1. Climate in China

In April 2014, the surface air temperature averaged over China was 12.1ºC, which was 1.1ºC above normal (11.0ºC), ranking the 5^th highest since 1961, and the mean temperature over Inner Mongolia ranked the 2^nd highest since 1961. The mean precipitation over China was 43.7 mm, which was 2.2% less than normal (44.7mm). It was only 6.2 mm over Northeast China, being 78.8% less than normal (29.2 mm) and being the least since 1951. In the month, extremes of High Temperature (HT), Daily Temperature Drop (DTD) and Consecutive Precipitation Days (CPD) were observed in China.

2. General Circulation Features

Northern Hemisphere: During April 2014, in the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered western Europe and Northeast Asia, with central values above 12 dam near the Lake Baikal. Meanwhile, negative anomalies below -4 dam covered northern North America, parts of the central-northern North Atlantic Ocean, regions from the Arctic to western Russia, and the northwestern North Pacific, with central values below -12 dam near the Arctic. For the month, the area of the northwestern Pacific subtropical high was larger than normal. Its strength was near normal, with its high ridge extending more northward and retreating more eastward.

Southern Hemisphere: For the month, in the 500hPa geopotential height anomaly field, positive anomalies over 4 dam were observed over the southern South Pacific and southern South Atlantic Ocean, with central values above 16 dam over the southern South Pacific Ocean. Meanwhile, negative anomalies below -4 dam were observed over the southwestern and southeastern South Indian Ocean, and regions near West Antarctica, with central values below -8 dam over the southeastern South Indian Ocean and near West Antarctica.

3. Tropical Features

Sea Surface Temperatures (SSTs): During April 2014, SSTs were 0.5℃ above normal in the eastern equatorial Pacific, the region around Date Line in the central equatorial Pacific, and the Northwestern pacific ocean. Niño1+2, Niño3, Niño4, Niño3.4 and NiñoZ indices were -0.5℃, 0.2℃, 0.6℃, 0.2℃ and 0.3℃, respectively. SSTs were near or slightly above normal in the tropical Indian Ocean. The Indian Ocean basin-wide index (IOBW) was 0.2℃. SSTs were slightly near or below normal in the tropical Atlantic Ocean.

Subsurface Temperatures: During April 2014, anomalously warm subsurface water dominated the most central and eastern equatorial Pacific, with central values at least 6℃ above normal.

Warm Pools: For the month, the areas of both the Indian Ocean and the western Pacific warm pools were larger than normal, with the intensity of the Indian Ocean warm pool slightly above normal and that of the western Pacific warm pool significantly above normal.

Wind Field: For the month, at lower troposphere (850hPa), westerly anomalies covered the most equatorial Pacific, with central value at least 4 m/s above normal near the Date Line. At upper troposphere (200hPa), easterly anomalies dominated the most central and western equatorial Pacific, with central value above 8 m/s.

Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 0.7.

Convections over the Tropics: During April 2014, the anomalous outgoing longwave radiation (OLR) showed that convection was active over the central and eastern equatorial Pacific, with central values below -30W/m². However, convection was suppressed near the Marine continent, with central values above 15W/m². Meanwhile, convection was slightly suppressed over the most equatorial Indian Ocean.

4. ENSO Monitoring and Outlook

Since the mid of March 2014, SSTs in the middle-eastern equatorial Pacific increased quickly, and in early May, sea surface temperature anomalies (SSTA) in all the Nino regions were positive, the averaged SSTA is above 0.5°C. The multiple west wind burst (WWB) in the tropical Pacific from January to present exerts the strong and eastward propagating Kelvin wave. The basin scale thermocline that tilted up to the west deepened. The subsurface temperature keeps increasing. The intensity of WWB and the status of ocean subsurface are similar to the previous strong El Nino events.

Based on the model prediction and the diagnostic analysis, a moderate-strong El Nino event is expected to occur in this summer.

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