Tropical Cyclone Interaction with the Ocean: The Role of High-Frequency (Subdaily) Coupled Processes (original) (raw)
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Tropical Cyclone (TC) systems affect global ocean heat transport due to mixing of the upper ocean and impact climate dynamics. A higher Sea Surface Temperature (SST), other influencing factors remaining supportive, fuels TC genesis and intensification. The atmospheric thermodynamic profile, especially the sea-air temperature contrast (SAT), also contributes due to heat transfer and affects TC's maximum surface wind speed (V max) explained by enthalpy exchange processes. Studies have shown that SST can approximately be used as a proxy for SAT. As a part of an ongoing effort in this work, we simplistically explored the connection between SST and V max from a climatological perspective. Subsequently, estimated V max is applied to compute Power Dissipation Index (an upper limit on TC's destructive potential). The model is developed using long-term observational SST reconstructions employed on three independent SST datasets and validated against an established model. This simple approach excluded physical parameters, such as mixing ratio and atmospheric profile, however, renders it generally suitable to compute potential intensity associated with TCs spatially and weakly temporally and performs well for stronger storms. A futuristic prediction by the HadCM3 climate model under doubled CO 2 indicates stronger storm surface wind speeds and rising SST, especially in the Northern Hemisphere.
Tropical Cyclone Intensity Change from a Simple Ocean–Atmosphere Coupled Model
Journal of the Atmospheric Sciences, 2001
The interaction between a tropical cyclone (TC) and the underlying ocean is investigated using an atmosphereocean coupled model. The atmospheric model is developed from the Pennsylvania State University (Penn State)-National Center for Atmospheric Research (NCAR) mesoscale model version 4 MM4 and the ocean model consists of a mixed layer and an inactive stagnant layer beneath. Coupling between the atmosphere and the ocean models is achieved through wind stress and surface heat and moisture fluxes that depend on the sea surface temperature (SST). In the absence of a background flow, the atmospheric component consists of only a predefined vortex with an initial central pressure and the radius of the 15 m s Ϫ1 wind. The basic control experiments demonstrate that the coupled model can simulate the development of a TC and its interaction with the ocean. Changes in TC intensity are sensitive to those of SST and the response is almost instantaneous. An SST of ϳ27ЊC is found to be the threshold for TC development. In addition, the initial depth of the ocean mixed layer has an appreciable effect on TC intensity, which also depends on the movement of the TC. Furthermore, the vertical structure of ocean (vertical temperature gradient in the stagnant layer and temperature differential between the two layers) plays a significant role in modulating TC intensity. In the presence of a warm core eddy (WCE), a TC intensifies before its center reaches the edge of the WCE. Although the TC attains maximum intensity at the center of the WCE, it does not weaken to its original intensity after leaving the WCE. During the entire passage of the TC, the SST at the center of the WCE decreases by about only 1ЊC, and the WCE generally maintains its original characteristics. However, two cold pools are observed around its periphery. A similar intensification process occurs when a TC moves over a sharp SST gradient and a locally deep ocean mixed layer. These results are explained by the interaction between the ocean and the TC circulation as well as the change in the total surface heat flux.
Numerical simulations of tropical cyclone-ocean interaction with a high-resolution coupled model
Journal of Geophysical Research, 1993
The tropical cyclone-ocean interaction was investigated using a high-resolution tropical cyclone ocean coupled model. The model design consisted of the NOAA Geophysical Fluid Dynamics Laboratory tropical cyclone prediction model which was coupled with a multilayer primitive equation ocean model. Coupling between the hurricane and the ocean models was carried out by passing into the ocean model the wind stress, heat, and moisture fluxes computed in the hurricane model. The new sea surface temperature (SST) calculated by the ocean model was then used in the tropical cyclone n'"'• A set of idealized numerical experiments were performed in ,•,hi(-h • tropical was embedded in both easterly and westerly basic flows of 2.5, 5, and 7.5 m s-• with a fourth experiment run with no basic flow specified initially. The profile of the tangential wind for Hurricane Gloria at 1200 UTC 22, September 1985 was used as the initial condition of the tropical cyclone for each of the experiments. The model ocean was initially horizontally homogenous and quiescent. To clarify the impact of the ocean response to the hurricane's behavior, analogous experiments were also carried out with the SST kept constant (control cases). The experiments indicated that the cooling of the sea surface induced by the tropical cyclone resulted in a significant impact on the ultimate storm intensity due to the reduction of total heat flux directed into the tropical cyclone above the regions of decreased SST. The sea surface cooling produced by the tropical cyclones was found to be larger when the storms moved slower. In the experiments run without an initial basic flow, the maximum SST anomaly was about-5.6øC with a resulting difference in the minimum sea level pressure and maximum surface winds of 16.4 hPa and-7 m s-1 , respectively. In contrast, in the experiments run with the 7.5 m s-1 basic flow, the maximum SST anomalies ranged from about 2.6 ø to 3.0øC with a difference in the minimum sea level pressure and maximum surface winds of about 7.3 hPa and-2.7 m s-1. The tropical cyclone-ocean coupling significantly influenced the storm track only for the case with no basic flow and the 2.5 m s-1 easterly flow. In these cases the storm with the ocean interaction turned more to the north and east (no basic flow) or the north (2.5 m s-1 easterly flow) of the experiments with constant SST. In the first case, the storm by 72 hours was located over 70 km to the east-southeast of the control case. A possible explanation for this track deviation is related to a systematic weakening of the mean tangential flow at all radii of the storm due to the interaction with the ocean and resulting alteration of the beta drift. 1.
