News & Views
ASTROBIOLOGY Volume 14, Number 8, 2014 ª Mary Ann Liebert, Inc. DOI: 10.1089/ast.2014.1404
A Sea Change in Exoplanet Climate Models? Peter L. Read
W
hat would Earth’s climate be like without the oceans? This is a question that is not easy to answer in detail, since it depends among other things upon what impact removing the oceans would have on the composition of the atmosphere and, in particular, on the concentration of greenhouse gases such as water vapor, atmospheric dust concentrations, and cloudiness. But one immediate consequence would be the likelihood of much more extreme geographic, seasonal, and diurnal variability in surface temperature, almost certainly rendering large areas of the planet virtually uninhabitable for advanced forms of life. The immense heat capacity of the oceans causes their surface temperature to respond only very slowly to seasonal changes in solar heating, while their near-global extent ensures that temperature swings across the planet are kept to tolerable levels. But the ocean circulation also carries a huge amount of heat energy from the steamy tropics towards the poles, thereby also moderating the extreme temperature contrast between equator and high latitudes. It is clear, therefore, that the oceans play a vital role in keeping much of Earth comfortably habitable. It is surprising, therefore, that modelers seeking to study and simulate habitable climates on Earth-like and superEarth extrasolar planets have tended so far to focus almost exclusively on their atmospheres (Showman et al., 2013) and have largely ignored the likely presence of extensive oceans on those planets that find themselves within their star’s habitable zone. This looks set to change, however, given the results presented by Cullum et al. (2014) in the current issue of this journal. These authors have used a modern ocean circulation model to simulate the pattern of ocean circulation on an idealized, ocean-covered, Earth-like planet in which they reduced the planetary rotation rate by a factor of up to 10. The results suggest that, in contrast to the atmosphere (in which the peak poleward heat transport and consequent equator-pole thermal contrast may be relatively insensitive to planetary rotation period, at least for rotation rates slower than the present Earth), the poleward heat transport carried by the oceans would increase by a factor *2 or even greater on a more slowly rotating planet. Such a strong increase in heat transport would be expected to have a major impact on the temperature distribution across such a planet, significantly reducing extremes of climate variability and potentially allowing a greater fraction of the planet to become habitable. The climatic consequences of combining a strong seasonal cycle with the absence of oceans on a terrestrial-style planet within a habitable zone can be clearly seen, for example, in
the atmosphere of Mars. Despite a vigorous atmospheric circulation on that planet, near-surface air temperatures at high latitudes are observed to swing (Read and Lewis, 2004) over a range of more than 100, to the extent that the polar regions are among the warmest places on the planet during summer (when the Sun never sets) but the coldest during winter. On tidally locked planets that continuously present the same face towards their parent star, the situation may be even more extreme, since the temperature on the nightside might then fall to very low temperatures, perhaps even cold enough to condense the atmosphere itself and cause atmospheric collapse ( Joshi et al., 1997). A dynamically active ocean might then be crucial in helping to even out temperature variations between dayside and nightside sufficiently to sustain a substantial habitable atmosphere (Hu and Yang, 2014). The strong increase in ocean heat transport with rotation period compared with the atmosphere arises because heat is transported by rather different mechanisms in the oceans than in a typical planetary atmosphere. In the oceans, motions are driven partly by direct buoyancy forcing (due to contrasts in solar heating and salinity) but also by mechanical stresses at the upper surface driven by atmospheric winds. In the absence of continental landmasses, Coriolis accelerations would inhibit the relatively sluggish ocean currents from forming a substantial direct overturning circulation to carry heat polewards, making the oceans even more dependent on baroclinic eddies to carry heat across the planet than the atmosphere. But the natural scale of baroclinic eddies in the oceans is much smaller than in the atmosphere, so long-range meridional eddy transport is relatively inefficient in the oceans compared with the atmosphere. However, as taken into account in the model simulations from Cullum et al., the presence of extensive north-south continental boundaries completely changes the ocean circulation pattern to one in which heat is transported by a combination of a direct, buoyancy-driven meridional overturning circulation and a pattern of wind-driven gyres. The Coriolis forces acting on the northward or southward flow in these gyres are balanced by an east-west pressure gradient that spans the entire ocean basin. But the circulation then needs to be closed by intense and very narrow boundary currents flowing in the opposite direction, such as the Gulf Stream in the Atlantic or the Kuroshio current in the Pacific Ocean, that ‘‘lean against’’ the western coastal boundary. The strength of the direct meridional overturning circulation is determined by the intensity of cross-isopycnal
Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, Oxford, UK.
