Arctic Sea Ice and Eurasian Climate:A Review

The Arctic plays a fundamental role in the climate system and has shown significant climate change in recent decades,including the Arctic warming and decline of Arctic sea-ice extent and thickness. In contrast to the Arctic warming and reduction of Arctic sea ice, Europe, East Asia and North America have experienced anomalously cold conditions, with record snowfall during recent years. In this paper, we review current understanding of the sea-ice impacts on the Eurasian climate.Paleo, observational and modelling studies are covered to summarize several major themes, including： the variability of Arctic sea ice and its controls; the likely causes and apparent impacts of the Arctic sea-ice decline during the satellite era,as well as past and projected future impacts and trends; the links and feedback mechanisms between the Arctic sea ice and the Arctic Oscillation/North Atlantic Oscillation, the recent Eurasian cooling, winter atmospheric circulation, summer precipitation in East Asia, spring snowfall over Eurasia, East Asian winter monsoon, and midlatitude extreme weather; and the remote climate response（e.g., atmospheric circulation, air temperature） to changes in Arctic sea ice. We conclude with a brief summary and suggestions for future research.

Effects of Large-Scale Climatic Oscillations on the Variability of the Indian Summer Monsoon Rainfall

In India,large-scale climatic oscillations have strong influences on the Indian summer monsoon rainfall(ISMR),which plays a crucial role in India’s agricultural production and economic growth.However,there are limited studies in India that explore the influences of decadal and multidecadal oscillations on the ISMR and associated El Niño–Southern Oscillation(ENSO).Therefore,in this study we carried out a comprehensive and detailed investigation to understand the influences of ENSO,Pacific decadal oscillation(PDO),and Atlantic multidecadal oscillation(AMO)on ISMR across different regions in India.The statistical significance of ISMR associated with different phases(positive/warm and negative/cold)of ENSO,PDO,and AMO(individual analysis),and combined ENSO–AMO,and ENSO–PDO(coupled analysis)were analysed by using the nonparametric Wilcoxon Rank Sum(WRS)test.The individual analysis results indicate that in addition to the ENSO teleconnection,AMO and PDO significantly affect the spatial patterns of ISMR.Coupled analysis was performed to understand how the phase shift of PDO and AMO has modulated the rainfall during El Niño and La Niña phases.The results indicate that the La Niña associated with a positive PDO phase caused excessive precipitation of about 21%–150%in the peninsular,west–central,and hilly regions compared to the individual effect of ENSO/PDO/AMO on ISMR;similarly,the west–central,coastal,and northwest regions received 15%–56%of excessive rainfall.Moreover,during the El Niño combined with PDO positive(AMO positive),above-normal precipitation was observed in hilly,northeast,and coastal(hilly,northeast,west–central,and coastal)regions,opposite to the results obtained from the individual ENSO analysis.This study emphasizes the importance of accounting the decadal and multidecadal forcing when examining variations in the ISMR during different phases of ENSO events.

Multi-scale variation characteristics of polar vortex at 10 hPa in the Southern Hemisphere

By use of 1948-2007 NCEP/NCAR reanalysis monthly geopotential data, a set of circulation indices are defined to characterize the polar vortex at 10 hPa in the Southern Hemisphere, including area-（S）, intensity-（P） and centre position-（λc , φc） indices. Sea-sonal variation, interannual anomalies and their possible causes of 10 hPa polar vortex in the Southern Hemisphere are analyzed by using these indices, the relationship between 10 hPa polar vortex strength and the Antarctic Oscillation are analyzed as well. The results show that： （1） the polar region at 10 hPa in the Southern Hemisphere is controlled by anticyclone （cyclone） from Dec. to Jan. （from Mar. to Oct.）, Feb. and Nov. are circulation transition seasons. （2） Intensity index （P） and area index （S） of anticy-clone （cyclone） in Jan. （Jul.） show a significant spike in the late 1970s, the anticyclone （cyclone） enhances （weakens） from ex-tremely weak （strong） oscillation to near the climatic mean before a spike, anticyclone tends to the mean state from very strong oscillation and cyclone oscillates in the weaker state after the spike. （3） There is significant interdecadal change for the anticyclone center in Jan., while markedly interannual variation for cyclone center in July. （4） The ozone anomalies can cause the interannual anomaly of the polar anticyclone at 10 hPa in the Southern Hemisphere in Jan. （positive correlation between them）, but it is not related to the polar cyclone anomalies. （5） There is notable negative correlation between the polar vortex intensity index P and the Antarctic Oscillation index （AAOI）, thus AAOI can be represented by P.

