Remarkable Link between Projected Uncertainties of Arctic Sea-Ice Decline and Winter Eurasian Climate

We identify that the projected uncertainty of the pan-Arctic sea-ice concentration(SIC) is strongly coupled with the Eurasian circulation in the boreal winter(December–March; DJFM), based on a singular value decomposition(SVD) analysis of the forced response of 11 CMIP5 models. In the models showing a stronger sea-ice decline, the Polar cell becomes weaker and there is an anomalous increase in the sea level pressure(SLP) along 60°N, including the Urals–Siberia region and the Iceland low region. There is an accompanying weakening of both the midlatitude westerly winds and the Ferrell cell,where the SVD signals are also related to anomalous sea surface temperature warming in the midlatitude North Atlantic.In the Mediterranean region, the anomalous circulation response shows a decreasing SLP and increasing precipitation. The anomalous SLP responses over the Euro-Atlantic region project on to the negative North Atlantic Oscillation–like pattern.Altogether, pan-Arctic SIC decline could strongly impact the winter Eurasian climate, but we should be cautious about the causality of their linkage.

Simulation by CMIP5 Models of the Atlantic Multidecadal Oscillation and Its Climate Impacts

This study focuses on the climatic impacts of the Atlantic Multidecadal Oscillation （AMO） as a mode of internal vari- ability. Given the difficulties involved in excluding the effects of external forcing from internal variation, i.e., owing to the short record length of instrumental observations and historical simulations, we assess and compare the AMO and its related climatic impacts both in observations and in the ＂Pre-industrial＂ experiments of models participating in CMIP5. First, we evaluate the skill of the 25 CMIP5 models＇ ＂Historical＂ simulations in simulating the observational AMO, and find there is generally a considerable range of skill among them in this regard. Six of the models with higher skill relative to the other models are selected to investigate the AMO-related climate impacts, and it is found that their ＂Pre-industrial＂ simulations capture the essential features of the AMO. A positive AMO favors warmer surface temperature around the North Atlantic, and the Atlantic ITCZ shifts northward leading to more rainfall in the Sahel and less rainfall in Brazil. Furthermore, the results confirm the existence of a teleconnection between the AMO and East Asian surface temperature, as well as the late withdrawal of the Indian summer monsoon, during positive AMO phases. These connections could be mainly caused by internal climate variability. Opposite patterns are true for the negative phase of the AMO.

Change in Sea Ice Cover is Responsible for Non-Uniform Variation in Winter Temperature over East Asia

Observed winter(December–February)surface air temperature over East Asia(0°–60°N,100–140°E)(TEA)shows non-uniform variation during 1979–2013,with cooling and weak warming north and south of40°N.To understand this,the authors perform statistical analysis(linear regression and composite)on the observed data.The results suggest that reduced(increased)autumn sea ice cover in the Barents-Kara Sea(BK-ASIC)lowers(warms)TEA over northern East Asia,which is consistent with previous studies.In comparison,increased(decreased)winter sea ice cover in the Sea of Okhotsk(O-WSIC),warms(cools)the air over southern East Asia.The mechanism can be described as follows:When the BK-ASIC decreases,the East Asian winter monsoon tends to be stronger with an intensified Siberian high,leading to cooling over northern East Asia.An O-WSIC increase is associated with cold anomalies north of 50°N,altering the meridional temperature gradient between the midlatitudes and tropics,and leading to a northward shift of the East Asian jet steam in the upper troposphere.In the low atmosphere,anomalous northeasterly winds prevail north of50°N and anomalous southerly winds control the southern coast of East Asia,contributing to the weak warming over southern East Asia.Version 3 of the Community Atmosphere Model also provides evidence for the impact of increased O-WSIC on the warm southern mode of TEA.

