A Simplified Model for the Baroclinic and Barotropic Ocean Response to Moving Tropical Cyclones: 1. Satellite Observations

Abstract

Changes of sea surface temperature and height, derived from 20‐day passive microwave and altimeter measurements for three tropical cyclones (TCs), Jimena, Ignacio and Kilo, during the 2015 Pacific hurricane season, sampling different stages of intensification, wind speeds, radii, Coriolis parameter, translation velocities, and ocean stratification conditions, are reported and analyzed. As triggered along the path of moving TCs, very large interior ocean displacements can occur to leave prominent sea surface height (SSH) anomalies in the TC wake. Resulting surface depressions can reach 0.3–0.5 m, depending upon size, translation speed, and ocean stratification conditions. These signatures can be quite persistent, that is, more than few weeks, to possibly be intercepted with satellite altimeters. To interpret sea surface temperature (SST) and SSH anomalies, a semiempirical framework is adopted, based on the heat and momentum conservations laws for the upper wind driven mixed layer. As interpreted, SSH anomalies provide direct estimates to evaluate the upwelling impact, that is, the upwelling amplification on the SST wake. For the reported cases, the influence of the upwelling is found rather moderate, of order 10–40%. More promising, the proposed bottom‐up approach can help document the resulting wind forcing and practical drag coefficient under extreme TC conditions. As found for these three TCs, a marked drag reduction for wind speed higher than 35 m/s is inferred to ensure consistency with the measured SSH and SST anomalies.

First Precise Spaceborne Sea Surface Altimetry With GNSS Reflected Signals

“…Furthermore, submesoscale features emerge or are magnified under calm conditions, especially just after a wind event has stirred the upper ocean, when the Ekman circulation interacts with the underlying mesoscale turbulence resulting in rich submesoscale structures. A special case of interest would be the study of hurricane wakes, which last several days after the pass of the cyclone [26].…”

Observed Ocean Surface Winds and Mixed Layer Currents Under Tropical Cyclones: Asymmetric Characteristics

“…According to Kudryavtsev et al. (2019), the OML thickness for a fast‐moving TC is approximately proportional to, $$$h\sim {U}_{10}{[{R}_{\text{max}}/(N{U}_{h})]}^{1/2}$$$ where, N is the Brunt‐Väisälä frequency for the seasonal pycnocline, U 10 is ocean surface wind speed at 10‐m height, R max is the radius of maximum wind, and U h is the TC's translation speed. The wind‐driven current speed in the OML for a moving TC (Price, 1983) can be approximately expressed as $$${U}_{\mathrm{s}}\sim \tau {R}_{\text{max}}/(h{U}_{h})$$$ where, τ is wind stress ($$\tau ={\rho }_{\text{air}}{C}_{d}{{U}_{10}}^{2}$$) which can be estimated from air density ( ρ air ), drag coefficient ( C d ) and U 10 .…”

“…The observed ageostrophic current speeds are found to increase linearly with increasing wind speeds (20-50 m/s). According to Kudryavtsev et al (2019), the OML thickness for a fast-moving TC is approximately proportional to,…”

Strength and Spatial Structure of the Perturbation Induced by a Tropical Cyclone to the Underlying Eddies

“…It has been observed that a TC can cause SSH depressions of up to 20–60 cm in satellite altimeter data (Ginis, 2002; Kudryavtsev et al, 2019), which has also been simulated in numerical ocean models (Shay et al, 1990). In spite of the strong near‐inertial contributions to these SSH depressions, geostrophic components, as shown in Figure 1, are also believed to be considerable.…”