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Tropical Teleconnections and Western Boundary Current Variability. Michael Alexander NOAA/Earth System Research Lab WBC Workshop January 2009. WBC Web page ( Obs. Model. DJF SLP Contour (1 mb); FMA SST (shaded º C). El Ni ñ o – La Ni ñ a Composite:.

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Tropical teleconnections and western boundary current variability

Tropical Teleconnections and Western Boundary Current Variability

Michael Alexander

NOAA/Earth System Research Lab

WBC Workshop January 2009

El ni o la ni a composite

Obs Variability


DJF SLP Contour (1 mb); FMA SST (shaded ºC)

El Niño – La Niña Composite:

Bridge related question 1
Bridge related question 1 Variability

  • What impact does the wind stress (curl) associated with ENSO have on the Kuroshio via Rossby wave generation?

    • If impact is modest (Schneider), why is this the case since it so strongly projects on PDO forcing? Frequency of forcing?

    • Role of ENSO in maintaining NPGO related wind stress forcing.

Correlation pdo sat

CCSM3 Variability

Correlation PDO & SAT


Correlation between enso and the pdo
Correlation between ENSO and the PDO Variability

Why do IPCC-class models under estimate the connection

between the PDO and the tropical Pacific (ENSO)?

Composite ni o ni a sst c shaded

FMA Variability

(yr 1)

Yr 0: Nino 87, 91, 97, 02, 06; Nina 88, 98, 99, 05


(yr 0)

Composite Niño - Niña SST (°C; shaded)

Mean Con Int 4°C

Bridge related question 2
Bridge-related question #2 Variability

  • What is impact of ENSO on WBCs in seasons other than winter?

    • e.g. Kuroshio in summer

    • Are there feedbacks on the atmosphere?

    • If so does it involve different processes than in winter?

  • While Atlantic signal has been known before high resolution SST data reveal its tightly confined to GS/North Atlantic Current.

Storm track trajectory counts of surface lows
Storm track Variabilitytrajectory counts of surface lows

Climatological Mean JFM

Niño -Niña

Alexander et al. 2006 J Climate

Composite el ni o la ni a jfm
Composite El Niño - La Niña JFM Variability

Alexander & Scott 2008 J Climate

Bridge related questions 3
Bridge Related Questions #3 Variability

  • Course resolution AGCM-MLM suggest a positive feedback of bridge-related SSTs in the Atlantic on the NAO

    • Is this robust?

    • Role for intense air-sea interactions over GS?

  • Non-linearity of ENSO response (not shown) much stronger response during La Niña then El Niño.

    • If robust, why?

    • Issues for studies that linearly remove ENSO before conducting statistical analyses.

Atlantic teleconnections

SST ML NDJ Variability

Specified SST

AGCM z500 Jan

AGCM + ML Jan z500

Atlantic teleconnections

Peng et al. 2005, 2006

Observed lead lag mca slp sst
Observed Lead-Lag MCA SLP SST Variability

  • Model studies suggest that tropical Atlantic impacts N. Atlantic in winter (with extratropical air-sea feedback) results from observations much less clear.

Frankignoul and Kestenare 2005

Indian ocean teleconnections
Indian Ocean Teleconnections Variability

Cor w GOGA


Z 500

Cor w GOGA



Hoerling et al. 2004 Climate Dynamics

Indian ocean nao teleconnection
Indian Ocean - NAO teleconnection Variability

  • How robust is this connection especially with poor match for circulation over the Pacific.

  • Model dependence - precipitation over the Indian ocean much strong in CCM (Hoerling et al) than in CAM (Deser and Phillips 2006).

  • Indian Ocean steadily warming (most likely due to global warming) - so getting this signal right could be very important.

  • If this teleconnection is realistic is there feedback from the N. Atlantic Ocean.

