Influence Of Local Air-sea Interaction On Ocean Temperature In The North Atlantic During Glacial Periods

Global climate change has been a hot research topic in recent years,and the millennium time-scale abrupt climate event(also known as Dansgaard-Oeschger event)in the last glacial has also been paid widespread attention.Previous numerical modeling studies show that the amplitude of interannual-interdecadal climate variability in North Atlantic high latitudes during the cold stadial was much greater than that during the warm interstadial,but its dynamic mechanism still remains unclear.In addition,paleoclimatic records show that the sea surface temperature(SST)changes in some high latitude regions in the Northern Hemisphere during stadial are similar to those in Antarctica and subtropical North Atlantic,indicating local differences in SST in the North Atlantic.In order to address the above two scientific questions,the study explores the amplification mechanism of interannual-interdecadal climate variability and the causes of the gradual signal in the North Atlantic Ocean by utilizing a coupled atmosphere-ocean general circulation model(COSMOS)and setting three typical sensitivity experiments of external forcing(linearly increasing atmospheric CO₂concentration,freshwater injection,and increasing the height of northern hemisphere ice sheets)at stadial states.The results reveal that the high-value areas of climate variability amplitude during the stadialare distributed in the southwestern Icelandic Sea,and high-value areas for the interstadial are located in the Nordic Sea.Seasonal sea ice is present in all high-value regions.The region with the most pronounced gradual signal is also located in southwestern Iceland,illustrating itis the most prominent feature in the linearly increasing atmospheric CO₂concentration experiment.Mechanistic studies of the above characteristics show that seasonal sea ice in the study area,driven by experimental external forcing,can influence the high-frequency climate variability amplitude by controlling the ocean-atmosphere heat transport.During the stadial,sea subsurface temperature gradually warms up due to the northward transport of tropical warm water masses with gradual signals from low latitudes,weakening the vertical density stratification of the ocean.And the increase in temperature led to an upwelling trend of warm water mass in the subsurface layer,which promotes sea ice melting and SST increase.The increasing SST in turn strengthened the sea surface low pressure.The updraft caused by sea surface heat exchange will stimulate the negative variable pressure anomaly,and the associated cyclonic wind shear can further accelerate the vertical mixing of seawater through Ekman pumping,forming a positive sea-ice-atmosphere feedback.As the heat release gradually ceases,the sea ice will regrow without the replenishment of warm water mass,isolating the ocean from the atmosphere.This process weakens the low-pressure system and stimulates anti-cyclonic wind anomaly,and promotes subsurface heat accumulation,providing conditions for the next feedback process.Since the study area is located in the center of Icelandic low,the vertical mixing is relatively intense,and therefore the SST in the southwestern Iceland is characterized by a local gradual signal.During the interstadial,the ocean can exchange heat with the atmosphere in a quasi-equilibrium state and the amplitude of climate variability decreases significantly with the disappearance of seasonal sea ice from southwestern Iceland.Therefore,seasonal sea ice plays a vital role in the accumulation and release of subsurface heat,shedding light on our understanding of amplification of interannual-interdecadal climate variability during cold stadial.The results also combine the climate changes on different timescales.As long as the model runs long enough,millennial-scale changes in extrinsic forcing will be accompanied by high-frequency climate variability,and the coupled ocean-atmosphere process is the link between them.In addition,the interannual-interdecadal climate variability in the study area explains the divergence in the reconstruction records of the paleoclimate during the stadial,and such SST fluctuations also prove the universality of the conclusions.These processes and feedbacks invoked here might provide a new insight for the future studies of sediments and palaeoceanographic ocean temperature reconstruction.