Impact of global sea surface temperature on the recent early winter Arctic tropospheric warming in coordinated large ensemble simulations

Coordinated numerical ensemble experiments with six different state-of-the-art atmosphere models were used to evaluate and quantify the impact of global SST(from reanalysis data)on the early winter Arctic warming during 1982-2014.Two sets of experiments were designed:in the first set(EXP1),OISSTv2 daily sea-ice concentration and SST variations were used as the lower boundary forcing,while in the second set(EXP2)the SST data were replaced by the daily SST climatology.In the results,the multi-model ensemble mean of EXP1 showed a nearsurface(~850 hPa)warming trend of 0.4℃/10 yr,which was 80%of the warming trend in the reanalysis.The simulated warming trend was robust across the six models,with a magnitude of 0.36-0.50℃/10 yr.The global SST could explain most of the simulated warming trend in EXP1 in the mid and low troposphere over the Arctic,and accounted for 58%of the simulated near-surface warming.The results also suggest that the uppertropospheric warming(~200 hPa)over the Arctic in the reanalysis is likely not a forced signal;rather,it is caused by natural climate variability.The source regions that can potentially impact the early winter Arctic warming are explored and the limitations of the study are discussed.

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.

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.

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.

A Sensitivity Study of Arctic Ice-Ocean Heat Exchange to the Three-Equation Boundary Condition Parametrization in CICE6

In this study,we perform a stand-alone sensitivity study using the Los Alamos Sea ice model version 6(CICE6)to investigate the model sensitivity to two Ice-Ocean(IO)boundary condition approaches.One is the two-equation approach that treats the freezing temperature as a function of the ocean mixed layer(ML)salinity,using two equations to parametrize the IO heat exchanges.Another approach uses the salinity of the IO interface to define the actual freezing temperature,so an equation describing the salt flux at the IO interface is added to the two-equation approach,forming the so-called three-equation approach.We focus on the impact of the three-equation boundary condition on the IO heat exchange and associated basal melt/growth of the sea ice in the Arctic Ocean.Compared with the two-equation simulation,our three-equation simulation shows a reduced oceanic turbulent heat flux,weakened basal melt,increased ice thickness,and reduced sea surface temperature(SST)in the Arctic.These impacts occur mainly at the ice edge regions and manifest themselves in summer.Furthermore,in August,we observed a downward turbulent heat flux from the ice to the ocean ML in two of our three-equation sensitivity runs with a constant heat transfer coefficient(0.006),which caused heat divergence and congelation at the ice bottom.Additionally,the influence of different combinations of heat/salt transfer coefficients and thermal conductivity in the three-equation approach on the model simulated results is assessed.The results presented in this study can provide insight into sea ice model sensitivity to the three-equation IO boundary condition for coupling the CICE6 to climate models.

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.

Diagnosing Ocean Tracer Transport from Sellafield and Dounreay by Equivalent Diffusion and Age

A simple approach for estimating the equivalent diffusion for diagnosing tracer transport is proposed. Two different expressions are derived; one is based directly on an analytical solution of the two-dimensional advection-diffusion equation, the other uses the variance of the tracer distribution. To illustrate some features of the equivalent diffusion and possible applications thereof, idealized releases of passive tracers from the nuclear fuel reprocessing plants at Sellafield in the Irish Sea and Dounreay on the northern coast of Scotland have been simulated with a regional isopycnic co-ordinate Ocean General Circulation Model. Both continuous and pulse releases are considered; the former being representative of the actual historical discharges from the reprocessing plants, the latter resembling an accidental scenario. Age tracers are included to calculate the mean time elapsed since the tracers left their source regions. It is found that in the Nordic Seas the age of tracers from Dounreay is approximately 2 years younger than the age from Sellafield. Although tracers from both sources eventually end up along the same transport routes, significant qualitative differences regarding the dispersion properties are found. It is argued that one single parameter, the equivalent horizontal diffusion, which is estimated to be in the range of 20 56 m^2 s^-1 from Sellafield and 170-485 m^2 s^-1 from Dounreay, determines these differences.

Modulation of the Aleutian–Icelandic Low Seesaw and Its Surface Impacts by the Atlantic Multidecadal Oscillation

Early studies suggested that the Aleutian–Icelandic low seesaw(AIS) features multidecadal variation. In this study, the multidecadal modulation of the AIS and associated surface climate by the Atlantic Multidecadal Oscillation(AMO) during late winter(February–March) is explored with observational data. It is shown that, in the cold phase of the AMO(AMO|-),a clear AIS is established, while this is not the case in the warm phase of the AMO(AMO|+). The surface climate over Eurasia is significantly influenced by the AMO’s modulation of the Aleutian low(AL). For example, the weak AL in AMO|-displays warmer surface temperatures over the entire Far East and along the Russian Arctic coast and into Northern Europe,but only over the Russian Far East in AMO|+. Similarly, precipitation decreases over central Europe with the weak AL in AMO|-, but decreases over northern Europe and increases over southern Europe in AMO|+.The mechanism underlying the influence of AMO|-on the AIS can be described as follows: AMO|-weakens the upward component of the Eliassen–Palm flux along the polar waveguide by reducing atmospheric blocking occurrence over the Euro–Atlantic sector, and hence drives an enhanced stratospheric polar vortex. With the intensified polar night jet, the wave trains originating over the central North Pacific can propagate horizontally through North America and extend into the North Atlantic, favoring an eastward-extended Pacific–North America–Atlantic pattern, and resulting in a significant AIS at the surface during late winter.

Stratospheric Precursor of Non-Uniform Variation in Early Spring Surface Temperature over Eurasia

The stratospheric influences on the non-uniform variation in early spring（March–April,MA）surface temperature over Eurasia is investigated based on the ERA-Interim,NCEP-1,and NCEP-2 reanalysis data for the period1980–2016.A lead–lag correlation is found between preceding winter（December–February,DJF）stratospheric polar vortex displacements（SPVD）and the MA west–east seesaw pattern in surface temperature over Eurasia.Further analysis reveals that the East Asian jet stream may act as a bridge linking DJF SPVD and MA surface temperature over Eurasia.A positive change in SPVD is associated with a decelerated polar jet stream and an accelerated East Asian jet stream in the troposphere in DJF.The East Asian jet stream signal can persist into MA.As a result,anomalous southerly/northerly winds prevail over western/eastern Eurasia,accounting for the west–east surface temperature seesaw over Eurasia.