1. Climate in China
In February 2015, the surface air temperature averaged over China was -0.2ºC, with 1.5ºC above normal (-1.7ºC), ranking the 6th highest since 1961. The mean precipitation was 16.3 mm over China, which was 6.3% less than normal （17.4mm). 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 February 2015, in the field of mean 500hPa geopotential height anomaly, negative anomalies below -4 dam covered the region from Barents Sea to eastern North American via Greenland, southwestern Europe and central North Pacific Ocean, with central values below -12dam over Greenland, central eastern North Pacific Ocean and eastern North America. Meanwhile, positive anomalies above 4 dam covered eastern Russia, the region from western Russia to western North America via northern North Pacific Ocean and western North Atlantic Ocean, with central values above 16dam over Okhotsk Sea. During 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 shifted further westward and southward than normal.
Southern Hemisphere: For the month, in the field of mean 500hPa geopotential height anomaly, positive anomalies over 4 dam were observed over western South Indian Ocean, parts of southwestern and southeastern South Pacific Ocean, with central values above 8 dam over parts of western South Indian Ocean. Meanwhile, negative anomalies below -4 dam covered most polar region.
3. Tropical Features
Sea Surface Temperatures (SSTs): In February 2015, SSTs were over 0.5℃ above normal in most of the central equatorial Pacific, especially above 1℃ in some parts. Indices of Niño1+2，Niño3，Niño4，Niño3.4 and Niño Z were -0.6℃，0.2℃，1.0℃，0.6℃ and 0.5℃, respectively. The Indian Ocean Dipole (IOD) index was -0.7, indicating the development of negative phase of Indian Ocean Dipole mode. The North Atlantic Triple (NAT) index was 1.0 and kept in the persistent positive phase.
Subsurface Temperatures: In February, the anomalously warm subsurface water shifted eastward to the equatorial central Pacific around 170°W. Meanwhile, the anomalously cool subsurface water upwelled in the equatorial eastern Pacific, with the central value below -2℃ in the eastern equatorial Pacific near 105ºW.
Warm Pools: For the month, the area and the intensity of the Indian Ocean warm pool were near normal. The area of the western Pacific warm pool was larger than normal and the intensity was greatly above normal.
Wind Field: For the month, at lower troposphere (850hPa), the westerly anomalies dominated over most equatorial Pacific. At upper troposphere (200hPa), the westerly anomalies prevailed over the equatorial Indian Ocean and the equatorial eastern Pacific, while the easterly anomalies ruled the equatorial central-western Pacific.
Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was -0.3.
Convections over the Tropics: During February 2015, the anomalous outgoing longwave radiation (OLR) showed that convections were active around the tropical western Indian Ocean, and the region of tropical central Pacific to the west of the dateline, with central values below -15W/m2. However, convections were suppressed around the tropical eastern Indian Ocean, Maritime Continent, and the region of tropical central Pacific to the east of the dateline, with central values above 15W/m2.
4. ENSO Monitoring and Outlook
Recently, another downwelling Kelvin wave with warm phase has developed and pushed eastward along the equatorial Pacific. Meanwhile, active Madden-Julian Oscillation and strong westerly anomalies have prevailed over the tropical western Pacific. Based on models’ prediction and diagnoses of the oceanic variations, this El Nino event will redevelop during the following months of 2015, with El Nino condition favored in summer 2015.
Therefore, we will closely monitor the development of ENSO conditions and update our ENSO wrap-up in time.