Annual Highlights

1. Climate in China

During 2011, annual mean surface air temperature of China was 9.3℃ which was 0.5℃ above  normal (8.8℃), being the 15th consecutive warm years since 1997. While temperatures were generally below normal in south-central Northeast China, parts of northern Xinjiang, South China, locations of Southwest China, temperatures were above normal in most of China, with 1-2℃ above normal in most of Northwest China, most of Southwest China, North China and parts of the Southern China.

In 2011, averaged annual precipitation over China was 556.8 mm, which was 9.1% less than normal and was the smallest value since 1951. Compared with normal, precipitations were 30-50% less than normal, with more than 50% above normal in locations, while precipitations were near or below normal in the remainder of China, with remarkably less than normal precipitations were observed in northeastern Inner Mongolia and most of Yunnan.

2. General Circulation and Monsoon

1) In the boreal winter of 2011/2012，the East Asia winter monsoon was abnormally stronger than normal

During the boreal winter (Dec. 2011-.Feb. 2012), in the field of 500hPa geopotential height anomaly, significant wave trains were observed from the Atlantic Ocean to Eurasia. The blocking activities were enhanced over the Ural and North Atlantic Ocean. The East Asia trough also was stronger than normal. Furthermore, the monitoring of daily Arctic Oscillation Index exhibited strong intra-seasonal variations, and the Siberian High kept stronger than normal features. The intensities of East Asian winter monsoon in 2011/12 and 1967/68 were the strongest since 1948. 

2) In the boreal summer, the northwestern Pacific subtropical high was weaker than normal

In the boreal summer of 2011, the Northwest Pacific Subtropical High was weaker than normal, with its ridge extending more eastward than normal. And, the area of the high was near the normal.

3) SCS summer monsoon onset was earlier than normal, the ending was later than normal, and its intensity was abnormally weaker than average

The South China Sea (SCS) summer monsoon (SCSSM) broke out in the 2nd pentad of May in 2011with two pentads earlier than normal, and it withdrew in the 3rd pentad of October with three pentads later than normal. The intensity of the SCS summer monsoon was -1.08, which was much weaker than normal (-0.5-0.5).

During the period from the onset of the SCS summer monsoon in the 2nd pentad of May to the 1st pentad of June, the front of the East Asian subtropical summer monsoon maintained over the region from South China to south of the Yangtze River Valley (YRV). In the 2nd pentad of June, with the northward advancement of the EASSM and the northward jump of the subtropical high over the western North Pacific (WNP), the front of the summer monsoon advanced to the middle and lower reaches of the YRV, and correspondingly the Meiyu period started. In the 6th pentad of June, the monsoonal rain belt in eastern China advanced northward to North China and then the rainy season over these regions started, which also signified the ending of Meiyu. In the 5th pentad of July, with the subtropical high over the WNP retreating southward, the rain belt in eastern China shifted southward from North China. Since the 2nd pentad of September, the autumn rain started in West China. In the 5th pentad of September, due to the abnormally active tropical cyclone during this period, the warm and wet air moved southward to region from the Jiangnan area to South China, which resulted in the persistent strong precipitations over South China and its neighboring areas. In the 3rd pentad of October, with the cold air from the North intruding into the coastal areas and the SCS, thermodynamic property of the air mass changed obviously over the SCS. Then, the front of summer monsoon began to withdraw from the SCS and then the SCS summer monsoon ended.

3. Tropical oceanic condition

The last La Nina weakened in the beginning of 2011 and ended in April 2011. During May-July 2011, the equatorial eastern Pacific experienced a short fluctuation of abnormally warm water, with the maximum warm anomaly located over the eastern part of the equatorial eastern Pacific. SSTA developed into a cold condition over most of the equatorial central-eastern Pacific in August, and reentered a La Nina condition in September. During October-December, the La Nina condition continued to develop and reached its peak in December 2011.