Why should we look to the ocean for low-frequency (> 1 season) variability?

1. Ocean Processes and Pacific Decadal Climate VariabilityMichael AlexanderEarth System Research LabPhysical Science DivisionNOAA

2. Why should we look to the ocean for low-frequency (> 1 season) variability? • Thermal Inertia • 4 m of ocean holds as much heat as atmosphere above • Water takes a long time to heat up and cool down • Temperature anomalies once created persist • Dynamical Processes • Some very slow • Currents slow (1 m/sec) • Advection of temperature anomalies can take many years • Adjustment of midlatitude currents (~5 years - decades) • Exchanges with the deep ocean can take decades to centuries! • Important Implications • Marine Ecosystems (fisheries) • Atmospheric Circulation • ( SSTs => Atmosphere?)

3. Midlatitude SST Variability • There are many ways that SST anomalies form • We will explore just a few mechanisms • Ones that are part of larger Pacific climate signals • Mechanisms for generating midlatitude SST anomalies • Surface heat fluxes - Climate Noise • Upper Ocean mixing processes • “Atmospheric Bridge”: Teleconnections with ENSO • Changes in ocean currents • Wind driven (through ocean Rossby waves) • Thermal/salt driven: Thermohaline (Atlantic) - next week

4. Heat fluxes associated with weather events, “random forcing” Ocean response to flux back heat which slowly damps SST anomalies Air-sea interface SST anomalies form Fixed depth ocean No currents Bottom Simple model for generating SST variability“stochastic model”

5. Log plot of SSTA Spectra No damping SSTA Variance Atm forcing 10 yr 1yr 1 mo Period The Simple Ocean’s SST Anomaly Variability Complex behavior with decadal anomalies! SSTA 10 yrs time SSTn+1 =  *SSTn +  l=constant;  = Random number

6. Simple Ocean Model: correspondence to the real world?Observed and Theoretical Spectra for a location in the North Atlantic Ocean Observed OWS Temperature Variance Theoreticalspectra of Simple ocean model (Hz) is the frequency period: 1 year 1 month Atmospheric forcing and ocean feedback estimated from data

7. Qnet’ Seasonal cycle of Temperature & MLD in N. PacificReemergence Mechanism • 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 MLD Alexander and Deser (1995, JPO), Alexander et al. (1999, J. Climate)

8. Reemergence in three North Pacific regions Regression between SST anomalies in April-May with monthly temperature anomalies as a function of depth. Regions

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

10. 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

11. Obs Model Impact midlatitude SSTs: modest ~2 mb SLP And complex - varies with season DJF SLP Contour (1 mb); FMA SST (shaded ºC) El Niño – La Niña Composite:

12. Subtropical Gyre Subtropical Gyre Ocean Surface Currents Surface currents mainly driven by wind

13. The Pacific Decadal Oscillation (PDO) - Phase K + Phase PC 1 SST North Pacific Mantua et al. (BAMS 1997) Leading Pattern (1st EOF) of North Pacific SST

14. Pacific Decadal Atmospheric Variability “NP” Index (Nov-Mar) 1900-2002 Trenberth and Hurrell (1994) EOF1 SLP Pacific/Arctic PC: Regressed on full field Independent of the Atlantic • Extratropical Signature • Tropical Linkages

15. Precipitation (land only) Wet Dry 180° Precipitation and Temperature Patterns Associated with NP Index Surface Air Temperature Warm Cold 180°

16. North Pacific Climate Indices (Winter) 1900 2000 25 47 77 SST PC 1 SLP (- NP Index) PRECIP Alaska – Japan AIR T Alaska/Canada Deser et al. (J. Climate, 2004)

17. What Causes the PDO? and Pacific Decadal Variability in General? • Random forcing by the Atmosphere • Aleutian low => underlying ocean • Signal from the Tropics? • Perhaps associated with decadal variability in the ENSO region • Midlatitude Dynamics • Shifts in the strength/position of the ocean gyres • Could include feedbacks with the atmosphere

18. SLP PC1 - SST correlation SLP PC1 - Qnet correlation EOF 1 SST (34%) Aleutian Low Impact on Fluxes & SSTs in (DJF)Leading Patterns of Variability AGCM-MLM EOF 1 SLP (50%)

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

20. 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

21. “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)

22. L Rossby Waves Wind Generated Rossby Waves Atmosphere Ocean ML Thermocline West East • After waves pass ocean currents adjust • Waves change thermocline depth, if mixed layer reaches that depth, cold water can be mixed to the surface

23. KE Region: 40°N, 140°-170°E SSTOBS T400 SSTfcst Observed Rossby Waves & SST Correlation Obs SST hindcast With thermocline depth anomaly March Forecast equation for SST based on integrating wind stress (curl) forcing and constant propagation speed of the (1st Baroclinic) Rossby wave Schneider and Miller 2001 (J. Climate)

