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 1958-1999

- HEAT only

- HEAT+WIND

• 1/12° North Atlantic

• climatological ECMWF forcing

• both: 45 z-level, rigid-lid, BBL, iso-

pycnal mixing, GM90

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 SEC

NEUCNBC

SEC

EUC

SEUC NEUC

SEC

EUCNBC

SEUCNEUC

SEC SEC

Mean zonal circulation at 35°W

EUC

SEUC

SICC

EIC

NICC

NBC

SEC SEC

NEUCNBC

SEC

EUC

SEUC NEUC

SEC

EUCNBC

SEUCNEUC

SEC 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 SvNBC: max. 60 cm/s, -32.2 Sv EIC: 10.2 Sv NEUC, SEUC > 10 cm/s

max. 60 cm/s, 15.7 Sv max. >60 cm/s, -27.4 Sv no EIC weak mean SEUC NEUC reaches surface

max. 80 cm/s, 15.9 Sv max. >60 cm/s, -27.2 Sv no EIC weak mean SEUC NEUC 0m to 700m

1/3° 1/12°obs.

EUC

SEUC

SICC

EIC

NICC

NBC

SEC SEC

NEUCNBC

SEC

EUC

SEUC NEUC

SEC

EUCNBC

SEUCNEUC

SEC 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 SEC

NEUCNBC

SEC

EUC

SEUC NEUC

SEC

EUCNBC

SEUCNEUC

SEC SEC

EUC variability

mean EUC at 0°N

26.2

24.4

26.025.5 • EUC bounded by the isopycnals = 24.4-26.2

• upwelling of this isopyc- nals into the mixed- layer eastward of 30°W

EUC variability

mean EUC at 0°N

HEATHEAT+WIND

26.2

24.4

26.025.5

Interannual variability of the EUC at 35°W

• EUC bounded by the isopycnals = 24.4-26.2

• upwelling of this isopyc- nals into the mixed- layer eastward of 30°W

HEAT+WINDHEAT• 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

24.426..2

• 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

24.426..2

• 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 1960-70, low from 1970-90, 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

HEAT HEAT+WIND

v‘T: meridional coherence ?

HEATHEAT+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) at35°W and the NBC (lower) in Sv

v‘T: meridional coherence ?

HEATHEAT+WIND

HEAT+WINDHEAT

• 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) at35°W and the NBC (lower) in Sv

Correlation of EUC and NBC anomalies

v‘T: meridional coherence ?

HEATHEAT+WIND

HEAT+WINDHEAT

• 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: 1964-65, 1968-80, 1993-94

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

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) withthe changes of TC-Index