Monthly Highlights

Climate in China

During May 2009, the monthly mean surface air temperatures were at least 1℃ above normal in Northeast China, eastern and northern North China, central and eastern Inner Mongolia, the eastern Huanghuai area, the eastern Jianghuai area and the eastern Jiangnan area, and parts of northeastern Southwest China, with temperatures at least 2-4℃ above normal in most Northeast China, northeastern North China and the Shangdong Peninsula, eastern Inner Mongolia and plateau area in western Sichuang. Temperatures were near normal in the rest areas of China. For the month, precipitations were observed 30-200% more than normal in southern North China, the most Huanghuai area, central Inner Mongolia, most Shaanxi, most Qinghai, central Xizhang, central and eastern North Xinjiang, with precipitations at least 200% more than normal in some places of Qinghai-Xizhang Plateau. Meanwhile, precipitations were observed 30-80% less than normal in most Northeast China, eastern Inner Mongolia, Beijing-Tianjin Region, the eastern Jiangnan area, eastern South China, parts of Southwest China and southern Xinjiang, with at least 80% less than normal in locations. Precipitations were near normal in the rest areas of China.

Climate in the World

During May 2009, in the temperature anomaly field, temperatures were 1-2℃ below normal in most Siberia, locations of southeastern Europe, central North Africa, northern North America, northwestern Brazil and northwestern part of southwestern Australia, with temperatures at least 4℃ below normal in locations. Meanwhile, temperatures were observed 1-2℃ above normal in southwestern and northern Europe, Russian Far East, most northeastern China and eastern Southwest China, most West Asia and western Central Asia, part of West Africa and western East Africa, southeastern North America and most of Alaska, central South America and part of the north, with temperatures at least 4℃ above normal in locations(Fig.1).

For the month, precipitations were observed 50-100% less than normal in parts of southwestern and northeastern Europe, northern part of the Central Siberian Plateau, parts of Northeast China and southern Xinjiang，most West Asia, parts of northwestern and eastern Africa, most Australia, parts of central and northwestern North America, and northern South America, with no precipitation in parts of above regions. Meanwhile, precipitations were 50-100% more than normal in central and southeastern Europe, most Siberia, the eastern Huanghuai area and the eastern Qinghai-Xizhang Plateau in China, the most India Peninsular and part of the Indo-China Peninsular, the central part of Central Asia, parts of West Africa and South Africa, parts of northeastern and southwestern North America, central South America and part of the south, locations of eastern Australia, with precipitations at least 200% more than normal in locations (Fig.2).

General Circulation Features

Northern Hemisphere: During May 2009, the mean 500hPa geopotential height field exhibited a 4-wave pattern at the mid-high latitudes. In the 500hPa geopotential height anomaly field, positive anomalies above 4 dam covered northeastern China, the northwestern North Pacific, the coasts and their adjacent sea area along the northwestern and northeastern North America, southwestern Europe and eastern West Asia, with central values above 16 dam over the Aleutian Islands. Meanwhile, negative anomalies below -4 dam covered central Siberia, the northeastern North Pacific, and regions from northern North America to the southern Greenland Island, with central values below -12 dam over the northern Canada (Fig.4). For the month, the area and the ridge position of the Northwest Pacific Subtropical High (NWPSH) was close to normal. The intensity was weaker than normal and the western end of the ridge contracted further eastward.

Southern Hemisphere: In May, in the 500hPa geopotential height anomaly field, most areas were near or above normal except the eastern part of the Southeast Basin, New Zealand and the sea area nearby, and part of the southeastern South Pacific with negative anomalies below -4 dam. Positive anomalies above 4 dam covered the most Antarctic and the middle-high latitude sea area, with central values above 32 dam over the South Pole.

Tropical Features

Sea Surface Temperatures (SSTs): During May, the SSTs were slightly above normal in most of the equatorial Pacific (Fig.5). In May, indices of NINO 1+2, NINO 3, NINO 4, NINO 3.4 and NINO Z were 0.4℃,0.3℃,0.3℃,0.3℃ and 0.3℃, with a rise of 0.0℃,0.3℃,0.4℃,0.5℃ and 0.3℃, respectively.

Subsurface Temperatures: During May, the anomalously warm subsurface water dominated most of the equatorial Pacific (Fig.6).

Warm Pools: During May, the warm pool area and intensity of either the western Pacific or Indian Ocean were both above normal.

Wind Field：For the month, at lower troposphere (850hPa), weak easterly anomalies prevailed over the central Pacific, while weak westerly anomalies covered the western equatorial Pacific (Fig.7). At upper troposphere (200hPa), weak westerly and easterly anomalies covered the western and central equatorial Pacific, respectively.

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

Convections over the Tropics: During May, the anomalous outgoing longwave radiation (OLR) in the equatorial Pacific showed that the convection was suppressed over most of the central-eastern equatorial Pacific, and enhanced over the vicinity of Philippines (Fig.8). In the lower troposphere (850hPa) divergent wind field, a convergent center was near 125ºE, 10ºN, and a divergent center was near 85ºW, 15ºS. At upper troposphere (200hPa), the corresponding divergent center was near 125ºE, 15ºS, and the convergent center was near100ºW, 15ºS.

ENSO Monitoring and outlook

During May 2009, monitoring results showed that ENSO-neutral conditions prevailed over the tropical Pacific by viewing the Nino indices, and the suppressed convections and the persistence of the easterly anomalies over the central-eastern equatorial Pacific, however anomalously warm subsurface water has been across the equatorial Pacific. On the other hand, the recent monitoring results during the first 2 weeks of June 2009, exhibited that the warm subsurface water in the eastern equatorial Pacific extended to the surface continuously, and the low-level westerly anomalies over the eastern equatorial Pacific extended westward across the dateline as a forced result of warmer-than-average SSTs in the central and eastern equatorial Pacific, so a likelihood of the transition from ENSO-neutral to El Nino conditions is increasing. Furthermore, a majority of ENSO models indicate above-average SSTs in the Nino 3.4 region through the remainder of 2009.

In a word, based on aforementioned monitoring about recently atmospheric and oceanic conditions in the tropical Pacific, and the predictions of statistical and dynamic models, within this summer, the SSTs in the central and eastern equatorial Pacific will be in warmer-than-average conditions, and a likelihood of the transition from ENSO-neutral conditions to El Nino conditions should be considered. For a last conclusion, we need to further monitor the developments of ENSO conditions and update our prediction in time.