Monthly Highlights

Climate in China

During April 2007, monthly mean surface air temperatures were observed 1-2℃ below normal in most of South China and eastern Yunnan, while temperatures were near or above normal in the remainder of China, with temperatures 2-4℃ above normal in Xinjiang and western Tibet.

For the month, precipitations were observed 30-80% below normal in northwestern and southwestern Northeast China, eastern and southern North China, Huanghuai, mid-northern Jiangnan, eastern Northwest China, southern Qinghai and southern Xinjiang. Meanwhile, precipitations were 30-200% above normal in central and northern Northwest China, western Inner Mongolia, the region from northern Inner Mongolia to southwestern Heilongjiang, with precipitations over 200% more than normal in western Gansu.

Climate in the World

During April 2007, monthly mean surface air temperatures were observed above normal in most of the world. Temperatures were more than 2℃ above normal in most of Russia, western China, central Asia, most of Europe and Alaska, with temperatures 8-10℃ above normal in northern Russia. Meanwhile, temperatures were more than 2℃ below normal in Turkey and southeastern USA, with temperatures 4℃ below normal in eastern Turkey.

For the month, precipitations were observed 50-100% less than normal in East Asia, most of Xinjiang in China, most of South Asia, central Russia and the Far East, most of Europe, most of Turkey, most of USA, northern Mexico, part of West Africa, northern and southeastern Australia, eastern Brazil and northern Argentina, with no precipitation in parts of above regions. Meanwhile, precipitations were 50-100% more than normal in eastern Northwest China, the region from Yunnan in China to the Indochina Peninsula, the region nearby the Ural Mountains, most of Spain, northeastern and part of southern USA, southern and northwestern Australia, with precipitations over 200% more than normal in locations.

General Circulation Features

Northern Hemisphere: During April 2007, the mean 500hPa geopotential height field exhibited a 4-wave pattern at the mid-high latitudes, with the troughs over the East Europe Plain, the offshore of eastern Asia, the northeastern North Pacific and the offshore of eastern North America. In the 500hPa height anomaly field, positive anomalies above 8 dam covered most of Siberia, central Asia and most of Europe, with central value above 16 dam over Europe. Meanwhile, negative anomalies covered the region from western Russia to West Asia, the northwestern North Atlantic Ocean and the northern North Pacific, with central value below -8 dam. Heights were near normal in other regions. For the month, the Northwest Pacific subtropical high was larger and stronger than normal, and extended farther west and north.

Southern Hemisphere: For the month, the mean 500hPa geopotential height field showed obvious wave-pattern at the mid-high latitudes, with the troughs over southern South America, the ocean southeast to New Zealand and the central South Indian Ocean. In the 500hPa height anomaly field, negative anomalies covered the trough regions mentioned above, with central value below -12 dam. Meanwhile, positive anomalies above 4 dam covered the southeastern South Pacific and the region from the ocean south to Australia to most of the Antarctica, with central value above 8 dam. Heights were near normal in other regions.

Tropical Features

Sea Surface Temperatures (SSTs): During April 2007, while SST anomalies in the central and eastern equatorial Pacific decreased in general, the pattern of “cold in the east and warm in the west” still kept in the equatorial Pacific. Positive SST anomalies above 0.5℃ shrunk in the western equatorial Pacific, while negative SST anomalies below -0.5℃ were in most of the eastern equatorial Pacific and the farthest southeastern tropical Pacific, with central values below -2.0℃ in the Pacific offshore of South America. NINO Z, NINO 4 and NINO 1+2 SST anomaly indices were -0.1℃, 0.3℃ and -1.2℃, with drops of 0.1℃, 0.2℃ and 0.4℃ from last month, respectively, while NINO 3.4 SST anomaly index was 0.1℃, with a rise of 0.1℃ from last month, and NINO 3 SST anomaly index was -0.3℃, same as the one in last month. Meanwhile, SST anomalies were near normal in most of the equatorial Indian Ocean.

Subsurface Temperatures: During April 2007, comparing with last month, anomalously cold subsurface waters were weakened in the eastern equatorial Pacific, while the area of anomalously warm subsurface waters was enlarged in the western equatorial Pacific.

Warm Pools: During April 2007, areas of the western Pacific and Indian Ocean warm pools were above normal and their intensities were both stronger than normal.

Wind Field：During April 2007, at the lower troposphere (850hPa), westerly anomalies controlled the eastern Pacific, while easterly anomalies covered most of the central and western equatorial Pacific with maximum values over -8 m·s^-1. The zonal winds were near normal over the equatorial Indian Ocean. At the upper troposphere (200hPa), westerly anomalies above 4 m·s^-1 prevailed between 140-160°W.

Southern Oscillation: For the month, Southern Oscillation Index (SOI) was 0.0, with a decrease of 0.1 from last month.

Convection over the Tropics: During April 2007, the anomalous outgoing longwave radiation (OLR) showed that convections were enhanced over the western equatorial Pacific and suppressed over the eastern equatorial Pacific. In the lower troposphere (850hPa) divergent wind field, a strong convergent belt was observed over the western equatorial Pacific with centers covering most of the western equatorial Pacific, and a weak divergent belt was over the southeastern tropical Pacific with a center located in 95ºW, 15ºS. Correspondingly, at the upper troposphere (200hPa), a strong divergent center was found over the western tropical Pacific with the center at 170°E, 10°S , and the weak convergent center was over 100ºW, 5ºS.

ENSO Monitoring and Prediction

① During April 2007, while SST anomalies in the central and eastern equatorial Pacific decreased in general. Positive SST anomalies above 0.5℃ shrunk in the western equatorial Pacific , while negative SST anomalies below -0.5℃ were in most of the eastern equatorial Pacific and the farthest southeastern tropical Pacific. Nino Z index was -0.1℃;

② During April 2007, comparing with last month, anomalously cold subsurface waters were weakened in the eastern equatorial Pacific, while the area of anomalously warm subsurface waters were enlarged in the western equatorial Pacific;

③ Southern Oscillation Index (SOI) was 0.0;

④ At the lower troposphere (850hPa), westerly anomalies controlled the eastern Pacific, while easterly anomalies covered most of the central and western equatorial Pacific;

⑤ Convections were enhanced over the western equatorial Pacific and suppressed over the eastern equatorial Pacific;

⑥ Most of models predict that SST anomaly in the mid-eastern Pacific will still be normal–cold phase during the following 1-3 months.

In conclusion, the anomalously SST in the equatorial Pacific showed a pattern of “warm in the west and cold in the east”, and the normal - cold conditions in the mid-eastern Pacific are expected to maintain in the following 1-3 months.