Dynamical Mechanisms Of Upper Oceanic Response To Tropical Cyclones

The mechanisms of oceanic response and feedback to tropical cyclones are hot topics in oceanic and atmospheric research,and hold a key for skillful forecasts of tropical cyclone intensity and associated marine environmental variations.With a combination of field observation,numerical simulation and theoretical analysis,this paper focuses on the characteristics and mechanisms of upper ocean response to tropical cyclones.Using buoy/mooring observations in the northern South China Sea during 2014 and 2016,several case studies(especially on typhoon Kalmaegi)were carried out,followed by theoretical analyses on the dynamical responses of the upper ocean to tropical cyclones under more general conditions.Typhoon Kalmaegi passed the observation array in the South China Sea in September 2014,bringing quickly increased local winds that turned clockwise(anti-clockwise)on the right(left)side of the typhoon track,along with depressed surface air pressure,increased surface relative humidity and caused persistent rainfall.During the passage of Kalmaegi,sea surface wave height and wave peak period increased,and both sea surface temperature and air temperature decreased with the former lagging the latter.The current response was primarily in the form of the first baroclinic mode,with a right-side intensification.The mixed layer currents decayed in 4-5 near-inertial periods,accompanied by downward propagaton of energy.The thermohaline response showed a three-layer anomaly structure,with the surface layer becaming cooler and saliter,the subsurface layer warmer and fresher,and the deeper layer cooler and saliter again.The 3DPWP model results indicates that the thermal response was controlled by vertical mixing and vertical advection.Surface heat fluxes were not important during the forced stage,but solar radiation was responsible for the recovery of sea surface temperature afterwords.Other case studies included typhoon Rammansun(2014),tropical storm Dianmu(2016),typhoon Sarika(2016)and typhoon Frances(2004).During Rammansun,air temperature,sea surface temperature and salinity all decreased,and the kinetic energy in mixed layer propogated downward to below 150 m.Tropial storm Diammu had a life span of only two days,it did not cause significant current response,while slightly increased local winds and decreased air pressure.During typhoon Sarika,there were strong near-inertial oscillations in the form of the first-baroclinc mode,with detectable signals at depths.In addition to the observation,we also did a ROMS model simulation of the ocean response to typhoon Frances(2004).In the forced stage,the center of current reponse was on the left-rear part of Frances;in the relaxization stage,the mixed layer currents oscillated between "single core" and "double core" patterns,with the strongest response at the 50 m to 80 m depth range.Upwelling took place right under the track of Frances,with the cold water upwelled into the surface layer being advected to the right side of track.Under the mixed layer,the upwelling caused intense coling beneath the typhoon track,while the compensating downwelling induced warm anomalies on the flanks of the track.The first part of therotical analyses was based on a series of idealized numerical model experiments.A two-layer model shows that when the translation speed of a tropical cyclone is slower than that of the first baroclinic mode,the response will be confined under the wind field;when the translation speed is greater than that of the first baroclinic mode,there will be a wake along the track,with the angle between the edge of the wake and the cyclone track being a function of the translation speed and the baroclinic mode speed.In the case of a continuously stratified model,when tropical cyclone is stationary,the oceanic response consists of the divergence and convergence of the Ekman transport,with upwelling within the radius of maximum wind and downwelling outside of it.When tropical cyclone moves at a speed greater than the baroclinic wave speed,there will be strong near-inerial oscillations in the wake of the cyclone,producing a net lift of the thermocline.The structure of the near-inerial oscillation depends on ocean stratification,and its energy is mainly contained in the surface mixed layer and the thermocline,while propagating vertically and horizontally as internal waves,finally being dissipated by dispersion.The second part of theoretical analyses focused on the "heat pump" and "cold suction"effects of tropical cyclones on the ocean.The intense wind forcing of a tropical cyclone deepens the surface mixed layer,causing a surface cooling and a subsurface warming.While the surface temperature recovers after typhoon passage,the subsurface warm anomaly is left behind.Thus a tropical cyclone acts as a "heat pump" if only its mixing effect is considered.On the other hand,the cyclone-induced upwelling can shallow the mixed layer depth and increase the entrainment at the base of the mixed layer,thus cooling the surface as well as the subsurface layers.Thus a tropical cyclone can also act to have a "cold suction" effect.Our theoretical analysis revealed the parameter range of the "heat pump" and "cold suction" effects,indicating that the subsurface temperature anomaly caused by tropical cyclone is controlled by these two effects,and that the "cold suction" effect can reduce the warm anomaly caused by the "heat pump" effect,or even reverse it to a cold anomaly.Our analysis quantifies the relative importance of the "heat pump" and "cold suction" effects,therefore challenging the popular theory that deems the "heat pump" as the primary effect of tropical cyclone on the ocean.