Ocean energetics in GCMs: how much energy is transferred from the winds to the thermocline on ENSO timescales? Alexey Fedorov (Yale) Jaci Brown (CSIRO) and Eric Guilyardi (IPSL) Funded by DOE, NSF, CNRS

Brown J., Fedorov, A.V., and Guilyardi, E., 2010: How well do coupled models replicate ocean energetics relevant to ENSO? Climate Dynamics, in press Brown J. and Fedorov, A.V., 2010: How much energy is transferred from the winds to the thermocline on ENSO timescales. J. Climate 23, 1563–1580. Brown J. and Fedorov, A.V., 2008: The mean energy balance in the tropical ocean, J. Marine Research 66, Fedorov, A.V., 2007: Net energy dissipation rates in the tropical ocean and ENSO dynamics. J.Climate 20, 1099–1108 Fedorov, A.V., Philander, S.G., Harper, S.L., B. Winter, B. and A. Wittenberg, 2003: How predictable is El Niño? Bull. Amer. Meteorol. Soc. 84,

Buoyancy Power acts to displace isopycnals Wind Power generated by wind stress acting on surface currents Available Potential Energy generated when isopycnals are distorted Kinetic Energy of ocean currents Atmosphere Ocean isopycnals OCEAN ENERGETICS Goddard and Philander 2000; Fedorov et al 2003

Questions: What fraction of power generated by the winds reaches the equatorial thermocline on ENSO timescales? How much damping occurs for thermocline anomalies? Can we use the energetics of the tropical ocean to compare different coupled models? 4

A shallow-water model 5  What are  and  in GCMs? Fundamental question: How (well) do GCMs describe the transfer of energy from the winds to the thermocline?

Wind stress  Surface Currents, U (m/s) U=U(x,y,t) – zonal velocity  (x,y,t) – zonal wind stress 6 U and  - same direction positive wind power negative wind power U and  - different direction Wind power W is generated when winds work on ocean currents

MOM4 Buoyancy power B controls vertical displacements of the thermocline. It is generated from the conversion of wind power. w – vertical velocity g – gravity  (x,y,z,t) – density  z) – reference stratification 7

High APE, La Niña: Low APE, El Niño: APE (denoted as E ) is generated when isopycnals are distorted, and is proportional to the thermocline slope along the equator!  (x,y,z,t) – density  z) –reference stratification APE variations are highly anti-correlated with the Nino3 SST, correlation up to -0.9

El Nino of Integration: tropical Pacific (15 o N - 15 o S,130 o E - 80 o W, 0-400m)

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11 E – the APE K – kinetic energy (negligible, less than 1% of E ) B – buoyancy power (describes the conversion of kinetic into potential energy W – wind power D 1, D 2 – viscous and diffusive dissipations

Wind Power Buoyancy Power APE (~thermocline slope) SST Wind Stress D 1 D 2 12

Coupled Models (from IPCC AR4): GFDL-CM2.1 (NOAA/GFDL) CCSM3 (NCAR) CSIRO-Mk3.5 (CSIRO.) MIROC3.2 (medres) (U. of Tokyo) IPSL-CM4 (IPSL) HadGEM1 (Hadley Centre) Ocean-only, data assimilating, and coupled GCMs 13 Ocean-Only Models: POP (LANL / NCAR) ORCA a,b (IPSL: NEMO / ORCA5) MOM4 (GFDL) Data Assimilations: ECCO-GODAE (MIT) GODAS (NOAA CPC) Data assimilating models Ocean models Coupled models

0 We introduce efficiency  = B/W That is, only a fraction of wind power W is converted to buoyancy power B What are the values of  ? How good is the assumption of proportionality? (our assumption)

15 Calculating efficiency 

16 Efficiency  versus correlation between Buoyancy and Wind powers Ocean-only and data- assimilating models:  Coupled models: 

17 Characteristic ENSO period versus efficiency 

(  -1 is the APE damping timescale 18 (our assumption)

19 Calculating damping timescale  

20 Ocean-only and data-assimilating models:  -1 = year Coupled models:  -1 = years Damping timescale   versus correlation between E and D 2

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Summary Ocean energetics of ENSO is characterized by two physical parameters  - the efficiency of the energy transfer from the winds to the thermocline. Most of coupled GCMs are less efficient (  =15-50%) than ocean-only and data assimilating models (  =50-60%).  - the APE damping rate. Most of coupled GCMs produce shorter damping timescales (  -1 =0.4-1 year) than ocean-only and data assimilating models (  -1 =1-1.2 year). The two parameters can be used as metrics for evaluating dissipative properties of the models and other model properties 22

Implications How models describe quantitatively the transfer of energy (and momentum) from the winds down the water column is important Efficiency 30% vs. 60% matters Energy e-folding damping 4 months vs. 1 year matters Error compensation in GCMs 23

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Decadal Variability Wind PowerAvailable Potential Energy Shift in late 1970s in Wind power and resulting APE Interannual variability consistent. 25

Wind Power Buoyancy Power APE (~thermocline slope) SST Wind Stress   26 Coupled models generate lower efficiency and stronger damping than ocean-only or data-assimilating models!

27 Annual-mean zonal averages between160 o E to 90 o W

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K – kinetic energy (small) B – buoyancy power; P – the rate of work of ageostrophic pressure; A M – advection of K away from the tropics D M – turbulent viscous dissipation; k MV, k MH – viscosities

B – buoyancy power Q – damping by surface heat fluxes A – advection of APE away from the tropics D – turbulent diffusive dissipation; k MV, k MH – diffusivities 31

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Wind Power Buoyancy Power APE (~thermocline slope) SST Wind Stress Dissipation 1 Dissipation 2 35

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Wind Power (TW) Buoyancy Power (TW) Efficiency of Wind Power to Buoyancy Power Transfer W B 37

38 APE damping timescales  -1 : ocean models and data assimilations:  -1 = 1 year coupled models:  -1 = years Correlation (between D 2 and E )

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Wind Power Buoyancy Power APE (Thermocline slope) Sea Surface Temperatures Wind Stress The energetics of the tropical ocean: D 1 D 2 40  E is highly anti-correlated with the SST in the eastern equatorial Pacific (r=-0.9)!

 E is highly anti-correlated with the SST in the eastern equatorial Pacific (r=-0.9)! High APE means La Niña; Low APE means El Niño 41 El Nino 1997

K – kinetic energy (small) B – buoyancy power; P – the rate of work of ageostrophic pressure; A M – advection of K away from the tropics D M – turbulent viscous dissipation; k MV, k MH – viscosities

K – kinetic energy (small) B – buoyancy power; P – the rate of work of ageostrophic pressure; A M – advection of K away from the tropics D M – turbulent viscous dissipation; k MV, k MH – viscosities

B – buoyancy power Q – damping by surface heat fluxes A – advection of APE away from the tropics D – turbulent diffusive dissipation; k MV, k MH – diffusivities 44

B – buoyancy power Q – damping by surface heat fluxes A – advection of APE away from the tropics D – turbulent diffusive dissipation; k MV, k MH – diffusivities 45

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Calculating damping rates  Available Potential Energy Dissipation Anomaly E 47 D2D2

48 Correlation (between D 2 and E )