Projected Changes in Eurasian and Arctic Summer Cyclones under Global Warming in the Bergen Climate Model

Using the coupled ocean-atmosphere Bergen Climate Model,and a Lagrangian vorticity-based cyclone tracking method,the authors investigate current climate summer cyclones in the Northern Hemisphere and their change by the end of the 21st century,with a focus on Northern Eurasia and the Arctic.The two scenarios A1B and A2 for increasing greenhouse gas concentrations are considered.In the model projections,the total number of cyclones in the Northern Hemisphere is reduced by about 3% 4%,but the Arctic Ocean and adjacent coastal re-gions harbour slightly more and slightly stronger summer storms,compared to the model current climate.This in-crease occurs in conjunction with an increase in the high-latitude zonal winds and in the meridional tempera-ture gradient between the warming land and the ocean across Northern Eurasia.Deficiencies in climate model representations of the summer storm tracks at high lati-tudes are also outlined,and the need for further model inter-comparison studies is emphasized.

Has the Problem of a Permanent El Nio been Resolved for the Mid-Pliocene?

The mid-Pliocene, the most recent warm geological period, is thought to be indicative of the fate of the Earth's climate under global warming. Earlier evidence has suggested that permanent El Nio-like conditions existed in the mid-Pliocene, though the concept of a permanent El Nio remains controversial. Here, the authors analyzed Nio 3.4 SST in pre-industrial and mid-Pliocene simulations with the low-resolution version of the Norwegian Earth System Model (NorESM-L). The simulated mid-Pliocene Nio3.4 SST, with a smaller standard deviation, indicated that a weaker ENSO existed in the mid-Pliocene relative to the pre-industrial experiment. Compared with earlier modeling studies, our simulations show that the problem of ENSO's standard deviations in the mid-Pliocene remains unresolved, although the mean and the period of ENSO in the mid-Pliocene have been resolved by earlier geological and modeling studies.

Subpolar Gyre Index and the North Atlantic Meridional Overturning Circulation in a Coupled Climate Model

The subpolar gyre index (SPG), derived from the analysis of sea surface height (SSH), is proposed to be a potential indicator for the North Atlantic Meridional Overturning Circulation (AMOC) based on observation as well as the Ocean General Circulation Model (OGCM). We investigated the correspondence between the SPG and the AMOC in a coupled climate model. Our results confirm that the SPG can be used as an early indicator for the AMOC in the subtropical North Atlantic. Changes in the SPG are closely related to variations in the air-sea heat exchange in the Labrador Sea, and variations in deep water formation and southward dense water transport with the deep western boundary current (DWBC) in the North Atlantic.

Influence of Low-frequency Solar Forcing on the East Asian Winter Monsoon Based on HadCM3 and Observations

In this study, we investigate the influence of low-frequency solar forcing on the East Asian winter monsoon（EAWM）by analyzing a four-member ensemble of 600-year simulations performed with Had CM3（Hadley Centre Coupled Model,version 3）. We find that the EAWM is strengthened when total solar irradiance（TSI） increases on the multidecadal time scale. The model results indicate that positive TSI anomalies can result in the weakening of Atlantic meridional overturning circulation, causing negative sea surface temperature（SST） anomalies in the North Atlantic. Especially for the subtropical North Atlantic, the negative SST anomalies can excite an anomalous Rossby wave train that moves from the subtropical North Atlantic to the Greenland Sea and finally to Siberia. In this process, the positive sea-ice feedback over the Greenland Sea further enhances the Rossby wave. The wave train can reach the Siberian region, and strengthen the Siberian high. As a result, low-level East Asian winter circulation is strengthened and the surface air temperature in East Asia decreases. Overall,when solar forcing is stronger on the multidecadal time scale, the EAWM is typically stronger than normal. Finally, a similar linkage can be observed between the EAWM and solar forcing during the period 1850–1970.

Simulated Spatiotemporal Characteristics of Climate Change in China during the Han Dynasty(1–200 A.D.)

In this study,a 2000-year simulation forced by transient,external forcings is carried out with the Community Earth System Model.The authors investigate the spatiotemporal features of climate change in the Han Dynasty(1–200 A.D.)using the empirical orthogonal function(EOF)method.The leading EOF mode of the annua mean temperature anomalies shows a uniform variation of temperature over the whole of China,while the second EOF mode indicates opposite variations of temperature between western and eastern China.For the annual mean precipitation anomalies,the first EOF mode indicates a meridional dipole pattern over eastern China,with increased(decreased)precipitation to the south of the Yangtze River and decreased(increased)precipitation to the north.The leading mode of the 850 h Pa winds and sea level pressure in summer exhibits a southwesterly(northeasterly)anomaly over South China,which is associated with a strengthened(reduced)meridional sea level pressure gradient.Compared to reconstructions,the model can capture the majority of features of climate changes in the Han Dynasty,though it underestimates the magnitude.

