How the ratio of meridional flux of fresh-water to flux of heat fixes the latitude where low salinity intermediate water sinks

Transcription

1 Tellus (198), 32, SHORT CONTRIBUTION How the rtio of eridionl flux of fresh-wter to flux of het fixes the ltitude where low slinity interedite wter sinks By HNRY STOMML, Woods Hole Ocenogrphic Institution, Woods Hole, Msschusetts, 2543 u.s.a. (Mnuscript received Mrch 11; in finl for My 19, 198) ABSTRACT Prescription of the eridionl fluxes of fresh-wter nd het fixes lower liits on the gnitude of eridionl volue fluxes, the direction nd reltive depths t which they occur, nd the ltitude t which low slinity interedite wter converges nd sinks. It lso revels tht the in trnsport occurs in two seprte eridionl cells with opposing sense of rottion, nd deterines the ltitude of the boundry between the. 1. The Southern Heisphere ocens The strong zonl circultion of the Antrctic Circupolr Current intins n S,T reltion which is lrgely siilr in ech of the southern ocens. Fig. 1 shows the S,T reltion for severl Discouery sttions t different ltitudes (ll ner 1 l5o longitude). As first pproxition we will ssue tht the S,T reltions re independent of longitude, nd tht the ltitudinl dependence of the S,T reltion shown in Fig. 1 is typicl of the entire southern heisphere. Ocenic eridionl fluxes of fresh-wter, F, in units of lo6 3 s-* (fro Bugrtner nd Reichel, 1975) nd of het, H, in units of 1 W (fro Hstenrth, 198) sued over different prllels of ltitude re given in Tble 1. The corresponding verge excess of slinity, ds, nd of teperture, dt, required for ll southwrd oving ocenic wter volues over the verge slinity nd teperture of those volues oving northwrd is lso tbulted ssuing stndrd plitude of wter volue exchnge of 3 x lo6 3 s-. If the ctul plitude of the volue exchnge is M, then the pproprite vlues for ds nd dt y be obtined by ultiplying the tbulted vlues by 3lM. The slope dt/ds (which we will cll the principl flux slope ) nd the length d indicted to the right of the S,T curve in ech pnel of Fig. 1, ws obtined by ssigning ech Discovery sttion noinl ltitude, t intervls of 5 degrees of ltitude, nd using the corresponding nubers in Tble 1. The line segents (or principl flux vectors ) cn be trnslted without rottion. The nuerl t ech end (positive is northwrd) gives the direction of the volue flux nd its ount in units of lo6 3 s-l. By trnsltion nd chnge of length the principl flux vectors cn be oved so tht both ends lie on the S,T curve or one end does nd the other lies on fesible ixture of two prts of the S,T curve. If the fit is so djusted tht xiu length cn be chieved, then we hve found the iniu possible ocenic volue exchnge which cn ffect both prescribed eridionl fluxes of fresh-wter nd het t tht ltitude. This ide hs been exploited by Stoel (198) to test proposed eridionl fluxes for individul southern heisphere ocens for consistency. It is unlikely /8/6562-5$2.5/ 198 Munksgrd, Copenhgen Tellus 32 (198), 6

2 2 W N h e \o co v QI b" 35 "S (35" 45 IS) I 36 5 "S (5Ooi9'S) -- 1 I /, I "S 2 6 "S (58"49'S) s (%o/ s (%o/ Fig. I. S, Tcurves for selected Discovery sttions ner 115O, noinl (nd ctul) ltitudes re both indicted bove the sttion nuber. The line segent to the right of ech curve is the coputed principl flux vector for stndrd volue flux of 3 (x lo6 3 s-i), nd other nuerls t ends of lines re volue fluxes in se units clculted grphiclly by lws of ixtures (northwrd is positive). Soe curves of constnt sig-t re lso shown. Sttion 2158 is shown dshed on plot for sttion 2156 to show length of isopycnl flux vector (see text). x 3 e z > r fa $ 6 $ 9 2 U k5 3 ul o! w

3 564 H. STOMML Tble 1. Southern ocen eridionl fluxes, principl flux slopes, nerest Discovery sttions, nd en sig-t slopes Nerest Men Sig-r Lt. F H ds dt dtlds Sttion T slope OS 16,tl3~-' 113 w "C OCl% No. O C OCI% tht the lower liits so coputed cn ctully be relized: first, the volue exchnges re unlikely to be confined to the extree tepertures t the ends of the longest possible principl flux vector; second there re likely to be other constrints (e.g. geostrophy) tht y dend higher lower liit. If the exchnge process cn operte ner the lower liit, it y be spoken of s "the ost efficient" configurtion of exchnge of wter-sses. Inspection of Fig 1 shows tht there is generl decrese in the principl flux slope nd length fro 35OS to 6"s. Ner 35's nd 6OoS the S,T curves hve long stright portions which re convenient plces, with pproprite slopes, for locting optilly fitted principl flux vectors, fct which cn hrdly be coincidentl. Curves of constnt surfce density (sig-t) re drwn on the S,T plnes in Fig. 1. Their slopes vry over the S,T plne. Ner 35" S the principl flux vector hs slope greter thn tht of the sig-t curve in the vicinity of the en teperture nd slinity of the S,T curve. This ens tht on the verge the wr slty wter of the ost efficient southwrd volue flow ust overlie its denser northwrd counterprt. At 6 S, however, the principl flux vector hs slope less thn tht of the sig-t curve in the vicinity of the verge slinity nd teperture of the S,T curve, nd consequently the ost efficient southwrd volue flow of wr slty wter ust, on the verge, underlie its less dense northwrd flowing counterprt. At n interedite ltitude ner 47O S, s shown in Fig. 2 where the two slopes re displyed s functions of ltitude, the principl flux vector iplies isopycnl exchnges (or ixing) s the ost efficient. Since this is lso ltitude where the north-south chnge of S,T reltion is ost rked, we ight ssue tht the true zonl '\. I slope 1 I 1 I J LAT "S Fig. 2. Ltitudinl dependence of principl flux slope nd slope of sig-t curve ner vrge teperture nd slinity of ech sttion. verge S,T reltion t this ltitude is relly very brod in the rnge of sig-t fro to 27.5, due to endering of the front, in which cse isopycnl ixing could be quite efficient. In this wy we locte the ltitude where Antrctic Interedite Wter sinks below the surfce on its northwrd course, nd find tht this is lso region where isentropic (isopycnl) exchnges or ixing y be expected to ccount for the locl eridionl flux of fresh-wter nd het, without invoking explicitly ny dynicl constrints such TeHus 32 (198), 6

