1. Climate in China
In May 2014, the surface air temperature averaged over China was 16.5ºC, which was 0.3ºC above normal (16.2ºC). The mean precipitation over China was 73.8 mm, which was 6.2% more than normal (69.5mm). In the month, extremes of High Temperature (HT), Daily Temperature Drop (DTD) and Daily Precipitation (DP) were observed in China.
2. General Circulation Features
Northern Hemisphere: During May 2014, in the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered Central Asia, regions from eastern Russia to Alaska via the northern North Pacific, northern Canada, and the northeastern North Atlantic Ocean, with central values above 12 dam near the Ural Mountains and the Bering Sea. Meanwhile, negative anomalies below -4 dam covered the central North Pacific, the polar region, and the northwestern North Atlantic Ocean, with central values below -8 dam near the central North Pacific and the polar region. For the month, the northwestern Pacific subtropical high was stronger than normal, with its area larger than normal and its high ridge extending more northward and slightly westward.
Southern Hemisphere: For the month, in the 500hPa geopotential height anomaly field, positive anomalies over 4 dam were observed over the Antarctic, with central values above 12 dam. Meanwhile, negative anomalies below -4 dam mainly covered southern parts of the South Pacific Ocean, the South Indian Ocean, and the South Atlantic Ocean, respectively, with central values below -8 dam over the southeastern South Pacific.
3. Tropical Features
Sea Surface Temperatures (SSTs): During May 2014, SSTs were over 0.5℃ above normal in most equatorial central-eastern Pacific, especially above 1℃ in the equatorial central Pacific and above 1.5℃ in the equatorial eastern Pacific along the coastal areas of South America. Indices of Niño1+2、Niño3、Niño4、Niño3.4 and Niño Z were 1.3℃, 0.6℃, 0.8℃, 0.5℃ and 0.7℃, respectively. El Niño conditions occurred in the equatorial Pacific in May 2015. SSTs were over 0.5℃ above normal in the tropical western Indian Ocean, the Bay of Bengal, the South China Sea and the tropical western Pacific, while they were near or below normal in the tropical Atlantic Ocean. The IOBW index was 0.3℃.
Subsurface Temperatures: During May 2014, anomalously warm subsurface water enhanced rapidly and upwelled to the surface in the central and eastern equatorial Pacific.
Warm Pools: For the month, the areas of both Indian Ocean and the western Pacific warm pools were larger than normal. And their intensities were both above normal significantly.
Wind Field: For the month, at lower troposphere (850hPa), westerly anomalies covered most equatorial central-eastern Pacific, with central values at least 4 m/s, while easterly anomalies controlled the region from the equatorial eastern Indian Ocean to western Pacific, with central values below -4 m/s. At upper troposphere (200hPa), easterly anomalies dominated over most equatorial Pacific, with central values above 8 m/s, while westerly anomalies were observed over the equatorial Indian Ocean and the Marine continent.
Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 0.5.
Convections over the Tropics: During May 2014, the anomalous outgoing longwave radiation (OLR) showed that convection was active over most tropical Pacific, especially in the equatorial eastern Pacific with central values below -30W/m2. However, convection was suppressed near the South China Sea, with central values above 15W/m2. Meanwhile, convection was slightly enhanced over most tropical Indian Ocean, with central values below -30W/m2 over the western Bay of Bengal.
4. ENSO Monitoring and Outlook
The strong Kelvin wave invoked by the multiple west wind burst has propagated eastward since January, 2014. As a result, SST in the central-eastern equatorial Pacific increased continually. In May, SST anomaly was above 0.5℃ in most central-eastern equatorial Pacific. The thermocline was deeper than normal and remained warm phase of Tilt mode. Recharge process became weak over the equator.
Based on the dynamical model prediction and the diagnostic analysis, the SST will increase in the central-eastern equatorial Pacific and warm status will persist in the following months. A moderate strength El Nino event is expected to appear.
Therefore, we will closely monitor the development of ENSO conditions and update our ENSO wrap-up in time.