Unprecedented Retreat in a 50-Year Observational Record for Petermann Glacier, North Greenland

Petermann Glacier is a marine-terminating outlet glacier that had a 70 km-long floating ice tongue prior to a～270 km2calving event that was observed from satellite sensors in August 2010,shortening the ice tongue by～27 km.Further,in July 2012,another 10 km was lost through calving.In order to understand these events in perspective,here the authors perform a long-term data analysis of Petermann Glacier calving-front variability and ice velocity for each year in the 1990s–2000s,supplemented by available observations from the previous three decades.Five major(on the order of 100 km2)calving events are identified,with～153 km2calved from1959 to 1961,～168 km2in 1991,～71 km2in 2001,～270 km2in 2010,and～130 km2in 2012—as well as～31km2calved in 2008.The increased frequency of major calving events in recent years has left the front terminus position retreated nearly 25 km beyond the range of observed in previous decades.In contrast,stable ice-dynamics are suggested from ice-velocity measurements made each year between 1993–2012,which are on average1063 m yr–1,with limited interannual variability and no significant trend;moreover,there is no apparent relationship between ice-velocity variability and calving events.The degree to which the massive calving events in 2010and 2012 represent natural episodic variability or a response to atmospheric and/or oceanic changes remains speculative;however,melt-induced weakening of the floating ice tongue in recent years is strongly suggested.

Impact of Global OceanicWarming on Winter Eurasian Climate

In the 20 th century, Eurasian warming was observed and was closely related to global oceanic warming(the first leading rotated empirical orthogonal function of annual mean sea surface temperature over the period 1901–2004). Here, large-scale patterns of covariability between global oceanic warming and circulation anomalies are investigated based on NCEP–NCAR reanalysis data. In winter, certain dominant features are found, such as a positive pattern of the North Atlantic Oscillation(NAO), low-pressure anomalies over northern Eurasia, and a weakened East Asian trough. Numerical experiments with the CAM3.5, CCM3 and GFDL models are used to explore the contribution of global oceanic warming to the winter Eurasian climate. Results show that a positive NAO anomaly, low-pressure anomalies in northern Eurasia, and a weaker-than-normal East Asian trough are induced by global oceanic warming. Consequently, there are warmer winters in Europe and the northern part of East Asia. However, the Eurasian climate changes differ slightly among the three models. Eddy forcing and convective heating from those models may be the reason for the different responses of Eurasian climate.

Toward Quantifying the Increasing Accessibility of the Arctic Northeast Passage in the Past Four Decades

Sea ice,one of the most dominant barriers to Arctic shipping,has decreased dramatically over the past four decades.Arctic maritime transport is hereupon growing in recent years.To produce a long-term assessment of trans-Arctic accessibility,we systematically revisit the daily Arctic navigability with a view to the combined effects of sea ice thickness and concentration throughout the period 1979−2020.The general trends of Navigable Windows(NW)in the Northeast Passage show that the number of navigable days is steadily growing and reached 89±16 days for Open Water(OW)ships and 163±19 days for Polar Class 6(PC6)ships in the 2010s,despite high interannual and interdecadal variability in the NWs.More consecutive NWs have emerged annually for both OW ships and PC6 ships since 2005 because of the faster sea ice retreat.Since the 1980s,the number of simulated Arctic routes has continuously increased,and optimal navigability exists in these years of record-low sea ice extent(e.g.,2012 and 2020).Summertime navigability in the East Siberian and Laptev Seas,on the other hand,varies dramatically due to changing sea ice conditions.This systematic assessment of Arctic navigability provides a reference for better projecting the future trans-Arctic shipping routes.

