Self - Induced Ekman Pumping Over Oceanic Mesoscale Eddies

Self-Induced Ekman Pumping overOceanic Mesoscale Eddies 

Peter Gaube* and Dudley Chelton 

Oregon State University 

• Two mechanisms for eddy self-induced Ekman pumping: 

1. From air-sea interaction as winds blow over eddy-induced perturbations of SST. 

- First shown for Gulf Stream rings by Park & Cornillon (2002) and Park et al. (2006). 

- The feedback effects of this SST-induced Ekman pumping on the ocean disrupt the 

coherent evolution of the mesoscale eddy field (Jin et al., 2009). 

2. From the influence of eddy surface currents on the relative wind, and therefore 

on the wind stress 

- The potential importance of the feedback effects of this surface current induced Ekman 

pumping on the evolution of mesoscale eddies was first noted by Dewar & Flierl (1987). 

- First shown for Gulf Stream rings by Cornillon & Park (2001) and Park et al. (2006). 

• Both mechanisms tend to attenuate eddies. 

• The surface current effects generate upwelling within anticyclonies and downwelling 

within cyclones (Martin and Richards, 2001; McGillicuddy et al., 2007). 

* The results presented here are part of the PhD thesis by Peter Gaube

Procedure for Composite Averaging SST, Wind Speed 

and Wind Stress Curl in Eddy-Centric Coordinates: 

Synergy Between 4 Complimentary Satellite Datasets 

• Identify mesoscale eddies by altimetry from their SSH signatures. 

• Composite average the other satellite datasets in an “eddy-centric” translating 

reference frame with (∆x,∆y) coordinates relative to the eddy centroid normalized by 

the radius L s of maximum rotational speed at each location along its trajectory. 

- AMSR+AVHRR measurements of SST (Reynolds OI2 analyses). 

- QuikSCAT measurements of wind speed and wind stress. 

- SeaWiFS estimates of oceanic chlorophyll. 

• Because the dominant mechanism for eddy-induced SST variability is horizontal 

advection by the rotational velocity of the eddy, SST and wind speed must be 

composite averaged in a coordinate system that is rotated by an amount determined 

from the large-scale background SST gradient.

1. Eddy Influence on SST and Wind Speed

Global Composite Averages of SST in Eddy-Centric Coordinates 

Normalized normalized Distance distance from from Eddy eddy Centroid centroid 

mposites 

udes 

degrees C 

SST Eddy Composites 

SST Eddy Composites SST Eddy Composites 

Regions of Northward ∇T Midlatitudes 

Regions of Southward ∇T 

Clockwise Midlatitudes 

Rotating horizontally Clockwise normalized Rotating and rotatedMidlatitudes 

ed and rotatedCounterclockwise 

Rotating 

ard 

normalized distance from eddy 

horizontally normalized and rotated 

Northward 

normalized distance degrees Cfrom 

eddy centroid 

Southward 

Northward 

degrees C 

Counterclockwise Rotating 

Southward 

Normalized Distance from Eddy Centroid 

degrees C 


Northward 

Southward 

degrees C 

Contour Interval 

is 0.05°C 

Southward

Global Composite Averages of Wind Speed in Eddy-Centric Coordinates 

Wind Speed Eddy Composites 

Wind Speed Eddy Composites Wind Speed Eddy Compos 

Regions of Northward ∇T Midlatitudes 

Regions of Southward ∇T 

Clockwise Midlatitudes 

Rotating horizontally Clockwise normalized Rotating and rotatedMidlatitudes 

Normalized normalized Distance distance from from Eddy eddy Centroid centroid 

y Composites 

udes 

m s -1 

ed and rotatedCounterclockwise 

Rotating 

ard 

normalized distance from eddy 


Northward 

m s-1 normalized distance from eddy centroid 

Southward 

Northward 

m s -1 

Counterclockwise Rotating 

Southward 

Normalized Distance from Eddy Centroid 

m s -1 


Northward 

Southward 

m s -1 


is 0.025 ms Southward 

-1

2. Ekman Pumping from Eddy-Related SST 

Influence on Wind Speed

The Influence Ekman Pumping of Wind from Direction Eddy Perturbations on SST-induced 

