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Cold Tongue Upwelling / Mixing Coupled Equatorial Feedbacks Engage the Global Climate

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Cold Tongue Upwelling / Mixing Coupled Equatorial Feedbacks Engage the Global Climate Cold tongue upwelling / mixing Coupled equatorial feedbacks engage the global climate. The thermocline carries the essential ocean memory. Upwelling/mixing enables surface-thermocline communication. Remote consequences of these feedbacks are the main reason people outside the tropical Pacific care about what we do. Upwelling controls property transports (BGC/nutrients/CO2). The vertical motion and exchanges produced by this circulation are now largely unconstrained by observations ... We must get this right. Walker Circulation El Niño precip teleconnections Up Trade Easterlies Winds East New WarmWarm Water heated by the sun Guinea by the sun Pool UpwellingUpwelling South America Cool lower water Thermocline Cool lower water W.S.Kessler, NOAA/PMEL 1 Meridional circulation and processes z y TIW x Green = Wind Brown = Fluxes Red = Solar Blue = Velocity = Mixing = Isotherms EUC Upwelling is O(m/day) vertical mixing must be very strong Consider the modeling challenge ... 2 MARCH 2001 JOHNSON ET AL. 845 How well can we describe this circulation? Observed v in the east-central Pacific: Two limitations MARCH 2001 JOHNSON ET AL. 845 Near-surface Ekman divergence drives upwelling, but ... Limitations: 1) Ship ADCP does not see upper 25m 2) TIW fluctuations Need near-surface Red=Northward currents to Blue=Southward describe the structure of the (Johnson et al. 2001) divergence. (Scatterometer winds also need referencing over Mean meridional current in the east-central Pacific FIG.5.Vertical–meridionalsectionsbasedoncentered28 lat linear fitsstrong to data currents.) taken from 1708 to 958W, regardless of longitude (see 22 21 22 21 text). (a) MeridionalShipboard velocity, ADCPy (10 overms 170°W-95°W); CI 2, positive (northward) shaded. (b) Standard error of y,ey (10 ms ); CI 1, |y|. ey shaded. 10-year average (8400km) surfaceFIG.5.Vertical–meridionalsectionsbasedoncentered2 divergence and thermocline convergence,8 lat and linearsignal-to-noise fits to data taken from ratio 170 for8 yto. 95In8W, contrast regardless to the of longitudeu field, (see the y 22 21 y 22 21 y . littletext). (a) cross-equatorial Meridional velocity, flow.(10 Meridionalms ); CI velocities 2, positive were (northward)y exceeded shaded. (b) their Standard standard error of errors,ey (10 (shadingms ); in CI Fig. 1, | | 5b)ey muchshaded. smaller than zonal velocities. Hence contour in- mainly near the surface3 and in the thermocline on either tervals for y were five times finer than for u. Poleward side of the equator. surfacevalues of divergencey were significant and thermocline to 40–50 convergence, m (278–258 andC). signal-to-noiseSince seawater ratio is nearlyfor y. In incompressible contrast to the andu field, vertical the littlePeak cross-equatorial poleward surface flow. speeds Meridional were 20.09 velocities6 0.02 were m yvelocity,exceededw, theiris negligible standard at errors the surface, (shading the in horizontal Fig. 5b) 21 21 muchs at smaller 4.28Sand0.13 than zonal6 0.03 velocities. m s Henceat 4.68 contourN. Surface in- mainlydivergence, nearu thex 1 surfacey y,canintheorybeintegrateddown- and in the thermocline on either tervalsdrifters for alsoy showwere five stronger times poleward finer than flow for inu. Poleward the north sideward of from the the equator. surface yielded estimates of the vertical valuesthan in the of y southwere from significant 1508 to 130 to8 40–50W(BaturinandNiiler m (278–258C). velocity,Since seawaterw. The horizontal is nearly incompressible divergence was and dominated vertical Peak1997), poleward despite the surface stronger speeds trades were in the2 Southern0.09 6 0.02 Hemi- m velocity,by the meridionalw, is negligible term, y aty,inmostplaces.However, the surface, the horizontal 21 21 ssphereat 4.2 (Hellerman8Sand0.13 and6 Rosenstein0.03 m s 1983).at 4.6 Equatorward8N. Surface divergence,the shoalingu ofx 1 they y EUC,canintheorybeintegrateddown- did result in a significant zonal drifterssubsurface also speeds show stronger were significant poleward flow in the in the Southern north wardterm, fromux,inlimitedregions. the surface yielded estimates of the vertical thanHemisphere in the south from from 60 150 m (248 to8C) 130 through8W(BaturinandNiiler 160 m (158C) velocity,The largestw. The feature horizontal in ux divergence(Fig. 6a) was was the dominated vertical 1997),and in despite the Northern the stronger Hemisphere trades in from the Southern 60 m (25 Hemi-8C) bydipole the centered meridional about term, they equatory,inmostplaces.However, with zonal divergence spherethrough (Hellerman 110 m (21 and8C). Rosenstein Interior equatorward 1983). Equatorward velocities theabove shoaling 105 m of and the convergence EUC did result from in 110 a significant to 270 m. zonal This 21 subsurfacepeaked at 0.05 speeds6 0.02 were m s significantat 1.48Sand in2 the0.04 Southern6 0.03 term,patternux was,inlimitedregions. due to the shoaling of the EUC. Equatorial 21 Hemispherems at 3.68 fromNat85m(near23 60 m (248C) through8C). 