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

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.

This fellowship will impact the U.K. Met Office (UKMO); international modelling centres using UKMO general circulation models (GCMs), including national weather services in Australia, India, Korea and Singapore; the European Centre for Medium-range Weather Forecasts (ECMWF); the insurance and energy-trading industries; the Australian government; and the general public.

The primary impacts of the fellowship will come through improvements in the representation of key aspects of tropical and extra-tropical variability in the GCMs employed in this project, which are used for daily--seasonal forecasting and climate projections. The sub-seasonal phenomena of interest are the Madden--Julian oscillation (MJO), monsoon intra-seasonal variability (ISV; active and break rainfall cycles) and extra-tropical blocking. The MJO and monsoon ISV cause floods and droughts throughout the tropics, including in India, Australia and southeast Asia; extra-tropical blocking is responsible for severe heatwaves and droughts in the Northern and Southern Hemisphere mid-latitudes.

The UKMO and ECMWF will benefit through improved assessments of the ability of their GCMs to simulate these key sub-seasonal phenomena, as well as through the identification of sources of GCM errors that hinder sub-seasonal prediction. Identifying and improving errors in the UKMO and ECMWF GCMs that degrade the representations phenomena will translate into improved sub-seasonal--seasonal prediction skill, as well as more reliable projections of the regional impacts of climate change, such as the statistics of droughts and floods.

International centres using UKMO models will benefit not only from the above improvements to the UKMO GCM, but also from the integration of the KPP modelling framework (used in this fellowship) into the UKMO modelling infrastructure. The framework is a powerful, flexible tool for understanding and predicting the impacts of local and remote air-sea interactions on regional, sub-seasonal weather and climate phenomena. The framework will be used in an international model inter-comparison, sponsored by the Working Group on Numerical Experimentation MJO Task Force, to diagnose the effects of air-sea interactions on the MJO, which demonstrates the framework's attractiveness for process-based understanding of weather and climate extremes. Research into and prediction of sub-seasonal variability globally will be enhanced by including the framework in the UKMO infrastructure.

The global insurance industry will benefit from financial savings through improved sub-seasonal and seasonal predictions of extreme weather and climate, such as floods and droughts, as well as more reliable projections of the impacts of climate change. Likewise, the energy-trading industry will benefit financially from improved forecasts of extreme weather that affect energy demand and supply from renewable sources (e.g., solar, wind).

The Australian government will benefit from improved predictions of summer-monsoon sub-seasonal variability, heatwaves and droughts that will allow them to implement more effective disaster-management strategies. The increased reliability of the projected impacts of climate change on extreme weather will inform the Australian's governments decisions on adaptation and mitigation, such as water management for irrigation and urban use.

The general public will benefit from financial savings from improved forecasting of regional weather and climate extremes, as well as from more effective decisions on adaptation and mitigation to climate change. The agricultural sector relies on UKMO and ECMWF forecasts to determine planting dates, as well as to mitigate and adapt to extreme events, such as heatwaves or droughts that can stress crops at critical stages during the growing season. Improved extended-range predictions and climate-change projections will allow forecast users to make better-informed decisions in response to extreme events.