1. Climate in China

In December 2014, the averaged surface air temperature was -3.4ºC in China, which was 0.2ºC below normal (-3.2ºC). The mean precipitation was 7.9 mm in China, which was 24.8% less than normal (10.5mm). For the month, extremes of Daily Temperature Drop (DTD) and Consecutive Temperature Drop (CTD) were observed in China.

2. General Circulation Features

Northern Hemisphere: During December 2014, in the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered the regions from the Siberian Plain to the western Arctic Ocean, central North America to the North Atlantic, and from the eastern Mediterranean Sea to the Caspian Sea, with central values above 16 dam in locations of the North Atlantic. Meanwhile, negative anomalies below -4 dam covered the regions from the Greenland to eastern North Europe, Northeastern China to Japan, and the northeastern Pacific, with central values below -8 dam in locations. During this month, the subtropical high in the northwestern Pacific region was stronger than normal with its area larger than normal, and its position shifted westward.

Southern hemisphere: During December 2014, in the 500hPa geopotential height anomaly field, positive anomalies above 4 dam were observed over the regions from the sea areas in the mid-northern South Atlantic, the mid-northern South Indian Ocean, the South Pacific around New Zealand and the south, east to southern Chile, with central values above 8 dam over southern New Zealand. Meanwhile, negative anomalies below -4 dam covered the regions over the Antarctica and its vicinity, mid-South Pacific, with central values below -8 dam.

3. Tropical Features

Sea Surface Temperatures (SSTs): In December 2014, SSTs were over 0.5℃ above normal in most of the central and eastern equatorial Pacific, especially above 1℃ in some parts. Indices of Niño1+2，Niño3，Niño4，Niño3.4 and Niño Z were 0.1℃，0.8℃，0.9℃，0.8℃ and 0.8℃, respectively. The Indian Ocean Basin-Wide SSTA variation index  (IOBW) was 0.3℃and kept in the persistent positive phase. The North Atlantic tripole index was 1.0, indicating the development of positive phase of North Atlantic tripole mode.

Subsurface Temperatures: In December, the anomalously warm subsurface water shifted eastward to the eastern equatorial Pacific around 110°W. Meanwhile, the new anomalously warm subsurface water developed in the western equatorial Pacific.

Warm Pools: For the month, the areas of both the Indian Ocean and the western Pacific warm pools were larger than normal. And their intensities were both above normal.

Wind Field: For the month, at lower troposphere (850hPa), the westerly anomalies dominated the most equatorial middle and eastern Pacific, while the equatorial western Pacific was controlled by easterly anomalies. At upper troposphere (200hPa), the westerly anomalies prevailed over the equatorial western Indian Ocean, the western Pacific and the coastal areas of South America over the eastern Pacific, while the easterly anomalies ruled the equatorial central-eastern Pacific.

Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was -0.4.

Convections over the Tropics: During December 2014, the anomalous outgoing longwave radiation (OLR) showed that convections were active around the tropical Indian Ocean, Maritime Continent, and dateline region, with central values below -15W/m². However, convections were suppressed near the region to east of Australian continent, with central values above 15W/m².

4. ENSO Monitoring and Outlook

At present, positive equatorial sea surface temperature anomalies persisted across most of the central and eastern Pacific Ocean. Recently, positive subsurface anomalies in the Pacific have weakened, negative subsurface anomalies expanded across the eastern Pacific, and westerly wind anomalies have weakened over the eastern equatorial Pacific. Based on models’ prediction and trend diagnoses of the oceanic variations, the El Nino event will reach a peak on the coming winter and may come to an end in spring/summer 2015.

 Therefore, we will closely monitor the development of ENSO conditions and update our ENSO wrap-up in time.