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

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 Outline

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

Mean currents on   =25.0 cm/s

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

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

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

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 1/3°1/12° obs. EUC SEUC SICC EIC NICC NBC SEC NEUC NBC SEC EUC SEUC NEUC SEC EUC NBC SEUC NEUC SEC

EUC variability mean EUC at 0°N EUC bounded by the isopycnals   = upwelling of this isopyc- nals into the mixed- layer eastward of 30°W

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

NBC variability mean NBC at 5°S 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

NBC variability mean NBC at 5°S Interannual variability of the NBC at 5°S 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 low variability in HEAT, high in HEAT+WIND phase-shift: high NBC- transport from , low from , high from 1991 in both expe- riments

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

Mean meridional overturning... on z-levels... on   -levels

Mean meridional overturning... on z-levels... on   -levels 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

Mean meridional overturning... on z-levels... on   -levels 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 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

Mechanisms examination of STC transport: layer between   =24.4 and 26.2 kg/m^3

Mechanisms examination of STC transport: layer between   =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')

Mechanisms examination of STC transport: layer between   =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 ?

changes in the strength of STC

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 Variability: where ? RMS of transport density changes in the STC density range HEATHEAT+WIND

Variability: where ?

v‘T: meridional coherence ? HEAT HEAT+WIND 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 Interannual variability of the EUC (upper) at 35°W and the NBC (lower) in Sv

v‘T: meridional coherence ? HEAT HEAT+WIND HEAT 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 Interannual variability of the EUC (upper) at 35°W and the NBC (lower) in Sv Correlation of EUC and NBC anomalies

v‘T: meridional coherence ? HEAT HEAT+WIND HEAT 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 interannual wind variability masks clear signal propagation from the subtropics to the tropics HEAT: meridional coherence to 0°S variability up to 0.4 Sv high correlations from ~12°S between EUC and NBC variability however:

causes of v‘T-signal ? correlation of  x in ATL3 and v‘ in NBC correlation of  x in ATL3 and v‘ high values (0.6) in the NBC south of 4°S 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  x between 0°S and 4°S (not shown) ATL3

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 correlation of  x in ATL3 and v‘ correlation of  x in ATL3 and v‘ in WBC ATL3

Anomaly propagation

Anomalies from the south: v T' propagating temperature anomalies on the isopycnal 25.2 kg/m^3 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 strongest anomalies between 16°S & 12°S (0.6°C) “mean“ signals are 0.3°C, same magni- tude as RMS of inter- annual SST 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: , , Anomalies from the south: v T'

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

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

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

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

Work in progress known: sources of equatorial upwelling: mainly NBC, small parts from NEC, unknown: sources of off-equatorial upwelling in the Guinea and Angola Domes Lagrangian analysis in 1/3° and 1/12° model with daily/monthly/annual snapshots: 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

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

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

The END

Not shown correlations Correlation of the NBC- variability (STC-part) with the changes of TC-Index