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

Volume 137, Issue 657, pages 1095-1101, 27 APR 2011 DOI: 10.1002/qj.814

http://onlinelibrary.wiley.com/doi/10.1002/qj.814/full#fig2

Accordingly, we classify a grid point

on a given daily map as potentially

unstable to deep (surface to

tropopause) moist convection if, at

that grid point

Stp-s0< 0

• where specific entropy of moist air s

Separation of mechanisms:

• Different mechanisms at different background wind speeds (Spall et al. 2007)

• Momentum budgets (O’Neill et al. , Takatama et al 2011 )

• Mean flow vs synoptic variability (Liu, Xie, pers. comm. 2012)

Fig. 15. Schematic showing boundary layer response to

an ocean frontal boundary. The flow is from cold to warm

SST in a situation with (a) strong background winds and

(b) weak background winds. Below each panel are

horizontal across-front profiles of SST and air temperature

Tair. Above is shown the sea surface, with wave height

increasing to the right of the SST front. Background winds

U at left are incident on the front. A mixed internal

boundary layer (gray shading) develops downstream.

Circular arrows indicate the presence of turbulent eddies.

Forces due to mixing (Fmix, term IV of Eq. (4a)) and

pressure (Fpres, term III) are indicated with thin arrows.

Due to these forces the wind profile becomes uniform with

height (thick black arrows). To the right are shown vertical

profiles of the downstream air temperature anomalies

(dashed) and pressure anomalies (solid).

Boundary layer changes across ocean front

From Small et al. 2008 and based on Spall et al. 2007.

Broader Impacts

• Feedback on ocean

• Response of storm tracks

• Climate response

Feedback onto the ocean

• SST gradients lead to gradients in surface stress which impact the ocean

• Over cool upwelling regions (e.g. Cal. Curr. System) stress is reduced. This may reduce upwelling (Perlin et al., Jin et al. 2009).

• Impact of ocean surface currents on stress have bigger impact on Ekman pumping (Chelton et al. )

• Changes in wind stress curl affect local Ekman pumping (Chelton et al. 2007) and may also affect gyre circulations (Hogg et al.).

Wind stress, its divergence

and curl, over Tropical

Instability Waves.

From Chelton et al. 2001

A schematic representation of how

varying SSTs near a strong front

influence wind stress, and how these

wind stress variations might generate

divergences and curls. Vectors of

surface wind stress are shown.

From Maloney and Chelton 2006

Wind stress curl and divergence

Ekman pumping anomalies due to action of current on surface stress

Figure 8. Ekman pumping (color, 10-6 ms-1cm-1)

and SSHA (contours), both regressed onto

SSHA at 120W, 4.5N.

a) using observations of sea surface height

anomalies (SSHA) from altimeters and

stress derived from 10 m neutral wind from

QuiKSCAT.

b) Fully coupled model.

c) as b) but with estimated Ekman pumping

derived from Exp. 1 vorticity (contours, 10-6

ms-1cm-1).

fxwE

0

1

Let’s analyse the component of stress

’ which is due to current.

0

10

0 2

3

fUCw D

aE

fxwE

0

1

Dewar and Flierl

1987.

From Small et al. (2009)

Storm tracks and ocean fronts • In midlatitudes, atmospheric synoptic eddies pass

along well-defined “storm tracks”.

• Ocean fronts can induce air temperature gradients above, and stability

– Affects the “Eady Growth rate” or baroclinicity

• Near surface, changes in stability across ocean fronts may alter structure of storm track

Surface Storm Track (Booth)

(a) The free-tropospheric storm track, defined as the standard deviation of the bandpass-filtered meridional winds at 850 hPa. Colors

and contours show the same data, with 0.5 (thick lines) and 0.25 m s−1 (thin lines) contour intervals. (b) The surface storm track,

defined as the standard deviation of the bandpass-filtered meridional winds at 10 m, with colors and contours as in (a). (c) Wintertime

mean for air–sea instability (SST minus SAT) of the atmospheric boundary layer. Color and contours show the same data, with

contour interval of 1°C. The blue and magenta boxes indicate the locations where the time correlations are calculated (see text). (d)

Estimate of the surface storm track as defined in the text (dark gray shading) and surface storm track [contours; same data as shown

in (b)]. The mean path of the Gulf Stream, derived from altimetry data, is shown in white in (b),(c), and (d).

