Annual Highlights

 

Tropical oceanic-atmospheric condition

La Niña condition terminated in spring and El Nino occurred in summer

During 2009，a significant transition from La Nina condition to El Nino event had occurred in the central and eastern equatorial Pacific as mentioned following: La Nina condition terminated in April 2009 (Fig.1), which began in the end of 2008, then SST anomalies in the central and eastern equatorial Pacific warmed gradually and developed into an El Nino event in June 2009(Fig.2).

 

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

During 2009, SCS summer monsoon onset and end was later than normal, and it is weaker than average.

The 2009 South China Sea (SCS) summer monsoon broke out in the 6th pentad of May, which was later than normal. After the full onset of the SCS summer monsoon, the front of the summer monsoon maintained over the region from South China to the south of the Yangtze River (Fig.3). Accompanying with quickly northward movement of the Northwest Pacific Subtropical High (NPSH) in the 6th pentad of June, the front of the summer monsoon advanced to the middle and lower reaches of the Yangtze River. In the 2nd pentad of July, the NPSH jumped northward and the front of the summer monsoon moved northward the Yangtze-Huaihe River Basins and the Yellow-Huaihe River Basins. In mid-July, the front of the summer monsoon extended to North China. Since late September, the warm and wet air swiftly retreated southward to regions south of 250N. In the 3rd pentad of October, the front of summer monsoon withdrew from the SCS and then the SCS summer monsoon ended, with three pentads later than normal.

Fig.3 Latitude-time section (110°E -120°E) of potential pseudo-temperatures (unit: K)

 

The SCS summer monsoon intensity index was -0.35 in 2009, near normal (the normal value ranging from -0.5 to 0.5). The pentad evolution of the intensity index indicated that the SCS summer monsoon had two-stage variations, with its intensity stronger than normal in the former stage and weaker in the latter stage. From the 6th pentad of May to the 2nd pentad of August, the SCS summer monsoon was obviously stronger than normal as a whole, while after the mid-August, it became weaker than normal in spite of some fluctuations (Fig.4).

Fig.4 Variation of the zonal wind index (unit: m/s) over the monitoring region

 (10ºN -20ºN, 110°E -120ºE），(Red bars for climatological normal)

 

Snow Cover

China snow area was obviously larger than normal from October to December

During 2009, the areas of snow cover in the Northern Hemisphere, Eurasia, China and the Tibetan Plateau were smaller than normal from January to September and larger than normal from October to December. Snow area of north hemisphere and Eurasia was 5%-20% higher than normal, snow area in China and the Tibet Plateau ranked the highest record in November and October, respectively (Fig.5).

 During Autumn (September-October-November), the number of days with snow cover (NDSC) was 30-60 days in northern Europe, most of the regions in Asia north to 50°N and the northwestern and eastern Tibet Plateau,  northern Mongolia, regions in North America north to 55°N. Anomalies of  NDSC were 10-40 days in the northwestern and the eastern Tibet Plateau, eastern and northern Northwest China,  eastern  and central Inner Mongolia, western North China , central  Northeast China, most Mongolia, part of northern Europe, parts of western Russia ,parts of northern North America. Meanwhile, negative NDSC anomalies were -10—-30 days in the southern Tibetan Plateau, locations in central Asia and northwestern North America (Fig.6) .

 

 

Fig.5 Regional Snow Cover Indices (Unit: 106km2)，(a) Northern Hemisphere，(b) Eurasia，  (c) China，(d) Tibetan Plateau。( solid line: monthly snow cover area, red: below climatology, blue: above climatology，climatology is from the 1973-2002 base period mean)

 

 

 

Fig.6. Number of days with snow cover (left) and the anomalies (right) in the Northern Hemisphere in Autumn 2009 (Unit: day)