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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 (http://www.cdc.noaa.gov/WBC). 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

Model

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

El Niño – La Niña Composite:
bridge related question 1
Bridge related question 1
  • 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 between enso and the pdo
Correlation between ENSO and the PDO

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

(yr 1)

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

ASO

(yr 0)

Composite Niño - Niña SST (°C; shaded)

Mean Con Int 4°C

bridge related question 2
Bridge-related question #2
  • 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 tracktrajectory 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

Alexander & Scott 2008 J Climate

bridge related questions 3
Bridge Related Questions #3
  • 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

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
  • 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

Cor w GOGA

Pacific

Z 500

Cor w GOGA

Atlantic

Precip

Hoerling et al. 2004 Climate Dynamics

indian ocean nao teleconnection
Indian Ocean - NAO teleconnection
  • 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.
experiment control

SST (°C)

NDJ(1)

Experiment - Control

Winds

&

SST

MJJ(0)

tropical extratropical interactions

CCSM3

Zhong et al. 2008

Tropical - Extratropical Interactions

Observation

NP

TP

Deser et al. 2004

slide23

Life Cycle of NPM

Subpolar Route

Subtropical Route(s)

SST

HT400

Regression on KOE SST

-16 yr

-12 yr

-8 yr

-4 yr

0 yr

4 yr

slide24

Origin of NPM: Subtropics vs. Subpolar

Modeling Surgery Experiment

Origin: Subploar Route !

extratropics tropics
Extratropics => Tropics

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?
slide26

A Relay Teleconnection

Fast Leg

Slow Leg

Return Atmospheric bridge

N. Pacific

(WES)

EQ

STC

Positive

Tropics

Extratropics

Feedback

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
  • 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

Responds to subtropical SSTs

Winter:

Intrinsic

atmospheric

variability

Winds drive ocean

Leads to ENSO

?

Upwelling +entrainment

Extratropical => Tropical Connections

Seasonal Footprinting Mechanism (SFM)

Subduction

Meridional cross section through the central Pacific

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

slide30

NPGO

maintenance?

Di Lorenzo et al.

subduction and the subtropical cell
Subduction and the Subtropical Cell

Transport at 9ºN & 9ºS

Convergence & SST

McPhaden and Zhang 2002 Nature

climate indices
Climate Indices

1900

2000

Indian Ocean SST

- SPCZ Rain

Tropical

(poleward side)

SPCZ Rain

(eq’ward side)

D Cloud

(C Eq Pac)

D SLP (“SOI”)

(Indian – Pac)

(Boreal Winter)

25

47

77

- NP Index

experiment design
Experiment Design
  • 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

SLP

Qnet

Exp n+2

Exp n+1

Exp n

Control

Nov

Nov

Nov

Mar

Mar

Mar

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”

Meridional cross section through the central Pacific

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

slide40

FIG. 10. The El Niño – 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).

slide41

FIG. 9. The El Niño – 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).

slide42

FIG. 3. The El Niño – 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

SST (°C)

NDJ(1)

Experiment - Control

Thermocline depth (m)

subduction

Central North Pacific

Subduction

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

heating

Latent heat

release in

thunderstorms

Horel and Wallace, Mon. Wea Rev. 1981

slide47

Mechanism Loop of NPM

SP SST +

Salinity =>

SP Conv. + Fbk

Oyashio

Ocean or Coupled Mode ?

KOE SST +

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

ni o ni a observed jf 1 wind stress sst
Niño - NiñaObserved JF(1) Wind Stress/SST

Qnet

(Wm-2)

Qek (Wm-2)

Flux form

slide50

Propagation

SSH (Color)

SSH_S

SSH_T

50N

Westward Propagation

45N

40N

35N

Westward signal clear in subpolar

Subpolar SSH dominated by salinity

x

change in subduction rate
Change in Subduction Rate

Transport at 9ºN & 9ºS

Convergence & SST

ad