26°N observations of Overturning vs the Horizontal Gyre: compensation & recirculation Eleanor Frajka-Williams Stuart Cunningham, Gerard McCarthy, Darren Rayner, Harry Bryden Louis Clement, Joel Hirschi, Aurelie Duchez, Zoltan Szuts, Shane Elipot Funding by the Natural Environment Research Council & NSF/NOAA

RAPID 26°N: several measurement components T = T gs + T ek + T umo

[moc_tseries_wedge.png] Extending the timeseries T MOC : the northward transport above the peak in the overturning streamfunction T umo = T geo +T wedge +T comp

[Johns et al, 2008] Wedge: current meter measurements GeostrophicWedge T umo = T geo +T wedge +T comp Deep western boundar y current (DWBC) Antilles current (AC)

Mid ocean transport v z = -g/ρL [ρe-ρw] T geo =∫ v z dz T umo = T geo +T wedge +T comp

[moc_tseries_wedge.png] Extending the timeseries T MOC : the northward transport above the peak in the overturning streamfunction T umo = T geo +T wedge +T comp

Western boundary [AVISO MADT 10cm, ETOPO2 bathymetry m]

Objective: To understand sources of variability of the MOC -- This talk: subannual variability & covariability at the boundary

Components of the MOC are not correlated -- each projects variance onto the MOC Kanzow et al, (2009) on the MOC through 2007.

[moc_tseries_wedge.png] Covariances? T umo = T geo +T wedge +T comp

[moc_tseries_wedge.png] Covariances? T umo = T geo +T wedge +T comp

[moc_tseries_wedge.png] Covariances? T umo = T geo +T wedge +T comp

Western boundary compensation In 2008, there were two fluctuations in the Gulf Stream transport that were replicated in the -UMO transport anomalies. This meant they were canceling out in the surface transport, and not appearing in the MOC. This is a signature of the horizontal gyre.

[anticorr_window.png] Windowed correlations: GS vs UMO 360-day windowed correlations, with ndof chosen based on the longest integral timescale for the two timeseries in the correlation, show that the UMO is 95% significantly anticorrelated with the GS. T umo,w = T geo,w +T wedge +T comp above 1000 m

Thermal wind, west contribution only v z,e ̅ = -g/ρL [ρ ̅ e ̅ -ρw] T geo,e ̅ =∫ v z,e ̅ dz T umo,wb = T geo, e ̅ +T wedge +T comp }

[anticorr_window.png] Windowed correlations: GS vs UMO 360-day windowed correlations, with ndof chosen based on the longest integral timescale for the two timeseries in the correlation, show that the UMO is 95% significantly anticorrelated with the GS. T umo,w = T geo,w +T wedge +T comp above 1000 m

Wedge & Gulf Stream: Since 2009 T gs ∝ T wedge

[antismo_wcrp.png] Changing relationships at the WB The UMOwb and GS were anticorrelated from July 2006 through Feb 2009; using centered 360-day windowed correlations. From Oct 2009 to May 2010, the GS and Wedge (to 25km offshore) were correlated.

[wbw_gs_2.png] Changing relationships at the WB Transport timeseries, (UMOwb inverted). Anomaly time series: low pass filtered but NOT scaled.

Time series are independent Three time series are (nearly) independently calculated: Gulf Stream transports from cable measurements by NOAA, Miami Wedge transports from current meter moorings by RSMAS, Miami UMO, wb from dynamic height moorings by NOC, UK and a barotropic compensation term. This barotropic term (hcomp) does not have the same time variability nor magnitude fluctuations as the signal here and won’t be considered further.

Changing relationships at the WB What have we learned? The GS west of the Bahamas can be correlated with transport east of the Bahamas, either negatively (with UMOwb) or positively (with wedge). The relationship is non-stationary.

Physical causes Local forcing - atmospheric? Changes at the Gulf Stream cause changes east of the Bahamas due to bottom pressure effects. (Lin & Greatbatch 2009)

Physical causes Local forcing - atmospheric? Changes at the Gulf Stream cause changes east of the Bahamas due to bottom pressure effects. (Lin & Greatbatch 2009)

Wind forcing [ssha_GS_corr.png] Wind stress + topography (island) Wind stress curl

Wind forcing, windowed correlations

Physical causes Local forcing - atmospheric? Changes at the Gulf Stream cause changes east of the Bahamas due to bottom pressure effects. (Lin & Greatbatch 2009)

Barotropic compensation at WB2 Bottom pressures at WB2 show local compensation to baroclinic structure. Once that is removed from the bottom pressure signal, the residual bottom pressure is weakly correlated with the Gulf Stream transport. [Bryden et al, 2009]

Some implications... So we’ve seen some covarying wiggles-- What does it mean? Does it matter?

Horizontal gyre vs MOC variability The anti-correlation between the GS and UMOwb suggests that most of the variability of the Gulf Stream is balanced by variability in the horizontal gyre return. This suggests that the part of the GS associated with the overturning has very different temporal characteristics.

What is the western boundary current of the subtropical gyre? Often, the Gulf Stream transport through Florida Straits is taken to be the primarily northward western boundary current in the subtropical gyre. However, fluctuations east of the Bahamas (wedge) can be the same magnitude (as well as in phase): Twice the variability of the northward, boundary intensified flow.

[gyre_map5.png] Other sources of variability? We are piecing together sources of variability in the MOC, at the eastern and western edges of the subtropical gyre.

[Clement et al, in prep] [Kanzow et al, 2009; Szuts et al, 2011; Meinen et al, in review] Westward propagating planetary waves/eddies Eddies and waves propagate across the basin, seen here in SSHA and dynamic height anomalies from the RAPID moorings.

[dh_tr.eps] Variability at WB2 due to westward propagating anomalies While variations at WB2 are higher frequency and lower amplitude than at WB5, there is some component of the variability at the same frequencies and vertical scales. [Clement et al, in prep]

[Elipot et al, in prep] Meridionally travelling boundary waves There is some evidence for meridionally propagating signals between latitudes along the east coast of the US.

[gyre_map5.png] Sources of variability at the east

[Chidichimo et al, 2011] MOC seasonality due to the Eastern boundary Wind driven upwelling filaments explain the deep (~1km) density variations that dominate the seasonal cycle of the MOC. [Castro, 2011 SOES MSc]

[gyre_map5.png] Summary We are piecing together sources of variability in the MOC, at the eastern and western edges of the subtropical gyre.