627
628
transport, which is commonly represented (albeit not very accurately) as a simple (vertical) diffusive process with a characteristic diffusion coefficient jv. The result suggests (Vallis, 2006) that meridional energy transport should scale as *(j2v tR)1/3, where tR is the planetary rotation period, much as found by Cullum et al. in their model [although somewhat curiously this tends to contradict evidence from laboratory experiments (Rayer et al., 1998), which suggest total heat transport should become independent of tR]. However, the actual mechanism for cross-isopycnal transport actually depends on more complex processes than simple diffusion, such as turbulent eddies and internal gravity waves, most of which cannot be resolved in most ocean models so have to be parameterized empirically with considerable uncertainty. As a result, and because the actual quantitative heat transport in the oceans is very difficult to measure directly, the magnitude of the contribution of the oceans to the total poleward heat transport in Earth’s climate system has been poorly constrained until relatively recently. For many years, it was assumed that the oceans transported roughly the same amount of heat as the atmosphere (a kind of agnostic equipartition hypothesis), perhaps even with a tendency for mutual compensation (Bjerknes, 1964) so that the total combined poleward transport by the atmosphere and oceans would remain roughly constant, even if either component varied. This continues to be a controversial topic, however, for ongoing research (Vallis and Farneti, 2009; Farneti and Vallis, 2013). In many respects, the study by Cullum et al. (2014) is just among the first tentative steps in a new phase of extrasolar planetary climate modeling. Although the ocean model they used is relatively advanced, it used rather coarse spatial resolution, and so was unable to properly resolve or parameterize either the vigorous mesoscale and smaller ocean eddies or the intense, narrow western boundary currents within the wind-driven gyres that are known to play a very important role in Earth’s oceans. Their model also included only a crude representation of the wind forcing of their ocean circulation. So it remains to be confirmed that the scaling of heat transport with tR that they have found will also apply in more comprehensive, coupled atmosphere-ocean-cryosphere systems that fully resolve or represent the complete range of relevant scales of motion. But it seems clear that any future complete assessment of the habitability of an Earthlike exoplanet must take proper account of the presence of oceans in moderating its climate.
READ References
Bjerknes, J. (1964) Atlantic air-sea interaction. Advances in Geophysics 10:1–82. Cullum, J., Stevens, D., and Joshi, M. (2014) The importance of planetary rotation period for ocean heat transport. Astrobiology 14, doi:10.1089/ast.2014.1171. Farneti, R. and Vallis, G. (2013) Meridional energy transport in the coupled atmosphere-ocean system: compensation and partitioning. J Clim 26:7151–7166. Hu, Y. and Yang, J. (2014) Role of ocean heat transport in climates of tidally locked exoplanets around M dwarf stars. Proc Natl Acad Sci USA 111:629–634. Joshi, M., Haberle, R., and Reynolds, R. (1997) Simulations of the atmospheres of synchronously rotating terrestrial planets orbiting M dwarfs: conditions for atmospheric collapse and implications for habitability. Icarus 129:450–465. Rayer, Q.G., Johnson, D.W., and Hide, R. (1998) Thermal convection in a rotating fluid annulus blocked by a radial barrier. Geophysical and Astrophysical Fluid Dynamics 87: 215–252. Read, P.L. and Lewis, S.R. (2004) The Martian Climate Revisited, Springer-Praxis, Berlin. Showman, A.P., Wordsworth, R.D., Merlis, T.M., and Kaspi, Y. (2013) Atmospheric circulation of terrestrial exoplanets. In Comparative Climatology of Terrestrial Planets, edited by S.J. Mackwell, A.A. Simon-Miller, J.W. Harder, and M.A. Bullock, University of Arizona Press, Tucson, pp 277–326. Vallis, G. (2006) Atmospheric and Oceanic Fluid Dynamics: Fundamentals and Large-Scale Circulation, Cambridge University Press, Cambridge, UK. Vallis, G. and Farneti, R. (2009) Meridional energy transport in the coupled atmosphere–ocean system: scaling and numerical experiments. Quarterly Journal of the Royal Meteorological Society 135:1643–1660.
