Air-Sea interaction over ocean fronts and eddies R. Justin Small National Center for Atmospheric Research (NCAR) Boulder USA Based on: Air-Sea interaction over ocean fronts and eddies, a review paper in Dyn. Atm. Oce., 2008, 45, 274-319. Authors Small, deSzoeke, Xie, O’Neill, Seo, Song, Cornillon, Spall, Minobe Updated with new studies from 2008-present
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Air-Sea interaction over ocean fronts and eddies
R. Justin Small
National Center for Atmospheric Research (NCAR)
Boulder USA
Based on: Air-Sea interaction over ocean fronts and
eddies, a review paper
in Dyn. Atm. Oce., 2008, 45, 274-319.
Authors Small, deSzoeke, Xie, O’Neill, Seo, Song, Cornillon, Spall, Minobe
Updated with new studies from 2008-present
A motivation: Strong wind frequency
Sampe and Xie, 2007. Mapping high sea winds from space’, BAMS.
Liu, Tang and Xie 2008. Wind power distribution over the ocean. GRL.
Wind power density
Frequency of 10m winds > 20m/s
%
% QuikSCAT scatterometer data
SST
Overview
• Early work, proposed mechanisms
• Case studies:
– 1. Equatorial Front
– 2. Gulf Stream
– Satellite data, Field experiments and modelling
• Broader Impacts:
– Feedback on ocean
– Storm Track response
– Climate response
– Modelling issues
(1) Role of pressure gradients
• Lindzen and Nigam (1987) noted that air temperature anomalies
were proportional to SST anomalies throughout the boundary layer in
the tropics.
• Ideal gas law and hydrostatic balance then implies that pressure
gradients are negatively correlated with SST gradients.
• Winds driven by pressure gradients lead to surface convergence and
convection.
WARM SST COLD SST
WARM
AIR
COLD
AIR
Low pressure High pressure gz
p
Lindzen-Nigam schematic
Front
Cold SST
Warm SST
Max SST P
y
SSTvTSSpAssume
dzy
pvsovdz
y
pLinearise
vVCddzy
pequatornearvVCd
hdz
y
p
hUf
dragpressureCoriolisbetweenbalance
layerboundaryoverIntegratebalancemomentumV
ellayerboundaryNigamandLindzen
1
10
.~0:.11
:,,
:
mod
Near equator –
Coriolis term
dropped
However… • Wallace et al (JCLI, 1989) noted that strongest meridional
winds are NOT observed at the SST front: instead there is wind divergence at the front.
• Confirmed by Chelton et al JCLI, 2001 using satellite scatterometer data.
(2) Role of vertical mixing • Wallace et al, Hayes et al (JCLI, 1989) proposed that
changes of near-surface stability across an SST front modify the wind profile.
Wind speed
Hei
ght
Wind or over warm SST
Wind or over cold water
COLD WARM
STABLE UNSTABLE
Hei
ght
Potential Temperature
Air flowing from cold to warm water
wuzt
U
0
1
Case 1. Equatorial Pacific
• Cold tongue/warm pool complex, ITCZ, cross-equatorial winds
• Data from Eastern Pacific Investigation of Climate Processes (EPIC)
Model vs Eastern Pacific Investigation of Climate Processes (EPIC) data
Dashed: cold tongue
Solid: north of front
Potential Temperature Profiles
Mean of 8 days on which EPIC NCAR C130 flights were run along 95W.
Model vs Eastern Pacific Investigation of Climate Processes (EPIC) data
Meridional Wind profiles
Dashed: cold tongue
Dots: north of front
Mean of 8 days on which EPIC NCAR C130 flights were run along 95W.
So far all agrees with momentum mixing hypothesis.
• However…
95° W: relationship of SST gradients to pressure gradients
Because of thermal advection, air temperature gradient is broader and more downstream than SST gradient …consequently pressure gradient is located downstream, at around 3-4 N.
Latitude
SST gradient (d/dy, per 100 km)
Surface Tv gradient
Pressure Grad. Negated hPa/100km
A simple thermal advection/surface heating balance model confirmed this mechanism (Small et al 2005).
