrawing further to South China by mid-September and lingered until early October (Fig.3). The summer monsoon withdrew from the SCS during the second pentad of October 2008, two pentads later than normal. Overall, the SCS summer monsoon index during 2008 was -1.37 and weaker than normal. Pentad intensities of the SCS summer monsoon were stronger than normal during May and the period from the third pentad of September to the first pentad of October, but were weaker than normal during most of the period June-August (Fig.4). Over eastern China, precipitation totals for summer (June-August) were above 30% more than normal in the Jianghan area, the eastern Huanghuai area and South China (Fig.5).

Fig.3 Time-latitude section of 850hPa θse (110 ºE～120ºE mean) (Unit:K)

Fig.4 Variation of pentad zonal wind index over monitoring region, red open bars stand for climatology (Unit: m/s)

Fig.5 Time-latitude section of pentad precipitation

percentage anomalies over 110 ºE～120ºE (Unit: %)

Snow Cover

The area of snow cover in China was larger than normal during 2007/2008 winter.

The areas of snow cover in the Northern Hemisphere and Eurasia were slightly larger than normal during winter (Dec.2007- Feb.2008) while all smaller than normal in spring, summer and autumn. The area of snow cover in China was nearly 40% larger than normal in winter, with over 50% in the Tibetan Plateau and over 40% in Northern Xinjiang. In autumn, snow cover extents China and the Tibetan Plateau were obviously larger than normal (Fig.6).

During winter, the number of days with snow cover (NDSC) was over 75 days in northern Europe, most of the regions in Asia north to 50°N and regions in North America north to 40°N. As for China, NDSC was over 75 days in northern Xinjiang, northeastern Inner Mongolia and parts of Heilongjiang. NDSC was observed 10-30 days more than average in western and central North America and the regions from southeastern Europe to northern China through West Asia. Meanwhile, NDSC was observed 10-30 days below normal in central Europe, the southern Tibetan Plateau and parts of northeastern China (Fig.7).

Fig.6 Regional snow cover anomaly indices (Unit: 10⁶km² )

(a) Northern Hemisphere (b) Eurasia (c) China (d) Tibetan Plateau

(dashed line: climatology, solid line: monthly snow cover area, red: monthly snow cover area is below climatology, blue: monthly snow cover area is above climatology)

Fig.7 Number of days with snow cover (left) and the anomalies (right) in the Northern Hemisphere during Dec.2007 to Feb. 2008 (Unit: day)