Four Types Of Baroclinic Instability:Global Distribution And Seasonal Varibility

Baroclinic instability（BCI）is one of the most important mechnism to generate eddies,and according to vertical structure,BCIs can be classified into four types,and they can imply the types of mesoscale eddies,for example,Charney＿s type BCI with maximum amplitude at surface and Phillips type BCI with maximum amplitude at subsurface,which can predict surface and subsurface eddies,respectively.Therefore,researches about BCI types may bring some new insight into eddies’researches.In this paper,we use the temperature and salinity data,as well as corresponding geostrophic flow from climatologic and seasonal climatologic World Ocean Atlas2013（WOA13）to research global BCIs and their seasonal variations.We first analyze global distributions of four types of BCIs.We find that Eady type BCIs,with maximum amplitude at surface or bottom and minimum amplitude in the middle depth,generally locate in Antarctic Circumpoler Current（ACC）and high latitudes in north hemisphere;the Charney bottom（short for Charney＿b,hereafter）type BCIs,with amplite intensifying at bottom and decaying towards surface,generally scatter around the Eady type BCIs as patches;the surface-intensified Charney surface（short for Charney＿s,hereafter）type BCIs,with amplite intensifying at surface and decaying towards bottom,distribute wildly but primarily occur at subtropics（±10°–35°）;the Phillips type BCIs,with amplitude intensifying at subsurface and decaying towards surface and bottom,primarily occur in low latitudes（±5°–20°）.Furthermore,the BCI types can imply types of mesoscale eddies.The locations of the Phillips type BCIs most overlap the locations of obsevred subsurface eddies,and depths of Phillips type BCIs’maximum amplitude have small difference with core depths of obsevred subsurface eddies.Baroclinic instabilities can propagate as waves,which propate eastward in ACC and mid-latitude（±25°–35°）,and propagate eastward in low latitude（30°S–30°N）and high latitude in north hemisphere（>50°N）.The BCI waves are the interaction of two counter-propagating Rossby waves（CRWs）under the Doppler shift of mean flow.The type of CRWs and depth range of mean flow vary with BCI type.The Eady type BCIs demand first baroclinic Rossby waves and topographic Rossby waves,as well as the full-depth mean flow;the Charney＿s type BCIs demand first baroclinic Rossby wave and internal Rossby wave,as well as mean flow ranging from surface to critical layer;the Charney＿b type BCIs demand topographic Rossby wave and internal Rossby wave,as well as mean flow ranging from bottom to 0.3（normalized depth）above critical layer;the Phillips type BCIs demand second baroclinic Rossby wave and internal Rossby wave,as well as mean flow ranging from 0.5 above and 0.5below critical layer.（Critical layer is right the depth where the two CRWs interact,and also the depth where the phase speed of BCI equals to the mean flow.）The seasonal variations of global BCIs are significant in most regions except ACC,including growth time,wavelenth and types.In this paper,we focus on three typical seasonality of BCIs:transition of dominant BCIs types in the Kuroshio Extension（KE）and Gulf Stream（GS）,seasonal migration of border between the Charney＿s and Phillips type BCIs in the tropical—subtropical system,and occurance of submesoscale BCIs in winter in the subtropical.In KE and GS,the transition of dominant BCI types is due to seasonal variation of upper stratification.The Charney＿s type BCIs previal in winter due to weak upper stratification,the Phillips type BCIs prevail in summer due to strong statification,and the Charney＿s and Phillips type coexist with similar portion in spring and autumn.Correspondingly,the baroclinic conversion rate（BCR）is strong at surface in winter,and it is strong at subsurface in summer,and the BCR at surface is comparable to that at subsurface in spring and autumn.Different with KE and GS regions,it’s the seasonal variations of geostrophic flow other than the upper stratification that dominate the seasonal variations in tropical—subtropical system.In tropical-subtropical system,the boundary of the Phillips type BCIs prevaling in tropical and the Charney＿s type BCIs prevailing in subtropical is generally determined by the boundary of North Equatorial Current（NEC）and Subtropical Countercurrent（STCC）.The boundary migrates equatorward in spring and summer,and retreats poleward in autumn and winter.The EKE induced by BCIs changes as well:the subsurface EKE independent of surface EKE is weak and shrinks in spring and summer,but it’s strong and extends in autumn and winter.The mesoscale BCIs discussed above are choosen when K<5K_def,however,under such criterion,there are submesoscale Charney＿s type BCIs in STCC.To distinguish submesoscale BCIs from mesoscale BCIs,we define a new criterion,K<3K_def.Furthermore,mesoscale BCIs grow fasters in spring and slowest in autumn,which leads the surface EKE by two months;submesoscale BCIs grow fasters in winter,which leads the surface EKE by a month.