Ocean Current Shear Research Articles

Ocean currents break up a tabular iceberg.

In December 2020, giant tabular iceberg A68a (surface area 3900 km2) broke up in open ocean much deeper than its keel, indicating that the breakage was not immediately caused by collision with the seafloor. Giant icebergs with lengths exceeding 18.5 km account for most of the calved ice mass from the Antarctic Ice Sheet. Upon calving, they drift away and transport freshwater into the Southern Ocean, modifying ocean circulation, disrupting sea ice and the marine biosphere, and potentially triggering changes in climate. Here, we demonstrate that the A68a breakup event may have been triggered by ocean-current shear, a new breakup mechanism not previously reported. We also introduce methods to represent giant icebergs within climate models that currently do not have any representation of them. These methods open opportunities to explore the interactions between icebergs and other components of the climate system and will improve the fidelity of global climate simulations.

Interannual variability of upper ocean stratification in Bay of Bengal: observational and modeling aspects

The annual cycle and interannual variability of stratification in Bay of Bengal (BoB) are studied using both observations and Global Ocean Data Assimilation System (GODAS) analysis during 2003–2012. Annual cycle of stratification and sea surface temperature (SST) evolve coherently, highlighting its role on modulating air-sea interaction over this climatologically important region. Spatial distribution of stratification shows strong seasonality in ARGO observations, whereas it is highly underestimated in GODAS with highest discrepancies during fall and spring. The annual cycle of sea surface salinity (SSS) in GODAS is out of phase with observations implying potential feedbacks. During La Nina years, SSS drop in fall and winter and are lesser than those reported during El Nino years. All these features are misrepresented in GODAS. As stratification modulates air-sea interaction over BoB especially during El Nino and La Nina years, such misrepresentation of ocean stratification may lead to unrealistic thermocline-SST coupling in the models. The mean stratification and its interannual variability in GODAS are weaker than observed even though interannual variability in freshwater flux (P-E) is higher in GODAS. Detailed analysis of GODAS with in situ observations reveals that upper ocean current shear (vertical) is overestimated in GODAS, leading to unrealistically strong mixing which is primarily responsible for the deeper penetration of surface warm and freshwater resulting weaker stratification. As GODAS is used to initialize the ocean component of the coupled forecasting system for seasonal prediction of Asian monsoon, proper representation of stratification is essential. This study advocates the need of accurate representation of upper ocean salinity in GODAS for improved stratification. We speculate that improved stratification and mixing in the BoB improve summer monsoon forecast.

Recurrent frontal slicks of a coastal ocean upwelling shadow

Marine ecosystems are greatly influenced by the structure and dynamics of fronts. In coastal upwelling systems, frontogenesis occurs frequently by upwelling and transport of cold water and warming in sheltered “upwelling shadow” retention sites. Monterey Bay, in the California Current upwelling system, hosts a dynamic upwelling shadow environment. Using a decade of satellite synthetic aperture radar (SAR) observations with ancillary remote sensing and in situ data, we describe recurrent surface slicks that develop along the seaward periphery of the Monterey Bay upwelling shadow, and we examine their relationships with fronts. Slick median dimensions, 17.5 km long and 0.8 km wide, describe their elongated structure. Although the typical pattern is a single slick, multiple slicks may concurrently develop in association with different types of fronts. Repeated volume surveys through a front, underlying a slick, revealed lateral mixing and interleaving of regional water types. Velocity fields from coastal HF radar show that slicks may coincide with a variety of surface circulation patterns, that they may extend contiguously across regions having very different surface velocity, and that they may be separated from the shear front of upwelling filaments by 5–10 km. Slicks occur in all seasons and may coincide with both upwelling and downwelling wind forcing. Surfactant accumulation in small‐scale convergence zones is indicated as the primary mechanism of slick formation, not ocean current shear or small‐scale air‐sea coupling. The results of this study emphasize the role of upwelling system fronts in creating small‐scale structure and dynamics that influence plankton ecology.