of SST 

for Winds from Various Directions Over a 1.0°C SST Anomaly at 30°S 

Ekman Pumping 

cm day -1 


is 3 cm day -1 

1.0 deg 

C temp 

anomaly

The Influence Ekman Pumping of Wind from Direction Eddy Perturbations on SST-induced 

of SST 

for Winds from Various Directions Over a 0.5°C SST Anomaly at 30°S 


cm day -1 



0.5 deg 

C temp 

anomaly

3. Ekman Pumping from Eddy Surface Currents 

for an Idealized Gaussian Eddy

Eddy-Induced Ekman Pumping 

Ekman Pumping from Eddy Surface Currents 

for an Idealized Anticyclonic Gaussian How it works: Eddy and Westerly Winds at 30°S 

• The surface circulation of an eddy can induce Ekman pumping in a uniform wind 

field. SSH Structure of 

Idealized Eddy Gaussian VorticityEddy 

Ekman Ekman Pumping Pumping Velocity 

l = −→ u a − −→ −→ 

u rel u= o 

−→ u a − −→ u o 

−→ 

τ = ρCD|urel| −→ u rel 

−→ u rel = −→ u a − −→ u o 

m s -1 per 100 km 


is 5 cm 

• 30°S 

• 20 cm amp. 

• 10 ms -1 wind 

• Max current 

40 cm s -1 

WE = 1 

ρf ∇×−→ τ 

cm day -1 


is 3 cm day -1

The Influence Ekman Pumping of Wind from Eddy Direction Surface on Currents Gaussian 

for an Idealized Anticyclonic Gaussian Eddy and Winds from Various Directions 

Eddy-induced at Ekman 30°S Pumping 

cm day -1 


is 3 cm day -1

4. Ekman Pumping from the Combined Effects of SST 

Influence and Eddy Surface Currents 

for an Idealized Gaussian Eddy

Ekman The Pumping Influence from of Eddy Wind Surface Direction Currents on and Gaussian 

SST Combined 

for an Idealized Anticyclonic Gaussian Eddy and Winds from Various Directions 

Eddy-induced and SST-induced Ekman Pumping 

over a 0.5°C SST Anomaly at 30°S 

cm day -1 



0.5 deg 

C temp 

anomaly

m s -1 per 

How does this eddy-induced Ekman pumping compare 

with Ekman pumping associated with the large-scale wind field? 

Combined SST and Eddy Induced 

−→ 

u a − −→ a − 

u o 

−→ u o 

D|urel| −→ • 30°S 

u • 20 cm amp. 

rel 

• 10 ms-1 wind 

• Max current 

40 cm s-1 W from Currents and 0.5 E oC SST Anomaly 


WE = 1 

ρf ∇×−→ τ 

cm day -1 

Normalized Distance Contour Interval 

is 3 cm day Risien and Chelton, 2008 

from Eddy Centroid 

• Ekman pumping due to eddy-induced SST anomalies is secondary to the 

upwelling induced by the eddy surface currents 

• Eddy-induced Ekman pumping is of the same order of magnitude as basin 

scale Ekman pumping 

-1 

Eddy-induced Ekman pumping is an Order-1 Perturbation 

of the Ekman pumping associated with the large-scale wind field. 

cm da 

0.5 deg

5. Is the previous Ekman pumping for an idealized 

Gaussian eddy observed in the QuikSCAT data?

5. Is the previous Ekman pumping for an idealized 

Gaussian eddy observed in the QuikSCAT data?

