1. Climate in China

In April 2015, the surface air temperature averaged over China was 11.6ºC, with 0.6ºC above normal (11ºC). The mean precipitation was 44 mm over China, which was near normal (44.7mm). For the month, extremes of Daily Temperature Drop (DTD) and High Temperature (HT) were observed in China.

2. General Circulation Features

Northern Hemisphere: During April 2015, in the field of mean 500hPa geopotential height anomaly, negative anomalies below -4 dam covered the regions over the northern North Pacific, northern North America and eastern Europe. Meanwhile, positive anomalies above 4 dam covered the regions from northern Russia and central Siberia to  parts of the Arctic Ocean around it, the central North Pacific and the northeastern North Atlantic, with central values above 12 dam over parts of the region to the north of Siberia. For the month, the northwestern Pacific subtropical high was weaker than normal, with the area smaller than normal. The position of the northwestern Pacific subtropical high extended further westward and shifted northward than normal.

Southern Hemisphere: For the month, in the field of mean 500hPa geopotential height anomaly, negative anomalies below -4 dam were observed over the most area around the Antarctica, parts of the Southeast Pacific and the Southwest Atlantic. Meanwhile, positive anomalies over 4 dam covered the regions to the south of Australia and parts of the Southeast Atlantic.

3. Tropical Features

Sea Surface Temperatures (SSTs): In April 2015, SSTs were more than 0.5℃ above normal in most of the eastern and central equatorial Pacific, with SSTs 1.5℃ above normal near the Dateline and the regions along the west coast of South America. Indices of Niño1+2，Niño3，Niño4，Niño3.4 and Niño Z were 1.3℃，0.7℃，1.3℃，0.8℃ and 1.0℃, respectively. The Indian Ocean Basin-Wide SSTA variation index (IOBW) was 0.3. The North Atlantic Triple (NAT) index was 1.9, indicating the slight weakening of positive phase of NAT mode.

Subsurface Temperatures: In April, the anomalously warm subsurface water strengthened and extended eastward, and covered the most of the eastern and central equatorial Pacific.  The center of the anomalously warm subsurface water was located around 130°W, with the central values above 6℃.

Warm Pools: For the month, the area of the Indian Ocean warm pool was larger than normal and its intensity was above normal. The area of the western Pacific warm pool was larger than normal and its intensity was prominently above normal.

Wind Field: For the month, at lower troposphere (850hPa), the westerly anomalies dominated the most eastern and central equatorial Pacific with the center around 150ºW. At upper troposphere (200hPa), the easterly anomalies prevailed over the most eastern and central equatorial Pacific, while the westerly anomalies controlled the regions from equatorial western Pacific to the maritime continent and parts of the equatorial eastern Pacific.

Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 0.1.

Convections over the Tropics: During April 2015, the anomalous outgoing longwave radiation (OLR) showed that convections were active around the region of the tropical central Pacific, with central values below -30 W/m². Meanwhile, convections were suppressed in the equatorial western Pacific, with central values above 30W/m².

4. ENSO Monitoring and Outlook

The current El Niño event is characterized by the central Pacific type of El Niño (CP El Niño), but the latest monitoring indicates that there is a tendency for the CP El Niño to transform into the EP El Niño (EP Pacific type of El Niño, namely canonical El Niño) in following months. In addition, the heat content anomalies in the central and eastern Pacific thermocline have decreased after reached a peak at late March 2015. Based on the diagnoses of precursory factors and predictions of dynamical and statistical models, the current El Nino event is expected to go through summer 2015. We will closely monitor the development of ENSO conditions and update our ENSO Outlook in time.

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