W B C F AS I: G S K E

flinkexistenceMechanics

Oct 27, 2013 (3 years and 10 months ago)

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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

uxes 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




KE

GS

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


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



A
tmosphere

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




E
nhancement
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


KE

GS

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
.)
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
o
r 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


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
𝜕
𝑡
=
𝑄
+
𝑎𝑑𝑣
.

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