WESTERN BOUNDARY CURRENTS AND FRONTAL
AIR–SEA INTERACTION:
GULF STREAM AND KUROSHIO EXTENSION
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. AIR–SEA INTERACTION AND WBCS: A BRIEF

a.)WBC temperature structure and air–sea fluxes
Gulf stream



OVERVIEW
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
KE

GS
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
2. AIR–SEA INTERACTION AND WBCS: A BRIEF

OVERVIEW
a.)WBC temperature structure and air–sea fluxes
KE
GS
Latent heat flux
Sensible heat flux
In march,
Latent heat exceed 200 W m-2 , sensible heat exceed 200 W m-2
2. AIR–SEA INTERACTION AND WBCS: A BRIEF


OVERVIEW
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
* marine BLD
－ SST
2. AIR–SEA INTERACTION AND WBCS: A BRIEF

OVERVIEW
b.) Boundary layer interactions and near-surface winds
Topography
Frequency of high wind event(>20 m-s)
White contour: SST
2. AIR–SEA INTERACTION AND WBCS: A BRIEF
OVERVIEW

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
KE
850hPa
GS
day-1
2. AIR–SEA INTERACTION AND WBCS: A BRIEF

d.) Deep atmospheric response to WBCs
shaded : vertical velocity
black: boundary layer height
contour: wind convergence
(b)(c)SST contour(black)
OVERVIEW
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)
3. OCEANOGRAPHY

OF THE
GS
AND
KE SYSTEMS
a.) Mean WBC properties
KE
GS
Steep topography
3. OCEANOGRAPHY

a.) Mean WBC properties
Warm core
 SRG

OF THE
GS
AND
KE SYSTEMS
3. OCEANOGRAPHY

OF THE
GS
b.) Path and transport statistics
stable
unstable
AND
KE SYSTEMS
Monthly path of KE from SSH
3. OCEANOGRAPHY

OF THE
GS
b.) Path and transport statistics
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)
3. OCEANOGRAPHY

OF THE
GS
AND
KE SYSTEMS
b.) Path and transport statistics
GS
interannual change
KE
decadal change
But path/transport correlation
is not significant in KE or GS
during altimeter obs.
3. OCEANOGRAPHY

OF THE
GS
AND
KE SYSTEMS
c.) SST signatures of path and transport anomalies
(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
Dark contours enclose
the regions where correlations with the
indices exceed 95% confidence level of
0.23(GS)/0.31(KE)
4. THERMODYNAMICS

AND DYNAMICS OF THE
WBCS
a.) Upper-ocean heat budget
In the upper 800 m
Heat storage rate
highly correlated with
Advection/diffusion
rather than
sfc. heating
4. THERMODYNAMICS

AND DYNAMICS OF THE
WBCS
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
Thick STMW
↔stable path
4. THERMODYNAMICS

AND DYNAMICS OF THE
WBCS
c.) Ocean forcing of air–sea fluxes
Correlation between the
turbulent fluxes and SSH
SSH(solid); turbulent flux(dash)
GS: SSH leads by 3
months
KE : not significant
5. DISCUSSION

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
 Difference
the cause & implication of STMW

MLD :GS 250m ; KE 150m
𝜕𝐻
= 𝑄 + 𝑎𝑑𝑣.
𝜕𝑡
5. DISCUSSION

a.) Ocean state
3. Less heat flux to the atmosphere
1. Less advection
2. STMW (heat storage)more
5. DISCUSSION


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 deeptropospheric 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