Impacts of the extratropical North Pacific on boreal summer Arctic circulation

北极夏季大气环流的一个显着特征是以北冰洋为中心的异常反气旋环流,与北极增暖存在紧密的联系.先前的研究已将其形成的潜在机制与早春欧亚融雪,热带太平洋海表温度异常联系起来.本研究发现北极夏季反气旋环流异常与春季副热带北太平洋海表温度异常(SSTA)存在显着联系,表明副热带北太平洋与北极之间存在遥相关.该SSTA的特点是北太平洋中纬度地区有暖的SSTA,,周围有明显的冷SSTA,类似于太平洋年代际振荡(PDO)的负位相,但热带地区没有明显的信号(本研究称之为类负位相PDO),5月份,这种类负位相PDO可以激发从白令海传播到北极的罗斯贝波,导致北极出现反气旋环流异常.这种反气旋环流异常可以持续到夏季.同时,这种类负位相PDO的SSTA可以持续到夏季,并在夏季引起白令海上空的低压异常.这种低层的低压异常会导致异常的上升运动,并引起高层的辐散异常,从而进一步加剧夏季北极对流层上层的反气旋环流异常。这种反气旋环流异常可迫使北极上空出现异常绝热下沉运动,导致北极出现显著的绝热加热.于是,北极上空出现显著的暖异常,其暖中心位于北极对流层中部。数值模拟试验证实了5月份类负位相PDO的SSTA与夏季北极反气旋环流异常之间的联系.

温盐环流反转及其对新生代气候的影响

温盐环流是由海水温度、盐度差异驱动的全球洋流循环系统。在气候系统中,它对全球热量输送起到了十分重要的作用。在亚轨道尺度(千年时间尺度)上,温盐环流的改变导致了一系列快速的气候变化,因此备受关注。在构造时间尺度(百万年时间尺度)上,古海洋记录和数值模拟揭示出,温盐环流的反转对新生代气候也产生了非常显著的影响。在新生代,温盐环流由"南大洋深层水主控型"向"北大西洋深层水主控型"反转。这一反转改变了全球的热量输送,使得南半球强烈变冷,并有可能导致南极东部永久冰盖的形成。在这一反转事件中,热带海道的作用更加重要。

Impacts of the Autumn Arctic Sea Ice on the Intraseasonal Reversal of the Winter Siberian High

During 1979–2015, the intensity of the Siberian high(SH) in November and December–January(DJ) is frequently shown to have an out-of-phase relationship, which is accompanied by opposite surface air temperature and circulation anomalies.Further analyses indicate that the autumn Arctic sea ice is important for the phase reversal of the SH. There is a significantly positive(negative) correlation between the November(DJ) SH and the September sea ice area(SIA) anomalies. It is suggested that the reduction of autumn SIA induces anomalous upward surface turbulent heat flux(SHF), which can persist into November, especially over the Barents Sea. Consequently, the enhanced eddy energy and wave activity flux are transported to mid and high latitudes. This will then benefit the development of the storm track in northeastern Europe. Conversely, when downward SHF anomalies prevail in DJ, the decreased heat flux and suppressed eddy energy hinder the growth of the storm track during DJ over the Barents Sea and Europe. Through the eddy–mean flow interaction, the strengthened(weakened)storm track activities induce decreased(increased) Ural blockings and accelerated(decelerated) westerlies, which makes the cold air from the Arctic inhibited(transported) over the Siberian area. Therefore, a weaker(stronger) SH in November(DJ) occurs downstream. Moreover, anomalously large snowfall may intensify the SH in DJ rather than in November. The ensemble-mean results from the CMIP5 historical simulations further confirm these connections. The different responses to Arctic sea ice anomalies in early and middle winter set this study apart from earlier ones.

