the physical processes underlying interannual variations in - ifremer

The physical processes underlying interannual variations in global mean sea level as revealed by Argo and their representation in ocean models Rory Bingham, Peter Clarke & Phil Moore Newcastle University

The steric component of global mean sea level from Argo

Steric component of global mean sea level (66S-66N; 0-900m) from Argo

The steric component of global mean sea level from Argo

Steric component of global mean sea level from Argo

Comparing total and steric global mean sea level

Argo GMSH

Altimetry GMSL (detrended)

A decomposition of global mean steric height into 100m layers

Interannual GMSH from Argo

Interannual GMSH per 100m layer from Argo

Cross correlation array

Global mean steric height from 0-100m and 100-300m layers

clear anti-correlation between

0-100m and 100-300m layers

variability below 300m much

weaker, but is cumulative

Layer 1 (0-100 m) Layer 2 (100-300m)

Contribution to GMSH from upper layer (0-100 m)

GMSH (0-900m) Layer 1 (0-100 m)

EOF analysis of steric height from 0-100m and 100-300m layers

Layer 1 (0-100 m) Layer 2 (100-300m)

Leading EOF from layer 1 (0-100m)

Leading EOF from layer 2 (100-300m)

Principal Components


Layer 1 (0-100 m) Layer 2 (100-300m)

GMSH (solid)

PCs (dashed)

Global mean steric height from layer 1 (0-100m) and NIN03.4

GMSH (0-900m) Layer 1 (0-100 m) Layer 1 (0-100 m) NIN03.4

mm

; S

ST

ºC

Global mean steric height and NIN03.4

GMSH (0-900m) Layer 1 (0-100 m)

GMSH (0-900 m) NIN03.4

mm

; S

ST

ºC

Steric height analysis for HadCM3

Global mean steric height in HadCM3 (green) largely determined by the

upper 100m (red)

GMSH Layer 1 (0-100 m)

Steric height analysis for HadCM3

Consequently interannual global mean steric height fluctuations in

HadCM3 (green) are largely driven by ENSO (NINO3 index - magenta)

GMSH NINO3

Steric height EOF analysis for HadCM3

EOF1 layer 1

EOF1 layer 2

Nino4 index

EOF Principal components (dashed) and corresponding layer global means (solid)

EOF1: layer 1 (0-100m) EOF1: layer 2 (100-300m)

Layer 1 (0-100 m) Layer 2 (100-300m)

The vertical structure of Argo steric height variability

Argo

HadCM3

OCCAM

Standard dev. of interannual steric height variability

Standard dev. of interannual steric height variability function of reference depth H

Conclusions: GMSH and ENSO

• Depth decomposition shows ENSO to be dominant mode of interannual steric sea level

• Responsible for the global means of the 0-100m and 100-300m layers

• 0-100m and 100-300m layers compensatory – ENSO leaves small direct trace in GMSL

• The strong El Nino/La Nina events of 2009/2010 are seen in GMSH – upper layer signal larger relative to lower

• ENSO dominant steric signal in models

• Upper layer GMSH not compensated by lower layer: – ENSO dominates GMSH

– Over-estimation of interannual GMSH fluctuations

The contribution to GMSH from below 300m

Interannual GMSH from Argo

Interannual GMSH per 100m layer from Argo

Cross correlation array


GMSH (0-900m) Layer 1+2 (0-300 m) Layer 3 (300-900m)

0-300m layer dominates 0-300m and 300-900 m

layers contrib. equally

Global mean steric height: equatorial vs. mid-latitude contributions

GMSH

Equatorial (20S-20N) contribution

Total – solid

0-300m – dashed

300-900m –dotted

GMSH

Mid-high lat. contribution

Total – solid

0-300m – dashed

300-900m –dotted

EOF analysis of steric height from 300-900m layer

Leading EOF

Principal component

GMSH (300-900 m)

Area mean steric height timeseries from 300-900m layer

North Pacific

South Atlantic

North Atlantic

Global mean steric height from 300-900 m layer

Principal component

GMSH (300-900 m)

Area mean (300-900m) steric height over 3 key regions

Area mean steric height timeseries from 0-900m layer

North Pacific

South Atlantic

North Atlantic

Area mean 0-900m steric height (solid) and sea level (dashed)

North Pacific

South Atlantic

North Atlantic

Global mean steric height from 0-900 m layer

Principal component

GMSH excluding equator (0-900 m)

Area means over 3 key regions (steric – solid; total - dashed)

Leading EOF of interannual atmospheric surface pressure

Principal component

Area means

steric – solid

total – dashed

Leading EOF of interannual atmospheric surface pressure

GMSH

Principal component

Global mean steric height: Northern vs. Southern hemispheres

GMSH (exc. eq.)

N. Hemisphere (>20N) contribution

Total – solid

0-300m – dashed

300-900m –dotted

GMSH (exc. eq.)

S. Hemisphere (>20S) contribution

Total – solid

0-300m – dashed

300-900m –dotted

Conclusions

• ENSO dominates IA density variations

• First period (2004-2008): – Net equatorial (ENSO) contribution to GMSH is small

– Mid-high latitude contribution dominates

– Spread equally between 0-300m and 300-900m layers

– Part of global atmospherically teleconnected mode which sees a slow fall/rise in total/steric sea level in three basins (10 cm in North Pacific)

– Primarily from Southern Ocean

– 4 mm drop in GMSL

• Second period (2009-2011): – Net mid-high latitude contribution to GMSH is weak

– Equatorial ENSO response dominates (strong event)

– Confined to upper 300 m layer

• Longer record required!

the physical processes underlying interannual variations in - ifremer

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