Climatic responses to the shortwave and longwave direct radiative effects of sea salt aerosol in present day and the last glacial maximum

Abstract

We examine the climatic responses to the shortwave (SW) and longwave (LW) direct radiative effects (RE) of sea salt aerosol in present day and the last glacial maximum (LGM) using a general circulation model with online simulation of sea salt cycle. The 30-year control simulation predicts a present-day annual emission of sea salt of 4,253 Tg and a global burden of 8.1 Tg for the particles with dry radii smaller than 10 μm. Predicted annual and global mean SW and LW REs of sea salt are, respectively, −1.06 and +0.14 W m−2 at the top of atmosphere (TOA), and −1.10 and +0.54 W m−2 at the surface. The LW warming of sea salt is found to decrease with altitude, which leads to a stronger net sea salt cooling in the upper troposphere. The changes in global mean air temperature by the present-day sea salt are simulated to be −0.55, −0.63, −0.86, and −0.91°K at the surface, 850, 500a, and 200 hPa, respectively. The emission of sea salt at the LGM is estimated to be 4,075 Tg year−1. Relative to present day, the LGM sea salt emission is higher by about 18% over the tropical and subtropical oceans, and is lower by about 35% in the mid- and high-latitudes in both hemispheres because of the expansion of sea ice. As a result of the weakened LGM water cycle, the LGM annual and global mean burden of sea salt is predicted to be higher by 4% as compared to the present-day value. Compared with the climatic effect of sea salt in present day, the sea-salt-induced reductions in surface air temperature at the LGM have similar magnitude in the tropics but are weakened by about 0.18 and 0.14°K in the high latitudes of the Southern and Northern Hemispheres, respectively. We also perform a sensitivity study to explore the upper limit of the climatic effect of the LGM sea salt. We assume an across-the-board 30% increase in the glacial wind speed and consider sea salt emissions over sea ice, so that the model can reproduce the ratio of sea salt deposition between the LGM and present day in the high latitudes of the Southern Hemisphere as suggested by the ice core records. This sensitivity run predicts a global emission of sea salt of 11,941 Tg year−1 with a global burden of 20.9 Tg. With such a high loading, sea salt aerosol at the LGM can have a net RE of −2.28 W m−2 at the TOA and lead to an annual and global mean cooling of 1.27°K at the surface.