Target Classification and Remote Sensing of Ocean Current Shear Using a Dual-Use Multifrequency HF Radar

In this paper, we describe a high-frequency (HF) radar capable of multifrequency operation over the HF band for dual-use application to ship classification and mapping ocean current shear and vector winds. The radar is based on a digital transceiver peripheral component interconnect (PCI) card family that supports antenna arrays of four to 32 elements with a single computer, with larger arrays possible using multiple computers and receiver cards. The radar makes use of broadband loop antennas for receive elements, and a number of different possibilities for transmit antennas, depending on the operating bandwidth desired. An option exists in the choice of monostatic or multistatic operation, the latter providing the ability to use several transmit sites, with all radar echo signal reception and processing conducted at a single master receiver site. As applications for such a multifrequency radar capability, we show measurement and modeling examples of multiple frequency HF radar cross section (RCS) of ships as an approach to ship target classification. Results of using 32 radar frequencies to measure the fine structure in ocean current vertical shear are also shown, providing evidence of one edge of a 1-3-m deep uniform flow masked at the surface by wind-driven current shear in a different direction. Other applications of current-shear measurements, such as vector wind mapping and volumetric current estimation in coastal waters, are also discussed

Estimation of the two‐dimensional ocean current shear field with a synthetic aperture radar

A double inverse method to convert SAR data to estimates of current shear was developed theoretically. Problems of applying the technique to real SAR data were examined. The first step of the inversion process involved elimination of noise in the ( ) wave spectra deduced from SAR images. The second step, estimation of current shear, required the existence of well defined swell and a favorable single of interaction between swell and ocean current. Application of the method in the Antarctic Circumpolar Current encountered sea conditions that both satisfied and violated these requirements. Where the wave‐current geometry was favorable, realistic surface current velocity fields were obtained, suggesting the presence of a substantial mesoscale eddy. Another attempt to apply the method in the Gulf Stream produced wave spectral estimates that showed no clear peak and hence produced no current estimates.

Acoustical effects of ocean current shear structures in the parabolic approximation

In a previous article [Robertson et al., J. Acoust. Soc. Am. 77, 1768–1780 (1985)], the authors developed a family of parabolic equations that includes effects due to the presence of a time-dependent, depth-dependent current. Some of these equations contain a new term that explicitly depends on current gradient. In this article, certain effects of this new term are studied. By transforming the parabolic equations, it is possible to convert them to forms that can be directly solved numerically using existing IFD or FFT implementations. Ocean currents with vertical fine structure present situations that can require these new types of parabolic approximations. Propagation in a shallow isospeed channel is examined, with both rigid and lossy bottoms, and use is made of a shear flow with features of an actual ocean current. The vertical current variation can cause changes in relative intensity that are substantial and that depend on bottom loss, source and receiver locations, and acoustic frequency. Intensity differences are demonstrated and examined for reciprocal sound transmissions.

Acoustical effects of ocean current shear in the parabolic approximation

In a previous study [J. S. Robertson, W. L. Siegmann, and M. J. Jacobson, J. Acoust. Soc. Am. 77, 1768–1780 (1985)], the authors developed a family of parabolic equations which include effects due to the presence of a steady, depth-dependent current. Three of these equations contained a new term which explicitly depended on current gradient. In this work, we study the effects of this new term. By appropriately transforming and simplifying, the equations are transformed into parabolic equations which can be integrated numerically with existing implementations. Presence of ocean current fine structure is one mechanism which can require new terms. Propagation is examined in a shallow isospeed channel with a lossy bottom and a variety of shear flows, some of which model actual ocean flows. Current fine structure can induce variations in intensity which are substantial and which depend on shear structure, source and receiver locations, and frequency. Finally, intensity differences are examined in reciprocal sound transmissions. [Work supported by ONR.]