Address correspondence to: Peter L. Read Atmospheric, Oceanic and Planetary Physics Clarendon Laboratory Parks Road Oxford, OX1 3PU UK E-mail: [email protected] Submitted 30 June 2014 Accepted 8 July 2014
ASTROBIOLOGY Volume 14, Number 8, 2014 ª Mary Ann Liebert, Inc. DOI: 10.1089/ast.2014.1404
A Sea Change in Exoplanet Climate Models? Peter L. Read
W
hat would Earth’s climate be like without the oceans? This is a question that is not easy to answer in detail, since it depends among other things upon what impact removing the oceans would have on the composition of the atmosphere and, in particular, on the concentration of greenhouse gases such as water vapor, atmospheric dust concentrations, and cloudiness. But one immediate consequence would be the likelihood of much more extreme geographic, seasonal, and diurnal variability in surface temperature, almost certainly rendering large areas of the planet virtually uninhabitable for advanced forms of life. The immense heat capacity of the oceans causes their surface temperature to respond only very slowly to seasonal changes in solar heating, while their near-global extent ensures that temperature swings across the planet are kept to tolerable levels. But the ocean circulation also carries a huge amount of heat energy from the steamy tropics towards the poles, thereby also moderating the extreme temperature contrast between equator and high latitudes. It is clear, therefore, that the oceans play a vital role in keeping much of Earth comfortably habitable. It is surprising, therefore, that modelers seeking to study and simulate habitable climates on Earth-like and superEarth extrasolar planets have tended so far to focus almost exclusively on their atmospheres (Showman et al., 2013) and have largely ignored the likely presence of extensive oceans on those planets that find themselves within their star’s habitable zone. This looks set to change, however, given the results presented by Cullum et al. (2014) in the current issue of this journal. These authors have used a modern ocean circulation model to simulate the pattern of ocean circulation on an idealized, ocean-covered, Earth-like planet in which they reduced the planetary rotation rate by a factor of up to 10. The results suggest that, in contrast to the atmosphere (in which the peak poleward heat transport and consequent equator-pole thermal contrast may be relatively insensitive to planetary rotation period, at least for rotation rates slower than the present Earth), the poleward heat transport carried by the oceans would increase by a factor *2 or even greater on a more slowly rotating planet. Such a strong increase in heat transport would be expected to have a major impact on the temperature distribution across such a planet, significantly reducing extremes of climate variability and potentially allowing a greater fraction of the planet to become habitable. The climatic consequences of combining a strong seasonal cycle with the absence of oceans on a terrestrial-style planet within a habitable zone can be clearly seen, for example, in
the atmosphere of Mars. Despite a vigorous atmospheric circulation on that planet, near-surface air temperatures at high latitudes are observed to swing (Read and Lewis, 2004) over a range of more than 100, to the extent that the polar regions are among the warmest places on the planet during summer (when the Sun never sets) but the coldest during winter. On tidally locked planets that continuously present the same face towards their parent star, the situation may be even more extreme, since the temperature on the nightside might then fall to very low temperatures, perhaps even cold enough to condense the atmosphere itself and cause atmospheric collapse ( Joshi et al., 1997). A dynamically active ocean might then be crucial in helping to even out temperature variations between dayside and nightside sufficiently to sustain a substantial habitable atmosphere (Hu and Yang, 2014). The strong increase in ocean heat transport with rotation period compared with the atmosphere arises because heat is transported by rather different mechanisms in the oceans than in a typical planetary atmosphere. In the oceans, motions are driven partly by direct buoyancy forcing (due to contrasts in solar heating and salinity) but also by mechanical stresses at the upper surface driven by atmospheric winds. In the absence of continental landmasses, Coriolis accelerations would inhibit the relatively sluggish ocean currents from forming a substantial direct overturning circulation to carry heat polewards, making the oceans even more dependent on baroclinic eddies to carry heat across the planet than the atmosphere. But the natural scale of baroclinic eddies in the oceans is much smaller than in the atmosphere, so long-range meridional eddy transport is relatively inefficient in the oceans compared with the atmosphere. However, as taken into account in the model simulations from Cullum et al., the presence of extensive north-south continental boundaries completely changes the ocean circulation pattern to one in which heat is transported by a combination of a direct, buoyancy-driven meridional overturning circulation and a pattern of wind-driven gyres. The Coriolis forces acting on the northward or southward flow in these gyres are balanced by an east-west pressure gradient that spans the entire ocean basin. But the circulation then needs to be closed by intense and very narrow boundary currents flowing in the opposite direction, such as the Gulf Stream in the Atlantic or the Kuroshio current in the Pacific Ocean, that ‘‘lean against’’ the western coastal boundary. The strength of the direct meridional overturning circulation is determined by the intensity of cross-isopycnal
Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, Oxford, UK.