24. Contours: geostrophic flow from change in wind stress Shading: vertically integrated temperature (0-450 m): 1982-90 – 1970-80 Ocean Response to Change in Wind Stress SLP 1977-88 - 1968-76 Deser, Alexander & Timlin 1999 J. Climate

25. Response to Midlatitude SST Anomalies CI = 0.5°C 2.5 SST Anomaly (°C) specified as the Boundary Condition in an AGCM Peng et al. 1997 J Climate; Peng and Whittaker 1999, J. Climate

26. 30 Heights 250 mb CI = 5m 30 40 30 200 30 Cross Section of heights along 40ºN CI = 5m 500 -20 1000 120E 120W 120E 120W Response to Midlatitude SST anomalies

27. PDO: Multiple Causes? • Newman, Compo, Alexander 2003, Schneider and Miller 2005, Newman 2006 (All in Journal of Climate) • Interannual timescales: • Integration of noise (Fluctuations of the Aleutian Low) • Response to ENSO (Atmospheric bridge) • Decadal timescales (% of Variance) • Integration of noise (1/3) • Response to ENSO (1/3) • Ocean dynamics (1/3) • Predictable out to (but not beyond) 1-2 years • We developed a statistical method gives skillful PDO prediction out ~1 year • Trend • Most Prominent in Indian Ocean and far western Pacific • Likely associated with Global warming

28. Curve Extrapolation Prediction of the PDO 1998 Transition? Monthly values PDO Index

29. Summary • Climate noise • Expect decadal variability when looking at SST time series • Atmospheric Bridge • Cause and effect well understood • Tropical Pacific => Global SSTs • Influence of air-sea feedback on extratropical atmosphere complex • PDO (1st EOF of North Pacific SST) • Thermal response to random fluctuations in Aleutian Low • A significant fraction of the signal comes from the tropics • Extratropical ocean integrates (reddens) ENSO signal • Decadal variability in tropics – impact atmosphere & ocean • Ocean currents & Rossby waves in western N. Pacific • extratropical air-sea feedback: modest amplitude • Other Processes/modes of variability • Other variability besdies PDO, focused on west Pacific • Extratropical => tropical interactions

30. 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)

31. Seasonal Footprinting Mechanism

32. Subduction and the Subtropical Cell Ekman Subtropical Cell McPhaden and Zhang 2002 Nature

33. Change in Subduction Rate Transport at 9ºN & 9ºS Convergence & SST

34. 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

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

36. Additional Information • Processes that influence SSTs • PDO verses ENSO • Reemergence as a function of time • Ocean Dynamics: • Rossby waves, • Ocean Rossby waves • Latif & Barnett Hypothesis for decadal variability • Subduction

37. 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

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

39. Model Experiments to Test Bridge Hypothesis Specified SSTs

40. Influence of Air-sea Feedback on the atmospheric response to ENSO

41. 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

42. Basin-wide Reemergence Alexander et al. 2001, Progress in Oceanography

43. Evolution of the leading pattern of SST variabilityas indicated by extended EOFanalyses No ENSO; Reemergence ENSO; No Reemergence Alexander et al. 2001, Prog. Ocean.

44. Upper Ocean: Temperature and mixed layer depth El Niño – La Niña model composite: Central North Pacific Alexander et al. 2002, J. Climate

45. Forecast Skill: Correlation with Obs SST Wave Model & Reemergence Wave Model Reemergence years Schneider and Miller 2001 (J. Climate)

46. PDO: The Latif and Barnett Hypothesis • Coupled atmosphere-ocean interaction in the extratropics causes variability with a period of ~20 years • Key processes: Atmosphere strongly responds to SST anomalies near Japan. Atmospheric circulation maintains SST through surface heat fluxes but drive changes in the ocean surface currents which reverse the SSTs ~5-10 years later. Time scale determined by oceanic Rossby waves.

47. Mechanisms for North Pacific Decadal Variability • Air-sea interaction within the North Pacific basin • stochastic forcing (null hypothesis, simple slab) • Ocean dynamics (Latif and Barnett 1994, 1996) Time scale set by changes in ocean currents (oceanic Rossby waves). Relies on strong atmospheric response to midlatitude SST anomalies. • Tropical-extratropical interactions • Subduction: ocean transport from N. Pacific to tropics; atmospheric teleconnections from Tropics to midlatitudes close the loop (Gu and Philander . Observations indicate this pathway is unlikely (Schneider et al., 1999). • Air-sea interaction within the Tropical Indo-Pacific basin, with atmospheric teleconnections to the North Pacific as a by-product • Tropical Ocean has ENSO + Reemergence => PDO (Null hypothesis II) • Tropical ocean has a mechanism for decadal variability

48. PDO SLP & SST Patterns of Pacific VariabilityWhat process are involved? ENSO Regressions: SLP – Contour; SST Shaded Mantua et al. 1997, BAMS

49. H wind Weakens warm Kuroshio Current ~5yrs to cross Schematic of the Latif and Barnett Hypothesis Positive air - sea feedback Warm SSTs Rossby Waves

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