Subseasonal transition of Barents-Kara sea-ice anomalies in winter related to the reversed warm Arctic-cold Eurasia pattern

“暖北极-冷欧亚”模态(WACE)的次季节反转会触发中国东部冬季的极端冷/暖转换以及春季沙尘暴.在这一显著的高纬度大气模态反转的驱动下,巴伦支海-喀拉海(BKS)的海冰异常也表现出次季节转换.当北极变暖以及乌拉尔山高压增强时,BKS异常向下的湍流热通量和增强的向下红外辐射有利于海冰融化;近地面的异常偏南风使海冰从薄冰区向多年冰区漂移,进一步扩大了开阔洋面.相反的机制出现在WACE反位相,导致BKS海冰异常偏多.当WACE次季节反转时,上述机制分别发生在前冬和后冬,使得BKS海冰异常出现显著的次季节转换.近10年随着WACE反转频率增多,BKS海冰前后冬的次季节变化增强.本文构建了WACE次季节反转的综合示意图,有助于更好地理解和预测中国东部极端气候.

Atmospheric responses over Asia to sea ice loss in the Barents and Kara seas in mid late winter and early spring:a perspective revealed from CMIP5 data

This study investigated atmospheric responses in mid late winter and early spring to sea ice loss in the Barents and Kara seas using regressions of the January March mean atmosphere on Barents and Kara sea ice area in November and December.Similar atmospheric circulation responses were obtained from reanalysis data and multimodel ensemble results from the Coupled Model Intercomparison Project Phase 5,i.e.,sea ice anomalies are the dominant factor driving the overlying atmosphere.The results showed that an Arctic Asia dipole structure,with opposite anomalies over the mid-latitudes of Asia and over the adjoining Arctic,appears to be the key atmospheric circulation anomaly influencing the East Asian climate in mid late winter and early spring.

Simulations and Projections of Winter Sea Ice in the Barents Sea by CMIP6 Climate Models

Dramatic changes in the sea ice characteristics in the Barents Sea have potential consequences for the weather and climate systems of mid-latitude continents,Arctic ecosystems,and fisheries,as well as Arctic maritime navigation.Simulations and projections of winter sea ice in the Barents Sea based on the latest 41 climate models from the Coupled Model Intercomparison Project Phase 6(CMIP6)are investigated in this study.Results show that most CMIP6 models overestimate winter sea ice in the Barents Sea and underestimate its decreasing trend.The discrepancy is mainly attributed to the simulation bias towards an overly weak ocean heat transport through the Barents Sea Opening and the underestimation of its increasing trend.The methods of observation-based model selection and emergent constraint were used to project future winter sea ice changes in the Barents Sea.Projections indicate that sea ice in the Barents Sea will continue to decline in a warming climate and that a winter ice-free Barents Sea will occur for the first time during 2042-2089 under the Shared Socioeconomic Pathway 585(SSP5-8.5).Even in the observation-based selected models,the sensitivity of winter sea ice in the Barents Sea to global warming is weaker than observed,indicating that a winter ice-free Barents Sea might occur earlier than projected by the CMIP6 simulations.

The Monsoon over the Barents Sea and Kara Sea

In the Arctic (mainly in its European sector) there is statistically detectable seasonal reversal wind pattern. The combination of seasonally warm (cold) land surfaces in arctic areas together with cool (cool) sea surface of Arctic seas not covered by ice is conducive to the formation of a monsoon like system. On the other hand, the predominance of the cyclonic regime during all seasons makes it difficult to answer the question of whether the Arctic region belongs to the monsoon type pattern. In this study, the monsoon features of atmospheric circulation over the Barents and Kara Seas were analysed. To extract specific monsoon signs, atmospheric circulation systems (separately for areas of each sea) were divided into ten weather types. Their appearance and statistics were compared with indicators of regional circulation. A significant part of intra-annual monsoon variability is associated with the configuration of such modes as the North Atlantic Oscillation and the Scandinavia teleconnection patterns. For example, during the winter season, the monsoon currents (from land to sea) occur only with a positive North Atlantic Oscillation index. With the prevalence of other modes of variability, the direction of the winds can be different, and the regular monsoon circulation pattern is changed by chaotic regime. In summer, northern streams (from sea to land) are realized on the western periphery of cyclones, regenerating and stabilizing over the Kara Sea. As for anomalies, the nature of the monsoons is manifested in the statistics of extreme winds even without selecting data on the regimes of variability. So, in winter, maximum speeds fall on the southern streams, and in the summer—on the northern ones. Large precipitation anomalies during all seasons, as one would expect, are encountered most often with the cyclonic type of circulation.

