The meridional coherence of the North Atlantic meridional overturning circulation

Rory Bingham Proudman Oceanographic Laboratory

Coauthors: Chris Hughes, Vassil Roussenov, Ric Williams

Motivation

Efforts to monitor/observe the MOC:

• RAPID-MOC (26N)

• RAPID-WAVE (Western boundary, 38-42N)

Questions:

• What are the dynamics of MOC variability on short timescales?

• What do measurement at one latitude tell us about MOC variability?

Concern regarding an MOC shutdown/slowdown and abrupt climate change

Presentation Outline

• Statistical analysis on meridional transport coherence

• Local dynamics of meridional transport variability at a given latitude

• Dynamical origins of meridional differences

OCCAM:

• 0.25° eddy permitting resolution• 66 vertical levels• ECMWF 6hrly forcing• 5 day mean fields• 1985-2003 period after spin up

Upper layer meridional transport variability

OCCAM North Atlantic MOC streamfunction (1985-2003)

This picture is suggestive of an MOC that varies as a coherent entity

Must be the case at long enough timescales

Short according to some theories of MOC adjustment (eg Johnson and Marshall 2003)

MOC adequately monitored at one latitude (eg 26N)

Depth integral of MT (100-1000m)

Upper layer meridional transport variability

OCCAM North Atlantic MOC streamfunction (1985-2003)

Low freq.dominates

High freq.dominates

• Examine the 100-1000m depth integral of the meridional transport

• Poleward of approx. 40N a interannual mode is clearly visible

• To the south higher frequency variability more dominant

• Short lived meridionally coherent signals apparent

• Radon transform indicates south propagation at 1.8ms-1

Depth integral of MT (100-1000m)

Upper layer meridional transport variability

OCCAM North Atlantic MOC streamfunction (1985-2003)

How well does interannual MT variability at one latitude correlate with the variability at other latitudes?

Statistical analysis: Cross correlation analysis

0-1000m

100-1000m

0-100m (Ekman)

For the 0-1000m MT integral clear separation at 40N. Mutually correlated north and south of 40N.

Due in part to meridional structure of zonal wind stress over NA

Excluding Ekman transport improves overall correlation between latitudes north and south of 40N, but still low

Suggests an underlying mode of interannual MT variability

Is there a coherent underlying mode of MT variability?

Statistical analysis: Empirical Orthogonal Functions

• Dominant interannual mode is a single overturning cell

• More intense to the north of 40N where it accounts for most of variance

• Becomes weaker and accounts for less of the variance to the south

• Represent meridionally coherent MT fluctuations of 0.8Sv RMS

1st mode(29%)

2nd mode(11%)

TF1: RedTF2: Blue

Contour int. = 0.2SV

x10-4Kgm-3

High-low density composite

MOC dynamics at 50N

Upper layer transport at 50N

• Low frequency mode has clearest expression at 50N -> examine dynamics at this latitude

x10-4Kgm-3

Density profile

• Strong association with density on the western boundary. Increased density leads to increased MT.

• Negligible signal on eastern boundary.

MOC dynamics at 50N

Western boundary

density profile

Anomalous bottom pressure (eq. cm) on

the western boundary

Density changes on the western boundary drive changes in bottom pressure

Anomalous meridional transport

Through geostrophy changes in the east-west pressure difference across that basin are associated with meridional transport variations

Dynamics: The geostrophic calculation at 50N

Assuming geostrophic balance, at depths below the Ekman layer the anomalous zonally-integrated northward mass flux is given by:

Inc northward flowInc southward flow

highwp

lowwp

Dynamics: The geostrophic calculation at 50N

Assuming geostrophic balance, at depths below the Ekman layer the anomalous zonally-integrated northward mass flux is given by:

Inc northward flowInc southward flow

highwp

lowwp

Dynamics: The geostrophic calculation at 50N

• At interannual timescales the meridional transport is well determined by western boundary pressure

• Knowledge of wb pressure variations may be sufficient to monitor to interannual variability of the MOC

• Need to understand density on the western boundary

Upper layer (100-1000m) transport; RMS error: 0.39Sv

ActualInferred from western boundary pressure

Lower layer (1000-3000m) transport; RMS error: 0.39Sv

Leading EOFs of interannual sea-surface height and bottom pressure

BP EOF1

SSHBP

SSH EOF1 • Strong association between leading bottom pressure and sea-level EOFs and low frequency MOC mode

• Pressure signal strongly constrained by bathymetry

• Consistent with geostrophic relationship

• Both account for most of variance on shelf and upper slope but little in the deeper ocean.

• Signal weakens to the south

Origin of meridional differences: Evolution of boundary density

P1

P2

P3

P1

P2

P3

Anomalous density along the 1000m isobath

Advection

Convection + advection+ waves

50N

42N

Advection+ waves

advection0.9cms-1

wave:1.8ms-1

• Seasonal cooling events associated with NAO are integrated to give low frequency mode clear at 50N

• 50N signal advected to lower latitudes, and degraded along the way

(E2)Model resolution: 1.4 degreesForcing: winds and surface fluxes from ECMWF

(E3)Model resolution: 0.23 degreesForcing: monthly climatological winds and surface fluxes from ECMWF, repeating each year.

(E1)Model resolution: 0.23 degreesForcing: winds and surface fluxes from ECMWF

Are the results robust to different model formulations and forcing scenarios?

Statistical analysis: Isopycnal model experiments

Observational evidence: Leading EOFs of interannual sea-surface height

Altimetry OCCAM

AltimetryOCCAM

Summary

• Clear difference in the nature of MOC variability north and south of 40N:– Low frequency variability dominates to the north– Higher frequency variability dominates to the south

• Suggests caution when interpreting “MOC” measurements from one latitude.

• We should also monitor the MOC north of the Gulf Stream

• This may be possible using measurements on the western boundary only

• Low frequency mode results from density variations in the western subpolar gyre. Extends to lower latitudes but with decreased amplitude.

• Results appear robust to model formulation and forcing