A novel procedure for pollen-based quantitative paleoclimate reconstructions and its application in China

Traditionally, the evaluation of pollen-based quantitative paleoclimate reconstructions focuses on the ability of calibration sets to infer present climatic conditions and/or the similarity between fossil and modern assemblages. Objective criteria for choosing the most appropriate climate parameter(s) to be reconstructed at a specific site are thus lacking. Using a novel approach for testing the statistical significance of a quantitative reconstruction using random environmental data, in combination with the advantageous large environmental gradients, abundant vegetation types and comprehensive modern pollen databases in China, we describe a new procedure for pollen-based quantitative paleoclimatic reconstructions. First, the most significant environmental variable controlling the fossil pollen assemblage changes is identified. Second, a calibration set to infer changes in this targeted variable is built up, by limiting the modern ranges of other environmental variables. Finally, the pollen-based quantitative reconstruction is obtained and its statistical significance assessed. This novel procedure was used to reconstruct the mean annual precipitation(Pann) from Gonghai Lake in the Lvliang Mountains, and Tianchi Lake in the Liupan Mountains, on the eastern and western fringe of the Chinese Loess Plateau, respectively. Both Pann reconstructions are statistically significant(p<0.001), and a sound and stable correlation relationship exists in their common period, showing a rapid precipitation decrease since 3300 cal yr BP. Thus, we propose that this procedure has great potential for reducing the uncertainties associated with pollen-based quantitative paleoclimatic reconstructions in China.

Teleconnection between Sea Ice in the Barents Sea in June and the Silk Road,Pacific–Japan and East Asian Rainfall Patterns in August

In contrast to previous studies that have tended to focus on the influence of the total Arctic sea-ice cover on the East Asian summer tripole rainfall pattern, the present study identifies the Barents Sea as the key region where the June sea-ice variability exerts the most significant impacts on the East Asian August tripole rainfall pattern, and explores the teleconnection mechanisms involved. The results reveal that a reduction in June sea ice excites anomalous upward air motion due to strong near-surface thermal forcing, which further triggers a meridional overturning wave-like pattern extending to midlatitudes.Anomalous downward motion therefore forms over the Caspian Sea, which in turn induces zonally oriented overturning circulation along the subtropical jet stream, exhibiting the east–west Rossby wave train known as the Silk Road pattern. It is suggested that the Bonin high, a subtropical anticyclone predominant near South Korea, shows a significant anomaly due to the eastward extension of the Silk Road pattern to East Asia. As a possible descending branch of the Hadley cell, the Bonin high anomaly ultimately triggers a meridional overturning, establishing the Pacific–Japan pattern. This in turn induces an anomalous anticyclone and cyclone pair over East Asia, and a tripole vertical convection anomaly meridionally oriented over East Asia. Consequently, a tripole rainfall anomaly pattern is observed over East Asia. Results from numerical experiments using version 5 of the Community Atmosphere Model support the interpretation of this chain of events.

Greenland Ice Sheet Contribution to Future Global Sea Level Rise based on CMIP5 Models

Sea level rise （SLR） is one of the major socioeconomic risks associated with global warming. Mass losses from the Greenland ice sheet （GrIS） will be partially responsible for future SLR, although there are large uncertainties in modeled climate and ice sheet behavior. We used the ice sheet model SICOPOLIS （Simulation COde for POLythermal Ice Sheets） driven by climate projections from 20 models in the fifth phase of the Coupled Model Intercomparison Project （CMIP5） to estimate the GrlS contribution to global SLR. Based on the outputs of the 20 models, it is estimated that the GrIS will contribute 0-16 （0-27） cm to global SLR by 2100 under the Representative Concentration Pathways （RCP） 4.5 （RCP 8.5） scenarios. The projected SLR increases further to 7-22 （7-33） cm with 2~basal sliding included. In response to the results of the multimodel ensemble mean, the ice sheet model projects a global SLR of 3 cm and 7 cm （10 cm and 13 cm with 2~basal sliding） under the RCP 4.5 and RCP 8.5 scenarios, respectively. In addition, our results suggest that the uncertainty in future sea level projection caused by the large spread in climate projections could be reduced with model-evaluation and the selective use of model outputs.

