Role of ocean circulation in the climate response to meridional - PowerPoint PPT Presentation

Role of ocean circulation in the climate response to meridional energy imbalances My main button ? Francis Codron LOCEAN/IPSL, Sorbonne-Université, Paris

Motivation : Coupling of energy transports by atmosphere and ocean in the Tropics Atmosphere: Hadley cells. Ocean: Shallow circulation cells, driven by trade winds / Ekman transport • Mass transport is coupled by wind stress. • Energy transport in the same direction (Schneider et al, 2014)

Slab ocean model F sens F lat F rad H QFlux • Oceanic surface mixed layer. 50-m depth • Temperature equation : !" # $ %& %' = ) *+,- + /)012 • F surf Surface fluxes • Qflux Ocean circulation. Prescribed or parameterized

Ekman transport parameterization (Codron 2012, Clim. Dyn) • 2 slab layers, prognostic temperatures T s and T d • Ekman mass transport (vertical integral over surface layer) : ! " = − 1 & '× ⃗ * + = − , (At the equator, ! " - . + * / ) Opposite return flow at depth •

Gent-McWilliams scheme (eddy diffusivity) H S H D Impact of eddy fluxes is to reduce the slope of isotherms. Ø Eddy-induced velocity, mass flux opposite in the 2 layers, proportionnal to the slope Ø Downgradient heat flux Ø Re-stratification

Implementation • LMDZ AGCM, 96x96 grid points, 39 levels • Ocean-planet geometry, no sea ice (water does not freeze) • Obliquity (seasons), no eccentricity • Ekman and G&M transport, plus horizontal diffusion • No additional heat flux (Q-flux = 0)

Mean northward energy transports 4 Ocean total Total (TOA) 3 6 Ekman Atmosphere 2 4 Eddies Diff. Ocean 1 2 PW PW 0 0 − 1 − 2 − 2 − 4 − 3 − 6 − 4 − 80 − 60 − 40 − 20 0 20 40 60 80 − 80 − 60 − 40 − 20 0 20 40 60 80 latitude latitude • Ocean energy transport dominates in the (deep) tropics • Ekman large, poleward in tropics • Equatorward, weaker mid-latitudes • Diffusive transport max in mid-latitudes

Sensitivity experiments: 1. add 1 PW northward transport between 40°S and 40°S (as additional prescribed Q-flux in the slab) 2. CO 2 doubling • Interactive Ekman and diffusive transports (INT) • Ekman and diffusion prescribed (as Qflux) to the control run seasonal cycle (FIX)

Northward energy transport difference, fixed ocean heat transport (OHT) 1 1 0.5 0.5 0 0 TOA TOA total Atmosphere Clear sky − 0.5 − 0.5 Ocean Clouds − 1 − 1 − 80 − 60 − 40 − 20 0 20 40 60 80 − 80 − 60 − 40 − 20 0 20 40 60 80 latitude latitude Ø Large compensation by radiative fluxes in mid-latitudes Ø Compensation by atmospheric transport in the Tropics (Clear-sky and cloud forcing responses cancel out) Ø North-South asymmetry

Atmospheric circulation response, fixed OHT case Meridional Streamfunction (10 9 kg/s) Zonal wind fixed OHT 200 200 200 pressure (hPa) pressure (hPa) 400 400 400 600 600 600 800 800 800 1000 1000 1000 − 80 − 60 − 40 − 20 0 20 40 60 80 − 80 − 60 − 40 − 20 0 20 40 60 80 − − − − − − − − − − − − − − − − − − − − latitude latitude − − − − − 160 − 120 − 80 − 40 0 40 80 120 160 − 10 − 5 0 5 10 Ø Northward jet shifts Ø Cross-equatorial Hadley cell & opposite trade wind changes

