western boundary currents and frontal air–sea interaction: gulf stream and kuroshio extension

WESTERN BOUNDARY CURRENTS AND FRONTAL AIR–SEA INTERACTION: GULF STREAM AND KUROSHIO EXTENSION

KATHRYN A. KELLY et al.2011/9/29Wen-Lin Lin

OUTLINE Introduction Air–sea interaction and WBCs: A brief overview Oceanography of the GS and KE systems Thermodynamics and dynamics of the WBCs Discussion

1. INTRODUCTION Western boundary current (WBC) systems ：

—the Gulf Stream(GS) in the North Atlantic and —the Kuroshio Extension(KE) in the North Pacific

There is a complex interaction between dynamics and thermodynamics and between the atmosphere and ocean.

The ocean’s heat is fluxed to the atmosphere through turbulent exchanges that fuel intense cyclogenesis over the regions.

(Hoskins and Hodges 2002;Nakamura et al. 2004; Bengtsson et al. 2006)

Variations in the GS and KE currents and in air–sea heat fluxes have been shown to be related to the dominant climate indices in each ocean.

(NAO; see Joyce et al. 2000; Qiu 2003;Kelly and Dong 2004; DiNezio et al. 2009)

1. INTRODUCTION Air–sea fluxes in the KE region is suggesting

predictability in the transfer of heat to the atmosphere.

(Kwon and Deser 2007)

Two WBC systems have similar dynamical and thermodynamical roles in the ocean but may differ somewhat in their air–sea interactions ！

2. AIR–SEA INTERACTION AND WBCS: A BRIEF OVERVIEW a.)WBC temperature structure and air–sea fluxes

Gulf stream Kuroshio Extension

Large SST gradient : more than 10℃ over just 200 km in the GS

Turbulent heat fluxes as large as 1000 W m-2 over the GS

North of the KE jet show mean values of more than 600 W m-2

2. AIR–SEA INTERACTION AND WBCS: A BRIEF OVERVIEW a.)WBC temperature structure and air–sea

fluxes

Air-sea temperature difference during winter timeKE is as large as that over the GSsuggesting that the Japan/East Sea does not appreciably warm the overlying air

KE GS

2. AIR–SEA INTERACTION AND WBCS: A BRIEF OVERVIEW a.)WBC temperature structure and air–sea

fluxesKE GS

In march, Latent heat exceed 200 W m-2 , sensible heat exceed 200 W m-2

Latent heat flux

Sensible heat flux

b.) Boundary layer interactions and near-surface winds

Air across warm water become more instable 1. increased vertical exchange of momentum 2. induce wind

2. AIR–SEA INTERACTION AND WBCS: A BRIEF OVERVIEW

b.) Boundary layer interactions and near-surface winds

SST fronts affect atmosphere: 1. the shear in the lower-atmosphere wind profile 2. changes in boundary layer height of up to 2 km

2. AIR–SEA INTERACTION AND WBCS: A BRIEF OVERVIEW

* marine BLD－ SST

b.) Boundary layer interactions and near-surface winds

2. AIR–SEA INTERACTION AND WBCS: A BRIEF OVERVIEW

Frequency of high wind event(>20 m-s)White contour: SST

Topography

b.) Boundary layer interactions and near-surface winds

Atmosphere affect SST:

Stratiform clouds exert POSITIVE feedback [form over cold water]

Convective clouds exert NEGATIVE feedback[form along the WBCs]

2. AIR–SEA INTERACTION AND WBCS: A BRIEF OVERVIEW

c.) Cyclogenesis and synoptic development

Enhancement of low-level baroclinicity by SST gradients will likely increase synoptic storm activity (Nakamura and Shimpo 2004)

Individual synoptic weather often enhanced when they pass over the strong SST gradients of the WBCs (Sanders and Gyakum 1980; Sanders 1986; Cione et al.1993)

2. AIR–SEA INTERACTION AND WBCS: A BRIEF OVERVIEW

c.) Cyclogenesis and synoptic development (Hoskins and Hodges 2002)

genesis density: the density of where systems originate

2. AIR–SEA INTERACTION AND WBCS: A BRIEF OVERVIEW

850hPa

day-1

KE

GS

d.) Deep atmospheric response to WBCs

shaded : vertical velocity (b)(c)SST contour(black)black: boundary layer heightcontour: wind convergence

