Dynamics of solitary waves observed over the North Indian Ocean during the Indian Ocean Experiment (INDOEX) 1999

Abstract

[1] During the Indian Ocean Experiment (INDOEX) experiment (January-March 1999), mesoscale solitary waves have been observed and tracked over the North Indian Ocean on a series of Meteosat-5 satellite images. These solitary waves have a horizontal wavelength of 10-15 km and propagate westward at low level at a speed of 10-18 m/s. Unlike similar wave phenomena observed mainly over land, they have a long lifetime, which can exceed 48 h. A key element explaining the existence and longevity of the solitary waves is the presence of an inversion layer, acting as a waveguide and separating the boundary layer into two sublayers: a lower layer over the ocean (marine boundary layer) and an upper layer originating from the Indian subcontinent (land plume layer). Profiles from radiosondes launched from the Ron Brown and from dropsondes from the Hercules C-130 airplane helped to determine this waveguide structure. A suggested mechanism leading to the generation of solitary waves is a collision between the sea breeze in the vicinity of the West Indian coast and the easterly/northeasterly winter monsoon winds, with a possible contribution of convection reinforced by topography (Western Ghats range), during the local afternoon. Another phenomenon related to sea breeze and local convection in this coastal area, the injection of "bubbles" of moisture into the drier upper boundary layer, has also been identified on European Centre for Medium-Range Weather Forecasts analyses. These bubbles form daily during the afternoon and drift westward over the North Indian Ocean at latitudes around 12°N and progressively subside and dissipate or become integrated into larger air masses.

Wave‐cloud lines over the Arabian Sea

“…The generation mechanism was identified in a simulation with the Met Office Unified Model of a wave‐cloud line that formed on 13 March 2011, which was similar in appearance to those observed by Szantai et al []. The model was initialized at 0530 IST, 12 March 2011, and run for 48 h with the grid spacing of the innermost domain equal to 4 km.…”

“…The case illustrated in Figure a is chosen because it is one of the most pronounced wave clouds identified in the climatology (section 2); it appears to be similar in structure to the wave clouds reported by Szantai et al . [], and it propagates toward the west, which is the direction most frequently observed.…”

“…Szantai et al . [] observed solitary waves on a number of days from a ship in the Arabian Sea during the Indian Ocean Experiment between January and March 1999. They tracked these waves using satellite imagery and found that they traveled westward at speeds of 10–18 m s −1 and lasted for up to 48 h. They suggest that the inversion layer above the marine boundary layer may have acted as a waveguide, thereby explaining the longevity of the waves.…”

“…Szantai et al . [] propose that the waves form near the west Indian coast through a collision between a sea breeze and the easterly/northeasterly offshore flow and that they could be reinforced by the topography [e.g., Zhang and Koch , ]. They do not, however, provide substantial direct evidence to support the proposed mechanism because their analysis is limited to satellite observations and 6‐hourly ERA‐Interim data.…”

“…A case study, similar to the waves observed by Szantai et al . [], is selected, and a limited‐area model simulation is performed to identify the formation mechanism and document the evolution of the wave (sections 4 and 5).…”