Dynamical response of horizontal temperature fields in the mid-latitude thermocline to the decadal variation of wind stress curl 風系の数十年変動に起因する中緯度温度躍層における水平温度場の力学的応答

博士論文

A major objective of this paper is to propose the mechanism that generates the variability in thermohaline structures, which was actually observed at intermediate depths of the North Atlantic subtropical-subpolar gyres, with interdecadal time scales. The observed variability is first compared with the climatological mean thermohaline structures and related to the observed variability in the wind stress curl over the North Atlantic. This data analysis yields two hypotheses: 1) a primary cause of the observed variability in thermohaline structures was the weakened wind stress cur,， and 2) the thermohaline variability caused by a uniform change in the wind stress curl was different between the western and eastern portions in both subtropical and subpolar gyres. In order to test these hypotheses, a four-level model, which is driven by wind stresses and surface buoyancy flux, is constructed with thermohaline structures represented by a temperature field. The model is used to examine responses of the subtropical-subpolar thermohaline field to a sudden and uniform change in wind stress curl. The model demonstratet that the uniformly weakened wind stress curl generates the thermohaline change with different tendencies between the western and eastem domains in both subtropical and subpolar gyres, similar to the observed pattern. The thermohaline change with different tendencies between the western and eastern domains is not related to a previous linear theory, which is associated with the local Ekman pumping and the long baroclinic Rossby wave. ln this study, the responses of the subtropical-subpolar thermohaline field are examined on the basis of a non-linear theory, which is associated with nonlinear advection in addition to above two mechanisms. The model solutions are analyzed using an analytical method: ie., the characteristic curves associated with both a linear wave propagation and nonlinear advection effects. The mechanism which generates the thermohaline change wi th different tendencies between the western and eastern domains is summarized as follows: the 1st baroclinic velocity field is weakened by the vertical shift of the temperature field in both subtropical and subpolar gyres. This process almost corresponds to the linear response. The weakened 1st baroclinic velocity field forms temperature anomalies in the thermocline, which have different tendencies between the western and eastern domains in both subtropical and subpolar gyres. The formation areas of temperature anomalies are related to the mean horizontal temperature structures, which are maintained by the wind stresses and surface buoyancy flux. The propagation of temperature anomalies along the characteristics generates the horizontal shift of temperature field.