Ocean feedback to tropical cyclones: climatology and processes
Climate Dynamics, 2014
This study presents the first multidecadal 1 and coupled regional simulation of cyclonic activity in 2 the South Pacific. The long-term integration of state-of 3 the art models provides reliable statistics, missing in 4 usual event studies, of air-sea coupling processes con-5 trolling tropical cyclone (TC) intensity. The coupling 6 effect is analyzed through comparison of the coupled 7 model with a companion forced experiment. Cycloge-8 nesis patterns in the coupled model are closer to ob-9 servations with reduced cyclogenesis in the Coral Sea.
Idealized Study of Ocean Impacts on Tropical Cyclone Intensity Forecasts
Monthly Weather Review, 2015
Idealized coupled tropical cyclone (TC) simulations are conducted to isolate ocean impacts on intensity forecasts. A one-dimensional ocean model is embedded into the Hurricane Weather Research and Forecasting (HWRF) mesoscale atmospheric forecast model. By inserting an initial vortex into a horizontally uniform atmosphere above a horizontally uniform ocean, the SST cooling rate becomes the dominant largescale process controlling intensity evolution. Westward storm translation is introduced by bodily advecting ocean fields toward the east. The ocean model produces a realistic cold wake structure allowing the sensitivity of quasi-equilibrium intensity to storm (translation speed, size) and ocean (heat potential) parameters to be quantified. The atmosphere provides feedback through adjustments in 10-m temperature and humidity that reduce SST cooling impact on quasi-equilibrium intensity by up to 40%. When storms encounter an oceanic region with different heat potential, enthalpy flux adjustment is governed primarily by changes in air-sea temperature and humidity differences that respond within 2-4 h in the inner-core region, and secondarily by wind speed changes occurring over a time interval up to 18 h after the transition. Atmospheric feedback always acts to limit the change in enthalpy flux and intensity through adjustments in 10-m temperature and humidity. Intensity change is asymmetric, with a substantially smaller increase for storms encountering larger heat potential compared to the decrease for storms encountering smaller potential. The smaller increase results initially from the smaller wind speed present at the transition time plus stronger limiting atmospheric feedback. The smaller wind speed increase resulting from these two factors further enhances the asymmetry.
Effects of surface heat flux-induced sea surface temperature changes on tropical cyclone intensity
Geophysical Research Letters, 2003
It is known that in deep and open oceans, the effect of sea surface sensible and evaporative heat fluxes on the tropical cyclone-induced sea surface cooling is small compared to that caused by turbulent mixing and cold water entrainment into the upper ocean mixed-layer. This study shows that tropical cyclone-induced surface heat fluxes dominate the surface cooling in near-coastal shallow ocean regions with limited or no underlying cold water. The thermal response of the ocean to the surface heat fluxes is nearly one dimensional through very quick vertical mixing in the ocean mixed layer. The flux-induced sea surface cooling may lead to appreciable reduction of storm intensity if the storm moves slowly. It is therefore important to account this negative feedback of ocean coupling in nearcoastal regions for more skillful forecasting of landfalling tropical cyclones.
Dependence of tropical cyclone intensification rate on sea‐surface temperature
Quarterly Journal of the Royal Meteorological Society, 2016
The dependence of tropical cyclone intensification rate on the sea-surface temperature (SST) is examined in the prototype problem for tropical cyclone intensification on an fplane using a three-dimensional, non-hydrostatic numerical model. The effects of changing the SST are compared with those of changing the latitude examined in a recent article. It is found that the dependence of intensification rate on latitude is largest when the SST is marginal for tropical cyclone intensification (26 • C) and reduces in significance as the SST is increased. Further, at a given latitude, intensification begins earlier and the rate of intensification increases with increasing SST, on account of a significant increase of surface moisture fluxes from the warmer ocean. These higher fluxes result in higher values of near-surface moisture and equivalent potential temperature, leading to a larger radial gradient of diabatic heating rate in the low to middle troposphere above the boundary layer. This larger radial gradient leads to a stronger overturning circulation, which in turn leads to a stronger radial import of absolute angular momentum surfaces and therefore more rapid spin-up. These arguments invoke the classical axisymmetric spin-up mechanism. Non-axisymmetric issues are touched upon briefly.
Geophysical Research Letters
This study examines two sets of high-resolution coupled model forecasts starting from no-tropical cyclone (TC) and correct-TC-statistics initial conditions to understand the role of TC events on climate prediction. While the model with no-TC initial conditions can quickly spin-up TCs within a week, the initial conditions with a corrected TC distribution can produce more accurate forecast of sea surface temperature up to 1.5 months and maintain larger ocean heat content up to 6 months due to enhanced mixing from continuous interactions between initialized and forecasted TCs and the evolving ocean states. The TC-enhanced tropical ocean mixing strengthens the meridional heat transport in the Southern Hemisphere driven primarily by Southern Ocean surface Ekman fluxes but weakens the Northern Hemisphere poleward transport in this model. This study suggests a future plausible initialization procedure for seamless weather-climate prediction when individual convection-permitting cyclone initialization is incorporated into this TC-statistics-permitting framework.