Northward energy transport difference, interactive ocean 1 Atmosphere Ocean 0.5 (PW) Fixed OHT 0 Inter. OHT − 0.5 − 1 − 80 − 60 − 40 − 20 0 20 40 60 80 latitude Ø In the tropics, large compensation by ocean (Ekman cells) Ø Weaker atmospheric transport Ø More symmetric structure

Response of Hadley cells (mean meridional streamfunction) fixed OHT interactive OHT 200 200 pressure (hPa) 400 400 600 600 800 800 1000 1000 − 80 − 60 − 40 − 20 0 20 40 60 80 − 80 − 60 − 40 − 20 0 20 40 60 80 latitude latitude − 160 − 120 − 80 − 40 0 40 80 120 160

Response of zonal winds (m/s) colors = differences, contours = control run fixed OHT interactive OHT 200 200 pressure (hPa) 400 400 600 600 800 800 1000 1000 − 80 − 60 − 40 − 20 0 20 40 60 80 − 80 − 60 − 40 − 20 0 20 40 60 80 latitude latitude − 10 − 5 0 5 10 • Weak Hadley cell-related changes • Northern hemisphere : jet weakening northward shift • Southern jet strengthening & northward shift

Precipitation response Control 10 1PW fixed OHT 1PW inter. OHT precip (mm/day) 5 0 − 80 − 60 − 40 − 20 0 20 40 60 80 latitude Ø Fixed OHT : more precip in warmer hemisphere + shift of the peak. Ø Interactive ocean : very small changes

Response to CO 2 doubling, fixed ocean transport Change in northward energy transport (PW) TOA 0.2 Atmosphere 0.1 0 − 0.1 − 0.2 − 80 − 60 − 40 − 20 0 20 40 60 80 latitude Ø Small changes overall (0.2 / 6 PW) Ø Very small changes in the Tropics

Circulation response, fixed ocean transport Control (contours), response to CO 2 (colors) Zonal wind Meridional streamfuction fixed OHT 200 200 pressure (hPa) pressure (hPa) 400 400 600 600 800 800 1000 1000 1000 − 80 − 60 − 40 − 20 0 20 40 60 80 − − − − − 80 − 60 − 40 − 20 0 20 40 60 80 latitude latitude • Upward and poleward expansion / jet shift − − − • Hadley cell weakening (tropics)

Response to CO 2 doubling : energy transport 0.2 0.1 (PW) 0 − 0.1 − 0.2 − 80 − 60 − 40 − 20 0 20 40 60 80 latitude Atmosphere, fixed OHT Ekman, control / 10 Atmosphere, inter. OHT Ekman, response to CO 2 Ocean Ø Tropics : opposite changes in atmosphere and ocean Ø Weaker Hadley cell : less poleward ocean transport Ø Poleward expansion : more poleward ocean transport

Response to CO 2 doubling : meridional streamfunction fixed OHT interactive OHT 200 200 pressure (hPa) 400 400 600 600 800 800 1000 1000 − 80 − 60 − 40 − 20 0 20 40 60 80 − 80 − 60 − 40 − 20 0 20 40 60 80 latitude latitude − 15 − 12 − 9 − 6 − 3 0 3 6 9 12 15 Ø Interactive ocean : • Less weakening near equator, more in subtropics • Stronger poleward shift

Response to CO 2 doubling : precipitation Control 1 CO 2 resp. fix OHT CO 2 resp. int OHT 0.75 mm / day 0.5 0.25 0 − 0.25 − 80 − 60 − 40 − 20 0 20 40 60 80 latitude Ø Precipitation increase in warmer climate Ø ITCZ moves poleward / equatorward with fixed / interactive ocean

Summary - Conclusion Adding Ekman heat transport significantly modifies the response (circulation, ITCZ) to changes in energy budget • Inter-hemispheric transport : weaker Hadley cell & ITCZ response (compensation by ocean energy transport) • CO 2 increase : total transport does not change (much), but compensating changes in atmosphere and ocean, due to Hadley cell weakening & poleward expansion. • Perturbations in the meridional energy transport = coupled problem