Abstract
Heat extremes over large continental regions are often intensified by depleted surface soil moisture, yet the role of land–atmosphere coupling over small island nations like New Zealand remains underexplored. Here, we present the results of bespoke regional climate model experiments designed to quantify the contribution of coincident dry soils to the severity of recently observed hot summer months over New Zealand. Specifically, we reproduce the broader weather conditions for four case study events in 1998, 2015, 2019 and 2020 but with prescribed soil moisture states: (1) a control experiment with the evolution of soil moisture kept consistent with observations; (2) a climatology experiment with soil moisture fixed to its long-term monthly mean; and (3) a wet experiment with soil moisture fixed to the 90th percentile of the monthly climatology. Results reveal strong land–atmosphere coupling is evident over important agricultural regions of the South Island, while the magnitude of this feedback varies depending on the case study and region considered in the North Island. Soil moisture deficits can intensify heat extremes by 3–5 °C compared to wet conditions across the most-affected regions, with the hottest days of the year intensified by more than 5 °C under the anomalously dry soil moisture conditions in the control experiment. These temperature anomalies are driven by enhanced sensible heat fluxes and suppressed latent heat fluxes, reinforcing surface heating. The presence of significant land–atmosphere coupling over a relatively small island nation with strong maritime influences suggests land–atmosphere feedbacks can be consequential for regions other than continental climate regimes under future warming.