Possible Impacts of Barents Sea Ice on the Eurasian Atmospheric Circulation and the Rainfall of East China in the Beginning of Summer

Possible influences of the Barents Sea ice anomalies on the Eurasian atmospheric circulation and the East China precipitation distribution in the late spring and early summer (May-June) are investigated by analyzing the observational data and the output of an atmospheric general circulation model (AGCM). The study indicates that the sea ice condition of the Barents Sea from May to July may be interrelated with the atmospheric circulation of June. When there is more than average sea ice in the Barents Sea, the local geopotential height of the 500-hPa level will decrease, and the same height in the Lake Baikal and Okhotsk regions will increase and decrease respectively to form a wave-chain structure over North Eurasia. This kind of anomalous height pattern is beneficial to more precipitation in the south part of East China and less in the north.

Evolution of the North Atlantic Current and Barents Ice Sheet as revealed by grain size populations in the northern Norwegian Sea during the last 60 ka

The grain size distribution of bulk sediment samples was decomposed in a core to reconstruct paleoceanographic evolution over the past 60 ka in the northern Norwegian Sea.The results show that sediments consisted of 3–4 grain populations derived from the North Atlantic Current(NAC)and Barents Ice Sheet(BIS).The grain size data suggest three palaeoceanographic evolution stages:(1)an environment affected by BIS and NAC and changed with the interstadial/stadial transition in phase with the Greenland ice-core record at 60–31 ka BP,during which discharge of icebergs and the content of the coarsest population containing ice-rafted debris(IRD)in the sediments increased significantly during stadial,while the fine silt population containing volcanic glasses increased with the enhancement of NAC during the interstadial;(2)an extreme environment controlled by BIS at31–13 ka BP.BIS reached to its maximum at about 31 ka BP and the turbid plumes that formed at the leading edge of BIS contributed to a significant increase in the clayey population in sediments.Icebergs drained into the northern Norwegian Sea with periodical calving of the BIS at 31–19 ka BP.Subsequently,the ablation of the BIS discharged massive floods with clayey sediments and icebergs into the Norwegian Sea at 19–13 ka BP,resulting in a constant increase in clay and IRD in sediments;and(3)a marine environment similar to the present one under the strong influence of NAC following the complete melting of the BIS after 13 ka BP,NAC is the dominant transport agent and no IRD occurred in sediments.The fine silt populations containing volcanic glasses transported by NAC significantly increased.

A numerical simulation of the generation and evolution of nonlinear internal waves across the Kara Strait

Nonlinear internal waves(NIWs) are ubiquitous around the Kara Sea, a part of the Arctic Ocean that is north of Siberia. Three hot spot sources for internal waves, one of which is the Kara Strait, have been identified based on Envisat ASAR. The generation and evolution of the NIWs through the interactions of the tide and topography across the strait is studied based on a nonhydrostatic numerical model. The model captures most wave characteristics shown by satellite data. A typical inter-packets distance on the Barents Sea side is about 25 km in summer, with a phase speed about 0.65 m/s. A northward background current may intensify the accumulation of energy during generation, but it has little influence on the other properties of the generated waves. The single internal solitary wave(ISW) structure is a special phenomenon that follows major wave trains, with a distance about 5–8 km. This wave is generated with the leading wave packets during the same tidal period. When a steady current toward the Kara Sea is included, the basic generation process is similar, but the waves toward the Kara Sea weaken and display an internal bore-like structure with smaller amplitude than in the control experiment. In winter, due to the growth of sea ice, stratification across the Kara Strait is mainly determined by the salinity, with an almost uniform temperature close to freezing. A pycnocline deepens near the middle of the water depth(Barents Sea side), and the NIWs process is not as important as the NIWs process in summer. There is no fission process during the simulation.

