Horizontal variability of near‐inertial oscillations associated with the passage of a typhoon

Abstract

[1] Surface currents to the east of Okinawa Island were observed by using HF (highfrequency) ocean radars in August-September 1995. During that period, a typhoon passed near the observation area, and the horizontal variability of near-inertial oscillations associated with the typhoon passage was estimated. In addition, the near-inertial amplitudes and horizontal wave number vectors were estimated. These estimations showed that the near-inertial amplitude was large in the offshore area and small near the coast. The maximum near-inertial wavelength was estimated to be in the order of 1000 km, and the near-inertial wave direction was southward and changed to northwestward on 23 August in 1995. A slab model, incorporating low-frequency currents, was used to calculate near-inertial currents. It was found that the amplitude of calculated near-inertial oscillations was also large in the offshore area. Moreover, the phase-propagation direction of the near-inertial waves was southward and changed to northwestward or westward on 22 August. The growth rate obtained from the dispersion relation was large in the offshore area, where the near-inertial amplitude was large.

Ocean wave directional spectra estimation from an HF ocean radar with a single antenna array: Observation

“…Figure 3 shows time series of significant wave heights ( H s ) and periods ( T s ) at Ky (Figure 1), as well as hourly wind vectors ( U I = ( U I cosθ I , U I cosθ I )) at I (Figure 1) during the HF radar observation period. The significant wave height H s = 4.5 m was the largest on 23 August 1995, and was associated with the passage of tropical cyclone (typhoon) Janis near the HF radar observation area [ Hisaki and Naruke , 2003]. The maximum wind speed was 14.5 m/s during this period.…”

“…It is important to be able to estimate ocean surface wave spectra for a wide variety of marine applications including uses in physical oceanography, wave forecasting, ship routines and coastal engineering. One promising technology for wave measurement is high‐frequency (HF) ocean radar, which radiates HF radio waves to the sea surface and measures ocean surface currents [ Barrick et al , 1977; Prandle and Ryder , 1985; Takeoka et al , 1995; Hisaki et al , 2001; Hisaki and Naruke , 2003] and surface waves [ Hisaki , 1996, 2002, 2004, 2005] by analyzing the Doppler spectra of backscattered signals.…”

Time interpolation of stationary and propagating surface disturbances for ocean modeling

“…For example, linear interpolation is inadequate when tropical storms, such as typhoons and hurricanes, are moving fast. The wind fields associated with tropical cyclones are expressed in a parametric form [e.g., Hisaki and Naruke , ] to investigate the response of the ocean to moving cyclones. There are no methods that can be applied to both stationary weather patterns and fast‐moving disturbances.…”

Directional distribution of the short wave estimated from HF ocean radars

“…One of the sensors for the estimation of a wave spectrum is HF ocean radar. HF ocean radar can estimate the wave directional spectrum [ Hashimoto et al , 2003; Hisaki , 1996, 2005, 2006; Lipa and Nyden , 2005; Wyatt , 1990] and ocean currents [ Hisaki et al , 2001; Hisaki and Naruke , 2003; Shay et al , 1998; Takeoka et al , 1995]. In particular, parameters characterizing wave directional distribution at the Bragg wavelength such as the s‐parameter can be easily estimated, because they are obtained from the first‐order scattering which is less affected by noise.…”