Research On Three-Dimensional Structure And Meridional Heat Transport Of Mesoscale Eddies

There are abundant mesoscale eddies all around the global ocean.Eddies usually have strong nonlinearity due to their rotational velocity in the horizontal direction,meanwhile can affect water layers with depth of tens to hundreds of meters vertically.Then eddies have crucial effects on the transports and the three-dimensional distributions of heat and salt in global ocean.However,due to the lack of data,researchers can't obtain the quantitative estimations of the three-dimensional distribution of global ocean heat transport on basis of existing observational materials.Our research analyzed the characteristics of mesoscale eddies in subtropical northwestern Pacific Ocean based on the Chelton global mesoscale eddy dataset.We found that eddies usually move westward,and meanwhile,they tend to move poleward in area south of about 21°N,and move equatorward in area north of about21°N.Besides,eddies'characteristics like their average radius,intensity,amplitude and rotational velocity all present high values at latitudes near 21°N,which is also caused by the local eddy propagation rules.Then we study mesoscale eddies in the region with deviation into two sub-regions according to the 21°N.Using AMSR-E sea surface temperature data and AVISO sea surface height data,we firstly composited the average sea surface eddy structure of temperature anomalies and geostrophic rotational velocities in sub-regions north of and south of 21°N.The compositing structure of eddy SST anomalies show significant asymmetry.Decompositing the average structure,we got a monopole structure and a residual structure close to a dipole pattern.Considering the mechanisms that eddy affecting heat distribution in water,the negative/positive SSTA monopole structures are mainly caused by cyclonic/anticyclonic eddies'elevation/depression of the thermocline,whereas the dipole structures of SSTA are largely caused by eddies rotational advection stirring the isotherms of surrounding water.We evaluated the two components'influence on the eddy-induced SSTA structure by using a definition of relative variance contribution.We found that for both cyclonic eddies and anticyclonic eddies,monopole component plays vital role in area with homogeneous background sea surface temperature field,however the relative variance contribution of dipole component increases in area with intense background sea surface temperature meridional variability.Especially,dipole component of cyclonic eddies show more advantages than monopole component in sub-area north of 21°N.To prove the hypothesis,we reclassified the eddies in the both sub-areas based on the local intensity of sea surface temperature gradient,and composited the eddy-induced SSTA structures.We found that the relative variance contribution of dipole SSTA does increase with the background SST gradient,especially for cyclonic eddies.Besides,we reclassified the eddies by month and recomposited,the result shows that as the background temperature field and horizontal and vertical range od western Pacific warm pool vary seasonally,the values of two SSTA components both show seasonal variabilities.In summer and autumn,the meridional range and thickness of warm pool are larger,and the SST in study region is more homogeneous,so the amplitude of dipole SSTA is smaller and the amplitude of SSTA caused by elevation/depression of thermocline is smaller.In winter and spring,the warm pool retreats southward,the thickness become thinner,the isotherms in study region get intenser and the SST gradient get larger.Then the amplitude of dipole SSTA is larger,and the amplitude of SSTA caused by elevation/depression of thermocline is larger.The conclusions all prove our deviation of eddy-induced SSTA structure and the physical mechanism they represent are reasonable.We did further research of compositing the three-dimensional structure of mesoscale eddies in study region by combining Argo float profiles,and then estimating the time-mean eddy-induced meridional heat transport in study region.We found that the core area of temperature anomaly in sub-area N is closer to sea surface,and show a weaker core structure in layers of about 400-600 meters.However in sub-area S,there is only one core structure of eddy-induced temperature anomaly in layers slightly deeper than sea surfaces.Besides,eddy-induced temperature anomalies and geostrophic velocity anomalies mostly confined within the range of two times of radius from the eddy center.We then estimated the eddy-induced meridional heat fluxes and time-mean heat transports based on the two mechanisms we have devided,we found that the eddy stirring-induced heat flux is about one order larger than trapping-induced heat flux based on the vertical profile of heat flux.The vertically-integrated time-mean meridional stirring-induced heat transport is 2~10times larger than trapping-induced heat transport,and is much larger in sub-area N than in sub-area S.We then composited the sea surface anomaly structures of mesoscale eddies in worldwide ocean,for the purpose of obtaining the global distribution feature of eddy structure.It is shown in the distribution pattern that the areas with high value of eddy-induced sea surface temperature anomalies structures mainly concentrate in the western boundaries of mid-latitude basins and areas associated with the Antarctic Countercurrent.Meanwhile the areas with high value of eddy sea surface rotational velocity also concentrate in these areas.The distribution rule is similar to the distribution of global eddy average amplitude concluded by Chelton.To estimate the eddy-induced sea surface meridional heat flux,which include the flux caused by eddy carrying water and propagate to a distance,and the flux caused by eddy rotational advection stirring the background isotherms.The final result indicated that the sea surface meridional heat flux caused by stirring effect is usually about 5 times of that caused by trapping effect.In region associated with Antarctic Countercurrent,trapping effect of cyclonic eddies and anticyclonic eddies both cause southward meridional heat flux.However in other basins,trapping effect of two kinds of polarities usually are with opposite directions,and largely offset each other.So considering all eddies,stirring-induced meridional heat flux plays vital role in most basins.Finally,we used refined methods,obtain an observation-based three-dimensional picture of how oceanic eddies contribute to the global heat transport by analyzing millions of high-quality Argo hydrographic profiles and high-resolution satellite altimetric data.We first presented the spatial differences of individual eddies by reconstructing254 representative eddies all over the ocean,and then calculated heat fluxes associated with eddies in 5°×5°boxes.It is revealed that all parameters of eddies vary significantly with both latitudes and longitudes,which is crucial in yielding spatially varying heat fluxes and transports.The eddies not only transport heat towards high latitudes(down-gradient),but also towards low latitudes(up-gradient),particularly at subsurface layers of mid-latitude northern Pacific Ocean and low-latitude Atlantic Ocean.The eddy heat transport is mainly confined in the upper 1000 m of the western part and mid-latitudes of the world's ocean basins,coinciding with maximum meridional temperature gradients.It peaks at 0.8 PW and 0.3 PW(1 PW=10~155 W)at45°S and 35°N,respectively,stronger than previous estimates based on model results,and accounts for about one half and one third of the estimated total oceanic heat transport at the same latitudes,respectively.In any location except for the areas associated with the Antarctic Circumpolar Current,the eddy stirring component is distinctly(1-10 times)larger than the eddy trapping component.