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Impact of a warm ocean eddy’s circulation on hurricane-induced sea surface cooling with implications for hurricane intensity Richard M. Yablonsky and Isaac Ginis University of Rhode Island 16th Conference on Air-Sea Interaction 13 January 2009
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Storm-core SST reduction
Evaporation from the sea surface provides the heat energy to intensify a hurricane, but the available energy is reduced if the SST decreases within the hurricane’s core SST can decrease in the hurricane’s core by: Vertical mixing/entrainment Upwelling Horizontal advection of a surface cold pool Hurricane propagation over cold pool Heat flux to the atmosphere Self-explanatory Small by comparison
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1) Vertical mixing/entrainment
O S P H E R Wind stress → surface layer currents Current shear → turbulence Turbulent mixing → entrainment of cooler water O C E A N Sea surface temperature decreases Warm sea surface temperature Cool subsurface temperature Subsurface temperature increases This is a 1-D (vertical) process
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2) Upwelling This is a 3-D process
M O S P H E R Cyclonic wind stress → divergent surface currents Cyclonic hurricane vortex Divergent currents → upwelling Upwelling → cooler water brought to surface O C E A N Warm sea surface temperature Cool subsurface temperature This is a 3-D process
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3) Horizontal advection of surface cold pool
M O S P H E R Preexisting cold pool is located outside storm core Cyclonic hurricane vortex Preexisting current direction is towards storm core Cold pool is advected under storm core by currents O C E A N Warm sea surface temperature Cool subsurface temperature This is a horizontal process
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Motivation Vertical mixing/entrainment is assumed to be the dominant mechanism for storm-core SST reduction Upwelling is neglected in coupled hurricane-ocean models that use a 1-D (vertical) ocean component Horizontal advection of a surface cold pool has rarely (never?) been considered a potential impact of a warm core ring (WCR) on storm-core SST… and it is neglected by 1-D (vertical) ocean models
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Warm Core Ring: Not just high heat content
Has anticyclonic circulation & is a thermocline feature Since warm water is deep in the ring, WCRs generally restrict hurricane-induced SST cooling, BUT… Can a WCR’s circulation increase storm-core SST cooling via advection? If so, only a 3-D ocean model (not 1-D) should be able to capture this result…
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WCRR WCRC WCRL CTRL < < < < < < < < < <
A T M O S P H E R Prescribed translation speed Cyclonic hurricane vortex < < < < < < < < O C E A N Homogeneous initial SST Vary position of ring relative to storm track < < < < Horizontally-homogeneous subsurface temperature Warm core ring evident in subsurface temperature field WCRR WCRC WCRL CTRL
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2.4 m s-1 4.8 m s-1 CTRL
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3-D 2.4 m s-1 1-D CTRL 4.8 m s-1 3-D 1-D
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SST & current vectors… storm is ~50 km past center of WCR… 3-D experiments
WCRC WCR Storm CTRL Storm Speed = 2.4 m s-1 WCRL WCR Storm WCRR WCR Storm
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SST cooling within 60-km radius of storm center
WCRR CTRL Mean ΔSST 1-D WCRR CTRL Mean ΔSST Speed = 2.4 m s-1 3-D WCRR SST – CTRL SST WCR Storm > < 1-D WCRR SST – CTRL SST WCR Storm > <
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SST cooling within 60-km radius of storm center
CTRL WCRR Mean ΔSST 1-D CTRL WCRR Mean ΔSST Speed = 4.8 m s-1 3-D WCRR SST – CTRL SST WCR Storm > < 1-D WCRR SST – CTRL SST WCR Storm > <
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Summary It is well-established that WCRs can impart a non-negligible influence on hurricane intensity by altering storm-core SST cooling BUT it is misleading to treat WCRs as simply regions of increased heat content without regard for: translation speed of the storm location of the ring relative to the storm track 3-D models are necessary to capture upwelling and horizontal advection of a surface cold pool
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Questions???
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Supplemental Slides…
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Hurricane Physics Evaporation from sea surface; strongest in storm’s core Evaporation from the sea surface provides heat energy to drive the hurricane
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