1. Role of the Gulf Stream and Kuroshio-Oyashio Systems in Large-Scale Atmosphere-Ocean Interaction: A Review
Authors:
YOUNG-OH KWON, MICHAEL A. ALEXANDER,NICHOLAS A. BOND, CLAUDE FRANKIGNOUL,
HISASHI NAKAMURA, BO QIU,AND LUANNE THOMPSON
Speaker: Yi-Ning Peng

2. Introduction Why western boundary current regions are so important?
The largest mean and variance at interannual and longer time scales of the net surface heat flux (Qnet) over the global ocean occurs in WBC regions. (Wallace and Hobbs 2006)
Oceanic changes are primarily communicated to the atmosphere via fluctuation in SST.
It is important to determine whether the WBCs have a significant influence on the large-scale atmospheric circulation.
強烈的海-氣fluxes of heat and moisture交互作用維持海表斜壓性, 加強暴風, 以及影響暴風路徑

3. Processes affecting the SST variability in the WBCs
Shift of oceanic fronts
Oceanic advection
Surface heat fluxes
Ekman transport
Remmergence mechanism
Remote wind stress curl forcing communicated via oceanic Rossby waves
Tropical atmospheric teleconnections

4. a. Shift of oceanic fronts
North Pacific
North Atlantic
北太平洋的較為複雜 因為其有兩個鋒面 還有兩鋒面混合之區域
Contours: Climatological mean

5. Vertical Structure OE front: surface KE front: below the sea 200m~600m
Black contours: temperature
Color shadings:
the changes in
5-winter mean temperature

6. b. Oceanic Advection
Variability in the heat content of the upper ocean in the WBCs is determined mainly by anomalous geostrophic advection.
Examples: In mid-1980s, intensification of the Oyashio and the southward displacement of its extension generated negative SSTAs along OE front.

7. c. Surface heat fluxes
Qnet in the WBCs undergoes a substantial seasonal cycle
Upward Qnet is strongest in winter and is dominated by latent heat flux along the KE and GS.
The when solar radiation dominates. Qnet is downward in summer

8. The presence of oceanic fronts yield tight cross-frontal
gradients in the sensible and latent heat fluxes.
↓ Sensible heat flux
↓ Latent heat flux

9. d. Ekman transport
Ekman transport responds quickly to changes in surface winds and can thus generate SSTAs with the passage of storms. (Xue et al. 1995)
They can also influence SSTs over a broader region. (Seager et al. 2001; Alexander and Scott 2008)
2. because of the high wind speed fluctuations over the central North Pacific and Atlantic associated with the dominant modes of basinwide atmospheric variability.

10. e. Reemergence mechanism
The solid line denotes
seasonal change of
the mixing layer depth
(MLD)
Above is a local process
typical for regions adjacent
to the WBCs and in the
central and eastern portion
of a basin.
MLD隨著季節之改變可導致中緯度SSTAs再發(此個冬季到下個冬季) 這個過程稱為reemergence mechanism
此項機制在北大西洋 北太平洋大區域常發生 在冬季15%~35%的SST在自是因為此

11. e. Reemergence mechanism-along WBCs
Strong advection by mean current can transport the anomalies over ~800km along the North Atlantic Current and ~4000km along the KE over a course of a year.

12. f. Remote wind stress curl forcing communicated via oceanic Rossby waves
SSH anomalies the wind-forced PDO SST index
baroclinic Rossby wave
當PDO是正的時候 他會冷卻中部北太平洋 並且產生負的SSH anomalies
North Pacific

13. g. Tropical atmospheric teleconnections
Atmosphere bridge
heat flux,
freshwater
flux,
momentum
flux
heat flux, freshwater flux, momentum flux 從赤道中轉換到中緯度
其會影響SST 鹽度 MLD 洋流..等等

14. WBCs and the basin-scale climate variability
Decadal SST variability in the WBCs
Oceanic dynamics and decadal variability??
Surface heat fluxes damping and implications
Decadal variability in atmosphere-ocean coupled general circulation models

