Aqua Planets

In an attempt to exploit the information and knowledge encoded in coupled climate models, we have begun to use them to explore some of the basic questions in climate such as what sets the pole-equator temperature gradient and the extent of polar ice caps.

Fig.1 shows a recent estimate of the meridional energy transport of atmospheric and oceanic circulation (the vertical arrows give an indication of the uncertainties). We see that the coupled system carries some 5.5PW poleward, atmospheric and oceanic contributions are of comparable magnitude, but that the northward energy transport of the atmosphere exceeds the ocean by a factor of ~4 in middle-to-high latitudes. Many questions come to mind. One wonders, for example, why the energy flux is partitioned in this way, could it have been very different in the past and what is its role in controlling the extent of polar ice caps?

In particular we have been studying partition of energy transport, and the extent of polar ice, in highly idealized worlds in which there is no land, or the land distribution is highly idealized comprising meridional barriers with and without gaps. Here we briefly describe results from calculations in which there is no land (Aqua), in which there is a meridional barrier extending from pole to pole separating the ocean in to one giant basin (Ridge) and in which there is an opening in the ridge around the south pole (Drake). These topographic constraints, sketched in Fig. 2, result in very different ocean circulation patterns and hence, as we shall see, its very different abilities to transport heat meridionally.

Millennial timescale simulations of a coupled atmosphere-ocean-ice system on Aqua (no barriers), driven by modern day orbital and CO₂ forcing, yield a climate in which icecaps reach down to 55^o of latitude or so. In the time-mean, both atmosphere and ocean comprise eastward- and westward-flowing jets whose structure is set by their respective hydro-dynamical (baroclinic) instabilities.

Fig.3 shows a snapshot of the solution after 5000 years (of order the ocean mixing timescale) of synchronous integration of the coupled system. We see atmospheric weather systems (which have a scale on the order of the radius of the planet – see Fig.3 top) sweeping warm air polewards and cold air equatorwards, thereby affecting meridional energy transport.

Despite the absence of planetary scale turbulent motions in the ocean (‘weather systems’ in the ocean have a scale of only 100km or so) and a predominance of zonal motion over meridional, the ocean is remarkably efficient at transporting heat poleward –compare Fig.4(top) with Fig.1. Such poleward heat transport is achieved by wind-driven meridional mass transport that is directed (because of Coriolis effects) at right-angles to the prevailing wind stress. Indeed, and remarkably, in a gross sense the partition of heat transport between the atmosphere and ocean is much the same as that of the present climate, with the ocean dominating in the tropics and the atmosphere dominating in middle-to-high latitudes. Subtleties, however, matter greatly. Notably, in this ocean without meridional barriers, ocean heat transport becomes very small polewards of 60^o, permitting the growth of large ice caps (see Fig.3, bottom) and, due to the ice-albedo feedback, a large pole-equator temperature gradient. But what happens if we introduce a meridional barrier in to the Aqua ocean, blocking zonal flow as in Ridge? In that case a gyral circulation with strong meridional flow in western boundary currents is set up which is much more efficient at carrying energy polewards in to high polar latitudes – compare Fig.4(middle) with Fig.4(top), leading to an equilibrium climate in which there are no polar ice caps and the pole-equator temperature gradient is much reduced.

In Drake, (inspired by the present-day Drake passage separating South America from Antarctica), in which we now introduce a gap in the barrier around the South Pole, circumpolar oceanic flow is set up, analogous to the Antarctic Circumpolar Current (ACC) of the present climate. Interestingly, an ice cap returns over the South Pole. Evidently the high-latitude circumpolar flow acts as an efficient barrier to meridional energy transport, permitting the growth of sea ice over the South Pole. This is clear by inspection of Fig.4c where we observe a marked inter-hemispheric asymmetry in oceanic meridional energy transport, which all but vanishes poleward of 60^o. This has obvious paleo implications concerning the tectonic separation of South America from Antarctica and the growth of the Antarctic ice sheet.

But what happens if we introduce a meridional barrier in to the Aqua ocean, blocking zonal flow as in Ridge? In that case a gyral circulation with strong meridional flow in western boundary currents is set up which is much more efficient at carrying energy polewards in to high polar latitudes – compare Fig.4(middle) with Fig.4(top), leading to an equilibrium climate in which there are no polar ice caps and the pole-equator temperature gradient is much reduced.

A summary of the climatologies of the Aqua, Ridge and Drake solutions is presented in Fig.5. It is important to realize that these very different climates (as different, for example, as that of the warm Eocene and the relative cold of the present day) are brought about entirely by changes in ocean circulation induced by geometrical constraints.

The presence or absence of ice is sensitive to high latitude ocean energy flux, which in turn is sensitive to topographic control on its circulation patterns. A more detailed discussion can be found in Enderton and Marshall (2009).

References
 

Campin, J-M., J. Marshall and D. Ferreira (2008). Sea ice-ocean coupling using a rescaled vertical coordinate z^*. Ocean Modelling, Vol 24, pp 1-14. (DOI. 10.1016/j.ocemod.2008.05.005)

Ferreira, D., J. Marshall, and J.-M. Campin, 2009: Role of
geometrical constraints on ocean circulation in setting the mean climate.
J. Climate. In press.

Enderton, D. and  J. Marshall, 2009 Explorations of atmosphere-ocean-ice climates on an aqua-planet and their meridional energy transports. J. Atmos. Sci, , 1593-1611. (DOI: 10.1175/2008JAS2680.1)
 

Marshall, J., D. Ferreira, J-M Campin and D. Enderton, 2007: Mean climate and variability of the atmosphere and ocean on an aquaplanet. J. Atmos. Sci, 64, 4270-4286. (DOI:10.1175/2007JAS2226.1)
 

Trenberth, K. and J. Caron, 2001: Estimates of meridional atmosphere and ocean heat transports. J. Climate, 14, 3433-3443. (DOI: 10/1175/1520-0442(2001)014<3433:EOMAAO>2.0.CO;2)
 

Wijffels, S.E., R.W. Schmitt, H.L. Bryden and A. Stigebrandt, 1992: Transport of freshwater by the oceans. J. Phys. Oceanogr., 22, 155-162.

Wunsch, C., 2005. The total meridional heat flux and its oceanic and atmospheric partition, J. Climate, 18, 4374-4380. (DOI: 10.1175/JCLI3539.1)