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How Gulf-Stream SST-fronts influence Atlantic winter storms: Results from a downscaling experiment with HARMONIE to the role of modified latent heat fluxes and low-level baroclinicity
Stockholm University, Faculty of Science, Department of Meteorology .
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Number of Authors: 52019 (English)In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 52, no 9-10, p. 5899-5909Article in journal (Refereed) Published
Abstract [en]

The strong horizontal gradients in sea surface temperature (SST) of the Atlantic Gulf Stream exert a detectable influence on extratropical cyclones propagating across the region. This is shown in a sensitivity experiment where 24 winter storms taken from ERA-Interim are simulated with HARMONIE at 10-km resolution. Each storm is simulated twice. First, using observed SST (REF). In the second simulation a smoothed SST is offered (SMTH), while lateral and upper-level boundary conditions are unmodified. Each storm pair propagates approximately along the same track, however their intensities (as measured by maximal near-surface wind speed or 850-hPa relative vorticity) differ up to +/- 25%. A 30-member ensemble created for one of the storms shows that on a single-storm level the response is systematic rather than random. To explain the broad response in storm strength, we show that the SST-adjustment modifies two environmental parameters: surface latent heat flux (LHF) and low-level baroclinicity (B). LHF influences storms by modifying diabatic heating and boundary-layer processes such as vertical mixing. The position of each storm's track relative to the SST-front is important. South of the SST-front the smoothing leads to lower SST, reduced LHF and storms with generally weaker maximum near-surface winds. North of the SST-front the increased LHF tend to enhance the winds, but the accompanying changes in baroclinicity are not necessarily favourable. Together these mechanisms explain up to 80% of the variability in the near-surface maximal wind speed change. Because the mechanisms are less effective in explaining more dynamics-oriented indicators like 850 hPa relative vorticity, we hypothesise that part of the wind-speed change is related to adjustment of the boundary-layer processes in response to the LHF and B changes.

Place, publisher, year, edition, pages
2019. Vol. 52, no 9-10, p. 5899-5909
Keywords [en]
Atlantic winter storms, Gulf Stream, SST-fronts
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:su:diva-169263DOI: 10.1007/s00382-018-4486-7ISI: 000465441400046OAI: oai:DiVA.org:su-169263DiVA, id: diva2:1323078
Available from: 2019-06-11 Created: 2019-06-11 Last updated: 2019-06-11Bibliographically approved

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