15. a. Decadal SST variability in the WBCs
The SST variance exhibits maxima in the central North Pacific and near 40N along the KOE.
In the North Atlantic, decadal SST variability is enhanced along the GS and its extension east of ~40W
Location
SSTAs variability
Central North Pacific
Interannual
KOE
decadal

16. a. Decadal SST variability in the WBCs
The pattern-based observational data analyses suggest that both the interannual and decadal time scales of the PDO are related to SSTAs in
the tropical Pacific. (Nitta and Yamada 1989; Deser and Blackmon 1995; Barlow et al. 2001; Newman et al. 2003; Alexander et al. 2002, 2008; Zhang et al. 1997; Guan and Nigam 2008)
PDO
同時 低頻的SST變異在北太平洋也受到西熱帶太平-印度洋之遙相關影響(decadal time)
Time scale
Be influenced by
Interannual time
Heat fluxes
Decadal time
Wind-driven ocean circulation

17. c. Surface heat fluxes damping and implications
Decadal WBC SSTAs are primarily forced by ocean dynamics, especially for time scales longer than a few years and damped by surface heat fluxes, implying anomalous heat transfer to the atmosphere.
The large-scale atmosphere circulation response appears to be more sensitive to the SSTAs in the WBC regions because of the proximity to the storm track. (Peng and Whitaker 1999)

18. d. Decadal variability in atmosphere-ocean coupled general circulation models
Extratropical climate variability can be difficult to assess using observations or single component models, such as OGCM hindcasts, because of the limited sampling.
Coupled climate models generally indicate that ocean–atmosphere interaction in WBC regions is a key factor in generating extratropical decadal variability.

19. Performance of climate models on simulation of WBC variability
High-resolution ocean simulations
Simulations of the Kuroshio and Gulf Stream at non-eddy-permitting resolution
Atmospheric simulations

20. a. High-resolution ocean simulations
SSH gradient
SST gradient
KE front
Sharp
Weak
OE front
The distinction between these two fronts can only
be found in high-resolution ocean models.
[e.g., see Nonaka et al.(2006), and the SSH from the Hybrid Coordinate Ocean Model (HYCOM) simulation discussed by Kelly et al.(2007)].
整體上 目前在對於WBCs的模擬上都還不是很完善
The interaction of the GS with the DWBC requires
that the nonlinear dynamics of the surface currents and
the formation regions, transport, properties, and
dynamics of the DWBC must be adequately represented.

21. b. Simulations of the Kuroshio and Gulf Stream at non-eddy-permitting resolution
North Pacific
North Atlantic
←observations
←coupled
version of CCSM3
Lack of advection of warm water from the boundary into the interior in CCSM3 results in errors in MLD along KOE.
←ocean-only
version of CCSM3
CCSM3: version 3 of Community Climate System Model (NCAR)

22. c. Atmospheric simulations
Atmospheric models used in coupled GCMs are suited for a variety of purposes ranging from understanding decadal-scale climate variability.

23. Outstanding issues
How are the frontal-scale and the basin-scale atmosphere-ocean interactions related?
What is the large-scale atmospheric circulation/wind stress curl response to the WBCs SSTAs?
Global warming and the WBCs
Relation between the Gulf Stream and the MOC
Connection between GS and KOE variability
多重尺度交互作用 2. 大尺度的大氣環流對於WBC SSTA的影響
3. 全球暖化&WBC 目前IPCC的模式太粗糙 模擬不出來
4. GS與MOC(徑向翻轉環流)之前的關係

24. Summary
WBCs are unique locations in the midlatitude ocean. WBCs are of potential importance to decadal climate variability.
Some climate model studies indicate that the basin-scale atmospheric circulation response is strong enough and of the correct spatial structure to generate decadal variability, even though current-generation climate models exhibit substantial deficiencies in reproducing WBCs, which can result in incorrect climate responses