Mechanisms for the link between onset and duration of open water in the Kara Sea

The sea ice conditions in the Kara Sea have important impacts on Arctic shipping,oil and gas production,and marine environmental changes.In this study,sea ice coverage(C_(R))less than 30%is considered as open water,its onset and end dates are defined as T_(open)and T_(close),respectively.The sea ice melt onset(T_(melt))is defined as the date when ice-sea freshwater flux initially changes from ice into the ocean.Satellite-based sea ice concentration(SIC)from 1989 to 2019 shows a negative correlation between T_(open)and T_(close)(r=-0.77,p<0.01)in the Kara Sea.This phenomenon is also obtained through analyzing the hindcast simulation from 1994 to 2015 by a coupled ocean and sea-ice model(NAPA1/4).The model results reveal that thermodynamics dominate the sea ice variations,and ice basal melt is greater than the ice surface melt.Heat budget estimation suggests that the heat flux is significant correlated with T_(open)(r=-0.95,p<0.01)during the melt period(the duration of multi-year averaged T_(melt)to T_(open))influenced by the sea ice conditions.Additionally,this heat flux is also suggested to dominate the interannual variation of the heat input during the whole heat absorption process(r=0.81,p<0.01).The more heat input during this process leads to later T_(close)(r=0.77,p<0.01).This is the physical basis of the negative correlation between T_(open)and T_(close).Therefore,the duration of open water can be predicted by T_(open)and thence support earlier planning of marine activities.

Changes in Barents Sea Ice Edge Positions in the Last 442 Years. Part 2: Sun, Moon and Planets

This is the second paper in a series of two, which analyze the position of the Barents Sea ice-edge (BIE) based on a 442-year long dataset to understand its time variations. The data have been collected from ship-logs, polar expeditions, and hunters in addition to airplanes and satellites in recent times. Our main result is that the BIE position alternates between a southern and a northern position followed by Gulf Stream Beats (GSBs) at the occurrence of deep solar minima. We decompose the low frequency BIE position variations in cycles composed of dominant periods which are related to the Jose period of 179 years, indicating planetary forcings. We propose that the mechanism transferring planetary signals into changes in BIE position is the solar wind (SW), which provides magnetic shielding of the Earth in addition to geomagnetic disturbances. Increase in the solar wind produces pressure which decelerates the Earth’s rotation. It also transfers electrical energy to the ring current in the earth’s magnetosphere. This current magnetizes the earth’s solid core and makes it rotate faster. To conserve angular momentum the earth’s outer fluid mantle rotates slower with a delay of about 100 years. In addition will geomagnetic storms, initiated by solar coronal mass ejections (CMEs) penetrate deep in the Earth’s atmosphere and change pressure pattern in the Arctic. This effect is larger during solar minima since the magnetic shielding then is reduced. The Arctic may then experience local warming. The transition of solar activities to a possibly deep and long minimum in the present century may indicate Arctic cooling and the BIE moving south this century. For the North Atlantic region, effects of the BIE expanding southward will have noticeable consequences for the ocean bio-production from about 2040.

A Parameterization Scheme for Wind Wave Modules that Includes the Sea Ice Thickness in the Marginal Ice Zone

The global wave model WAVEWATCH III®works well in open water.To simulate the propagation and attenuation of waves through ice-covered water,existing simulations have considered the influence of sea ice by adding the sea ice concentration in the wind wave module;however,they simply suppose that the wind cannot penetrate the ice layer and ignore the possibility of wind forcing waves below the ice cover.To improve the simulation performance of wind wave modules in the marginal ice zone(MIZ),this study proposes a parameterization scheme by directly including the sea ice thickness.Instead of scaling the wind input with the fraction of open water,this new scheme allows partial wind input in ice-covered areas based on the ice thickness.Compared with observations in the Barents Sea in 2016,the new scheme appears to improve the modeled waves in the high-frequency band.Sensitivity experiments with and without wind wave modules show that wind waves can play an important role in areas with low sea ice concentration in the MIZ.

Impact of interannual variations of spring sea ice in the Barents Sea on East Asian rainfall in June

This study reveals a significant relationship, on the interannual timescale, between a dipole mode, the second leading mode, of spring sea-ice anomalies in the Barents Sea and the following-summer rainfall in East Asia. Related to the dipole mode, with the heavier sea ice in the north and lighter sea ice in the southeast Barents Sea in spring, the East Asian summer subtropical rainy belt tends to move northward. The significant relationship is established through a wave train over northern Eurasia in the lower troposphere in June. The wave train enhances the northern East Asian low, which induces more rainfall to the north of the East Asian subtropical rainy belt and then attracts the subtropical rainy belt to move northward. This study suggests that the dipole mode of spring sea-ice anomalies in the Barents Sea may be a good precursor for the prediction of East Asian summer rainfall.