Sfm model experiment additional heat flux forcing wm 2 ndjfm 0

SFM Model Experiment - VariabilityAdditional Heat Flux Forcing (Wm-2) NDJFM(0)

SST (°C)

Experiment control

SST (°C) Variability


Experiment - Control





Di Lorenzo et al. Variability

Tropical extratropical interactions

CCSM3 Variability

Zhong et al. 2008

Tropical - Extratropical Interactions




Deser et al. 2004

Life Cycle of NPM Variability

Subpolar Route

Subtropical Route(s)



Regression on KOE SST

-16 yr

-12 yr

-8 yr

-4 yr

0 yr

4 yr

Origin of NPM: Subtropics vs. Subpolar Variability

Modeling Surgery Experiment

Origin: Subploar Route !

Extratropics tropics
Extratropics => Tropics Variability

What processes are important?

  • Atmospheric (e.g Kwon’s talk)

    • Direct response to SSTs?

    • WES

    • Shift of ITCZ to the warm hemisphere

    • Several could be involved with SFM

  • Ocean

    • Rossby waves +> Kelvin waves on western boundary

    • Subduction

      • Appears to be overturning rate rather than advection of anomalies

  • How important relative to tropical only variability?

A Relay Teleconnection Variability

Fast Leg

Slow Leg

Return Atmospheric bridge

N. Pacific








Figure 11: Schematic diagram of the extratropical-tropical relay teleconnection. A persistent strengthening ( weakening) of the Aleutian Low creates

cooling (warming) in the western-central North Pacific and warming (cooling) in the eastern subtropical Pacific; the latter rapidly propagates to the

tropics through the WES feedback to create warming ( cooling) in the equatorial region nearly synchronously; this fast surface coupled process

operates annually and creates persistent westerly (easterly) anomalies in the subtropics, leading to a weakening ( strengthening) of the STC to

further sustain the warming ( cooling) in the tropics. As a result, the warming (cooling) in the tropics can further intensify ( reduce) the Aleutian

Low through return atmospheric bridge, forming a positive feedback loop.

Wu et al. (2007, Climate Dynamics)

Issues directions
Issues/Directions Variability

  • Getting tropical-extra tropical connections (e.g. ENSO-PDO) is critical

    • As key as any local WBC metric for WBC variability

    • New datasets that suggest teleconnections can be WBC focused (e.g. ENSO => Gulf Stream).

      • What causes this.

    • Non-normal/nonlinear impact of remote teleconnections

      • Sampling, are they robust?

      • What causes nonlinearities

      • Methods to remove ENSO

    • Model experiments to isolate forcing and feedbacks

      • (w/wo) air-sea interaction

      • Model surgery

      • Consider a set of model experiments across models

Extratropical tropical connections

Spring-Summer: atmosphere Variability

Responds to subtropical SSTs





Winds drive ocean

Leads to ENSO


Upwelling +entrainment

Extratropical => Tropical Connections

Seasonal Footprinting Mechanism (SFM)


Meridional cross section through the central Pacific

(SFM: Vimont et al. 2003; Subduction: Schneider et al. 1999 JPO)

NPGO Variability


Di Lorenzo et al.

Subduction and the subtropical cell
Subduction and the Subtropical Cell Variability

Transport at 9ºN & 9ºS

Convergence & SST

McPhaden and Zhang 2002 Nature

Climate indices
Climate Indices Variability



Indian Ocean SST

- SPCZ Rain


(poleward side)


(eq’ward side)

D Cloud

(C Eq Pac)


(Indian – Pac)

(Boreal Winter)




- NP Index

Experiment design
Experiment Design Variability

  • Model:

    • AGCM: CCM3

    • Reduced Gravity Ocean (Cane-Zebiak) Model 30S-30N in Pacific.

    • Slab model over remainder of the ocean

    • Models are anomaly coupled

  • 100-year Control run

  • SFM Experiment

    • Add additional heat flux forcing associated with the NPO

      • 20°S-60°N; similar results when forcing > 10°N

    • Initiate 60 heat flux anomaly runs from Nov in control run.

