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Ocean Processes and Decadal Teleconnections Michael Alexander Climate Diagnostics Center, NOAA

Ocean Processes and Decadal Teleconnections Michael Alexander Climate Diagnostics Center, NOAA. Broad Questions About Midlatitude SSTs Part I: What is the role of the seasonal cycle in SST evolution? What causes extratropical SST anomalies to form?

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Ocean Processes and Decadal Teleconnections Michael Alexander Climate Diagnostics Center, NOAA

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  1. Ocean Processes and Decadal TeleconnectionsMichael AlexanderClimate Diagnostics Center, NOAA Broad Questions About Midlatitude SSTs Part I: • What is the role of the seasonal cycle in SST evolution? • What causes extratropical SST anomalies to form? • How does ocean processes influence the evolution of SSTs? • Do the (extratropical) SSTs influence the atmosphere?

  2. SLP & SST Patterns of Variability ENSO PDO Regressions: SLP – Contour; SST Shaded From Mantua et al. 1997, BAMS

  3. Time scales of variability Pacific Climate Indices • North Pacific – • variability decadal? • ENSO - • Variability interannual? • Both impact marine biology • Our main focus today to better understand the processes that generate anomalies shown on the right

  4. Specific Topics • SST Characteristics • Mean seasonal cycle • Patterns of Variability • Heat and Energy Budgets • Surface Temperature tendency equation • Simplest model for SST variability • Weather forcing of a slab ocean • Mixed Layer Dynamics • Winter-to-winter reemergence of SST anomalies • Atmospheric bridge • ENSO’s impact on extratropical SSTs • Midlatitude Ocean Dynamics • Subduction • Rossby waves

  5. Mean Seasonal Cycle of Temperature and MLDas a function of Depth in Central N. Pacific • Upper ocean well mixed • Uniform temperature of ML • SST’ spread over ML • Strong seasonal thermocline in summer • SST lags solar forcing by ~3 months: • Large heat capacity • 2.5 m holds as much heat as the entire atmosphere above • Thick mixed layer • Max in Aug-Sep, Min March • A lot of energy required to create an SST anomaly: • ~100 W/m2 => 1°C/month over 60m ML MLD

  6. Annual Mean SST & SST Range (ºC) Long term mean August – March SST (ºC)

  7. Ratio of Standard Deviations:Monthly/Seasonal Cycle

  8. Interannual Standard Deviation of SST(ºC) March September

  9. SST Tendency Equatione.g. Frankignoul (1985, Reviews of Geophysics) Variables Tm – mixed layer temp (SST) Tb – temp just beneath ML Qnet – net surface heat flux Qswh – penetrating shortwave radiation h – mixed layer depth w – mean vertical velocity we – entrainment velocity v - velocity (current in ML) vek – Ekman + vg - geostrophic A – horizontal eddy viscosity coefficient

  10. Process that Influence SST Vek important on all time scales Vg associated with eddies (~50km) & large-scale Rossby waves

  11. Simplest Model of Midlatitude SST Anomalies: Weather forcing of a slab ocean Spectra of Temperature Variance • “Stochastic model” (Frankignoul and Hasslemann 1977 Tellus) Observed OWS If no damping Assume: Q white noise > ~ 1 month l damping is linear. After Fourier transform Spectra of SST’ variance is: Theoreticalspectra l=4.5mon-1, h=100m () period: 1 year 1 month () is the frequency

  12. PDO or slab ocean forced by noise? From David Pierce 2001, Progress in Oceanography

  13. Reemergence Mechanism Qnet’ • Winter Surface flux anomalies • Create SST anomalies which spread over ML • ML reforms close to surface in spring • Summer SST anomalies strongly damped by air-sea interaction • Temperature anomalies persist in summer thermocline • Re-entrained into the ML in the following fall and winter

  14. Reemergence in three North Pacific regions Regression between SST anomalies in FMA with monthly temperature anomalies as a function of depth. Regions

  15. Reemergence in the East Pacific:Regressions with SST in Feb-May

  16. Basin-wide Reemergence

  17. “The Atmospheric Bridge” Meridianal cross section through the central Pacific

  18. Model Experiments to Test Bridge Hypothesis Specified SSTs

  19. DJF SLP Contour (1 mb); FMA SST (shaded ºC) El Niño – La Niña Composite: Observations Model: AGCM-MLM

  20. “Decadal” variability in the North Pacific: tropical (ENSO) Connection? Observed SST Nov-Mar (1977-88) – (1970-76) MLM SST Nov-Mar (1977-88) – (1970-76)

  21. Air-sea Feedback of the atmospheric response to ENSO

  22. Atmospheric Response to ENSO over the North Pacific El Niño – La Niña 30-day Running Mean Composite 500 mb height anomaly (176ºE-142ºW; 32ºN-48ºN) Aleutian Low

  23. Gu and Philander (Science, 1997) Hypothesis for Decadal Variability: 35N Upwelling + entrainment Subduction Subduction: Process where water leaves the surface layer and enters the permanent thermocline, where it can flow, relatively undisturbed over large distances.

  24. Central North Pacific Subduction Colored contours -0.3C anomaly isotherms for 3 different pentads Black lines – mean isopychnal surfaces (lines of constant density) Averaged over 170ºW-145ºW

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

  26. PDO: The Latif and Barnett Hypothesis H Positive air-sea feedback Warm SSTs wind Gyre spins down Weakens Kuroshio Rossby Waves ~5yrs to cross

  27. Latif & Barnett revisited • Atmospheric response to midlatitude SST anomalies (200 m) ~5-10 x too large • Negative or at best weak positive air-sea feedback • Ocean Rossby wave response to to winds are reasonable ~5years to cross ocean • But gyre not only spins-up but moves south resulting in cold (not warm) SST anomalies east of Japan Shading: vertically integrated temperature; Contours: flow predicted by change in wind stress integrated across the basin Based on observations see Deser et al. 1999 J. Climate

  28. Midlatitude influence on the tropics at Decadal timescales A-O GCM Observatons Data filtered to Retain periods > 10 years. From Pierce et al. 2000 J. Climate.

  29. The impact of Air-Sea Interaction on Atmospheric Variability EOF 1 500 mb heights Correlation of SSTs with EOF 1 From D. Pierce 2001

  30. Impact of Ocean Currents on the Atmosphere SLP (mb x 100) Prescribed ocean heat flux convergence in a slab ocean model coupled to a AGCM 60N 500 mb (m) 30N 15 Wm-2 eq Mimics ocean heat transport anomalies in Kuroshio region From Yulaeva et al., 2001, J Climate

  31. PDO influence on ENSO SLP signal over the US? a) El Niño b) La Niña -4.5 c) El Niño High PDO d) La Niña High PDO 3.5 -8.5 e) El Niño High PDO f) La Niña Low PDO -1.5 1.5

  32. Summary of Processes • Mean seasonal cycle and mixed layer physics • Reemergence • Atmospheric Bridge • Cause and effect well understood • Tropical Pacific => Global SSTs • Influence of air-sea feedback on extratropical atmosphere complex • PDO (1st EOF of N. Pacific SST) • Ocean integration of noise + Atmosperhic Bridge + Ocean dynamics • Realistic: ocean Rossby waves • Unrealistic: strong positive extratropcial air-sea feedback • Atmospheric changes associated with PDO • Impact on ENSO? • Impact on high frequency variability like storms? • Subduction • Influence on tropical thermocline and ENSO?

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