New methods for processing and interpreting marine magnetic anomalies:Application to structure,oil and gas exploration,Kuril forearc,Barents and Caspian seas

New methods are presented for processing and interpretation of shallow marine differential magnetic data, including constructing maps of offshore total magnetic anomalies with an extremely high reso- lution of up to 1-2 nT, mapping weak anomalies of 5-10 nT caused by mineralization effects at the contacts of hydrocarbons with host rocks, estimating depths to upper and lower boundaries of anom- alous magnetic sources, and estimating thickness of magnetic layers and boundaries of tectonic blocks. Horizontal dimensions of tectonic blocks in the so-called ＂seismic gap＂ region in the central Kuril Arc vary from 10 to 100 km, with typical dimensions of 25-30 km. The area of the ＂seismic gap＂ is a zone of intense tectonic activity and recent volcanism. Deep sources causing magnetic anomalies in the area are similar to the ＂magnetic belt＂ near Hokkaido. In the southern and central parts of Barents Sea, tectonic blocks with widths of 30-100 kin, and upper and lower boundaries of magnetic layers ranging from depths of 10 to 5 km and 18 to 30 km are calculated. Models of the magnetic layer underlying the Mezen Basin in an inland part of the White Sea-Barents Sea paleorift indicate depths to the lower boundary of the layer of 12-30 km. Weak local magnetic anomalies of 2-5 nT in the northern and central Caspian Sea were identified using the new methods, and drilling confirms that the anomalies are related to concentrations of hydrocarbon. Two layers causing magnetic anomalies are identified in the northern Caspian Sea from magnetic anomaly spectra. The upper layer lies immediately beneath the sea bottom and the lower layer occurs at depths between 30-40 m and 150-200 m.

Changes in Barents Sea ice Edge Positions in the Last 440 years: A Review of Possible Driving Forces

This is the first report of the Barents Sea Ice Edge (BIE) project. The BIE position has varied between latitude 76°N and above 82°N during the last 440 years. During the period 10,000 to 6000 years ago, Arctic climate was significantly warmer than today. We review various oceanic and atmospheric factors that may have an effect on the BIE position. The Gulf Stream beat with respect to alternations in flow intensity and N-S distribution plays a central role for the changes in climate and BIE position during the last millennium. This occurred in combination with external forcing from total solar irradiation, Earth’s shielding strength, Earth’s geomagnetic field intensity, Earth’s rotation, jet stream changes;all factors of which are ultimately driven by the planetary beat on the Sun, the Earth and the Earth-Moon system. During the last 20 years, we see signs of changes and shifts that may signal the end of the late 20th century warm period. The BIE position is likely to start advancing southward in next decade.

Impact of the Barents Sea SST in Autumn on the Winter Climate in Northeast China

We studied effects of sea surface temperature anomaly (SSTA) in the Barents Sea in autumn on the atmospheric circulation in northeast China in winter, using the NCEP reanalysis data and sea surface temperature (SST) data of the Hadley Center. The results show that the ocean thermal conditions in the Barents Sea in autumn can be used as an important reference factor for predicting the cold air activity in China. When the sea surface temperature anomaly of the Barents Sea elevated in the autumn, the sea-level pressure anomaly elevated in eastern China on December, northeast China and southeastern Russia on January and February. In the years when the SSTA of the Barents Sea elevated in the autumn, the abnormal high-pressure ridge developed over Europe, and the geopotential height in western China appeared negative anomaly at 500 hPa. At 1000 hPa, the Mongolia high-pressure increased and the northerly airflow strengthened the cold high-latitude air broke out to the south, which was easy to affect northeast and north of China. In negative SSTA years, the high-pressure ridge was west to the north Atlantic, and the geopotential height in central and northern Siberia appeared negative anomaly at 500 hPa;the Mongolia high-pressure was weakened at 1000 hPa.