627
628
transport, which is commonly represented (albeit not very accurately) as a simple (vertical) diffusive process with a characteristic diffusion coefficient jv. The result suggests (Vallis, 2006) that meridional energy transport should scale as *(j2v tR)1/3, where tR is the planetary rotation period, much as found by Cullum et al. in their model [although somewhat curiously this tends to contradict evidence from laboratory experiments (Rayer et al., 1998), which suggest total heat transport should become independent of tR]. However, the actual mechanism for cross-isopycnal transport actually depends on more complex processes than simple diffusion, such as turbulent eddies and internal gravity waves, most of which cannot be resolved in most ocean models so have to be parameterized empirically with considerable uncertainty. As a result, and because the actual quantitative heat transport in the oceans is very difficult to measure directly, the magnitude of the contribution of the oceans to the total poleward heat transport in Earth’s climate system has been poorly constrained until relatively recently. For many years, it was assumed that the oceans transported roughly the same amount of heat as the atmosphere (a kind of agnostic equipartition hypothesis), perhaps even with a tendency for mutual compensation (Bjerknes, 1964) so that the total combined poleward transport by the atmosphere and oceans would remain roughly constant, even if either component varied. This continues to be a controversial topic, however, for ongoing research (Vallis and Farneti, 2009; Farneti and Vallis, 2013). In many respects, the study by Cullum et al. (2014) is just among the first tentative steps in a new phase of extrasolar planetary climate modeling. Although the ocean model they used is relatively advanced, it used rather coarse spatial resolution, and so was unable to properly resolve or parameterize either the vigorous mesoscale and smaller ocean eddies or the intense, narrow western boundary currents within the wind-driven gyres that are known to play a very important role in Earth’s oceans. Their model also included only a crude representation of the wind forcing of their ocean circulation. So it remains to be confirmed that the scaling of heat transport with tR that they have found will also apply in more comprehensive, coupled atmosphere-ocean-cryosphere systems that fully resolve or represent the complete range of relevant scales of motion. But it seems clear that any future complete assessment of the habitability of an Earthlike exoplanet must take proper account of the presence of oceans in moderating its climate.
READ References
Bjerknes, J. (1964) Atlantic air-sea interaction. Advances in Geophysics 10:1–82. Cullum, J., Stevens, D., and Joshi, M. (2014) The importance of planetary rotation period for ocean heat transport. Astrobiology 14, doi:10.1089/ast.2014.1171. Farneti, R. and Vallis, G. (2013) Meridional energy transport in the coupled atmosphere-ocean system: compensation and partitioning. J Clim 26:7151–7166. Hu, Y. and Yang, J. (2014) Role of ocean heat transport in climates of tidally locked exoplanets around M dwarf stars. Proc Natl Acad Sci USA 111:629–634. Joshi, M., Haberle, R., and Reynolds, R. (1997) Simulations of the atmospheres of synchronously rotating terrestrial planets orbiting M dwarfs: conditions for atmospheric collapse and implications for habitability. Icarus 129:450–465. Rayer, Q.G., Johnson, D.W., and Hide, R. (1998) Thermal convection in a rotating fluid annulus blocked by a radial barrier. Geophysical and Astrophysical Fluid Dynamics 87: 215–252. Read, P.L. and Lewis, S.R. (2004) The Martian Climate Revisited, Springer-Praxis, Berlin. Showman, A.P., Wordsworth, R.D., Merlis, T.M., and Kaspi, Y. (2013) Atmospheric circulation of terrestrial exoplanets. In Comparative Climatology of Terrestrial Planets, edited by S.J. Mackwell, A.A. Simon-Miller, J.W. Harder, and M.A. Bullock, University of Arizona Press, Tucson, pp 277–326. Vallis, G. (2006) Atmospheric and Oceanic Fluid Dynamics: Fundamentals and Large-Scale Circulation, Cambridge University Press, Cambridge, UK. Vallis, G. and Farneti, R. (2009) Meridional energy transport in the coupled atmosphere–ocean system: scaling and numerical experiments. Quarterly Journal of the Royal Meteorological Society 135:1643–1660.
Address correspondence to: Peter L. Read Atmospheric, Oceanic and Planetary Physics Clarendon Laboratory Parks Road Oxford, OX1 3PU UK E-mail: [email protected] Submitted 30 June 2014 Accepted 8 July 2014
