Upper Ocean Temperature Response To Tropical Cyclones

The upper ocean response to tropical cyclones has attracted considerable attention since strong air-sea interaction was identified.Based on Argo observation,ideal model,and theoretical analysis,this thesis studies the characteristics and mechanisms of temperature and salinity anomalies caused by tropical cyclones in the upper ocean.More focus is paid on the contribution of upwelling to temperature response.Composite structures of ocean temperature and salinity anomalies caused by tropical cyclones(TC)or typhoons in the western North Pacific Ocean were obtained from Argo data.These structures were used to analyze ocean responses to typhoons and the dynamic mechanisms inherent in those responses with a particular focus on upwelling.TC-induced cooling is often strongly rightward-biased in the surface layer,and shifts toward the typhoon track at depths exceeding roughly 100 m.In the central water column within approximately 75 km of the typhoon track,subsurface warming predicted by vertical mixing is restrained and replaced by cooling due to upwelling.Upwelling contributes 15%on average to temperature cooling in the near surface layer(10～30 m),84%in the subsurface layer(30～250 m)and 94%in the deep layer(250～600 m)during the period of 0.5～2.5 days after the typhoon’s passage.It is suggested that the sea surface cooling effect of vertical mixing can be enhanced by the upwelling.The effect of upwelling is also prominent in the salinity response to typhoons.The composite results from the Argo data clearly reveal basic ocean responses to typhoons and indicate the important role of upwelling therein.Based on ideal model,the Argo observation results were verified and expanded.The three-dimensional structure of temperature anomaly obtained by the ideal model is similar to that of Argo observations,except there is no subsurface cooling core.It is found that the subsurface cooling core exists in weak and strong tropical cyclones,which does not exist in super-strong tropical cyclones.The translation speed of tropical cyclones will affect the range of subsurface cooling,especially on the right side of the track.The faster the tropical cyclones,the narrower the subsurface cooling range.In addition to intensity,translation speed of tropical cyclone,and the pre-storm oceanic conditions,the magnitude of the temperature anomaly is also related to the maximum wind radius of the tropical cyclone.Greater maximum wind radius of tropical cyclone would cause greater sea surface temperature drop and larger cooling range.The magnitude of the upwelling caused by tropical cyclones is also related to the intensity,translation speed and maximum wind radius of tropical cyclones.Stronger,slower and larger tropical cyclones would induce a larger magnitude of upwelling.Based on the one-dimensional mixed layer model,a formula of sea surface cooling induced by tropical cyclone is derived,including maximum wind speed,maximum wind radius,moving speed,and initial ocean stratification and initial mixed layer depth.The sea surface cooling formula is verified based on the results of ideal model.Then,a method is proposed to derive the initial ocean temperature profile for one-dimensional mixed layer model from real temperature profile,and the contribution of upwelling is discussed.Finally,this study investigates upper ocean response to Typhoon Soulik(2013)in the western North Pacific during July 8 to July 15,2013,using remote sensing observations and a coupled ocean-atmosphere model(ROMS-WRF).Compared with uncoupled results,simulations of wind fields associated with the typhoon are improved in the coupled model.Meanwhile,the model temperature and salinity profiles are in good agreement with the Argo profiles.The observations show that the surface cold wake after Soulik was not widely-reported band structure,but a three-core structure.The coupled model successfully reproduced this structure.It is found that the threecore structure was associated with preexisting mesoscale cyclonic eddies aligned with the path of Typhoon Soulik.A series comparative experiments were conducted to quantify and examine the influence of the preexisting cyclonic eddies on the upper ocean response to Typhoon Soulik.The results indicated that the sea surface cooling(cold wake)was enhanced by eddy-related larger vertical temperature gradient and horizontal advection,although the contribution of mixing was suppressed simultaneously.The cold wake that located northeast to the typhoon center strengthened the magnitude of zonal temperature gradient and then increased the zonal advection effect of eddy-related cyclonic circulation on the sea surface cooling.