Monthly Highlights

Climate in China

During October 2007, monthly mean surface air temperatures were above or near normal in most of China, with temperatures more than 1-2℃ above normal in northwestern Northeast China, Jianghuai, central and eastern Jiangnan, central South China and most of Tibetan Plateau, local regions with temperatures more than 2-4℃ above normal, but temperatures were slightly below normal in western and northern Xinjiang, western Inner Mongolia and eastern northwest China. For the month, precipitations were observed 30-100% above normal in northeastern Heilongjiang, southern Liaoning, most of North China, central and western Inner Mongolia, eastern Northwest China, northern and southwestern Xinjiang, western Yunnan and coasts of southeastern China, with more than 100% above normal in parts of the above-mentioned locations. Meanwhile, precipitations were 30-50% below normal in most of Northeast China, central and eastern Inner Mongolia, most of Xinjiang, most of the Tibetan Plateau, most of the regions south to the Yellow River, with more than 80% below normal in locations of above regions.

Climate in the World

During October 2007, monthly mean surface temperatures were above or near normal in most of the globe. Temperatures were 1-2℃ above normal in most Asia, eastern Europe, most Australia, most North America, central-eastern South America and northwestern Africa, with more than 4℃ above normal in locations of above areas. Meanwhile, temperatures were about 1-2℃ below normal in northeastern West Asia, northern Mongolia and parts of western Africa.

For the month, precipitations were observed 50-100% more than normal in southern Europe, southeastern Siberia, parts of central Asia, most of northern China, parts of eastern and western Africa, southern Africa, central and northern North America, with above 200% more than normal in locations. Meanwhile, precipitations were above 50% less than normal in West Asia, Central Asia, northern South Asia, most of the regions south to the Yangtze River, parts of northern Africa, most Australia, eastern South America, southwestern North America and Alaska, with no precipitation in locations of above regions.

General Circulation Features

Northern Hemisphere: During October 2007, the mean 500hPa geopotential height field exhibited a 4-wave pattern at the mid-high latitudes, with the troughs over eastern Europe, northern Asia, central-eastern North Pacific and northern North Atlantic Ocean. In the 500hPa height anomaly field, positive anomalies above 4 dam covered central-northern Europe, northwestern Asia, eastern North America and northeastern North Pacific, with center above 12 dam over northwestern Europe and the Ural mountains. Meanwhile, negative anomalies below -4 dam covered Bering Sea and the polar region north to Europe. For the month, the Northwest Pacific subtropical high was larger and stronger than normal, and withdrew westward and its ridge line position was normal.

Southern Hemisphere: For the month, zonal circulation prevailed at most of the mid-high latitudes. In the 500hPa height anomaly field, positive anomalies above 4 dam covered central South Atlantic Ocean, central South Pacific and Ross Sea. Meanwhile, negative anomalies below -4 dam covered the seas south to South America, southern South Pacific and central-southern South India Ocean.

Tropical features

Sea Surface Temperatures (SSTs): During October 2007, the pattern “cold in the east and warm in the west” for the sea surface temperature anomalies (SSTa) in the equatorial Pacific further strengthened. Compared with last month, SSTa in the central and eastern equatorial Pacific decreased significantly, and the -1.0℃ contours of SSTa expanded westward obviously. NINO Z, NINO 3.4, NINO 4, NINO 3 and NINO 1+2 were -1.2℃, -1.4℃, -0.6℃, -1.5℃ and -2.1℃, with drops of 0.2℃, 0.5℃, 0.2℃, 0.2℃ and 0.3℃ from last month, respectively. Meanwhile, SSTs in most of the equatorial Indian Ocean were slightly higher than normal.

Subsurface Temperatures: Compared with last month, anomalously cold subsurface water center in the eastern equatorial Pacific shifted westward to 120ºW, while its intensity enhanced with the minimum below -5℃. Meanwhile, the intensity of warm subsurface water in the western equatorial Pacific enhanced.

Warm Pools: During October 2007, both the western Pacific and Indian Ocean warm pools enlarged slightly in areas and strengthened in intensity compared with normal.

Wind Field：For the month, at lower troposphere (850hPa), easterly anomalies still covered the central equatorial Pacific but weakened slightly compared with last month. The zonal winds were near normal over the eastern equatorial Pacific, however the distinct westerly anomalies appearing over the regions north to it. At upper troposphere (200hPa), weak westerly anomalies dominated the western and central tropical Pacific, and easterly anomalies prevailed over the eastern tropical Pacific.

Southern Oscillation: For the month, Southern Oscillation Index (SOI) was 0.8, with a rise of 0.6 from last month.

Convections over the Tropics: During October 2007, the anomalous outgoing long wave radiation (OLR) showed that convections were enhanced over the region from the northern Indian Ocean to the tropical western Pacific along 10ºN, while convections were suppressed over the central equatorial Pacific. For the tropical Pacific, in the divergent wind field at lower troposphere (850hPa), a weak convergent center and a divergent center located at 145ºE，10ºN, and 120ºW, 10ºS, respectively, while the corresponding divergent and convergent centers at upper troposphere (200hPa) were over 135ºE，15ºN, and 120ºW，15ºS, respectively.

ENSO Monitoring and Prediction

(1) Compared with last month, SSTa in the central and eastern Pacific decreased obviously, and the NINO Z index dropped to -1.2℃, showing a developing trend of La Nina conditions.

(2) Anomalously cold subsurface water in the central and eastern equatorial Pacific enhanced and its center shifted westward.

(3) Southern Oscillation Index (SOI) was 0.8 during this month, with a rise of 0.6 from last month.

(4) At lower troposphere (850hPa), weakened easterly anomalies covered the central equatorial Pacific.

(5) Convections were suppressed over the central equatorial Pacific.

(6) Most ENSO models predicted La Nina conditions are likely to persist through early 2008.

In a word, La Nina conditions are now well established across the central and eastern equatorial Pacific, and will reach its peak in the two coming months, and probably persist through early 2008 and show a weak -to- moderate intensity.