My overarching goal is to study climate as an evolving but surprisingly stable dynamic system, one in which interactions of the cryosphere, hydrosphere, atmosphere, and biosphere are considered in an integrated fashion. The role of Tropical atmosphere-ocean circulations in the global climate of the present and implications for their role throughout Earth's history have impressed themselves upon me. Specifically, the question that I would like to address via modeling and present and paleo-data analysis is the role of dynamic constraints on the Tropical ocean-atmosphere system in determining the large-scale climate patterns within the Tropics and heat export from the Tropics under "hothouse" scenarios. (For my recent work on the subject click here).
Understanding the interaction of radiative and dynamic processes at work in the Tropics is essential for evaluating the effects of ENSO, predicting climatic responses to anthropogenic CO2 increases and more generally, in shedding light on the problem of the long term stability of the Tropical climate. While isotopic and biologic data, and model results often do not agree with each other with respect to the specific history of temperature change they do tend to lend support to the idea that the tropical climate has experienced significant but bounded variation of up to 5° C . A large body of evidence exists to support the notion that the state of the Hadley circulation, including the variation through the phases of ENSO, is strongly coupled to extratropical heat transports. If variation of temperature in the Tropics is bounded, the view may be taken that the Tropics constrain the evolution of the extratropical climate.
Two theoretical tools have become available over the past twenty years to aid in understanding the Tropical atmosphere. The first is that the circulation may be understood as nearly inviscid and angular momentum conserving. The second is that over large time and space scales, the thermodynamic structure is roughly in agreement with the predictions of convective quasi-equilibrium. The implications for these atmospheric constraints as the top boundary condition on both the wind-driven and thermohaline circulation of the ocean have yet to be sufficiently explored. The question of what determines tropical-extratropical heat transports probably involves both these ocean effects coupled with the changes to the baroclinic atmospheric heat fluxes associated with the strength of the jet streams.
There exists a wide body of evidence including Ocean Anoxic Events and results from GCM (and box model) experiments indicating that during warm climate, 'greenhouse' regimes there have been major reorganization s of the ocean circulation. It has been hypothesized that under conditions of strong salinity forcing in the Tropics and weak temperature forcing near the poles a weak 'halo-thermal', salinity driven equatorial sinking mode ensues. This of course, suggests a substantially different atmospheric Tropical circulation than todays (one strong enough to create a dominant salinity forcing) and possibly a proportionally much greater atmospheric tropical-extratropical heat transport. The proxy data record , i.e., the record of wind contained in the eolian sediment record, the record of sea surface temperatures and ocean chemical composition contained in deep sea cores, and the paleo-botanical and paleolynological record of ancient land temperatures and rainfall distributions, provide a means of evaluating these hypotheses.
To address these issues in my research I am focusing on one of the "warmest" intervals in the Earth's past, the early Eocene, and I am exploring the general model-produced patterns of wind, rainfall, temperature, current, and heat transport. After comparision of the results based on different modeling assumptions, model configurations, and domain types the results are being compared with those inferred from the paleontological record. In this way the the dependencies of climate change predictions on the details of model type, boundary condition, and physical parameterization can be elucidated. and the theory of climate change would be advanced.
One important result that has already become evident from this study is that the temperatures of extratropical continental interiors are extremely sensitive to tropical SSTs, but only during the winter season. While the reasons for this sensitivity are relatively straightforward (a la work by Lindzen and Hou), the important implication of this work is that the "equable" climate problem may potentially be solved if current tropical SST reconstructions have a cool bias. We suggest that tropical SSTs may have been significantly warmer than modern during "hothouse" climates in the past, and that therefore the existence of a tropical thermostat is unlikely.