The role of air-sea interactions in sub-seasonal variability

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This fellowship aims to understand the mechanisms by which air-sea interactions control the development and intensification of high-impact weather extremes, and to improve the ability of numerical models to simulate those mechanisms.Daily-monthly regional variations in weather and climate influence lives and livelihoods by affecting agriculture, hydrology and infrastructure. These sub-seasonal variations are controlled by high-impact phenomena including tropical cyclones; "blocking" high-pressure systems that cause droughts and heatwaves; and broad, organised areas of enhanced or reduced tropical thunderstorm activity that cause active and suppressed periods of monsoon rainfall.Analysis of field and satellite observations has suggested that transfers of energy and moisture between the atmosphere and the sea surface influence the location and intensity of these phenomena. These transfers result in short-lived (1-2 weeks) changes to sea-surface temperatures (SSTs), which can influence the atmosphere; tropical thunderstorms tend to favour warmer waters, for example. It is not possible to distinguish forcing from response using observations alone, however, preventing understanding of how air-sea feedbacks influence sub-seasonal phenomena. Many short- and medium-range (1-14 days) forecasts use numerical models of only the atmosphere, neglecting potentially critical air-sea interactions. Atmosphere-ocean coupled models that represent these interactions are used for seasonal-to-decadal forecasts and climate-change projections, but often struggle to simulate sub-seasonal variability. These failings limit predictions of regional weather and extremes, create uncertainty in regional climate-change projections and prevent scientists from using these models to understand air-sea feedbacks.These failings will be addressed through a novel modelling framework of an atmospheric model coupled to a simplified ocean model, which improves simulated short-lived SST variations; minimises errors in the model's mean climate that inhibit the simulation of high-impact phenomena; and allows air-sea feedbacks to be simulated in only certain regions of the globe or at certain times of year, to aid understanding of how these feedbacks influence high-impact phenomena. The framework will used with models from the Met Office, the European Centre for Medium-range Weather Forecasts and the Center for Multiscale Modelling of Atmospheric Processes (U.S.). Using models that differ considerably in their simulated high-impact phenomena permits more thorough testing of hypotheses about the impacts of air-sea interactions.This framework allows the simulated effects of air-sea interactions on high-impact phenomena to be more cleanly separated from the effect of errors in the simulation of the mean climate. Previous studies have conflated these effects, creating uncertainty about the role of air-sea interactions in sub-seasonal variability. In this framework, variations among models in how high-impact phenomena respond to air-sea interactions will be caused only by variations in the formulations of the atmospheric models. This will inspire experiments to alter these formulations and investigate how the representation of key atmospheric processes, such as the relationship between atmospheric moisture and precipitation, affects the simulation of high-impact phenomena. Re-forecasts of past high-impact phenomena will allow close comparisons of simulations and observations and permit experiments that test the effects of individual processes. Experiments in which model errors in the simulated mean climate are introduced in particular regions, or times of year, will identify those errors that most inhibit sub-seasonal variability. This fellowship will improve understanding of air-sea interactions and their role in sub-seasonal variability, predictions of weekly-monthly variations in weather and climate, and regional projections of climate change.