PowerPointPrsentation

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Interannual variability of the tropical-subtropica
l connections in the Atlantic
Sabine Hüttl, IFM-GEOMAR Kiel
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Outline

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

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Models configurations

• FLAME-model configurations
• 1/3 Atlantic
• forcing NCEP 1958-1999
• - HEAT only
• - HEATWIND
• 1/12 North Atlantic
• climatological ECMWF forcing
• both 45 z-level, rigid-lid, BBL, iso-
• pycnal mixing, GM90

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Mean currents on sq25.0
cm/s
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Mean zonal circulation at 35W
observational mean 1/3
november mean 1/12 november mean
(Schott et al., 2003)
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Mean zonal circulation at 35W
observational mean
1/3 mean 1/12 mean
(Schott et al., 2003)
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Mean zonal circulation at 35W
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
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Mean zonal circulation at 35W

• 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

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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 30W

25.5
26.0
26.2
mean EUC at 0N
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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 30W

25.5
26.0
26.2
mean EUC at 0N

• nearly no variability in HEAT
• (RMS
• wind variability creates ampli- tudes up to 2
  Sv

Interannual variability of the EUC at 35W
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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 5S
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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 5S

• low variability in HEAT, high in HEATWIND
• 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 5S
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...bringing it together... interannual
variability of the STC
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Mean meridional overturning
... on z-levels
... on sq-levels
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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
... on sq-levels
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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
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Mechanisms

• examination of STC transport layer between
  sq24.4 and 26.2 kg/m3

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Mechanisms

• examination of STC transport layer between
  sq24.4 and 26.2 kg/m3
• causes of interannual variability ?
• variations in the strength of the STC (vT)
  may caused by
• changes in equatorial divergence ("pull")
• changes in volume of subducted water ("push")
• advection of temperature anomalies from the
  subtropics (v T')

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Mechanisms

• examination of STC transport layer between
  sq24.4 and 26.2 kg/m3
• causes of interannual variability ?
• variations in the strength of the STC (vT)
  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 ?

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changes in the strength of STC
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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 HEATWIND signal of NBC retroflection

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Variability where ?
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vT meridional coherence ?

• amplitudes 1 Sv
• anomalies meridional coherent to 4S
• signal needs
• decadal variation of NBC-transports
• NBC and EUC-anomalies normally not in phase
  for regions south of 4S

HEAT HEATWIND
Interannual variability of the EUC (upper)
at35W and the NBC (lower) in Sv
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vT meridional coherence ?

• amplitudes 1 Sv
• anomalies meridional coherent to 4S
• signal needs
• decadal variation of NBC-transports
• NBC and EUC-anomalies normally not in phase
  for regions south of 4S

HEAT HEATWIND
Correlation of EUC and NBC anomalies
Interannual variability of the EUC (upper)
at35W and the NBC (lower) in Sv
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vT meridional coherence ?

• amplitudes 1 Sv
• anomalies meridional coherent to 4S
• signal needs
• decadal variation of NBC-transports
• NBC and EUC-anomalies normally not in phase
  for regions south of 4S

HEAT HEATWIND
however

• HEAT meridional coherence to 0S
• variability up to 0.4 Sv
• high correlations from 12S between EUC and
  NBC variability

interannual wind variability masks clear signal
propagation from the subtropics to the tropics
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causes of vT-signal ?
correlation of tx in ATL3 and v
ATL3

• high values (0.6) in the NBC south of 4S

correlation of tx in ATL3 and v in NBC

• high values in all latitudes south of 4S
• correlation breaks down in the region of the
  southern TC
• strength of TC is highly correlated with tx
  between 0S and 4S (not shown)

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causes of vT-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
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Anomaly propagation
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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/m3
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Anomalies from the south v T'

• strongest anomalies between 16S 12S
  (0.6C)
• mean signals are 0.3C, 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

propagating temperature anomalies on the
isopycnal 25.2 kg/m3
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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 4S/N and equator

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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 4S/N and equator
• interannual variability strongest at the
  boundary and weak in the interior
• transport anomalies coherent south of 4S
• decadal fluctuation of the NBC-transports
• no simple connection between transport anomalies
  in the NBC and the EUC

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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 4S/N and equator
• interannual variability strongest at the
  boundary and weak in the interior
• transport anomalies coherent south of 4S
• 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 12S), a
  fluctuating NBC
  results in fluctuating TCs

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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 4S/N and equator
• interannual variability strongest at the
  boundary and weak in the interior
• transport anomalies coherent south of 4S
• 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 12S), a
  fluctuating NBC
  results in fluctuating TCs
• propagating temperature anomalies are o(0.3C)
  and often do not reach the equatorial
  upwelling-zone, varying TC-transports blur the
  anomaly signals

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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 20W in May,backward in
time integration after 1 year
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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
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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
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The END
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? Not shown correlations ?
Correlation of the NBC- variability (STC-part)
withthe changes of TC-Index