2. AIR–SEA INTERACTION AND WBCS: A BRIEF OVERVIEW

d.) Deep atmospheric response to WBCs

Deep convection is occurring over the GS and that planetary waves may consequently be excited by the deep heating, with far-field effects extending to Europe. (Minobe et al. 2008)

2. AIR–SEA INTERACTION AND WBCS: A BRIEF OVERVIEW

a.) Mean WBC properties3. OCEANOGRAPHY OF THE GS AND KE SYSTEMS

KE GS

Steep topography

a.) Mean WBC properties

Warm core SRG

3. OCEANOGRAPHY OF THE GS AND KE SYSTEMS

b.) Path and transport statistics3. OCEANOGRAPHY OF THE GS AND KE SYSTEMS

Monthly path of KE from SSH

stable

unstable

b.) Path and transport statistics3. OCEANOGRAPHY OF THE GS AND KE SYSTEMS

Monthly path of GS from SSH

The standard deviation of path latitude for the KE is nearly twice as large as for the GS (0.268 versus 0.468)

b.) Path and transport statistics3. OCEANOGRAPHY OF THE GS AND KE SYSTEMS

GSinterannual change

KEdecadal change

But path/transport correlation is not significant in KE or GS during altimeter obs.

c.) SST signatures of path and transport anomalies

3. OCEANOGRAPHY OF THE GS AND KE SYSTEMS

Dark contours enclosethe regions where correlations with the indices exceed 95% confidence level of 0.23(GS)/0.31(KE)

(a)(c) GS & KE bothnorthward path anomaly ↔positive SST anomaly

GS has SST dipoleKE path more latitude anomalies than GS

Shaded: SST anomalies

a.) Upper-ocean heat budget4. THERMODYNAMICS AND DYNAMICS OF THE WBCS

In the upper 800 m

Heat storage rate highly correlated with Advection/diffusion rather than sfc. heating

b.) STMW(subtropical mode water): The intersection of dynamics and thermodynamics

A thick layer of STMW corresponds to low ocean stratification (low PV), low heat content, and low surface temperatures (Kwon 2003)

Wintertime deep boundary layer subducted into thermocline part of them advected or dissipated

4. THERMODYNAMICS AND DYNAMICS OF THE WBCS

b.) STMW(subtropical mode water): The intersection of dynamics and thermodynamics

4. THERMODYNAMICS AND DYNAMICS OF THE WBCS

Thick STMW ↔stable path

c.) Ocean forcing of air–sea fluxes4. THERMODYNAMICS AND DYNAMICS OF THE WBCS

Correlation between the turbulent fluxes and SSH

SSH(solid); turbulent flux(dash)

GS: SSH leads by 3 monthsKE : not significant

a.) Ocean state

GS & KE have the same …◎ stronger jet, meandering (stability less)◎ transport anomalies associated with change in NRG◎ Changes in the volume of STMW are clearly linked to air–sea interaction

Differencethe cause & implication of STMW

MLD :GS 250m ; KE 150m

5. DISCUSSION

a.) Ocean state

5. DISCUSSION

1. Less advection

2. STMW (heat storage)more

3. Less heat flux to the atmosphere

b.) Impact on atmosphere

Several aspects of the WBCs may contribute to the air–sea interaction.

-the strength and location of the SST gradient of the WBC itself

-the land–sea contrast

The impact of the WBCs may depend on the atmospheric state

The SST fronts of the WBCs modify atmospheric stability and enhance the low-level baroclinicity.

Nakamura et al. (2004) Changes in the strength and stability of the WBCs may

be important in determining low level baroclinicity.

5. DISCUSSION

b.) Impact on atmosphere

Atmospheric circulation patterns modify the jet stream and the relative location of the jet stream and WBC.

(S. Businger2007, personal communication) How WBCs themselves induce a deep-

tropospheric response? -frontal-scale effects over the GS may be felt well above the boundary layer -the planetary wave response may be energetic

5. DISCUSSION

c.) Ocean–atmosphere coupling

The possibility of a coupled response remains an unresolved issue for midlatitude air–sea interaction.Ex: the impact of wind on the KE is simple, but complex on the GS

Use of SST in many climate studies is convenient, but problematic

5. DISCUSSION

d.) The way forward

WBC anomalies Marine boundary layer Storm and atmosphere impact layer Modeling and observations

5. DISCUSSION