    • Apply Heat flux anomaly during first Nov-Mar

    • Then let model evolve with unperturbed fluxes for 12 more months.

  • Compare ENSO evolution in perturbation and control runs.

  • Note: model already includes SFM

Additional sfm forcing

2nd EOF SLP & Qnet Nov-Mar Variability



Exp n+2

Exp n+1

Exp n








Additional SFM Forcing

  • NPO from AGCM

    • With Climatological SST

      • Isolates intrinsic variability

    • 2nd EOF of SLP EOF in North Pacific in Winter

    • Regress Sfc Heat flux on PC

    • double flux values

      • Max values of ~30 Wm-2

  • Add identical/constant forcing in each of the experiments

The atmospheric bridge
“The Atmospheric Bridge” Variability

Meridional cross section through the central Pacific

(Alexander 1992; Lau and Nath 1996; Alexander et al. 2002 all J. Climate)

FIG. 10. The El Ni Variabilityño – La Niña composite of U200 (m/s) during JFM(1) for (a) EKM, and (b) Δ. The shading (contour) interval in (a) is 1.5 (3) m/s. In panel (b), the shading indicates 95% and 99% confidence limits for the Δ U200 (contour interval 0.5 m/s).

FIG. 9. The El Ni Variabilityño – La Niña composite of Z500 (m) during JFM(1) for (a) observed (1950-1999), (b) EKM, (c) MLM, and (d) Δ. The shading (contour) interval is 5 (10) m in (a)-(c). In panel (d), the shading indicates 95% and 99% confidence limits for the Δ Z500 (contour interval 5 m).

FIG. 3. The El Ni Variabilityño – La Niña composite of SST (˚K) during JFM(1) for (a) observed (1950-1999), (b) EKM, (c) MLM, and (d) EKM-MLM (Δ). The shading (contour) interval is 0.1 (0.5) ˚K in (a)-(c). In panel (d), the shading indicates 95% and 99% confidence limits for the Δ SST (contour interval 0.1 ˚K). The large box (panels (b)-(d)) indicates the region of prescribed SST forcing and the smaller boxes (panel (d)) in the N. Pacific and N. Atlantic will be used for regional averaging (see FIG. 8).

Experiment control1

Winds & Qlh Variability

SST (°C)


Experiment - Control

Thermocline depth (m)


Central North Pacific Variability


Colored contours -0.3C anomaly

isotherms for 3 different pentads

Black lines – mean isopycnal surfaces

(lines of constant density)

Averaged over 170ºW-145ºW

Mechanism for atmospheric circulation changes due to el nino southern oscillation
Mechanism for Atmospheric Circulation Changes due to El Nino/Southern Oscillation

Atmospheric wave

forced by tropical


Latent heat

release in


Horel and Wallace, Mon. Wea Rev. 1981

Mechanism Loop of NPM Nino/Southern Oscillation


Salinity =>

SP Conv. + Fbk


Ocean or Coupled Mode ?


Atmos. Resp

S. Curl + =>

M-lat. Thm Fbk +

Sfc wind -- =>

WES Fbk +

N. Curl -- =>

SP Thm Fbk --

SP Rossby Wave Delay –

Strong enough to break conv

Enso noaa high res uwnd composite10s 60 n across the n pacific and n atlantic
ENSO NOAA High Res Uwnd Nino/Southern Oscillation composite10S-60 N across the N. Pacific and N. Atlantic

Ni o ni a observed jf 1 wind stress sst
Niño - Niña Nino/Southern OscillationObserved JF(1) Wind Stress/SST



Qek (Wm-2)

Flux form

Propagation Nino/Southern Oscillation

SSH (Color)




Westward Propagation




Westward signal clear in subpolar

Subpolar SSH dominated by salinity


Change in subduction rate
Change in Subduction Rate Nino/Southern Oscillation

Transport at 9ºN & 9ºS

Convergence & SST