by Judith Curry
A paper in press in the Journal of Climate provides some insight into the interaction of cloud feedback with ocean heat transport.
Climate sensitivity to changes in ocean heat transport
Marcelo Barreiro and Simona Masina
Abstract: Using an atmospheric general circulation model coupled to a slab ocean we study the effect of ocean heat transport (OHT) on climate prescribing OHT from zero to two times the present-day values. In agreement with previous studies an increase in OHT from zero to present-day conditions warms the climate by decreasing the albedo due to reduced sea-ice extent and marine stratus cloud cover and by increasing the greenhouse effect through a moistening of the atmosphere. However, when the OHT is further increased the solution becomes highly dependent on a positive radiative feedback between tropical low clouds and sea surface temperature. We found that the strength of the low clouds-SST feedback combined with the model design may produce solutions that are globally colder than Control mainly due to an unrealistically strong equatorial cooling. Excluding those cases, results indicate that the climate warms only if the OHT increase does not exceed more than 10% of the present-day value in the case of a strong cloud-SST feedback and more than 25% when this feedback is weak. Larger OHT increases lead to a cold state where low clouds cover most of the deep tropics increasing the tropical albedo and drying the atmosphere. This suggests that the present-day climate is close to a state where the OHT maximizes its warming effect on climate and pose doubts about the possibility that greater OHT in the past may have induced significantly warmer climates than that of today.
The paper is in press in the Journal of Climate. A complete online version of the paper is found [here].
Introduction
The overall all problem is laid out in the Introduction, which provides an excellent literature review on the topic. A few excerpts:
The oceans absorb heat mainly in the tropical regions where cold water upwells to the surface and lose it in high latitudes where cold and dry winds blow over warm currents during winter time. This implies a net heat transport by the oceanic circulation from the equator to the polar regions that contributes to remove the surplus of heat received in the tropics. Averaged over long times the ocean must gain and lose equal amounts of heat in order to maintain a steady state. The oceanic heat transport is largest in the tropical region and becomes very small poleward of 45° (Trenberth and Caron 2001). At higher latitudes the heat transported by the atmosphere, due mainly to the presence of energetic eddies, is the main contributor to total poleward heat transport.
The circulation of the oceans likely changed over the course of Earth’s history, due to changes in external forcings, e.g, insolation and greenhouse gases, and changes in continental configuration. Thus, a change in ocean heat transport is a common explanation in studies of past climates. For example, Rind and Chandler (1991) propose that 46% greater ocean heat transport during the Jurassic period (200144 million years ago, Ma) would have warmed the climate by 6 K. They also suggest that 68% greater ocean heat transport during the Cretaceous (14465 Ma) would have warmed the climate by 6.5 K. Barron et al. (1993) studied the impact of oceanic heat transport in the Cretaceous using an atmospheric model coupled to a slab ocean. Imposing presentday zonally averaged heat transport but distributed differently among oceans due to a different continentaconfiguration they found that increased ocean heat transport warms the climate. Moreover, they found that the warming is not linearly related to the value of oceanic heat transport: increasing from 0 to present day heat transport increases the surface temperature by 2.6 K, but only 0.6 K from present day to two times present day values. Closer to the present and already with the same continental configuration, Dowsett et al. (1996, 2009) argue that the warmer high latitude ocean temperatures during the midPliocene (~3 Ma) can be explained by a more vigorous North Atlantic Deep Water formation and thermohaline circulation. Finally, Romanova et al. (2006) found using an atmospheric general circulation model that reduced ocean heat transport contributed to global cooling during the Last Glacial Maximum. In general, patterns of decreased equatortopole temperature gradients due to a large extratropical warming, as in the case of the Eocene, are explained as due to enhanced ocean heat transport: larger ocean heat transport decreases sea ice in high latitudes leading to an icealbedo feedback that warms these regions. The tropics may cool or stay close to present values, so that there is overall global warming. In recent years, other studies have suggested that increased ocean heat transport cannot fully explain the decrease in the meridional temperature gradient during the Eocene. Alternative explanations involving high latitude convection feedbacks have been proposed to explain the high latitude warming of past climates.
The undergoing changes in climate caused by human activities will probably affect the oceanic circulation and its heat transport, which then may feed back onto theatmosphere and climate. Nevertheless, the connection between atmospheric and oceanic heat transports is not yet well understood. For example, is it possible to change one component without changing the other one? Everything else being equal (e.g. constant greenhouse concentration), this would result in changes in the albedo of the planet because the total heat transport by the oceanatmosphere system will be different, and thus the system has to gain heat differently at each latitude.
