W ESTERN B OUNDARY C URRENTS AND F RONTAL A IR –S EA I NTERACTION : G ULF S TREAM AND K UROSHIO E XTENSION KATHRYN A. KELLY et al. 2011/9/29 Wen-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. A IR – SEA INTERACTION AND WBC S : 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. A IR – SEA INTERACTION AND WBC S : A BRIEF OVERVIEW a.)WBC temperature structure and air–sea fluxes Air-sea temperature difference during winter time KE is as large as that over the GS  suggesting that the Japan/East Sea does not appreciably warm the overlying air KEGS

2. A IR – SEA INTERACTION AND WBC S : A BRIEF OVERVIEW a.)WBC temperature structure and air–sea fluxes KE GS In march, Latent heat exceed 200 W m -2, sensible heat exceed 200 W m -2 Latent heat flux Sensible heat flux

2. A IR – SEA INTERACTION AND WBC S : A BRIEF OVERVIEW a.)WBC temperature structure and air–sea fluxes KE GS Standard deviation of daily wintertime (January–March) Possibly owing to higher synoptic activity in the Pacific farther downstream from the western boundary than in the Atlantic

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. A IR – SEA INTERACTION AND WBC S : 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. A IR – SEA INTERACTION AND WBC S : A BRIEF OVERVIEW * marine BLD － SST

b.) Boundary layer interactions and near-surface winds 2. A IR – SEA INTERACTION AND WBC S : 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. A IR – SEA INTERACTION AND WBC S : 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. A IR – SEA INTERACTION AND WBC S : A BRIEF OVERVIEW

c.) Cyclogenesis and synoptic development (Hoskins and Hodges 2002) genesis density: the density of where systems originate 2. A IR – SEA INTERACTION AND WBC S : 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 height contour: wind convergence 2. A IR – SEA INTERACTION AND WBC S : 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. A IR – SEA INTERACTION AND WBC S : A BRIEF OVERVIEW

a.) Mean WBC properties 3. O CEANOGRAPHY OF THE GS AND KE SYSTEMS KEGS Steep topography

a.) Mean WBC properties Warm core SRG 3. O CEANOGRAPHY OF THE GS AND KE SYSTEMS

b.) Path and transport statistics 3. O CEANOGRAPHY OF THE GS AND KE SYSTEMS Monthly path of KE from SSH stable unstable

b.) Path and transport statistics 3. O CEANOGRAPHY 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 statistics 3. O CEANOGRAPHY OF THE GS AND KE SYSTEMS GS interannual change KE decadal change But path/transport correlation is not significant in KE or GS during altimeter obs.

c.) SST signatures of path and transport anomalies 3. O CEANOGRAPHY OF THE GS AND KE SYSTEMS Dark contours enclose the regions where correlations with the indices exceed 95% confidence level of 0.23(GS)/0.31(KE) (a)(c) GS & KE both northward path anomaly ↔positive SST anomaly GS has SST dipole KE path more latitude anomalies than GS Shaded: SST anomalies

a.) Upper-ocean heat budget 4. T HERMODYNAMICS AND DYNAMICS OF THE WBC S In the upper 800 m Heat storage rate highly correlated with Advection/diffusion rather than sfc. heating

b.) Wind forcing of the WBCs 4. T HERMODYNAMICS AND DYNAMICS OF THE WBC S

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. T HERMODYNAMICS AND DYNAMICS OF THE WBC S

b.) STMW(subtropical mode water): The intersection of dynamics and thermodynamics 4. T HERMODYNAMICS AND DYNAMICS OF THE WBC S Thick STMW ↔stable path

c.) Ocean forcing of air–sea fluxes 4. T HERMODYNAMICS AND DYNAMICS OF THE WBC S Correlation between the turbulent fluxes and SSH SSH(solid); turbulent flux(dash) GS: SSH leads by 3 months KE : not significant

5. D ISCUSSION

a.) Ocean state 5. D ISCUSSION 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. D ISCUSSION

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. D ISCUSSION

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. D ISCUSSION

d.) The way forward WBC anomalies Marine boundary layer Storm and atmosphere impact layer Modeling and observations 5. D ISCUSSION