Meridional velocity also peaks at this latitude suggesting that here
dzy
pv
vdzy
pLinearise
vVCddzy
p
equatornear
vVCdh
dzy
p
hUf
topatfluxtentrainmenneglect
surftoph
dzy
p
hUf
dragpressureCoriolisbetweenbalanceLN
layerboundaryoverIntegrate
zy
pfu
Dt
Dv
balancemomentumV
yy
1
0
.~0
.11
)(
//11
~
:,,:
11
Latitude
SST gradient (d/dy, per 100 km)
Surface Tv gradient
Pressure Grad. Negated hPa/100km
Meridional Velocity
What is the atmospheric response to transients
(ocean mesoscale eddies)?
Wind response to Equatorial Front (2)
Tropical Instability Waves: •Affect winds (Hayes et al 1989, Xie et al 1998, Liu et al 2000, Chelton et al 2001, Small et al 2003)
•Affect clouds (Deser et al 1993)
•Affect heat budget of cold tongue through horizontal advection and vertical mixing -Jochum and Murtugudde 2006, Menkes, Vialard et al 2006, Moum et al. 2009)
•Affect biology (Yoder et al)
•Coupled processes and feedbacks on ocean(Seo et al 2007, Small et al 2009)
•Affect mean climate (Jochum et al 2005, Seo et al 2006) …and climate variability (Jochum, Deser and Philips 2007)
From Chelton et al. 2001, J. Cli.
Tropical Instability Waves
MODEL.
Regional atmospheric
model simulation.
Response to daily
evolving, realistic SST.
Small et al 2003, JCL.
OBSERVATIONS.
2 month record of SST,
neutral 10 m winds, from
TMI and QuiKSCAT,
filtered and regressed onto
SST.
Hashizume et al 2001, JGR
Surface winds are driven by the pressure
Small et al 2003, JCLI.
Observations from TAO moorings (Cronin et al 2003, JCLI) confirmed the downstream pressure response.
An analysis of wind response to SST in
oceanic eddies: 40°S to
40° N
Cross-spectral analysis reveals a consistent relationship between SST and wind speed in the Atlantic, pacific and Indian Oceans.
Based on an analysis of QuikSCAT neutral 10 m winds and TRMM TMI SST.
Small et al 2005b(JGRO)
See also Chelton et al 2001JCLI, 2004Sci..
Xie 2004BAMS
Wind speed response in m/s/K
Phase difference between wind speed and SST
Positive correlation between SST and wind speed on ocean mesoscales
Small et al 2008: “A review of air-sea interaction over ocean fronts and eddies.” Dynamics of Atmospheres and Oceans. Contains descriptions of all proposed mechanism of atmospheric response, and coupled effects. These processes are now seen in coupled models e.g. Bryan et al., CCSM, thus confirming observations…
Case 2. Gulf Stream • Important supplier of heat to atmosphere – Trenberth and Caron 2001,
Wunsch 2005
• Helps warm Europe and downstream regions? (but see Seagur et al 2002, and Rhines and Hakkinen 2008)
• Part of Meridional Overturning Circulation – may reduce in future climate change (e.g. Dai et al 2005, Hurrell et al. 2006)?
• Important to storm tracks -diabatic heating at ocean fronts help maintain atmospheric storm tracks (Hoskins and Valdes 1990, Nakamura et al 2008) and annular modes (Nakamura et al 2008).
• A number of numerical modeling studies suggest that smoothing out the SST gradients in western boundary currents leads to much reduced storm track activity (Minobe et al 2008, Taguchi et al 2009)
• Effect on individual storms, bombs (Sanders 1986) –e.g. Cione et al 1993, Jacobs et al 2005 relate storm growth to temperature contrast from coast to Gulf Stream north wall (surface Baroclinicity).
Annual mean Mean heat fluxes
From Yu and Weller 2007 – WHOI analysed flux datasets.
Change of surface stability across Gulf
Stream north wall. a) photograph, b)
SAR image, c) Ta-SST. From Chelton et
al. 2006, Sikora et al 1995. Reproduced
in Small et al. 2008.
Roughness changes across Gulf Stream
From Minobe et al., 2008. Relationship of surface wind convergence, surface
pressure, deep vertical motion and the Gulf Stream front. Ascent linked to
Laplacian of sea level pressure (Lindzen-Nigam type model).
