1. Climate in China
In December 2013, the surface air temperature averaged over China was -2.8ºC, which was 0.4ºC above normal (-3.2ºC). Monthly mean surface air temperatures were generally above normal in most of Northern China, while below normal in most of Southern China. Monthly mean air temperatures were 1~2ºC below normal in some locations of central-eastern Qinghai and southeastern Heilongjiang, eastern Jiangnan, most of South China, central-northern Sichuan, eastern Yunnan, and southwestern Guizhou, with negative anomalies over 2ºC in some locations. Meanwhile, air temperatures were near or above normal in the rest of China. Temperatures were 2~4ºC above normal in northern Xinjiang, northeastern Inner Mongolia, and northwestern Heilongjiang, with positive anomalies over 4ºC in some locations.
For the month, the mean precipitation over China was 15.4 mm, which was 46.7% more than normal (10.5mm). The precipitation was 50～80% less than normal in most of China, except for the precipitation of 50～200% more than normal in eastern and southern Jiangnan, South China, most of Yunnan, and southern Guizhou, with precipitation over 200% more than normal in some locations. Meanwhile, precipitation was over 80% less than normal in southwestern and eastern Xinjiang, northwestern Gansu, western and central Inner Mongolia, Ningxia, Shanxi, most of North China, Huanghuai, northern Jianghuai, and Jianghan.
In December, extremes of Daily Minimum Temperature (DMT), Daily Temperature Drop (DTD), and Consecutive Temperature Drop (CTD) were observed in China. DMT extremes occurred in 36 stations, which are mainly located in Yunnan, Guizhou, Sichuan, Xizang, Guangxi, and Anhui. DTD extremes occurred in 20 stations, which are mainly located in Yunnan, Qinghai, Heilongjiang, Jilin, and Hebei, with new records in Tangshan station (13.3 ºC) and Bazhou station (13.2 ºC) of Heibei and Tianlin station (13.8 ºC) of Guangxi. At the same time, CTD extremes occurred in 27 stations, which are mainly located in Yunnan, Sichuan, and Heilongjiang, with new records in Jiangcheng station (14.7ºC) of Yunnan and other 4 stations.
2. Climate in the World
During December 2013, monthly mean air temperatures were 1~4℃ above normal in most of Europe and Russia, northern Central Asia, central-northern India, northern East Asia, eastern and southwestern Australia, southwestern and southeastern parts of the United States of America, western Alaska, parts of central and southern South America, parts of western and central-eastern Africa, with temperatures 4~8℃ above normal in some locations. Meanwhile, mean air temperatures were 1~4℃ below normal in northern West Asia, western South Asia, southern China, Indo-China Peninsula, most of North America, locations of northeastern South America, and parts of northwestern and southern Africa, with temperatures 4~8℃ below normal in eastern Alaska and central-northern Canada.
For the month, precipitation was at least 50% above normal in parts of northern Europe, northern West Siberia, the Far East of Russia, central-western West Asia, Central Asia, eastern Mongolia, southern China, northern Indo-China Peninsula, northwestern Australia, most of Canada, northern Central America, central South America, parts of southern and northern Africa, with precipitation 100~400% above normal in some locations. Meanwhile, precipitation was 50~80% below normal in southwestern East Europe, central-eastern West Asia, South Asia, northern China, western Mongolia, northeastern Australia, central-western and central-northern North America, parts of northwestern and southern South America, and most of northern Africa, or even over 80% below normal in some locations.
3. General Circulation Features
Northern Hemisphere: During December 2013, in the field of mean 500hPa geopotential height anomaly, negative anomalies below -4 dam covered northern North America, northern North Atlantic Ocean, East Europe, and the region from northeastern East Asia to western North Pacific, with central values below -12dam over parts of northern North Atlantic Ocean. Meanwhile, positive anomalies above 4 dam covered western and southern Europe, the region from central Siberia to northern North Pacific and parts of southwestern North Atlantic Ocean, with central values above 16dam over parts of northern North Pacific. During the month, the northwestern Pacific subtropical high was weaker than normal, with the area smaller than normal. The position of its west ridge shifted further eastward than normal.
Southern Hemisphere: For the month, in the field of mean 500hPa geopotential height anomaly, positive anomalies over 4 dam were only observed over southern South Pacific Ocean and parts of southern South Indian Ocean, while most parts of the Southern Hemisphere were covered by negative anomalies, with central values below -8dam over parts of southwestern South Atlantic Ocean.
4. Tropical Features
Sea Surface Temperatures (SSTs): During December 2013, SSTs were above normal over most of the western equatorial Pacific to the west of dateline, while SST anomalies were near normal over the most central and eastern equatorial Pacific. Niño1+2, Niño3, Niño4, Niño3.4 and NiñoZ indices were -0.3℃、-0.1℃、0.1℃、-0.1℃ and 0.0℃, with increases of 0.1℃ being both observed over Niño1+2 and Niño3, decreases of 0.2℃ and 0.1℃ over Niño4 and Niño3.4, respectively, and no change over NiñoZ from last month.
Subsurface Temperatures: During December 2013, invasion of anomalously cold subsurface water in central tropical Pacific weakened the eastward movement of warm subsurface water in the western equatorial Pacific. Meanwhile, anomalously cold subsurface water weakened and retreated eastward in the central and eastern equatorial Pacific.
Warm Pools: During December 2013, the area of the Indian Ocean warm pool was slightly larger than normal and its intensity was near normal. The area and intensity of the western Pacific warm pool were larger and stronger than normal, respectively.
Wind Field: During December 2013, at lower troposphere (850hPa), the easterly wind anomalies covered the western and central equatorial Pacific, while westerly wind anomalies were observed over northern part of the eastern equatorial Pacific, and weak easterly anomalies were observed over the southern part. At upper troposphere (200hPa), westerly wind anomalies covered parts of regions to the west of the dateline, and easterly wind anomalies covered most central and eastern equatorial Pacific.
Southern Oscillation: For the month, the Southern Oscillation Index (SOI) was 0.3, with a decrease of 0.6 from last month.
Convections over the Tropics: During December 2013, the anomalous outgoing long wave radiation (OLR) showed that convections enhanced from the Indo-China Peninsula to northern Australia, while convections were suppressed in the central equatorial Pacific Ocean. At lower troposphere (850hPa) divergent wind field, a convergent center was around 120ºE，0ºN，and the divergent center was over 110ºW，5ºS. At upper troposphere (200hPa), a corresponding divergent center was near 110ºE，5ºS, and the convergent center was over 110ºW, 20ºS.
5. ENSO Monitoring and Outlook
ENSO-neutral conditions continue at present. Positive ocean heat content anomaly in the central-eastern equatorial Pacific decreased substantially to near normal in Dec 2013. Furthermore, the western equatorial Pacific was under the control of easterly wind anomalies at 850-hPa level in the past two months, which is conducive to descending ocean heat content further in the coming month.
So considering the multiple timescale variations of the recent sea-air conditions in the tropical Pacific, and the consensus forecast from dynamical and statistical models, ENSO-neutral is expected to continue into the Northern Hemisphere spring 2014, with slightly cooler and warmer than normal conditions in the early and late spring, respectively.
Therefore, we will closely monitor the development of ENSO conditions and update our ENSO wrap-up in time.