Climate of an Earth- like Aquaplanet: the high- obliquity case and the <dally- locked case

Transcription

1 Climate of an Earth- like Aquaplanet: the high- obliquity case and the <dally- locked case David Ferreira, John Marshall Paul O Gorman, Sara Seager, Massachusetts Institute of Technology, Harriet Lau (Imperial College)

2 Outline 1 Why is the problem interesting from a climate dynamics perspective? 2 Our approach: Earth-like planet with an atmosphere, an ocean and possibility of ice,...but no land! + Coupled GCM 3 Climate of an aquaplanet at high obliquity 4 Tidally-locked aquaplanets Conclusions

3 1. 1 Why is the problem interesting from a climate dynamics perspective? Annual mean Incoming solar radia<on At high obliquity the poles are warmed more than the equator January Extreme seasonal cycle Expect a reversal of pole-equator temperature gradient!! If polar temperatures are not to wildly fluctuate, heat must be stored or carried there. Likely key role for the ocean

4 Insolation for a tidally-locked Earth-like planet Large and steady insolation contrast, Outgoing long wave on night side. è requires a transport from day side to night side: Atmosphere and Ocean 0 W/m^ W/m^2 è no role for storage in ocean

5 Key climate ques<ons What is the role of the ocean in modulating extremes of temperature (through storage and transport)? What determines the pattern of surface winds? (i.e. atmos angular mtm transport) - critical to ocean circulation What determines the meridional energy transport and its partition between the atmosphere and ocean? Earth-like planet but without geometrical constraints Aqua Coupled GCM Atmosphere-only work: Joshi et al., 97; Joshi, 03; Williams and Kasting, 97; Williams and Pollard, 03; Merlis and Schneider, 2010; Showman et al. 2009; Heng and Vogt, 2011.

6 2 MIT GCM: Coupled Ocean-Atmosphere-Sea ice: Primitive equation models, Cube-sphere grid: ~3.75º, Same grid for ocean and atmosphere Fully coupled: no adjustments Synoptic scale eddies in the atmosphere, Gent and McWilliams eddy parameterization in the ocean, Poles well represented Simplified atmospheric physics (SPEEDY, Molteni 2003), Conservation to numerical precision (Campin et al. 2008) Temperature snap-shot at 500 mb.

7 Aquaplanet at 23.5 obliquity

8 W W Climate of Aquaplanet Pressure θ A 500mb T E U at obliquity of 23 o Depth θ O U SST & sea ice 60N 30N 30S Equator Circumpolar currents everywhere 60S Pattern of surface winds Zonal jets in ocean Aquaplanet solution discussed in Marshall, Ferreira et al. (2007, J. Atmos. Sci.)

9 Energy transports in an Aquaplanet Today s Earth climate Aquaplanet at 23.5 obliquity Atm Total +4 PW Ocn 0 PW -4 - Patterns and magnitudes of transports are well captured in an Aquaplanet è to first order, continents are not necessary to explore the climate of an Earth-like planet

10 3 Climate at high obliquity Annual means φ = 90 o Atmosphere Potential Temperature φ = 23.5 o Atmosphere 25 ºC Ocean 2 ºC 4 ºC 32 ºC Ocean

11 Winds Annual means φ = 90 o φ = 23.5 o U E U W W E W W Surface westerlies in middle latitudes, easterlies in tropics τ S = top bottom y [u ' v ' ]dz Convergence of Surface wind stress = eddy momentum fluxes

12 Asymmetries between easterly and westerly sheared flows W φ = 90 o E W φ = 23.5 o W E W u'v' U (colors) (contours) Eliassen-Palm fluxes: " EP = u'v', f v'θ ' % $ ' # & θ z turbulence waves q y β >>1 waves Eq turbulence Eq q y β 1

13 Seasonal Cycle θ A Convective index Depth [m] θ O Depth [m] 3000 Ψ O Atmosphere Ocean Strong surface temperature gradient in summer hemisphere (~40K) and weak gradient in winter hemisphere (~10K) Seasonal variations restricted to top 200m --- amplitude of ~12K at pole --- almost steady and ~2K at equator

14 January winds φ = 90 o φ = 23.5 o U E U W E W W 250 Sv Ocean overturning circulation -250 Sv OHT ρ O C P Ψ O ΔT OHT dominated by Eulerian wind-driven circulation

15 Atmos and Ocean energy transport at high obliquity Total Atm Ocn PW Atm January Annual mean -4 0 Ocn January 10 _ Eddy _ 8 Total Atmosphere Atmosphere and Ocean heat transports are achieved seasonally: large in the summer hemisphere nearly vanish in the winter hemisphere Equatorward transport everywhere down large-scale temperature gradient

16 Climate at 54 degree obliquity PW Annual 300 Ocean and atmosphere energy 950 Transports [PW] Annual January Ocn Potential temperature in K, Aqua Obli54 C24 Cpl362 March 320 Total Annual mean: weak gradients March and weak transports Atm PW OHT950 AHT Total Fig. 8: Idem to Fig. 2 but for a 54 obliquity. 10 January May Hadley cell May Wind-driven overturning 5 5

17 4 Tidally-locked Aquaplanet Rossby radius NH! = =" # f 800 km (Atm) 50 km (Ocean) at T=1 day km 1000 km at T=20 days

18 Surface air temperature: T = 1 day T = 20 days ΔT = 63 K TOA OLR: ΔT = 38 K Latitude OLR In W/m^2 Longitude : Substellar point

19 Surface winds T = 1 day T = 20 days Surface ocean currents Merlis and Schneider, 2010; Showman and Polvani 2011, Heng and Vogt, 2011

20 Ocean Surface heat flux (W/m^2) Positive = into the atmosphere T = 1 day T = 20 days ~220 ~220 Heat budget ~220 ~220 Atm ~150 Atm ~85 Ocean ~70 In W/m^2 Ocean ~135

21 Surface air temperature: T = 1 day T = 20 days Coupled GCM Ice covered night side Atm GCM + Slab ocean

22 Conclusions At high obliquity: Surface climates are rather mild despite extreme summer insolation and long polar nights seasonal cycle between 10 and 35K. Baroclinic eddies are the primary heat transport mechanism Hadley cell plays lesser role, Ocean plays an important role in heat transport, carrying about 1/3 of the total Wind-driven middle-latitude Ekman cells are the primary mechanism subtropical/equatorial cells play a lesser role Heat is stored in the ocean in the summer and delivered to the atmosphere in the winter, keeping it warm and somewhat moist. Tidally locked case: Surface climates are rather mild despite extreme, Both ocean and atmosphere transport energy from the day to the night side, as the rotation rate decreases, nigth-day heat transport moves to the ocean, the ocean is more efficient at smoothing out the night-day temperature constrast.