Intensity of Level Ice Simulated with the CICE Model for Oil-Gas Exploitation in the Southern Kara Sea, Arctic

Sea ice is the predominant natural threat to marine structures and oil-gas exploitation in the Arctic.However,for ice-resistant structural design,long-term successive level ice thickness measurements are still lacking.To fill this gap in the southern Kara Sea,the Los Alamos Sea Ice Model(CICE)is applied to achieve better simulation at the local and regional scales.Based on the validation against ice thickness observations in March and April in 1980-1986,the statistical root-mean-square error is determined to be less than 0.2 m.Then,based on the hindcast data,the spatiotemporal distributions of level ice thickness are analyzed annually,seasonally,and monthly,with thicker level ice of 1.2-1.5 m in spring and large ice-free zones in September and October.For floating platforms,a novel ice grade criterion with five classifications,namely,excellent,good,moderate,severe,and catastrophic,is pro-posed.The first two grades are most suitable for offshore activities,particularly from August to October,and the moderate grade is acceptable if with ice-resistant protections.Furthermore,hostile ice conditions are discussed in terms of the generalized extreme value distribution.The statistics reveal that at a return period of 100 yr,extreme level ice is primarily between 0.6 m and 1.0 m in December.The present investigation could be a useful reference for a feasibility study of the potential risk analysis and ice-resistant operation of oil-gas exploitation in the Arctic.

Extreme Values of Wind Speed over the Kara Sea Based on the ERA5 Dataset

Extreme values of wind speed were studied based on the highly detailed ERA5 dataset covering the central part of the Kara Sea. Cases in which the ice coverage of the cells exceeded 15% were filtered. Our study shows that the wind speed extrema obtained from station observations, as well as from modelling results in the framework of mesoscale models, can be divided into two groups according to their probability distribution laws. One group is specifically designated as black swans, with the other referred to as dragons (or dragon-kings). In this study we determined that the data of ERA5 accurately described the swans, but did not fully reproduce extrema related to the dragons;these extrema were identified only in half of ERA5 grid points. Weibull probability distribution function (PDF) parameters were identified in only a quarter of the pixels. The parameters were connected almost deterministically. This converted the Weibull function into a one-parameter dependence. It was not clear whether this uniqueness was a consequence of the features of the calculation algorithm used in ERA5, or whether it was a consequence of a relatively small area being considered, which had the same wind regime. Extremes of wind speed arise as mesoscale features and are associated with hydrodynamic features of the wind flow. If the flow was non-geostrophic and if its trajectory had a substantial curvature, then the extreme velocities were distributed according to a rule similar to the Weibull law.

Assessment of Arctic sea ice simulations in CMIP5 models using a synthetical skill scoring method

The Arctic sea ice cover has declined at an unprecedented pace since the late 20th century. As a result, the feedback of sea ice anomalies for atmospheric circulation has been increasingly evidenced. While climatic models almost consistently reproduced a decreasing trend of sea ice cover, the reported results show a large distribution. To evaluate the performance of models for simulating Arctic sea ice cover and its potential role in climate change, this study constructed a reasonable metric by synthesizing both linear trends and anomalies of sea ice. This study particularly focused on the Barents Sea and the Kara Sea, where sea ice anomalies have the highest potential to affect the atmosphere. The investigated models can be grouped into three categories according to their normalized skill scores. The strong contrast among the multi-model ensemble means of different groups demonstrates the robustness and rationality of this method. Potential factors that account for the different performances of climate models are further explored. The results show that model performance depends more on the ozone datasets that are prescribed by the model rather than on the chemical representation of ozone.

Links between winter dust over the Tibetan Plateau and preceding autumn sea ice variability in the Barents and Kara Seas

The Tibetan Plateau(TP)is characterized by heavily local dust activities,however,the mechanism of interannual variations of winter dust frequency over the TP remain poorly understood.Previous studies showed the autumn Arctic sea ice could significantly influence the winter climate over Eurasia.Whether autumn sea ice affects winter dust activity over the TP or not?Here,we used an integrated surface database to investigate possible mechanisms for interannual variability in the frequency of winter dust events above the TP.This variability,which is thought to be mainly caused by local dust emissions,shows significant correlations with sea ice concentration(SIC)in the Barents and Kara Seas during the preceding autumn.Low Barents-Kara SIC is accompanied by reduced snow depth over northern Eurasia between autumn and winter,which can enhance the Eurasian mid-latitude westerly jet stream.This strengthening increases the cyclogenesis and occurrence of strong surface wind speeds in winter,especially over the TP.In addition,a lower SIC is closely associated with reduced precipitation and snow cover in late autumn and winter over the TP,which in turn enhances warming of the land surface and reduces the area of frozen ground.These anomalies in atmospheric circulation patterns and local surface conditions promote dust events above the TP during winter.The ensemble means of Atmospheric Model Intercomparison Project experiments from Phase 6 of the Coupled Model Inter-comparison Project and the Community Atmosphere Model version 4 can generally reproduce the atmospheric circulation anomalies associated with decreased Barents-Kara SIC.This study reveals the crucial effect that SIC anomalies in the Barents and Kara Seas have on winter dust activities over the TP.

