Annual high lights

ENSO monitoring

A weak warm water fluctuation occurred during the period from September 2004 to February 2005. SST anomalies exhibited near normal condition in the equatorial Pacific from February to December 2005.

In the early of 2005, SST anomalies decreased quickly in the central and eastern equatorial Pacific, and negative SST anomalies were observed in the eastern Pacific (Fig.1). During February, SST index of Nino Z (Nino 1+2+3+4) dropped to 0.2^oC (Fig.2), which showed that the weak warm water fluctuation ended. SST is a little above normal in the most equatorial Pacific from February to August 2005. During September, the cold water in the eastern Pacific westward spread rapidly and the intensity increased. The condition continued to the end of 2005 (Fig.1).

Fig.1 Time-longitude section of equatorial Pacific (5^oN ~ 5^oS) SST anomaly (Unit: ℃)

2003 2004 2005

Fig.2 Variations of NINO indices (Unit: ℃) and SOI

Monsoon monitoring

The South China Sea (SCS) summer monsoon onset in the 6th pentad of May 2005, about 2 pentads behind the normal date. Southwesterly at 850hPa settled in South China before the 4th pentad of June which was stronger than normal. In the last dekad of June, southwesterly advanced to the region between the reaches of Yellow River and the Huaihe River along with the subtropical high quickly northward shifted. During the 2nd dekad of August, 340K isoline of θse (potential pseudo-equivalent temperature) retreated southward quickly then reached to 30ºN nearby, and lasted there until the middle of September. In the 6th pentad of September, 340K isoline of θse withdrew from the SCS, which indicated the warm and humid air had withdrawn eastern China (Fig.3). Meanwhile, wind direction of the monitoring region in SCS also changed from southwesterly to northeasterly stably. This meant SCS summer monsoon withdrew in the 6th pentad of September, which was near normal.

The SCS summer monsoon index was -1.42 in 2005, which was weaker than normal. Intensity of SCS monsoon was weaker than normal during summer except for the periods from the 6th pentad of May to the 3rd pentad of June and from the 2nd to 3rd pentad of August (Fig.4). Precipitation was more than average in most of South China in June and in reaches of the Huaihe River from July to September.

Fig.3 Latitude-time section of zonal averaged (110-120ºE) potential

pseudo-temperature (θ_se) (Unit: K) at 850hPa

Fig.4 Variation of zonal wind index over SCS (Unit: m/s)

Snow monitoring

During 2005, areas of snow cover (AOSC) as a whole were observed obviously less than normal in the Northern Hemisphere, with slightly smaller than normal in Eurasia, distinctly larger than normal in China and slightly smaller than normal in the Tibet Plateau (Fig.5).

During boreal winter (2004-05), the whole AOSC was more than normal in the Northern Hemisphere, with obviously larger than average in Eurasia, China and the Tibet Plateau. Number of days with snow cover (NDSC) was found above 75 days in most of mid-high latitudes of Eurasia and North America north to 45^oN. NDSC was above 10 days more than average in Northeast China, Inner Mongolia, northern Xinjiang, the eastern Tibet Plateau, parts of West Asia, western Europe and parts of western and eastern US, with 20 days more than average in north China and parts of the eastern Tibet Plateau. During spring 2005, AOSC were obviously less than normal in the Northern Hemisphere and little less than average in Eurasia and more than average in China and the Tibet Plateau. Meanwhile, AOSC were less than average in the Northern Hemisphere, Eurasia, northern China and the Tibet Plateau during summer. AOSC were less than average in the Northern Hemisphere and Eurasia, and near normal in China and more than average in the Tibet Plateau during autumn.

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Fig.5 Regional Snow Cover Anomaly Indices (Unit: 10⁶km²)

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

Fig.6 Number of Days with Snow Cover (left) and Anomalies (right) in the Northern Hemisphere during Dec.2004~Feb.2005 (Unit: day).

Remark: Original data are from http://www.cpc.ncep.noaa.gov/data/snow/.Analyse: Climate System Diagnostics and Prediction Division/National Climate Center/CMA. Climatology is from 1973 to 2002 base period mean.