Annual Highlights

ENSO Monitoring

A La Niña event came into being since August 2007.

After the 2006/2007 El Niño event ended, sea surface temperatures anomalies (SSTA) in the central and eastern equatorial Pacific have gradually decreased since February 2007. The negative SSTA in the east-central Pacific have obviously been extending westward since July 2007 (Fig. 1). During August, NINO Z and NINO 3.4 indices dropped to -0.6℃ and -0.5℃ respectively (Fig. 2), which signified the beginning of La Nina conditions, and La Nina conditions enhanced quickly during the following four months. During 8-12 months, Nino Z index number was respectively -0.6℃, -1.0℃, -1.2℃, -1.5℃ and -1.3℃ (Fig. 2), continuously below -0.5℃ for 5 months, and their accumulation was -5.6℃, which was below -4.0℃ (the standard of La Nina). Therefore, A La Nina event came into being since August 2007.

The Nino Z index both was -1.2℃ in January and February 2008 (Fig. 2). SSTA in most of eastern equatorial Pacific still was below -1.0℃ (Fig. 1). Observation showed the La Nina status still keeps on.

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

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

East Asian monsoon monitoring

During 2007, East Asian summer monsoon onset was later than normal, came to end later than normal also and it is weaker than average.

The onset of the South China Sea (SCS) summer monsoon began in the fifth pentad of May, which was later than normal. After that, southwesterlies swiftly occupied the SCS-South China and warm and wet air controlled the SCS (Fig. 3). During the mid-June, southwesterlies moved northward to the Jianghuai area together with northward move of the northwest Pacific subtropical high. The Meiyu also prevailed in the area, and warm and wet air controlled south of the Huanghuai area. During late July, warm and wet air moved northward to North China. After mid-August, warm and wet air swiftly withdrew southward, moved back to the South China during early September and lingered until the third pentad of October when the air withdrew from the SCS and northeasterlies controlled the area. The SCS summer monsoon came to the end by then.

The SCS summer monsoon index was -1.3 in 2007, which was weaker than average. Pentad intensities of the SCS summer monsoon were weaker than average during the most of time May-July (Fig. 4). However they showed strong-weak-strong wave later. During summer (JJA) 2007, blocking was inactive and cold air was weak in Asia. The main rainy bands were observed in the Huanghuai-Jianghuai area, eastern Southwest China and western South China with rainfalls 30% to 50 % more than normal (Fig. 5).

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

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

Fig.5 Time-latitude cross section of pentad total precipitation

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

Sonw monitoring in northern hemisphere

During 2007, area of snow cover in northern hemisphere as a whole was observed smaller than normal.

During boreal winter (2006/2007), area of snow cover in northern hemisphere was slightly smaller than normal, with slightly smaller than average in Eurasia, larger than average in China and the Tibet Plateau. During spring, summer and autumn, the area of snow cover in northern hemisphere was smaller than normal in the Northern Hemisphere, with smaller than average in Eurasia, China and the Tibet Plateau (Fig. 6).

During boreal winter (2006/2007), the Number of days with snow cover (NDSC) was above 75 days in most of Eurasia and North America north to 50ºN. In China, NDSC was above 75 days in most of Heilongjiang, northeastern Inner Mongolia, and northwestern Xinjiang. Meanwhile, NDSC was 10-40 days more than average in part of central North America, part of West Asia, Mountain Tianshan, central-western Inner Mongolia and eastern and northern the Tibet Plateau and NDSC was above 10-40 days less than average in central-eastern Europe, part of Central Asia, part of western and southern the Tibet Plateau and southern Northeast China (Fig.7).

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

(a) Northern Hemisphere, (b) Eurasia, (c) China, and (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 its anomalies (right) in the Northern Hemisphere during Dec.2006-Feb.2007 (Unit: d)