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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

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
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

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.

Atlantic winter storms, Gulf Stream, SST-fronts
0930-7575
1-11
Vries, Hylke de
ab19b1d5-679d-49d2-9e75-7fbdc44c31f2
Scher, Sebastian
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Haarsma, Rein
a1c40e8b-9dce-4e33-8610-7589461c5166
Drijfhout, Sybren
a5c76079-179b-490c-93fe-fc0391aacf13
Delden, Aarnout van
0d032079-b44e-4fb5-a0be-fe38eea5c07f
Vries, Hylke de
ab19b1d5-679d-49d2-9e75-7fbdc44c31f2
Scher, Sebastian
6769d761-8016-48a0-b55f-480d9343eec8
Haarsma, Rein
a1c40e8b-9dce-4e33-8610-7589461c5166
Drijfhout, Sybren
a5c76079-179b-490c-93fe-fc0391aacf13
Delden, Aarnout van
0d032079-b44e-4fb5-a0be-fe38eea5c07f

Vries, Hylke de, Scher, Sebastian, Haarsma, Rein, Drijfhout, Sybren and Delden, Aarnout van (2018) 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. Climate Dynamics, 1-11. (doi:10.1007/s00382-018-4486-7).

Record type: Article

Abstract

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.

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Accepted/In Press date: 5 October 2018
e-pub ahead of print date: 12 October 2018
Keywords: Atlantic winter storms, Gulf Stream, SST-fronts

Identifiers

Local EPrints ID: 426325
URI: http://eprints.soton.ac.uk/id/eprint/426325
ISSN: 0930-7575
PURE UUID: edd1eda1-5b26-4c89-aabd-12268595a17d
ORCID for Sybren Drijfhout: ORCID iD orcid.org/0000-0001-5325-7350

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Date deposited: 22 Nov 2018 17:30
Last modified: 16 Mar 2024 07:16

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Contributors

Author: Hylke de Vries
Author: Sebastian Scher
Author: Rein Haarsma
Author: Aarnout van Delden

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