The Relationship between Melt Season Sea Ice over the Bering Sea and Summer Precipitation over Mid-Latitude East Asia

Independent datasets consistently indicate a significant correlation between the sea ice variability in the Bering Sea during melt season and the summer rainfall variability in the Lake Baikal area and Northeastern China.In this study,four sea ice datasets(HadISST1,HadISST2.2,ERA-Interim and NOAA/NSIDC)and two global precipitation datasets(CRU V4.01 and GPCP V2.3)are used to investigate co-variations between melt season(March−April−May−June,MAMJ)Bering Sea ice cover(BSIC)and summer(June−July−August,JJA)East Asian precipitation.All datasets demonstrate a significant correlation between the MAMJ BSIC and the JJA rainfall in Lake Baikal−Northeastern China(Baikal−NEC).Based on the reanalysis datasets and the numerical sensitivity experiments performed in this study using Community Atmospheric Model version 5(CAM5),a mechanism to understand how the MAMJ BSIC influences the JJA Baikal−NEC rainfall is suggested.More MAMJ BSIC triggers a wave train and causes a positive sea level pressure(SLP)anomaly over the North Atlantic during MAMJ.The high SLP anomaly,associated with an anti-cyclonic wind stress circulation anomaly,favors the appearance of sea surface temperature(SST)anomalies in a zonal dipole-pattern in the North Atlantic during summer.The dipole SST anomaly drives a zonally orientated wave train,which causes a high anomaly geopotential height at 500 hPa over the Sea of Japan.As a result,the mean East Asian trough moves westward and a low geopotential height anomaly occurs over Baikal−NEC.This prevailing regional low pressure anomaly together with enhanced moisture transport from the western North Pacific and convergence over Baikal−NEC,positively influences the increased rainfall in summer.

A Preliminary Study on the Relationship Between Arctic Oscillation and Daily SLP Variance in the Northern Hemisphere During Wintertime

In the present study, the authors investigated the relationship between the Arctic Oscillation (AO) and the high-frequency variability of daily sea level pressures in the Northern Hemisphere in winter (November through March), using NCEP/NCAR reanalysis datasets for the time period of 1948/49-2000/01. High-frequency signals are defined as those with timescales shorter than three weeks and measured in terms of variance, for each winter for each grid. The correlations between monthly mean AO index and high-frequency variance are conducted. A predominant feature is that several regional centers with high correlation show up in the middle to high latitudes. Significant areas include mid- to high-latitude Asia centered at Siberia, northern Europe and the middle-latitude North Atlantic east of northern Africa. Their strong correlations can also be confirmed by the singular value decomposition analysis of covariance between mean SLP and high-frequency variance. This indicates that the relationship of AO with daily Sea Level Pressure (SLP) is confined to some specific regions in association with the inherent atmospheric dynamics. In middle-latitude Asia, there is a significant (at the 95% level) trend of variance of-2.26% (10 yr)-1. Another region that displays a strong trend is the northwestern Pacific with a significant rate of change of 0.80% (10 yr)-1. If the winter of 1948/49, an apparent outlier, is excluded, a steady linear trend of +1.51% (10 yr)-1 shows up in northern Europe. The variance probability density functions (PDFs) are found to change in association with different AO phases. The changes corresponding to high and low AO phases, however, are asymmetric in these regions. Some regions such as northern Europe display much stronger changes in high AO years, whereas some other regions such as Siberia show a stronger connection to low AO conditions. These features are supported by ECMWF reanalysis data. However, the dynamical mechanisms involved in the AO-high frequency SLP variance connection have not been well understood, and this needs further study.

Variation in Principal Modes of Midsummer Precipitation over Northeast China and Its Associated Atmospheric Circulation

This study documents the first two principal modes of interannual variability of midsummer precipitation over Northeast China(NEC) and their associated atmospheric circulation anomalies. It is shown that the first principal mode exhibits the largest amount of variability in precipitation over the south of NEC(referred to as the south mode), whereas the second principal mode behaves with the greatest precipitation anomaly over the north of NEC(referred to as the north mode). Further findings reveal that, through modulating moisture transportation and upper-and lower-troposphere divergence circulation as well as vertical movement over NEC, the anomalous northwestern Pacific anticyclone and the anticyclone centered over northern NEC exert the dominant influence on the south and north modes, respectively. Additionally, it is quantitatively estimated that water vapor across the southern boundary of NEC dominates the moisture budget for the south mode, while the north mode has a close connection with moisture through NEC's northern and western boundaries. Furthermore, the north(south) mode is strongly related to the intensity(meridional shift) of the East Asian westerly jet.