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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 Michael Alexander NOAA/Earth System Research Lab WBC Workshop January 2009
Obs Model DJF SLP Contour (1 mb); FMA SST (shaded ºC) El Niño – La Niña Composite:
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.
CCSM3 Correlation PDO & SAT Obs
Correlation between ENSO and the PDO Why do IPCC-class models under estimate the connection between the PDO and the tropical Pacific (ENSO)?
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 • 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 tracktrajectory counts of surface lows Climatological Mean JFM Niño -Niña Alexander et al. 2006 J Climate
Composite El Niño - La Niña JFM Alexander & Scott 2008 J Climate
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.
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 • 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 Cor w GOGA Pacific Z 500 Cor w GOGA Atlantic Precip Hoerling et al. 2004 Climate Dynamics
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.
SFM Model Experiment - Additional Heat Flux Forcing (Wm-2) NDJFM(0) SST (°C)
SST (°C) NDJ(1) Experiment - Control Winds & SST MJJ(0)
CCSM3 Zhong et al. 2008 Tropical - Extratropical Interactions Observation NP TP Deser et al. 2004
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
Origin of NPM: Subtropics vs. Subpolar Modeling Surgery Experiment Origin: Subploar Route !
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?
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 • 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
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)
NPGO maintenance? Di Lorenzo et al.
Subduction and the Subtropical Cell Transport at 9ºN & 9ºS Convergence & SST McPhaden and Zhang 2002 Nature
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 • 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
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” Meridional cross section through the central Pacific (Alexander 1992; Lau and Nath 1996; Alexander et al. 2002 all J. Climate)
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).
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).
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).
Winds & Qlh SST (°C) NDJ(1) Experiment - Control Thermocline depth (m)
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
Do subducting anomalies reach the equator and influence ENSO? a) b) c) d) Latitude Year
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
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
ENSO NOAA High Res Uwnd composite10S-60 N across the N. Pacific and N. Atlantic
Niño - NiñaObserved JF(1) Wind Stress/SST Qnet (Wm-2) Qek (Wm-2) Flux form
Propagation SSH (Color) SSH_S SSH_T 50N Westward Propagation 45N 40N 35N Westward signal clear in subpolar Subpolar SSH dominated by salinity x