Link between land-ocean warming contrast and surface relative humidities in simulations with coupled climate models

Simulations of warming climates with coupled climate models exhibit strong land-ocean contrasts in changes in surface temperature and relative humidity, but little land-ocean contrast in changes in equivalent potential temperature. A theory that assumes equal changes in equivalent potential temperature over land and ocean captures the simulated land-ocean warming contrast in the tropics if changes in relative humidity and ocean temperature are taken as given. According to the theory, land relative humidity changes and the land-ocean contrast in the control climate contribute equally to the tropical warming contrast, while ocean relative humidity changes make a smaller (but also positive) contribution. Intermodel scatter in the tropical warming contrast is primarily linked to land relative humidity changes. These results emphasize the need to better constrain land relative humidity changes in model simulations, and they are also relevant for changes in heat stress over land. =