Composite Averages of Eddy-Induced Vorticity and 

Ekman Pumping Velocity in the Hawaiian Island Region 

Composite Averages of Current-Induced Ekman Pumping 

Geostrophic Vorticity 

from Altimetry 

geostrophic vorticity current-induced pumping observed Ekman pumping 

region around the Hawaiian Islands in the Central Pacific 

m s-1 per 100 km 

cm day-1 anticyclones cm day cyclones 

-1 

contour interval 

2 cm 


Surface Current-Induced 

Ekman Pumpming 



from QuikSCAT 

region around the Hawaiian Islands in the Central Pacific 

cm day-1 anticyclones cyclones 


0.5 cm day -1 


0.5 cm day -1 

The regions of strong upwelling 

and downwelling are zonally 

elongated because the winds 

are predominantly nearly zonal 

(easterly) in this region. 

The somewhat weaker upwelling 

velocities in the QuikSCAT 

composites are consistent with 

small mislocations of the eddy 

centroids because of noise in 

the altimeter SSH fields.

Composite Averages of Eddy-Induced Vorticity and 

Ekman Pumping Velocity in the South Indian Ocean 


Geostrophic Vorticity 


geostrophic vorticity current-induced pumping observed Ekman pumping 

South Indian Ocean 

m s-1 per 100 km 

cm day-1 anticyclones cm day cyclones 

-1 


2 cm 


Surface Current-Induced 




from QuikSCAT 

South Indian Ocean 

cm day-1 anticyclones cyclones 


0.5 cm day -1 


0.5 cm day -1 

The nearly circular regions of 

strong upwelling and downwelling 

are indicative of highly variable 

direction of the wind stress. 

The somewhat weaker upwelling 

velocities in the QuikSCAT 

composites are again consistent 

with small mislocations of the 

eddy centroids because of noise 

in the altimeter SSH fields.

6. Influence of Ekman Pumping on Oceanic Biology

Eddies Spawned from the Leeuwin Current 

a total of 734 anticyclonic and 818 cyclonic long-lived eddies were tracked from 2000-2008 

mg m -3

eddy scale 

Eddy eddy Radius scale 

AL.: EDDY-INDUCED EKMAN PUMPING X - 43 

eddy scale eddy scale 

 

mg m -3 

 

• Seasonally (May - September), we observed enhanced CHL at the cores of anticyclonic eddies. 

• Negative CHL anomalies 

eddy scale 

are a persistent feature of cyclonic eddies in this 

eddy 

region. 

scale 

 


1 cm day-1 Eddy-Induced Ekman Pumping Can can Sustain Phytoplankton Blooms 

filtered SeaWiFS chlorophyll with contours of QuikSCAT Ekman pumping 


1 cm day-1 Contour Interval 

1 cm day-1 Filtered wintertime SeaWiFS chlorophyll with contours of QuiksCAT Ekman pumping 

Eddy Radius 

mg m -3 

Eddy Radius 

• Seasonally (May - September), we observed enhanced CHL at the cores of anticyclonic eddies. 

• Negative CHL anomalies are a persistent feature of cyclonic eddies in this region. 

Eddy Radius

Conclusions 

1) Eddy-induced SST variability consists of horizontal advection by the 

azimuthal velocity of the eddies that is consistent with the coupling found 

previously between SST and wind speed along meandering SST fronts. 

2) The Ekman pumping associated with these SST anomalies over the 

eddy interiors is usually secondary to the Ekman pumping associated 

with eddy surface currents. 

• This Ekman pumping is clearly evident in the surface winds measured by QuikSCAT. 

• The small differences between the Ekman pumping velocity deduced from QuikSCAT data 

and the geostrophic velocity computed from altimeter data are consistent with noise in the 

SSH fields that results in small mislocations of the eddy centroids and hence misalignment of 

the Ekman pumping velocity fields. 

3) Ekman pumping over anticyclones appears to sustain blooms of 

phytoplankton within the cores of eddies in the South Indian Ocean 

during wintertime. 

• The reason that this is limited to wintertime is thought to be that the mixed layer is sufficiently 

deep to reach the nutricline, thus allowing the injection of nutrients ino the eddy interior where 

they can be utilized by phytoplankton trapped in the cores of anticyclones.

Self - Induced Ekman Pumping Over Oceanic Mesoscale Eddies

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