Oxygen, Carbon, Heat: Explorations in Atmosphere-Ocean Interaction

Abstract

This thesis describes three novel mechanisms of air-sea interaction. The first two chapters focus on the amplification of the hydrological (water) cycle as the climate system warms in response to a CO2 increase, which is realized as an amplification of freshwater flux (precipitation - evaporation) patterns. We find that this freshwater flux pattern leads to a redistribution of oxygen and carbon in the ocean, which modifies the previously recognized changes due to the atmospheric CO2 increase, warming, and circulation changes. The change in oxygen concentrations results from the change in sea surface salinity patterns which modifies the ocean circulation and heat uptake. The change in carbon concentrations results from the dilution or concentration of carbonate species (in parallel to salinity), and the same changes in heat uptake. The redistributions of oxygen and carbonate species in response to hydrological cycle amplification are comparable to the effect of global warming, which decreases both oxygen and carbon concentrations throughout the ocean. The third chapter provides a causal mechanism, for the first time, to link the change in sea surface temperature that develops during El Niño events to the change in temperature of the tropical free troposphere. The temperature of the free troposphere is primarily determined by the temperature profile followed by moist convection over the ocean (rising air above high sea surface temperatures). We isolate the part of the sea surface associated with convection (which is determined on thermodynamic rather than geographic grounds) and demonstrate that the El Niño surface temperature increase is driven by a decrease in surface wind speed which damps the evaporation rate. This result suggests a possible relationship between the zonal symmetry of the tropical atmospheric circulation, temperature of the free troposphere, and top-of-atmosphere energy budget.