Atmospheric response to SST fronts: a Review

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Atmospheric response to SST fronts a Review

• Justin Small and many contributors

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Direct Effects Surface Stress

• Change in surface stress due to change in surface
  stability AND change in wind speed
• (Sweet et al MWR1981, Chelton et al JCLI2001)
• AND surface current variation
• (Kelly et al GRL2001, Cornillon and Park
  GRL2001).
• Positive correlation between wind stress
  anomalies and SST anomalies.
• Hashizume et al JGR2001, Xie BAMS2004, Liu et al
  GRL2000
• Wind stress curl and divergence related to
  crosswind and alongwind components of SST
  gradient.
• Chelton et al JCLI2001, SCI2004

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Surface roughness over Gulf Stream
SAR image showing convective cells over Gulf
Stream smooth waters over cool water. Sikora et
al 1995.
Photo taken over Gulf Stream looking towards
cooler shelf waters. Courtesy P. Chang and D.
Chelton.
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Positive correlation between SST and wind speed
on ocean mesoscales
Small et al 2007, accepted A review of air-sea
interaction over ocean fronts and eddies. Dyn.
Atm. Ocean.
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Schematic showing the relationship of wind stress
curl (hashed) and divergence (stipled) to flow
across an SST front (thick isotherm). (From
Maloney and Chelton (2006)).
SST effects on wind stress divergence and curl.
From Chelton et al (2004). Shown are binned
scatter plots of spatial high-pass filtered
fields of the wind stress divergence from
QUIKSCAT as a function of the downwind SST
gradient (top row) from AMSR-E and the wind
stress curl as a function of the crosswind SST
gradient (bottom row) for four geographical
regions the Southern Ocean (60S to 30S, 0 to
360E), the eastern tropical Pacific (5S to 3N,
150W to 100W), the Kuroshio Extension (32N to
47N, 142E to 170W), and the Gulf Stream (35N
to 55N, 60W to 30W).
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Direct Effects - Stratification

• Change in surface heat flux due to change in
  surface stability AND change in wind speed
• Over 1200Wm-2 in Gulf Stream (Doyle and Warner
  MWR1990,
• Correlation of heat fluxes with TIWs (Thum et al
  JCLI2002).
• Changes in stratification sometimes formation
  of internal boundary layer
• (Hsu 1984, Rogers 1989, Anderson 2001)
• Change in boundary layer height several hundred
  meters higher on warm side of Gulf Stream
• (Sweet et al MWR1981, Wayland and Raman BLM1989).
• Change in cloud height
• (Holt and Raman MWR1992).

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Eastern Pacific Investigation Climate Processes
(EPIC)
EPIC 2001 observations of the MABL across the
Equatorial Front at 95 W. Potential temperature
(a) from representative dropwinsoundes in the
cold tongue (dot-dash) and north of the front
(solid). C), Cross-section composited from in
situ data from 8 flights by the NCAR C-130
aircraft. Adapted from deSzoeke et al. (2005).
WARM WATER
COLD TONGUE
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Direct Effects Winds and secondary circulations

• Change in wind profile due to momentum mixing and
  pressure gradients
• Changes in turbulent momentum flux as air crosses
  front, momentum from upper levels passed to
  surface (Sweet et al MWR1981, Hayes et al
  JCLI1989,
• Changes in thermal structure lead to hydrostatic
  pressure anomalies which force winds (Small et al
  JCLI2003, Cronin et al JCLI2003)
• Secondary circulations (akin to land-sea breeze)
• (Hsu JGR1984, Wai and Stage QJRMS1989, Warner et
  al MWR1990, Sublette and Young MWR1996)

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Tropical Instability Waves
Small et al 2003, JCLI. Model/satellite data
intercomparison.
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Tropical Instability Waves
Observations from TAO moorings (Cronin et al
2003, JCLI) confirmed the downstream pressure
response.
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Indirect Effects Observations

• Atmospheric fronts can form over Gulf Stream
  thermal gradient and can influence cyclogenesis
• (Doyle and Warner MWR1990).
• Holt and Raman MWR1992
• Rapidly growing synoptic storms (bombs) can
  intensify over, and track along, the Gulf Stream
• (Colucci BAMS1977, Sanders MWR1986, Businger et
  al 2005., Jacobs et al MWR2005)
• Intense storms (bombs) can have warm cores with
  bent-back warm fronts
• (Neiman and Shapiro MWR1993, Businger et al
  MAP2005).

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Atmospheric shallow Fronts and Gulf Stream
Holt and Raman 1992 a coastal front aligned
with Gulf Stream
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Indirect Effects Theory and Models

• Synoptic storm development optimally comprises an
  upper level trough (or potential vorticity
  anomaly) and surface temperature gradient
  (baroclinicity).
• Cyclonic flow induced by upper vorticity anomaly
  will form a temperature anomaly at surface via
  advection (Holton Textbook 2004).
• Can induce mutually reinforcing PV anomalies at
  surface and upper levels (Hoskins, McIntyre and
  Robertson QJRMS1985, Stoelinga MWR1996).

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Indirect Effects Theory and Models

• Synoptic storm development optimally comprises an
  upper level trough (or potential vorticity
  anomaly) and surface temperature gradient
  (baroclinicity).

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Indirect Effects Theory and Models

• Differential diabatic heating can enhance storm
  growth
• Surface fluxes very important at very beginning
  of growth stage (Kuo et al MWR1992,
• Latent heating gives rise to a low level (800 mb
  ) PV anomaly (Stoelinga MWR1996).
• Diabatic heating induces an upper level mass flux
  divergence to enhance storm and can destabilise
  atmosphere (Businger MAP2005.)
• Surface sensible heating can act to enhance or
  reduce low level thermal structure (fronts)
• (Doyle and Warner MWR1990, Stoelinga MWR1996).

q is Ertels PV, ? is absolute vorticity, ? is
density and ? is potential temperature. d ?/dt is
diabatic heating. (Stoelinga 1996)