Free tropospheric storm track

• Literature unresolved as to what effect ocean fronts has on real storm tracks:

– Ocean fronts essential to eddy variability associated with polar front jet (Nakamura et al 2008)

– Ocean dynamics shifts location of storm track (Wilson et al. 2009, Brayshaw et al. 2009)

– Self-maintenance, eddies and mean jet, no role of ocean (Robinson 2006)

• Let’s show two contrasting viewpoints

Coupled model (coarse resolution)

From Wilson et al. 2009. Storm tracks in four very different realisations of the FORTE

model: (a) control state with mountains and ocean dynamics; (c) with mountains and

without ocean dynamics. The storm tracks are shown by the square root of the high-pass

filtered eddy kinetic energy density (shaded, metres per second) at 250mbar, over ten

winters.

ALL PROCESSES.

NO OCEAN DYNAMICS.

Aquaplanet Model

From Nakumura, Sampe et al. 2008, Sampe et al. 2010. Meridional profiles of the

mean states of (a) SST, b) SST gradient, c) longitudinal variance of 250-hPa

meridional wind fluctuations (m2/s2) associated with subweekly disturbances. Control

(CTL, realistic SST) and No Front (NF, heavily smoothed SST) experiments shown.

Dashed lines – no front

experiment.

c)

Climate Impacts

• Shifts in ocean currents lead to SST anomalies and to heating anomalies in atmosphere

– Shallow and deep

• Linear response to heating (e.g. Hoskins and Karoly 1981, Palmer and Sun 1985)

– Tropical vs mid-latitude

• Non-linear, eddy response (e.g. Peng et al. papers, see Kushnir et al. 2002)

– Eddy fluxes affect the mean circulation

– Eddy effect on annular modes (Lorenz and Hartmann 2001)

Climate Impacts of Oyashio, Kuroshio Extension shifts

• Frankignoul

Estimated response in geopotential height

at (top) 250 hPa, (middle) SLP, and

(bottom) Ekman pumping to a unit value of

the OEI (typical northward displacement),

assuming d = 2 months, based on lag 3.

White (gray) contours are for negative

(positive) values. The dark contours

indicate 10% significance.

Estimated response to a unit value of the

KEI, based on a lag of 2 seasons,

assuming d = 1 season. White (gray)

contours are for negative (positive) values.

The thick continuous line indicates the 10%

significance level.

Frankignoul, C., N. Sennéchael, Y.-O. Kwon, and M.A. Alexander, 2011: Atmosphere-

ocean variability associated with Kuroshio and Oyashio Extension fluctuations. J.

Climate, in-press.

Typical latitudinal profiles of 250-hPa [U] (m/s) for the positive (solid) and negative

(dashed) phases of the annular mode for the (a) winter and (b) summer hemispheres in

the CTL experiment. (c) and (d): As in Figures 4a and 4b, respectively, but for the NF

experiment. Based on the composites for 17~29 events in which the mode index (PC1)

exceeds its 3 standard deviations in magnitude. Circles denote [U] anomalies significant

at the 95% confidence level.

More active storm track over ocean fronts affects annular modes

Nakamura et al. 2008.

Lag-correlation (color shaded) and

regression coefficients (contoured) of

January anomalies of (a) SST, (b) SLP, and

(c) Z250 with the November SAFZ-SST

index. The index is based on the ICOADS

data, while SST anomalies in (a) are based

on HadISST data. SLP and Z250 are

based on the NCEP–NCAR reanalysis.

(d)–(f) As in (a)–(c), but for February. The

contour intervals for the regression

coefficients are shown at the lower-right

corner of each with units of K K−1, hPa K−1,

and m K−1 for SST, SLP, and Z250,

respectively.

Climate Impacts of MERGED Oyashio, Kuroshio Extension shifts

Taguchi, B., H. Nakamura, M. Nonaka, N. Komori, A. Kuwano-Yoshida, and A. Goto, 2012. Seasonal evolution of atmospheric

response to decadal SST anomalies in the North pacific subarctic frontal zone: observations and a coupled model simulation.

Submitted to J. Climate. Available at http://www-aos.eps.s.u-tokyo.ac.jp/nakamura_lab/en/index.php?Publications

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.