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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