4 FFCTS OF MRIDIONAL FLUXS OF FRSH-WATR AND HAT 565 s kn puping due to wind-stress. In contrst to ny ides bout ocenic circultion which depend upon surfce boundry condition on oentu flux, the ides developed here depend upon surfce boundry conditions on fresh-wter nd het flux (the eridionl fluxes re equivlent to the, by integrtion of conservtion equtions). The S,T curves in the southern ocen re, of course, only pproxitely independent of longitude, nd fro one ocen to nother. It will, in the future, be helpful to prepre creful census deterintions of the ount of ech wter type ( bivrite distribution of S nd T) on certin fixed prllels of ltitude. A brief discussion of these differences, nd of the possible differences of the eteorologiclly deterined ocenic fluxes of het nd fresh-wter, is given in Stoel(198). 2. Clcultion of gnitude of the iniu volue fluxes in the Southern Heisphere vector, trnslted, nd with length djusted, is lso shown on the S,T curve. Where both ends cnnot be plced upon the curve, one end is plced on the curve (point 3) nd the other on fesible ixture of low slinity wter t sig-t of nd of deep wter t sig-t 27.8 (points 2 nd 1 respectively). The nuerls fixed t points 1, 2, nd 3 re the volue fluxes deterined by lws of ixture nd re surized in Tble 2. These fluxes need not necessrily be ssocited with stedy currents; the rther lrge vlues t 4'- 45OS y relly be ssocited with strong isentropic ixing t these ltitudes s entioned bove. lsewhere the gnitudes see to fll within the rnge of conventionl wisdo. It is striking how continuous the northwrd stre of low slinity (type 2) wter is. At the ltitude where it sinks beneth the surfce the sense of the in eridionl cell reverses. 3. Acknowledgeents Grphicl solutions for the iniu volue I hve hd the benefit of severl good discussions fluxes t ech noinl ltitude re indicted in Fig. with Dr M. S. McCrtney nd other collegues t 1. On the right the principl flux vector is shown for Woods Hole, nd the support of Ntionl Science stndrd volue flux 3 x lo6 3 s-l. The Foundtion Grnt No. OC This is nuerls t the ends of the vector indicte ount contribution nuber 4589 fro the Woods Hole nd direction of flow of the volue flux. This Ocenogrphic Institution. Tble 2. Minil volue fluxes by constructions in Fig. I (units lo6 rn's-') ~ Type Sig-I 6's 55's 5's 45's 4's 35's * * These lrge volue fluxes y be ssocited with isopycnl fluxes due to eddies on the front (see text). RFRNCS Bugrtner, A. nd Reichel, The world wter between the S,T curve nd globl het nd blnce. lsevier, 179 pp. tospheric wter trnsports. Journl of Geophysicl Hstenrth, S Het budget of tropicl ocen nd Reserch 85, C 1, tosphere. Journl of Physicl Ocenogrphy 1, Stoel, H Asyetry of interocenic fresh wter nd het fluxes. Proc. Nut. Acd. Sci. 77, Stoel, H. nd Csndy, G. T A reltion Tellus 32 (198), 6

5 566 H. STOMML KAK OTHOUHM MPMAMOHAflbHOrO IlOTOKA IlPCHOfi BOAbI K IlOTOKY TIIJIA QMKCMPYT UHPOTY, HA KOTOPOM OIlYCKA OTCd ITPOMXYTOYHbI BOAbI HM3KOfi COflHOCTM 3AHXe MepHAHOHWlbHblX IIOTOKOB IlpKHOfi BOAbI K Te 4HKCHpyeT HHXHHe IlPt?AeJIbI H BenHWHbI MePWViOHWbHbIX IIOTOKOB o6be~, HIIpBneHHe H othocenbhyw, y6~~y, H KOTOPO~~ OHM BCT- PeYIOTCII, H IIIHpOTy, H KOTOpOfi IlpOMeXYTOYHLde BOAbI HH3K8 COJIeHOCTH CXOARTCII H OnyCKIOTCX BHH3.OHO TKXe IlOK3blBeT, IT OCHOBHOfi IlepeHOC IIpOHCXOAHT B AByX OTAeJIbHbIX MePHAHOHWbHblX RYefiKX C IIpOTHBOIIOJIOXHbIMH 3HKMH BpLUeHHIl H onpeennet LUHPOTY rphhubr ps~en Mey HHMH. Tellus 32 (198), 6