NUMERICAL STUDY OF THE SHORT-TERM CLIMATIC OSCILLATION FORCED BY EL NINO DURING NORTHERN WINTER

Using a nine-layer global spectral model, numerical schemes with two different SST distributions in January (control case and abnormal case) have been tested to study the climatic effect, propagation charateris- tics and the maintenance mechanism of the short-term climatic oscillation caused by El Nino during northern winter. The main results are as follows: (1) During northern winter, there exist two wave trains because of the influence of El Nino. One is similar to PNA pattern, and the other is similar to EUP pattern. (2) The PNA-like wave train caused by the anomalous SST forcing in central and eastern equatorial Pacific Ocean is due to the response of ultralong wave and long wave components of Rossby mode, and the EUP-like wave train crossing Eurasia is mainly due to the wave component of Rossby mode. (3) During northern winter, the warm water region in central equatorial Pacific Ocean is the source of forced wave trains. (4) In northern winter, the energy source for maintaining the short-term climatic oscillation is from the interaction between eddies, and between eddy and zonal flow.

A NUMERICAL STUDY OF SHORT-RANGE CLIMATIC OSCILLATION WITH VARIOUS TIME SCALES FORCED BY EL NINO DURING THE NORTHERN SUMMER

The normal mode method is adopted to decompose the differences between simulations with SST(sea surface temperature)anomahes over centra-eastern Pacific and normal SST by use of a nine-layer global spec- tral model in order to investigate short-range climatic oscillation with various time scales forced by El Nino during the northern summer.Investigation shows that El Nino may have the following influence on atmosphere on various space-time scales.Extra-long wave components of Rossby mode forced by convective anomaly over equatorial western Pacific resulting from El Nino produce climatic oscillation on monthly(sea- sonal)time scale in middle-high latitudes of Southern and Northern Hemispheres;extra-long wave components of Kelvin mode forced by SST anomalies propagate along the equator,resulting in 30—60 day oscillation of tropical and subtropical atmosphere;and its long waves move eastward with westerly,resulting in quasi-biweek oscillation.

FORMATION MECHANISM AND PROPAGATION CHARACTERISTICS OF THE EQUATORIAL THERMALLY-FORCED SHORT-TERM CLIMATIC OSCILLATION DURING THE NORTHERN SUMMER

In this study, in order to investigate the global climatic oscillations forced by sea surface temperature (SST) anomalies over equatorial central-eastern Pacific, two numerical schemes with different SST distri-butions (normal and anomalous cases) are tested by using a nine-layer global spectral model. Experiment results show that (i) in northern summer, a wave train that is similar to the teleconnection pattern suggested by Nitta (1987) and Huang (1987) in the Northern Hemisphere and another one in the Southern Hemisphere are reproduced; (ii) simulated results suggest that the response of atmosphere in middle-high latitudes of both hemispheres to an anomalous heating source is more sensitive in tropical western Pacific than in equatorial central-eastern Pacific; and (iii) in northern summer, the formation of low-frequency oscillations on monthly (seasonal) time scale results from energy dispersion as well as interactions between eddies and zonal flow; and between eddies.

CLIMATIC JUMP FROM LATE 1970S TO EARLY 1980S AND ITS EFFECT

By use of the moving T test to do research on the interdecadal climate jump of the Northern Hemisphere sea level pressure.500 hPa height and North Pacific sea surface temperature,we found that in recent 50 years an obvious interdecadal climate jump existed at the late 1970s to the early 1980s.There is significant difference before and after the jump in terms of the Northern Hemisphere sea level pressure,500 hPa height and North Pacific sea surface temperature. Furthermore,the focus is then placed on the effect of the jump on temperature and rainfall in China.