Characterization of frontal air-sea interaction by spectral transfer functions

Niklas Schneider1, Bunmei Taguchi2, Masami Nonaka2 and Akira Kuwano-Yoshida21International Pacific Research Center, University of Hawaii, Honolulu, USA

2Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan

Imprint of ocean mesoscale on extratropical atmospherewarm SST associated with high wind speed

SST

con

tour

inte

rval

1K

Chelton and X

ieO

ceanography, 2010

July 2002, monthly average, spatially high pass filtered

win

d sp

eed

m/s

International Ocean Vector Winds Science Team Meeting, Hokkaido University, Sapporo, Japan, May 17-19, 2016

Air-sea interaction at weak SST frontsSchneider and Qiu, JAS, 2015

BackgroundEkman spiral

inversion

2

SST perturbation

Air-sea interaction at weak SST frontsSchneider and Qiu, JAS, 2015

BackgroundEkman spiral

inversion

warm

2

Vertical mixing mechanism (Wallace et al. 1989, Hayes et al. 1989, Samelson et al. 2006)

SST perturbation

Air-sea interaction at weak SST frontsSchneider and Qiu, JAS, 2015

BackgroundEkman spiral

inversion

warm

2

Vertical mixing mechanism (Wallace et al. 1989, Hayes et al. 1989, Samelson et al. 2006)

SST perturbation

Air-sea interaction at weak SST frontsSchneider and Qiu, JAS, 2015

BackgroundEkman spiral

inversion

warm

2

Vertical mixing mechanism (Wallace et al. 1989, Hayes et al. 1989, Samelson et al. 2006)

SST perturbation

Air-sea interaction at weak SST frontsSchneider and Qiu, JAS, 2015

BackgroundEkman spiral

inversion

warm

2

Pressure effect (Lindzen and Nigam 1987)

Vertical mixing mechanism (Wallace et al. 1989, Hayes et al. 1989, Samelson et al. 2006)

SST perturbation

Air-sea interaction at weak SST frontsSchneider and Qiu, JAS, 2015

BackgroundEkman spiral

inversion

warm

2

Wind&inversion responseadvection rotation

back pressure vertical mixing

continuity

Pressure effect (Lindzen and Nigam 1987)

Air-sea interaction at weak SST frontsSchneider and Qiu, JAS, 2015

BackgroundEkman spiral

inversion

warm

weak SST perturbation

3

wind&inversion responseadvection rotation

back pressure vertical mixing

continuity

Air-sea interaction at weak SST frontsSchneider and Qiu, JAS, 2015

BackgroundEkman spiral

inversion

warm

weak SST perturbation

3

advection by background winds only

wind&inversion responseadvection rotation

back pressure vertical mixing

continuity

Air-sea interaction at weak SST frontsSchneider and Qiu, JAS, 2015

BackgroundEkman spiral

inversion

warm

weak SST perturbation

3

vertical mixing mechanism acts on background shear onlybackground mixing acts on frontally induced winds

advection by background winds only

wind&inversion responseadvection rotation

back pressure vertical mixing

continuity

Air-sea interaction at weak SST frontsSchneider and Qiu, JAS, 2015

weak SST perturbation

4

vertical mixing mechanism acts on background shear onlybackground mixing acts on frontally induced winds

advection by background winds only

A! !k Φ! !k = D! !k T! !kdynamics u, v, w, h, Θ SST forcing

linear Rossby adjustment problem with coefficients constant in x, y

Air-sea interaction at weak SST frontsSchneider and Qiu, JAS, 2015

weak SST perturbation

4

vertical mixing mechanism acts on background shear onlybackground mixing acts on frontally induced winds

advection by background winds only

A! !k Φ! !k = D! !k T! !kdynamics u, v, w, h, Θ SST forcing

linear Rossby adjustment problem with coefficients constant in x, y

Φ! !k = A! !k−1D! !k T! !k

Transfer functiondependent on background wind speed, direction

mixing formulation

Transfer function for surface wind speed

Transfer function for surface wind speed

background surface wind direction

Transfer function for surface wind speed

downwind wave-number /Rossby Radius-1

background surface wind direction

Transfer function for surface wind speed

downwind wave-number /Rossby Radius-1

cros

swin

d w

ave-

num

ber

/Ros

sby

Radi

us-1

background surface wind direction

Transfer function for surface wind speed

downwind wave-number /Rossby Radius-1

cros

swin

d w

ave-

num

ber

/Ros

sby

Radi

us-1

Ug /gravity wave speed

Transfer function for surface wind speed

downwind wave-number /Rossby Radius-1

cros

swin

d w

ave-

num

ber

/Ros

sby

Radi

us-1

… in phase with SST

Frontally induced surface wind speed …

Ug /gravity wave speed

Transfer function for surface wind speed

downwind wave-number /Rossby Radius-1

cros

swin

d w

ave-

num

ber

/Ros

sby

Radi

us-1

… in phase with SST

… 90° phase-shifted with SST

Frontally induced surface wind speed …

Ug /gravity wave speed

Surface wind speed, best fit linear model

downwind wave-number/Rossby Radius-1

cros

swin

d w

ave-

num

ber

/Ros

sby

Radi

us-1

Surface wind speed, best fit linear model

downwind wave-number/Rossby Radius-1

cros

swin

d w

ave-

num

ber

/Ros

sby

Radi

us-1

downwind wave-number/Rossby Radius-1

cros

swin

d w

ave-

num

ber

/Ros

sby

Radi

us-1

Surface wind speed, best fit linear model

/0.1

ms-

1 K-1

QuikSCAT

downwind wave-number/Rossby Radius-1

cros

swin

d w

ave-

num

ber

/Ros

sby

Radi

us-1

Conclusions

• A linearized model for the atmospheric boundary layer response to ocean mesoscale sea surface temperatures is tested.

• Spectral transfer functions of the linear model and based on an AGCM (AFES) and QuikSCAT observations compare favorably in the Southern Ocean, and suggest that the linear model captures the underlying physics.

• Additional physics can be tested to improve fit between linear model, AGCMs and observations.

Schneider, N. and B. Qiu, 2015: The atmospheric response to weak sea surface temperature fronts. J. Atmos. Sci., 72, 3356-3377.

Cross-wind

downwind wave-number/Rossby Radius-1

cros

swin

d w

ave-

num

ber

/Ros

sby

Radi

us-1

QuikSCAT

/0.1

ms-

1 K-1

downwind wave-number/Rossby Radius-1

cros

swin

d w

ave-

num

ber

/Ros

sby

Radi

us-1

Cross-wind