Nonlocal effect of internal waves on the ocean surface

Abstract

The effects of nonlocal connection between an internal wave field and centimeter-and decimeter-scale waves were studied using the JUSREX-92 data. Centimeter-scale wave parameters were measured by means of a ship-mounted, X-band side-looking radar, decimeter-scale wave parameters were measured with a two-dimensional optical device (the wave spectrum analyzer) and parameters of the internal waves were measured by a towed, profiling batfish operated by the Atlantic Division of the Shirshov Institute of Oceanology. We will show that disturbed regions of centimeter-and decimeter-scale surface waves exist at distances up to 1 km away from the region of internal waves. The spatial period of the surface wave spectral density variations can differ from the internal wave length, but the magnitude of this difference depends on the relative direction of surface wave propagation. These variations in the spectral density of decimeterscale surface waves can be explained in terms of a standard kinematic model of the action of internal waves on wind waves. The variations in the spectral density of centimeter waves is a result of a cascade mechanism: the surface flow caused by the internal waves transforms the decimeter-scale waves, which then cause the centimeter-scale wave variations.It is the most widespread opinion, that surface manifestations of internal waves (IW) look like strips along the internal wave crests having the IW spatial period and propagating with the velocity equal to the IW phase velocity. However, it was obtained, during the expedition on the scientific-research vessel "Vityaz" in 1982 that in the decimeter wavelengths of the surface waves structure of manifestation of short internal wave trains on the sea surface is more complicated (Basovich,l986). In particular, the following peculiarities were registered: the scale of variation in the spectral density of the surface waves smaller than the internal wave period, existence of a forerunner of surface anomalies, the variation in the intensity of the wind waves anomalies along the internal wave trains. Later, these effects were registered at centimeter wavelengths as well (Basovich,l988). In this paper, some peculiarities, which were obtained during the JUSREX-92 field experiment, of such unlocal connection of the internal wave field with the structure of their surface manifestations are described. ApparatusTo determine the relative variations of spectral density of dm surface waves two-dimensional optical spectrum analyzer (DOSA) was used, created in the Institute of Applied Physics. The distance between the hull of the vessel and the sea surface region to be observed was 30 meters, what allowed to eliminate the distortions due t o the waves induced by the vessel. The wavelength range of the analyzed surface waves was from 3.9 to 140 centimeters. The spectral density variations of the surface waves was analyzed in sector 120" wide and was registered in 480 spectral points: 24 directions of the surface wave propagation and 20 frequency channels in each directio...

Stokes waves modulation by internal waves

“…[37] The account of SW nonlinearity allows constructing of the uniformly valid stationary solutions to the problem of the SW -IW interaction in the vicinity of their group velocity resonance and to describe a number of observed experimental effects. The basic properties of the solution correspond to the results of observations of SW modulations by large amplitude internal waves [Kropfli et al, 1999;Bakhanov and Ostrovsky, 2002] and by the relatively short internal waves [Basovich et al, 1987;Bakhanov et al, 1994].…”

“…This means that the relatively long SW ( c < 1/2), after crossing the region of internal wave current, keeps the low frequency amplitude modulations. That property is confined by field measurements [ Basovich et al , 1987; Bakhanov et al , 1994].…”

“…(4) The distance between the bands usually corresponds to the IW length. (5) A lot of experiments [ Basovich et al , 1987; Bakhanov et al , 1994] demonstrated the wind waves anomalies on the long (about 1km) distance from the space filled by the internal waves.…”

Exposure of internal waves on the sea surface

“…(i) Forerunner of internal waves. A significant number of experiments (see Basovich et al 1987;Bakhanov et al 1994) demonstrated surface wave anomalies at an extended distance (about 1 km) from the regions where internal waves exist, that is IW forerunner arises. Significant sea surface anomalies can be observed ahead of an IW train.…”

“…(iv) No uniformity in SW space modulations. Spectral density variations may have a characteristic period that is shorter than that of periodic IWs (see Basovich et al 1987;Bakhanov et al 1994). The SW spectral density along a uniform periodic IW train grows on average from the train's end to its head; the spectral density of decimetre-length SWs at the train head can exceed several times the unperturbed value.…”