Decadal Change of the Linkage between Sea Ice over the Barents–Kara Seas in November– December and the Stratospheric Polar Vortex in Subsequent January

The linkage between the sea ice concentration(SIC)over the Barents–Kara Seas in November–December(SIC_BKS_ND)and the stratospheric polar vortex(SPV)in subsequent January(SPV_Jan)is investigated.It is found that SIC_BKS_ND is positively(negatively)correlated with SPV_Jan for the period 1979–1995(1996–2009).Further analyses reveal that,during 1979–1995(1996–2009),SIC_BKS_ND is relatively higher(lower),accompanied by smaller(larger)interannual variability with its center shifting northwest(southeast).Meanwhile,the polar front jet waveguide is relatively stronger(weaker).The simultaneous anomalous eastward-propagating Rossby waves excited by anomalously low SIC_BKS_ND are stronger(weaker),which results in the stronger(weaker)negative–positive–negative wave-train structure of geopotential height anomalies over Eurasia,with the location of these anomalous height centers shifting remarkably westward(eastward).Such changes tend to enhance(suppress)vertically propagating tropospheric planetary waves into the lower stratosphere at high-latitude via constructive(destructive)interference of anomalous tropospheric wave-train structure with the climatological planetary waves,subsequently weakening(strengthening)SPV_Jan.However,in conjunction with anomalously high SIC_BKS_ND,the interference of the tropospheric wave-train structure anomalies and their climatologies shows an opposite distribution to that of low SIC_BKS_ND anomalies,which leads to a strong(weak)SPV_Jan anomaly during 1979–1995(1996–2009).

The marine environmental evolution in the northern Norwegian Sea revealed by foraminifera during the last 60 ka

Both planktonic and benthic foraminifera were identified in a sediment core collected from the northern Norwegian Sea to reconstruct the paleoceanographic evolution since the last glaciation.The assemblages and distribution patterns of dominant foraminiferal species with special habitat preferences indicated that three marine environments occurred in the northern Norwegian Sea since 62 ka BP:(1)an environment controlled by the circulation of the North Atlantic Current(NAC);(2)by polynya-related sinking of brines and upwelling of intermediate water surrounding the polynya;(3)by melt-water from Barents Sea Ice Sheet(BSIS).At 62-52.5 ka BP,a period with the highest summer insolation during the last glaciatial period,intensification of the NAC led to higher absolute abundances and higher diversity of foraminiferal faunas.The higher abundance of benthic species Cibicidoides wuellerstorfi indicates bottom water conditions that were well-ventilated with an adequate food supply;however,higher abundances of polar planktonic foraminiferal species Neogloboquadrina pachyderma(sin.)indicate that the near-surface temperatures were still low.During mid-late Marine Isotope Stage(MIS)3(52.5-29 ka BP),the marine environment of the northern Norwegian Sea alternately changed among the above mentioned three environments.At 29-17ka BP during the last glacial maximum,the dominant benthic species Bolivina arctica from the Arctic Ocean indicates an extreme cold bottom environment.The BSIS expanded to its maximum extent during this period,and vast polynya formed at the edge of the ice sheet.The sinking of brines from the formation of sea ice in the polynyas caused upwelling,indicated by the upwelling adapted planktonic species Globigerinita glutinata.At 17-10 ka BP,the northern Norwegian Sea was controlled by melt-water.With the ablation of BSIS,massive amounts of melt water discharged into the Norwegian Sea,resulting in strong water column stratification,poor ventilation,and an oligotrophic bottom condition,which ledto a drastic decline in the abundance and diversity of foraminifera.At 10-0 ka BP,the marine environment was transformed again by the control of the NAC,which continues to modern day.The abrupt decrease in relative abundance of Neogloboquadrina pachyderma(sin.)indicates a rise in near-surface temperature with the strengthening of the NAC and without the influence of the BSIS.