160 m (158C) zonalThe convergence largest feature below in u 270x (Fig. m was 6a) thewas result the vertical of the andMeridional in the Northern geostrophic Hemisphere velocities from estimated 60 m from (25 ver-8C) dipolegrowth centered of the westward-flowing about the equator with EIC zonal west divergence of 1408W throughtical integrals 110 m of (21 zonal8C). Interior density equatorward gradients (not velocities shown) above(Fig. 3a). 105 Zonal m and convergence convergence in from the NECC 110 to and 270 SEC m. This was peakedwere generally at 0.05 6 equatorward0.02 m s21 at over 1.48Sand the entire20.04 latitude6 0.03 patterndue to slight was due eastward to the weakening shoaling of of the the EUC. eastward-flow- Equatorial msrange.21 at They 3.68 wereNat85m(near23 also surface intensified8C). and stronger zonaling NECC convergence and slight below westward 270 m was strengthening the result of of the inMeridional the south. This geostrophic hemispheric velocities asymmetry estimated was from consis- ver- growthwestward of flowing the westward-flowing SEC, as expected EIC from west the of Sverdrup 1408W ticaltent with integrals the direct of zonal measurements density gradients of equatorward (not shown) trans- (Fig.balance. 3a). Zonal Zonal convergences convergence in centered the NECC near and 230 SEC m, 6 was38 wereports. Sincegenerally the equatorward equatorward geostrophic over the velocities entire latitude were duelatitude to slight were eastward tightly linked weakening to divergences of the eastward-flow- 60–80 m range.surface They intensified, were also the surface wind-driven intensified poleward and stronger Ekman ingshallower NECC and and about slight 28 westwardpoleward. strengtheningThis pattern was of due the incomponent the south. must This have hemispheric persisted asymmetry to at least was where consis- the westwardto the shoaling flowing and SEC, poleward as expected shift fromof the the NSCC Sverdrup and tentmaximum with the in direct equatorward measurements flow was of equatorward observed, roughly trans- balance.SSCC as Zonal they convergences move eastward centered (Johnson near 230 and m, Moore638 ports.85 m. Since This penetration the equatorward depth geostrophic was well below velocities the mixed were latitude1997). A were more tightly local estimate linked to of divergencesux from the slope 60–80 of m a surfacelayer depth intensified, (Figs. 2c the and wind-driven 2d). poleward Ekman shallowerthird-order and polynomial about 28 evaluatedpoleward. at This 1368 patternWsignificantly was due componentThe standard must errors have of persistedy (Fig. 5b)to at were least also where surface the toincreased the shoaling the magnitude and poleward of the shift dipole of the on the NSCC equator and maximumintensified in with equatorward off-equatorial flow maxima was observed, and dropped roughly be- SSCC(Fig. 7a, as dash–dotted they move line), eastward and other(Johnson features and off Moore the 21 21 85low m. 0.01 This m penetration s in places. depth The was maxima well below were 0.04 the mixed m s 1997).equator A (not more shown), local but estimate was not of appropriateux from the for slope a large- of a layerat 4.58 depthNand0.03ms (Figs. 2c and21 at 2d). 18S, and were likely related third-orderscale estimate polynomial of w. evaluated at 1368Wsignificantly toThe tropical standard instability errors wave of y activity(Fig. 5b) on were both also sides surface of the increasedSince uk they magnitudeand y y k u ofx, y they (Fig. dipole 6b) on was the contoured equator intensifiedequator, but with somewhat off-equatorial weaker maxima in the south and dropped (Chelton be- et (Fig.at five 7a, times dash–dotted coarser intervals line), and than otherux.Ontheequator features off the 21 21 lowal. 2000). 0.01 m While s in places. the mean The meridional maxima were currents 0.04 m were s equatorsurface meridional(not shown), divergence, but was noty appropriatey,wasstrong(Fig.7b) for a large- atnearly 4.58Nand0.03ms an order of magnitude21 at 18S, smaller and were than likely the related mean scaleat 60 ( estimate620) 3 of10w.28 s21.Meridionalconvergenceinthe 28 21 tozonal tropical currents, instability the y wavestandard activity errors on were both still sides half of the thermoclineSince uk reachedy and y 30y k (6u10)x, y y3(Fig.10 6b)s wason the contoured equator. equator,magnitude but of somewhat those for u. weakerThis combination in the south of (Chelton energetic et atThe five transition times coarser from meridional intervals than divergenceux.Ontheequator to conver- al.transient 2000). variability While the and mean a weak meridional mean resulted currents in a were low surfacegence occurred meridional at 50 divergence, m. Near-surfacey y,wasstrong(Fig.7b) meridional con- nearly an order of magnitude smaller than the mean at 60 (620) 3 1028 s21.Meridionalconvergenceinthe zonal currents, the y standard errors were still half the thermocline reached 30 (610) 3 1028 s21 on the equator. magnitude of those for u. This combination of energetic The transition from meridional divergence to conver- transient variability and a weak mean resulted in a low gence occurred at 50 m.
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