Sharp Frontal Interfaces in the Near-Surface Layer of the Ocean in the Western Equatorial Pacific Warm Pool

Abstract

During the TOGA COARE rich horizontal temperature and salinity variability of the near-surface layer of the ocean in the western Pacific warm pool was observed. High-resolution measurements were made by probes mounted on the bow of the vessel in an undisturbed region at ϳ1.7-m depth during four COARE cruises of the R/V Moana Wave. The authors observed several tens of cases of periodic sharp frontal interfaces of width 1-100 m and separation 0.2-60 km. The sharp frontal interfaces were often found in frontal regions and on the periphery of freshwater puddles. Maneuvers of the ship were conducted to determine the spatial orientation of a sharp frontal interface. The interfaces revealed anisotropy with respect to the wind direction. They were most sharp when the wind stress had a component along the buoyant spreading of the front. A possible origin of the sharp frontal interfaces is discussed. These interfaces may develop by nonlinear evolution of long-wave disturbances on the near-surface pycnocline that is often observed in the warm pool area. A shallow-water model may describe some features of the observations. A dimensionless number of the Reynolds type is a criterion of transition from wave train solution to dissipative shock-wave structure. The model predicts spatial anisotropy depending on the relative angle between the wind stress and horizontal density gradient.

Fine-scale features on the sea surface in SAR satellite imagery – Part 2: Numerical modeling

“…As a near-surface low-density plume propagates into the stratified environment, internal waves can be excited in the thermocline. This can lead to a resonant interaction between the internal waves and the near-surface current, which results in a fragmentation of the buoyancy-driven current at the surface as observed in laboratory experiments (Simpson, 1987) and in the open ocean (Soloviev and Lukas, 1997). The fragmentation can become apparent as a banding pattern on the sea surface, as seen in the velocity field in our simulations (Fig.…”

“…This can lead to a fragmentation of the density current, as seen in laboratory experiments by Simpson (1987) and open-ocean observations by Soloviev and Lukas (1997). A buoyancy-driven current propagating into a two-layer stratified ambient environment in the coastal ocean may result in a resonant generation of an upstream undular bore (Grimshaw and Yi, 1991;Nash and Moum, 2005;White and Helfrich, 2012).…”

Three-Dimensional Dynamics of Freshwater Lenses in the Ocean’s Near-Surface Layer

“…The asymmetry of freshwater plumes relative to the wind speed direction was observed in the western equatorial Pacific during TOGA-COARE (Soloviev and Lukas, 1997;Soloviev et al, 2002). Soloviev and Lukas (1997) suggested that this asymmetry can be explained by the mechanism of Stommel's (1993) Overturning Gate, schematically shown in Figure 7.…”

“…The water flow in the leading edge of the gravity current and trailing fluid contains a complex pattern of three-dimensional motions (Özgökmen et al, 2004). Gravity currents may also interact with ambient stratification in a resonant way, leading to fragmentation of the near-surface lens, and with the surface wind stress, resulting in lens asymmetry in structure and mixing (Simpson, 1987;Soloviev and Lukas, 1997;Matt et al, 2014). The patterns in gravity currents are inherently three dimensional.…”

“…Based on the laboratory results of Simpson (1987), Soloviev and Lukas (1997) hypothesized that the freshwater lens in the ocean's near-surface layer may interact with environmental stratification. A small thickness of the near-surface anomalies is favorable for the development of nonlinear interactions.…”