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Interannual variability of the tropical-subtropical connections in the Atlantic

Interannual variability of the tropical-subtropical connections in the Atlantic. Sabine Hüttl, IFM-GEOMAR Kiel. Outline. mean state at 35°W , EUC, NBC interannual variability in the STC-regime what spatial patterns ? what amplitudes & timescales ? what mechanisms ?

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Interannual variability of the tropical-subtropical connections in the Atlantic

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  1. Interannual variability of the tropical-subtropical connections in the Atlantic Sabine Hüttl, IFM-GEOMAR Kiel

  2. Outline • mean state at 35°W, EUC, NBC • interannual variability in the STC-regime • what spatial patterns ? • what amplitudes & timescales ? • what mechanisms ? • changes in the strength of the STC (v‘T) • changes by advection of temperature anomalies (v T') • role of NEUC/SEUC for the supply of the off-equatorial upwelling regions

  3. Models & configurations • FLAME-model configurations: • 1/3° Atlantic: • forcing: NCEP 1958-1999 • - HEAT only • - HEAT+WIND • 1/12° North Atlantic • climatological ECMWF forcing • both: 45 z-level, rigid-lid, BBL, iso- • pycnal mixing, GM90

  4. SEC SEC SEC SEC SEC SEC NBC EUC EUC EUC NBC NBC NEUC NEUC NEUC SEUC SEUC SEUC EIC SICC NICC Mean zonal circulation at 35°W observational mean 1/3° november mean 1/12° november mean (Schott et al., 2003)

  5. SEC SEC SEC SEC SEC SEC NBC EUC EUC EUC NBC NBC NEUC NEUC NEUC SEUC SEUC SEUC EIC SICC NICC Mean zonal circulation at 35°W observational mean 1/3° mean 1/12°mean (Schott et al., 2003)

  6. SEC SEC SEC SEC SEC SEC NBC EUC EUC EUC NBC NBC NEUC NEUC NEUC SEUC SEUC SEUC EIC SICC NICC 1/3° 1/12° obs. Mean zonal circulation at 35°W max. 80 cm/s, 15.9 Sv max. >60 cm/s, -27.2 Sv no EIC weak mean SEUC NEUC 0m to 700m max. 60 cm/s, 15.7 Sv max. >60 cm/s, -27.4 Sv no EIC weak mean SEUC NEUC reaches surface EUC: max. 75 cm/s, 20.9 Sv NBC: max. 60 cm/s, -32.2 Sv EIC: 10.2 Sv NEUC, SEUC > 10 cm/s

  7. SEC SEC SEC SEC SEC SEC NBC EUC EUC EUC NBC NBC NEUC NEUC NEUC SEUC SEUC SEUC EIC SICC NICC 1/3° 1/12° obs. Mean zonal circulation at 35°W • complex structure of zonal currents is already resolved in the 1/3° (isopycnic) model, higher resolution(1/12°) gives a sharper horizontal structure, but in the mean no currents like the EIC, NICC, SICC

  8. EUC variability 24.4 • EUC bounded by the isopycnals sq= 24.4-26.2 • upwelling of this isopyc- nals into the mixed- layer eastward of 30°W 25.5 26.0 26.2 mean EUC at 0°N

  9. HEAT+WIND HEAT HEAT HEAT+WIND EUC variability 24.4 • EUC bounded by the isopycnals sq= 24.4-26.2 • upwelling of this isopyc- nals into the mixed- layer eastward of 30°W 25.5 26.0 26.2 mean EUC at 0°N • nearly no variability in HEAT • (RMS <0.5 Sv) • wind variability creates ampli- tudes up to 2 Sv Interannual variability of the EUC at 35°W

  10. NBC variability • NBC-core in the den- sity range of EUC • northward transport of 24.3 Sv, in the STC • 8.5 Sv • broad southward recirculation (3.6 Sv) of the NBC with core near 200m 24.4 26..2 mean NBC at 5°S

