Climate implications of frontal scale air-sea interaction

The role of back pressure in frontal air-sea interactionAir-sea interaction at weak SST fronts

Niklas Schneider, Bo Qiu and Thomas KilpatrickInternational Pacific Research Center and Department of Oceanography

University of Hawaii at Manoa, Honolulu, HI, USA

Climate implications of frontal scale air-sea interaction, Boulder, CO, August 5-7, 2013

Small et al. 2008

Impact of SST fronts on lower troposphere

Downstream of a warm SST front:1. a “Vertical Mixing” increase in mixing entrains higher momentum from aloft (Wallace et al. 1989, Hayes et al. 1989)1. b “Boundary Layer Height Stress” deeper boundary layers accelerate the winds in presence of barotropic forcing (Samelson et al. 2006)2. “Pressure Effect” imprint of SST gradient on boundary layer virtual temperature and pressure gradients (Lindzen and Nigam 1987)3. “Back Pressure” and “Spindown” transport convergences (Ekman pumping) affects inversion height (Feliks et al. 2004)

ECMWF analysis

Jan-Mar 2008, daily

ECMWF analysis

UG10

τ

Jan-Mar 2008, daily

ECMWF analysis

UG10

τ

Jan-Mar 2008, daily

ECMWF analysis

UG7

UG10

τ

Jan-Mar 2008, daily

ECMWF analysis

UG7

UG10 -UG7UG10

τ

Jan-Mar 2008, daily

ECMWF analysis

Jan-Mar 2008, daily

Ekman spin-down

Feliks et al. 2004

Frontal Ekman spin-down

u(0), v(0) Ekman spiralΘ(0) constant

Idealized model for air-sea interaction at SST front

Ug

h(0)

inversion, ΔΘ, no flux

no ocean current, constant SST

• Reduced gravity model capped by sharp inversion

• Changes in cross frontal direction only

• Forced by barotropic tropospheric pressure gradient

• Background Ekman spiral

• Linear response to weak SST front

Background Ekman spiral

Air-sea interaction at weak SST front

Air-sea interaction at weak SST front

Air-sea interaction at weak SST front

Air-sea interaction at weak SST front

Air-sea interaction at weak SST front

Hei

ght

cold warm

u(0)

Cross-frontal distance in Rossby Radii

Strong across front background transport

Hei

ght

cold warm

u(0)

Cross-frontal distance in Rossby Radii

Strong along front background transport

warm

u(0)

cold

The role of back pressure

weak background

wind

surface pressure

baroclinic pressure

back pressure gradient

warm

u(0)

cold

The role of back pressure

strong background

wind

surface pressure

baroclinic pressure

back pressure gradient

Conclusions

• Introduced a model for the interaction of the atmospheric boundary layer with weak SST fronts linearized about a background of a bottom Ekman layer

• Dynamics of response depends on cross frontal background winds: inertia-gravity waves for u(0)>1, spin-down for u(0)<1

• Pressure effect and back-pressure are key to low background cross-frontal winds

• Solution is highly sensitive to the functional dependence on stability and magnitude of the background winds

SST gradient along geostophic wind

acro

ss

wind stress divergence

curl

Coupling coefficientsC

helto

n et

al.

2004

EXTRA SLIDES

Cross-frontal distance in Rossby Radii

Hei

ght

cold warm

u(0)

Kilpatrick 2013

Adjustment time

10-3m

s -1

Chelton and X

ie, Oceanography, 2010

Chelton et al. 2004