The representation of clouds is one of the main weaknesses of current climate models . In particular, the parameterization of boundary layer stratus clouds has proved to be very difficult and has been a major area of research in the last decade. These clouds have a very weak greenhouse effect, but strongly reflect incoming shortwave radiation, thus modulating the albedo of the Earth. Bony and Dufresne (2005) have shown that the simulation of marine low level clouds is a large source of uncertainty in tropical cloud feedbacks and of climate sensitivity, suggesting that the simulation of tropical responses to different forcings will strongly depend on the parameterization of these clouds, and that results need to be tested using different cloud schemes.
Conclusion
For decreased ocean heat transport we have found very similar results as previously reported by W03 and H05: a decrease in the heat transported by the ocean cools the climate by increasing the sea ice extent and the low oceanic cloud cover, thus increasing the albedo. Moreover, the tropical regions become narrower thus decreasing the moistening of the subtropical atmosphere and thus the greenhouse trapping. These atmospheric changes are such that the atmospheric heat transport tends to compensate for the decreased OHT: there is almost complete compensation in the deep tropics while in the extratropics the total poleward transport of heat is smaller when the ocean circulation is absent. We propose that these changes are robust across models mainly because decreasing the ocean heat transport does not fundamentally alter the circulation of the presentday atmosphere, it essentially represents a small deviation from today’s conditions.
The climatic response for larger than presentday values of ocean heat transport is very different from previous studies and it is highly dependent on the parameterization of low clouds. Taking equatorial regions warmer than the subtropics as a plausibility criterion for the solution, the results are that an increase in OHT tends to warm the climate and that this warming is largest when the tropical region is widest. However, the cloud scheme dictates how much can the OHT increase before the solution becomes unphysical. A highly sensitive scheme suggests that our current climate is very close to the maximum positive effect of the ocean heat transport on climate (less than a 15% increase away); another cloud scheme suggests that the climate can further warm 0.6 K for a 25% increase in OHT. For OHT increases larger than 25% of presentday values, a strong positive radiative feedback between tropical low level clouds and sea surface temperature works, always leading to an unphysical cold climate. In this state, low level clouds tend to cover the tropics which increases the albedo enormously. At the same time, the Hadley circulation reverses, inducing subsidence over the tropics which inhibits convection and dries the atmosphere, thus cooling it further due to decreased greenhouse trapping. As a consequence the tropical atmosphere transports heat equatorward resulting in decreased total ocean+atmosphere heat transport when the OHT increases.
Thus, as long as the cloud cover parameterizations are correct, the results presented here do not support the hypothesis that larger OHT may have led in the past to warmer than presentday climates without changing the total poleward heat transport, as has been suggested in the literature. We argue that the results of Barron et al. (1993) are due to the use of an atmospheric model with simpler physical parameterizations. To test this we repeated the experiment of increasing the OHT using the International Centre for Theoretical Physics (ICTP) AGCM, an atmospheric model with an horizontal resolution of T30 and 8 vertical levels and simpler parameterizations of the physical processes. In this model cloud cover is defined diagnostically from the values of relative humidity in the air column (excluding the boundary layer) and the total precipitation, and cloud albedo is proportional to the total cloud cover. We found that this model warms 0.8 K when the OHT is increased from 0 to present day values and 0.4 K from presentday to two times presentday heat transport. The sensitivity is much smaller than that of ECHAM5, and even compared to that of the model of Barron et al. (1993). However, as in the latter case, an increase in ocean heat transport always warms the climate, and in a nonlinear way. Taken together, the results of this work suggest that the simpler the cloud cover scheme and the cloudalbedo relationship the less sensitive is the model to changes in ocean heat transport. This is mainly due to differences in the parameterization of low level clouds, and their interaction with radiative fluxes.
A caveat of our results is the lack of ocean dynamical adjustment which may act as a negative feedback opposing cloudSST feedback that leads to the large simulated tropical cooling, in a similar way as found by Hazeleger et al (2005). Note that this caveat applies not only for increased values of the OHT, but also for decreased values because all solutions involve changes in the surface winds. Other possibilities include that the schemes used in today’s models are missing important physics to represent correctly the behavior of low clouds, as has been suggested previously , and so past climates could be used as test for models.
To date our understanding of the climatic response to changed OHT comes mainly from atmospheric models coupled to fixed oceans. Our results point that not only is the lack of dynamical adjustment an important issue when using these models, but also the parametrization of low clouds that result in cloudSST radiative feedbacks of different strengths. In the end, only through the use of coupled models that allow the interaction between these processes will be possible to address this question fully. Nonetheless, we believe the results presented here can serve as a guide for future explorations of the role of the oceans in climate.
JC comment: This paper addresses the very interesting problem of cloud feedback and climate sensitivity in response to ocean heat transport, which relates to some of the ideas that Roy Spencer has been developing. The paper uses climate models in the manner in which they are probably most useful: to conduct experiments using different forcing data and model parameterizations to increase understanding of both how the climate system works and the limitations of climate models.