  11. NBC variability • NBC-core in the den- sity range of EUC • northward transport of 24.3 Sv, in the STC • 8.5 Sv • broad southward recirculation (3.6 Sv) of the NBC with core near 200m 24.4 26..2 mean NBC at 5°S • low variability in HEAT, high in HEAT+WIND • phase-shift: high NBC- transport from 1960-70, low from 1970-90, high from 1991 in both expe- riments Interannual variability of the NBC at 5°S

  12. ...bringing it together... interannual variability of the STC

  13. Mean meridional overturning ... on z-levels ... on sq-levels

  14. Mean meridional overturning • deep MOC of >15 Sv • southern STC (~3 Sv) & TC (~2 Sv), • northern TC (~11 Sv) • equatorial upwelling: 16 Sv • most of upwelling associatedwith TCs ... on z-levels ... on sq-levels

  15. Mean meridional overturning • deep MOC of >15 Sv • southern STC (~3 Sv) & TC (~2 Sv), • northern TC (~11 Sv) • equatorial upwelling: 16 Sv • most of upwelling associated with TCs ... on z-levels • transports in density classes are lower because of isopycnal recirculation in the TCs (Kröger, 2001) • in the EUC-density range nearly no supply of northern hemispheric water ... on sq-levels

  16. Mechanisms • examination of STC transport: layer between sq=24.4 and 26.2 kg/m^3

  17. Mechanisms • examination of STC transport: layer between sq=24.4 and 26.2 kg/m^3 • causes of interannual variability ? • variations in the strength of the STC (v‘T) may caused by: • changes in equatorial divergence ("pull") • changes in volume of subducted water ("push") • advection of temperature anomalies from the subtropics (v T')

  18. Mechanisms • examination of STC transport: layer between sq=24.4 and 26.2 kg/m^3 • causes of interannual variability ? • variations in the strength of the STC (v‘T) may caused by: • changes in equatorial divergence ("pull") • changes in volume of subducted water ("push") • advection of temperature anomalies from the subtropics (v T') • questions: • concentrated at the boundary ? • meridional coherence? • signal propagating speeds ?

  19. changes in the strength of STC

  20. HEAT HEAT+WIND RMS of transport density changes in the STC density range Variability: where ? • highest variability in both experiments concentrated at the western boundary • variability intensity increases about 10 times if interannual winds are used • wind variations create small fluctuations in the interior which are in the order of heat flux- • driven variations in the boundary current • in HEAT+WIND signal of NBC retroflection

  21. v‘T: meridional coherence ? • amplitudes ~1 Sv • anomalies meridional coherent to 4°S • signal needs < 1 year from ~16°S to 4°S • decadal variation of NBC-transports • NBC and EUC-anomalies normally not in phase for regions south of 4°S HEAT HEAT+WIND Interannual variability of the EUC (upper) at35°W and the NBC (lower) in Sv

  22. HEAT+WIND HEAT v‘T: meridional coherence ? • amplitudes ~1 Sv • anomalies meridional coherent to 4°S • signal needs < 1 year from ~16°S to 4°S • decadal variation of NBC-transports • NBC and EUC-anomalies normally not in phase for regions south of 4°S HEAT HEAT+WIND Correlation of EUC and NBC anomalies Interannual variability of the EUC (upper) at35°W and the NBC (lower) in Sv

  23. HEAT+WIND HEAT v‘T: meridional coherence ? • amplitudes ~1 Sv • anomalies meridional coherent to 4°S • signal needs < 1 year from ~16°S to 4°S • decadal variation of NBC-transports • NBC and EUC-anomalies normally not in phase for regions south of 4°S HEAT HEAT+WIND however: • HEAT: meridional coherence to 0°S • variability up to 0.4 Sv • high correlations from ~12°S between EUC and NBC variability interannual wind variability masks clear signal propagation from the subtropics to the tropics

  24. causes of v‘T-signal ? correlation of tx in ATL3 and v‘ ATL3 • high values (0.6) in the NBC south of 4°S correlation of tx in ATL3 and v‘ in NBC • high values in all latitudes south of 4°S • correlation breaks down in the region of the southern TC • strength of TC is highly correlated with tx between 0°S and 4°S (not shown)