Deep motion over Gulf Stream
Czaja 2011
• Fraction of days (in percent) for which the criterion stp − so < 0 is met poleward of 20° for (a) the Northern winter of 2003–2004 and (b) the Southern hemisphere winter of 2004. The calculation was not carried out over continent (black) and sea-ice (fraction of days set to zero) covered grid points. This figure is available in colour online at wileyonlinelibrary.com/journal/qj
Areas of Deep convection in mid-latitudes
Quarterly Journal of the Royal Meteorological Society
Modelling Issues • Most reanalyses and models underestimate
coupling between ocean SST and overlying stress, even when high-resolution SST is used (Maloney and Chelton 2006).
• This may underestimate the impact on the deeper atmosphere, & remote teleconnections
• Problem may lie with the boundary layer schemes (Song et al. 2009), vertical/horizontal resolution, or with representation of the background state.
• Investigations by e.g. NCAR, OSU.
Bibliography • D.B. Chelton, M.G. Schlax, M.H. Freilich, R.F. MilliffSatellite measurements reveal persistent small-scale features in ocean
winds Science, 303 (2004), pp. 978–983
• S.-P. XieSatellite observations of cool ocean–atmosphere interaction Bull. Am. Meteor. Soc., 85 (2004), pp. 195–208
• Nakamura, H., T. Sampe, Y. Tanimoto, and A. Shimpo, 2004: Observed associations among storm tracks, jet streams and midlatitude oceanic fronts. Geophys. Mono. Ser, 147, 329-345.
• Nakamura, H., Sampe, T., Goto A., Ohfuchi, W. and S.-P. Xie, 2008. On the importance of midlatitude frontal zones for the mean state and dominant variability in the tropospheric circulation. Geophys. Res. Letts., 35, L15709, doi:10.1029/2008GL034010
• L.W. O’Neill, D.B. Chelton, S.K. Esbensen, F.J. WentzHigh-resolution satellite measurements of the atmospheric boundary layer response to SST variations along the Agulhas Return Current. J. Climate, 18 (2005), pp. 2706–2723
• Small, R. J., S. P. DeSzoeke, S. P. Xie, L. O’Neill, H. Seo, Q. Song, P. Cornillon, M. Spall and S. Minobe, 2008: ‘Air-sea interaction over ocean fronts and eddies’, Dyn. Atmos. Ocean. 45, 274–319. doi.org/10.1016/j.dynatmoce.2008.01.001
• Booth, J. F., L. Thompson, J. Patoux, K. A. Kelly and S. Dickinson, 2010. The signature of midlatitude tropospheric storm tracks in the surface winds. J. Climate, 23, 1160-1174.
• Kelly, K., Small, R. J., Samelson, R. M., Qiu, B., Joyce, T., Kwon, Y.-O., and Cronin, M., 2010. Western boundary currents and Frontal Air-Sea Interaction: Gulf Stream and Kuroshio Extension., J. Climate, 23, 5644-5667, doi: 10.1175/2010JCLI3346.1.
• Kushnir, Y., W.A. Robinson, I. Bladé, N.M.J. Hall, S. Peng, and R. Sutton, 2002: Atmospheric GCM response to extratropical SST anomalies: synthesis and evaluation. J. Climate, 15, 2233-2256.
• Kwon, Y.-O., M.A. Alexander, N.A. Bond, C. Frankignoul, H. Nakamura, B. Qiu, L. Thompson, 2010: Role of Gulf Stream and Kuroshio-Oyashio Systems in Large-Scale Atmosphere-Ocean Interaction: A Review. J. Climate, 23, 3249–3281.
• Chelton, D. B., and S.-P. Xie 201. 0Coupled ocean-atmosphere interaction at oceanic mesoscale. Oceanography, 23, 52-69.
• Bryan, F. O. , R. Tomas, J. M. Dennis, D. B. Chelton, N. G. Loeb, and J. L. McClean, 2010. Frontal scale air-sea interaction in high-resolution coupled climate models. J. Climate, 23, 6277–6291. doi: 10.1175/2010JCLI3665.1.
• Minobe, S., Kuwano-Yoshida, A., Komori, N., Xie, S.-P., and R. J. Small, 2008. ‘Influence of the Gulf Stream on the troposphere’. Nature, 452, 206-209.