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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. 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

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  1. Tropical Teleconnections and Western Boundary Current Variability Michael Alexander NOAA/Earth System Research Lab WBC Workshop January 2009

  2. WBC Web page (http://www.cdc.noaa.gov/WBC)

  3. Obs Model DJF SLP Contour (1 mb); FMA SST (shaded ºC) El Niño – La Niña Composite:

  4. 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.

  5. CCSM3 Correlation PDO & SAT Obs

  6. Correlation between ENSO and the PDO Why do IPCC-class models under estimate the connection between the PDO and the tropical Pacific (ENSO)?

  7. 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

  8. 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.

  9. Storm tracktrajectory counts of surface lows Climatological Mean JFM Niño -Niña Alexander et al. 2006 J Climate

  10. Composite El Niño - La Niña JFM Alexander & Scott 2008 J Climate

  11. 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.

  12. SST ML NDJ Specified SST AGCM z500 Jan AGCM + ML Jan z500 Atlantic teleconnections Peng et al. 2005, 2006

  13. 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

  14. Indian Ocean Teleconnections Cor w GOGA Pacific Z 500 Cor w GOGA Atlantic Precip Hoerling et al. 2004 Climate Dynamics

  15. 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.

  16. Tropics <=> Extratropics

  17. SFM Model Experiment - Additional Heat Flux Forcing (Wm-2) NDJFM(0) SST (°C)

  18. SST (°C) NDJ(1) Experiment - Control Winds & SST MJJ(0)

  19. Di Lorenzo et al.

  20. CCSM3 Zhong et al. 2008 Tropical - Extratropical Interactions Observation NP TP Deser et al. 2004

  21. 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

  22. Origin of NPM: Subtropics vs. Subpolar Modeling Surgery Experiment Origin: Subploar Route !

  23. 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?

  24. 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)

  25. 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

  26. 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)

  27. NPGO maintenance? Di Lorenzo et al.

  28. Subduction and the Subtropical Cell Transport at 9ºN & 9ºS Convergence & SST McPhaden and Zhang 2002 Nature

  29. 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

  30. 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

  31. Seasonal Footprinting Mechanism

  32. 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

  33. Deser and Phillips 2006

  34. Bader and Latif 2005

  35. “The Atmospheric Bridge” Meridional cross section through the central Pacific (Alexander 1992; Lau and Nath 1996; Alexander et al. 2002 all J. Climate)

  36. 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).

  37. 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).

  38. 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).

  39. Winds & Qlh SST (°C) NDJ(1) Experiment - Control Thermocline depth (m)

  40. 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

  41. Do subducting anomalies reach the equator and influence ENSO? a) b) c) d) Latitude Year

  42. 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

  43. 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

  44. ENSO NOAA High Res Uwnd composite10S-60 N across the N. Pacific and N. Atlantic

  45. Niño - NiñaObserved JF(1) Wind Stress/SST Qnet (Wm-2) Qek (Wm-2) Flux form

  46. Propagation SSH (Color) SSH_S SSH_T 50N Westward Propagation 45N 40N 35N Westward signal clear in subpolar Subpolar SSH dominated by salinity x

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