  25. causes of v‘T-signal ? • possible explanation: • stronger easterlies at ATL3 force stronger upwelling at the equator • stronger upwelling needs more inflow from the south via NBC • the stronger NBC strengthens the TC (and more north the NBC-retroflection), i.e. a stronger south- ward component near the boundary develops (corr. not shown) • the TCs decouple the equatorial circulation changes from the changes more south ATL3 correlation of tx in ATL3 and v‘ correlation of tx in ATL3 and v‘ in WBC

  26. Anomaly propagation

  27. Anomalies from the south: v T' • model reveals • clear anomalies • that propagate • to the western • boundary and • after that north- • ward propagating temperature anomalies on the isopycnal 25.2 kg/m^3

  28. Anomalies from the south: v T' • strongest anomalies between 16°S & 12°S (0.6°C) • “mean“ signals are 0.3°C, same magni- tude as RMS of inter-annualSST variability ! • most anomalies fade away on the way to the equator • propagation in the NBC needs ~2 years • some anomalies are visible in the EUC: 1964-65, 1968-80, 1993-94 propagating temperature anomalies on the isopycnal 25.2 kg/m^3

  29. Conclusions STC variability • 1/3°-model shows a detailed equatorial zonal current system • equatorial upwelling of 16 Sv • southern STC-transport: 3 Sv (without TC !), no mean northern STC • strong TCs between 4°S/N and equator

  30. Conclusions STC variability • 1/3°-model shows a detailed equatorial zonal current system • equatorial upwelling of 16 Sv • southern STC-transport: 3 Sv (without TC !), no mean northern STC • strong TCs between 4°S/N and equator • interannual variability strongest at the boundary and weak in the interior • transport anomalies coherent south of 4°S • decadal fluctuation of the NBC-transports • no simple connection between transport anomalies in the NBC and the EUC

  31. Conclusions STC variability • 1/3°-model shows a detailed equatorial zonal current system • equatorial upwelling of 16 Sv • southern STC-transport: 3 Sv (without TC !), no mean northern STC • strong TCs between 4°S/N and equator • interannual variability strongest at the boundary and weak in the interior • transport anomalies coherent south of 4°S • decadal fluctuation of the NBC-transports • no simple connection between transport anomalies in the NBC and the EUC • possible reason: wind stress variability changes the (eq.) upwelling, because of continuity this causes transport changes in the NBC (visible to 12°S), a fluctuating NBC results in fluctuating TCs

  32. Conclusions STC variability • 1/3°-model shows a detailed equatorial zonal current system • equatorial upwelling of 16 Sv • southern STC-transport: 3 Sv (without TC !), no mean northern STC • strong TCs between 4°S/N and equator • interannual variability strongest at the boundary and weak in the interior • transport anomalies coherent south of 4°S • decadal fluctuation of the NBC-transports • no simple connection between transport anomalies in the NBC and the EUC • possible reason: wind stress variability changes the (eq.) upwelling, because of continuity this causes transport changes in the NBC (visible to 12°S), a fluctuating NBC results in fluctuating TCs • propagating temperature anomalies are o(0.3°C) and often do not reach the equatorial upwelling-zone, varying TC-transports blur the anomaly signals

  33. Work in progress • Lagrangian analysis in 1/3° and 1/12° model with daily/monthly/annual snapshots: known: sources of equatorial upwelling: mainly NBC, small parts from NEC,unknown: sources of off-equatorial upwelling in the Guinea and Angola Domes Pathways of synthetic floats launched in the EUC of the 1/12° model at 20°W in May,backward in time integration after 1 year

  34. Work in progress • In this mean picture: • northern STC reaches to the NECC/NEUC-system which feeds the Guinea Dome • only few floats came from the south Guinea Dome 1/3° annual mean

  35. Work in progress • In this mean picture: • northern STC reaches to the NECC/NEUC-system which feeds the Guinea Dome • only few floats came from the south Guinea Dome 1/3° annual mean Guinea Dome • BUT with monthly mean forcing: • no inflow from northern hemisphere • nearly all water originates from the tropical regions and from the NBC ! • WHY ??? 1/12° monthly mean, launch: may

  36. The END

  37.  Not shown correlations  Correlation of the NBC- variability (STC-part) withthe changes of TC-Index

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