NASA-GISS AGCM Simulations of Glacial Climate Sensitivity to Ocean Heat Transport Forced Changes in Atmospheric Water Vapor Content

R S Webb (NOAA-NGDC Paleoclimatology Program, 325 Broadway, Boulder, CO 80303 USA; 303-497-6967; e-mail: robert.s.webb@noaa.gov),D H Rind (NASA-GISS, 2880 Broadway, New York, NY 10025 USA; 212-678-5593, e-mail: cddhr@giss.nasa.gov),R Healy (Woods Hole Oceanographic Institute, Woods Hole, MA 02543 USA; 508-289-3514; email: rhealy@whoi.edu),C Charles (Scripps Institute of Oceanography, La Jolla, CA, 92093 USA; 619-534-5911; email: ccharles@ucsd.edu)

In a series of paleoclimate sensitivity experiments using the 8x10 (latitude versus longitude) NASA-GISS Model II AGCM, we examine the role of different configurations of ocean heat transports (OHTs), and associated ocean heat convergences (OHCs), amongst otherwise Last Glacial Maximum (LGM) boundary conditions. We made AGCM simulations using OHTs calculated for 1) a modern climate but with glacial land-sea distributions, 2) a glacial climate with CLIMAP LGM SSTs, 3) a glacial climate with CLIMAP LGM SSTs minus 2 C everywhere, and 4) a cool tropics glacial climate with all CLIMAP LGM SSTs greater than 18 C assigned an average of the SST value and 18 C. We also considered the glacial climate response to changes from modern to LGM atmospheric CO2 levels in the sensitivity experiments. Our results indicated that the dominant atmospheric feedback contributing to LGM global surface air temperature cooling was regulated by increases/decreases in evaporation over the subtropical Pacific, an area that is a significant source of atmospheric water vapor today. The steep gradient of CLIMAP LGM SSTs between the subtropics and mid-latitudes of the Pacific gave rise to high OHCs in the subtropics relative to modern. Evaporation rates increased in the subtropics to balance the increased CLIMAP LGM OHCs, and this increase in evaporation over the subtropical Pacific compensated for decreases elsewhere, moderating changes in atmospheric water content in response to cooler glacial conditions. In contrast, maintaining modern OHTs under LGM boundary conditions greatly reduced OHCs and evaporation in the subtropical Pacific, supplied less water vapor to the atmosphere, and effectly cooled the mid to low latitudes. The cool tropics LGM experiment showed similar enhanced coolings related to reductions in OHTs and evaporation in the subtropical Pacific, and a comparable reduction in atmospheric water content, whereas the CLIMAP LGM SSTs minus 2 C exhibited moderate cooling, about halfway between the prescribed CLIMAP LGM SSTs experiment and either the LGM experiment with modern OHTs or the cool tropics LGM experiment.