Unifying Perspectives on Atmosphere-Ocean Interactions during Cyclone Development

Extratropical cyclones are a key feature of the mid-latitude climate and weather, where air-sea interactions play a crucial role in their genesis and intensification, yielding model uncertainties and forecast challenges. Understanding these interactions between ocean sea surface temperature fronts and the atmosphere is thus essential for understanding the role of diabatic processes in extratropical cyclones and constraining uncertainty. While recent studies highlight the importance of air-sea interactions for the development and intensification of cyclones, we are still lacking a theoretical framework unifying moist baroclinic under the influence of air-sea interaction and diabatic processes. During the last year, the researchers within UNPACC attended the online EGU conferences with three presentations. The group?s numerical modelling framework has been used for idealized studies of extratropical cyclones with specifically prescribed sea surface temperature (SST) that were moved underneath the cyclone. Several sensitivity experiments have been conducted and the analysis indicates a strong response of the cyclone development with respect to the underlying SST and its gradient, such that cyclones develop stronger over higher SST and intensify less strongly once they move over lower SST. Sensible heat fluxes were found to dampen cyclone development, while latent heat fluxes contribute to a further intensification. Two papers were published on these findings in 2020 and one more is in preparation for 2021 about extended idealized simulations. The ERA-Interim data has been used to classify cyclones with respect to their movement along or across the SST front in the Atlantic and Kuroshio region. The results indicate that cyclones respond most strongly to the SST front when their trajectory crosses the front from the warmer to the colder SST side. While the land-sea contrast plays an appreciable role in the Gulf Stream region, the jet position and intensity in the Kuroshio has a more dominant role. These results have been published in two papers in the Quarterly Journal of the Royal Meteorological Society. Moreover, we tested the importance of the SST front by using model simulations with a realistic and smooth SST front. The control run confirmed our analysis with ERA-Interim data. Furthermore, we found that the contribution of the surface fluxes in the Gulf Stream and Kuroshio region were most influenced during times when no cyclones were present, supporting that the SST front might not have a direct influence on cyclone development. Regarding the interactions between atmospheric and oceanic fronts, we have developed a new theoretical framework on how the underlying SST gradients affect low-level atmospheric frontogenesis, which has been submitted for publication in 2020 and is currently under review. The model will be extended to include vertical motion forced by the interaction of fronts with sea surface temperature gradients. The calculations will be completed and a paper describing the results written in the coming months. We also investigated case studies for which enhanced baroclinicity is left behind by a cyclone. For the extreme storm Dagmar, we found a direct route from diabatic available potential energy generation to eddy kinetic energy. We compared the isentropic slope distribution to the temporal distribution of extratropical cyclone occurrence and thereby propose a new hypothesis for the formation of clustering events. This work was published in 2020. We also started compiling composites of clustered and non-clustered cases, to see if other clustering cases behave in a similar way as the Dagmar case. First results indicate that diabatic heating along the trailing cold front is a general property of clustering. We are currently preparing a climatological study for submission in the coming months.

Researchers within UNPACC made great progress in understanding the interactions between the atmosphere and the ocean and their implications for cyclone development. The researchers worked efficiently and published eight papers in international journals and two more are currently under review with two further papers in preparation. The main finding is that that SSTs can play a significant role in cyclone development, such that the additional moisture provided over higher SSTs can yield stronger development of storms. On the other hand, an increase in the gradient of the SST, for example along the Gulf Stream or Kuroshio, does not play a significant direct role in the development of storms in the respective region. These findings have ramifications for weather and climate, where it appears more crucial to represent eventual SST anomalies than variations in the gradient of the SST.

Extratropical cyclones are a key feature of the mid-latitude climate and weather, where unresolved mesoscale air-sea interactions are thought to play a crucial role in their genesis and intensification, yielding model uncertainties and forecast challenges. Understanding these mesoscale interactions between ocean sea surface temperature fronts, ocean eddies, and the atmosphere is thus essential for understanding the role of diabatic processes in extratropical cyclones and constraining uncertainty. While recent studies highlight the importance of mesoscale air-sea interactions for the development and intensification of cyclones, we are still lacking a theoretical framework unifying moist baroclinic and frontal-wave instability under the influence of air-sea interaction and diabatic processes. How do oceanic fronts and eddies influence the genesis and intensification of cyclones? What is the role of mesoscale air-sea interaction processes for the upscale growth of instabilities? Can such instabilities trigger or inhibit the intensification of extratropical cyclones? What are the underlying mechanisms for diabatic amplification of energy conversion in extratropical cyclones? We will establish an innovative collaboration in atmospheric dynamics and air-sea interactions by combining key and complementary expertise from the Universities of Bergen, Monash (Australia), East Anglia (UK), Texas A&M (USA), Tokyo (Japan) as well as ECMWF (UK). The central goal of the collaboration is to elucidate and quantify the mechanisms responsible for extratropical cyclone intensification associated with mesoscale air-sea interactions. Our approach utilizes novel detection routines on high-resolution coupled model datasets and reanalyzes as well as idealized and real case simulations. As the integrated framework is grounded in fundamental dynamics, we expect UNPACC to yield a unified framework for moist frontal-baroclinic instability theory including the influence of air-sea interactions.