5:15 PM - 6:45 PM
[ACG40-P06] Linear response of deep ocean to a moving tropical cyclone
Keywords:Linear continuously-stratified ocean model, Modal decomposition, Near-inertial waves, Sea-floor pressure anomaly
To explore the impacts of a moving tropical cyclone (TC) on deep ocean, a linear continuously stratified model is solved by solving for the temporal and horizontal structure of each vertical mode. The response of the barotropic mode to the TC's pressure gradient is an isostatic balance, where the sea level rise almost completely cancel the atmospheric low pressure. The response of the barotropic mode to the winds is a permanent sea level drop behind the TC. The horizontal extent of this response is determined by the distribution of the weak negative wind curl outside the core of the strong positive curl. The baroclinic response to the TC's low pressure is negligibly small and that to the winds is dominated by the well-known train of near-inertial oscillation behind the TC. The first peak of upwelling comes a quarter wavelength behind the TC center. In addition to the oscillation, there is a mean upwelling and a resultant cooling. The lateral scale of the first upwelling is determined by the size of the TC's positive curl core; further behind in the TC's wake, this feature spreads laterally at the group speed of inertio-gravity waves for the mode.
The three-dimensional structure is then constructed by superposing these vertical modes. The position of the first upwelling peak coincides between the baroclinic modes; this vertical alignment results in a vertical column of upwelling, which linearly increases from zero at the bottom to its maximum at the bottom of the mixed layer. Further down the wake, this coherence is gradually lost because of slight difference in the streamwise wavelength between different modes. Likewise, the meridional spread also depends on the group speed of inertio-gravity waves, and as a result, lower vertical modes dominate further away from the TC track in the lateral direction. A uniform-density ocean also shows a similar columnar upwelling and downwelling pattern as a response to the same wind curl and the columnar structure can be explained as due to a "delayed Ekman pumping".
The pressure anomaly field at the ocean bottom is dominated by the barotropic response to winds, which is modified by the baroclinic response. The near-inertial oscillation reaches the bottom quickly because of the columnar response.
The three-dimensional structure is then constructed by superposing these vertical modes. The position of the first upwelling peak coincides between the baroclinic modes; this vertical alignment results in a vertical column of upwelling, which linearly increases from zero at the bottom to its maximum at the bottom of the mixed layer. Further down the wake, this coherence is gradually lost because of slight difference in the streamwise wavelength between different modes. Likewise, the meridional spread also depends on the group speed of inertio-gravity waves, and as a result, lower vertical modes dominate further away from the TC track in the lateral direction. A uniform-density ocean also shows a similar columnar upwelling and downwelling pattern as a response to the same wind curl and the columnar structure can be explained as due to a "delayed Ekman pumping".
The pressure anomaly field at the ocean bottom is dominated by the barotropic response to winds, which is modified by the baroclinic response. The near-inertial oscillation reaches the bottom quickly because of the columnar response.