Coupled Atmosphere-Ocean Feedbacks

ICTP-CLIVAR Summer School on EBUS

July 17, 2019

Art Miller

Scripps Institution of Oceanography

University of California, San Diego

Eddy-Wind Interaction in the California Current System:

Dynamics and Impacts

ICTP-CLIVAR Summer School on EBUS

July 17, 2019

Hyodae Seo1, Art Miller2 and Joel Norris2

1Woods Hole Oceanographic Institution 2Scripps Institution of Oceanography

Journal of Physical Oceanography (2016)

Coupled Atmosphere-Ocean Feedbacks

Outline

1) Ocean Eddy Influences on Surface Fluxes- SST effects and Current effects

2) Regional Coupled Modeling Experiments for the CCS- Isolating the effects of SST and U

3) Dynamical Diagnostics of the Eddy-Stress interaction- EKE and Ekman Pumping

4) Further Work- Ocean Influence on Atmospheric States

Coupled Atmosphere-Ocean Feedbacks

Initial Considerations….

1. Observations indicate mesoscale SST alters surface fluxes

2. Atmospheric response to these ocean-mediated fluxes is not obvious

3. Oceanic response to these ocean-mediated fluxes is not obvious

4. Regional coupled ocean-atmosphere modeling

may help shed on light on possible feedbacks

Driving Question

• What impact do mesoscale-eddy generated

surface flux anomalies have on the

atmosphere and ocean?

Our previous work involved modeled

mesoscale-SST anomaly influences on

surface fluxes

SST gradient impact on wind stress curl

and divergence

Scripps Coupled Ocean-Atmosphere Regional

(SCOAR) Model

Ocean

(Regional Ocean Modeling

Systems - ROMS)

Atmosphere

(Regional Spectral Model -

RSM)

SST-Flux Coupler

Mesoscale Air-sea

Coupling Experiment

Control

SCOAR

Control run

composed of fully-

coupled SCOAR

run for 2005-2010

Smoothed

SCOAR

SCOAR run with

daily, mesoscale

smoothing of SST

and/or U-ocean at

every coupling step Putrasahan et al. 2013

Seo, Miller and

Norris, 2016

Seo, Miller and Roads

J. Climate, 2007

Regional Coupled Ocean-Atmosphere Feedbacks in the

California Current Sector

Wind Stress

divergence

Latent

heat fluxSST, winds

Wind Stress

curl

• Coupling of SST

with Atmospheric

Boundary Layer is

observed and

modeled in the

CCS region over

eddy scales

•How does this

coupling affect

statistics of

ocean eddies,

and the

overlying

atmospheric

flows?

RSM

Atmos

model:

16 km

ROMS

Ocean

model:

7 km

SCOAR simulation

Seo, Miller and Roads

(2007, J. Climate)

Peru-Humboldt Current System:

Do coupled feedbacks of SST-anomaly-induced

mesoscale surface fluxes (wind stress, heat)

alter eddy statistics?

Putrasahan, Miller and Seo (2013, Ocean Dynamics)

Very small impacts – perhaps SST gradients are simply too weak

RMS SST

Peru-Humboldt Current System:

Do coupled feedbacks of SST-anomaly-induced

mesoscale surface fluxes (wind stress, heat)

alter eddy statistics?

Putrasahan, Miller and Seo (2013, Ocean Dynamics)

Very small impacts – perhaps SST gradients are simply too weak

RMS surface vorticity

Driving Question

• What impact do mesoscale-eddy generated surface flux anomalies have on the atmosphere and ocean?

Today, focus is on mesoscale eddy surface velocity impacts on surface stress, relative to the mesoscale eddy SST effects in the California Current System

Mean State Differences: JJAS SST vs. Obs and Smoothed Cases

- Upwelling too strong + Interesting patterns of eddies

Rectified SST patterns when turning off eddy coupling effects:

Downscaled

equatorward

wind stress

too strong

Mean State Differences: JJAS Wind stress vs. Obs and Smoothed Cases

- Stress patterns match interesting patterns of eddy-induced SST

Pattern of rectified SST difference spatially positively correlated with the wind stress difference

Mean State Differences: JJAS Latent Heat Flux

vs. Obs and Smoothed Cases

- Latent heat flux

is a response to SST

(not atmospheric state)

- Patches of SST difference associated with changes insemi-permanent eddies and filaments and associatedlateral eddy heat advection

Drifter Obs, rms:

Centurioni et al.

Cross-shore distribution of EKE:

- Peaks at 150 km offshore

- Remains high without inclusion of eddy current feedback on stress

Summary

• Examined the relative importance of τSST vs τcur in EKE and Ekman pumping

velocity in the CCS using a regional coupled model.

• Surface EKE is weakened (~25%) due to mesoscale current effects on τ

- Mesoscale SST has very little direct impact on EKE

• In the EKE budget: eddies both enhance eddy drag (by zonal current)

and weaken the wind work (by meridional current).

• Eddies modify the Ekman pumping velocity.

- via eddy SST through positive relationship between crosswind SST gradient and

wind stress curls

- via eddy Current through the gradient of surface vorticity

• SST- and Current-induced Ekman pumping have comparable spatial scales and

amplitudes but different patterns, indicative of their different feedback processes

Further Work (unpublished)

• Ocean eddy influences on Atmospheric statistics

• Ocean anomalous state influences on Atmospheric

Rivers (deterministic effects)

• Ocean anomalous state influences on the diurnal

cycle, sea breeze, and marine layer

DJF averaged rainfall

2005-2010

DJF mean precipitation

mm/day

DJF mean precipitation difference

mm/day

DJF mean precipitation percent difference (%)

e.g., (CTL-noTe)/CTL X 100

DJF Mean SST Difference

Deg C

Rainfall response to eddies

• Rectified mean SST difference (due to

eddies) drives changes in mean rainfall up

to 10% of the winter mean

• Some signatures of oceanic patterns extend

over land (coastal ranges, ~100km)

• Mainly convective precip, not large-scale

Coupled Atmosphere-Ocean Feedbacks

Outline

1) Ocean Eddy Influences on Surface Fluxes- SST effects and Current effects

2) Regional Coupled Modeling Experiments for the CCS- Isolating the effects of SST and U

3) Dynamical Diagnostics of the Eddy-Stress interaction- EKE and Ekman Pumping

4) Further Work- Ocean Influence on Atmospheric States

Thanks!

Art Miller

SIO/UCSD

La Jolla, CA

ICTP-CLIVAR Summer School on EBUS

July 17, 2019