How do groundwater dynamics influence heatwaves in southeast Australia?

Groundwater plays an integral role in land-atmosphere interactions by connecting the sub-surface storage of water to transpiration via interactions with the root zone. The role of groundwater in moderating heatwave intensity has rarely been examined in coupled climate simulations, as many models overlook this key component of the water cycle. We use a coupled land-atmosphere model with an explicit representation of groundwater dynamics to analyze three major heatwave events in southeast Australia. Groundwater moistens and cools the land surface, with impacts extending to the top of the atmospheric boundary layer during heatwaves. Associated with the cooling, an increase of net radiation and a reduction in the atmospheric boundary layer height occurred, primarily over areas with a shallow water table depth and woody vegetation. The maximum air temperatures were reduced by up to 3 °C at the surface and up to 1 °C through the atmospheric boundary layer. Our results point to an important influence of groundwater on heatwave intensity, implying the necessity of considering groundwater dynamics in climate models in the future heatwave predications.