Gordon Mertz

Interpretations of the JEBAR Term

Abstract In diagnostic calculations of the oceans circulation, the so-called JEBAR (joint effect of baroclinicity and relief) term may induce a significant depth-average current field, as has been noted in the oceanographic literature. Here we present two consistent interpretations of this term. In the equation governing the vorticity of the depth-averaged current, a topographic vortex-stretching term proportional to the dot product of depth-averaged velocity and the depth gradient arises. We show that JEBAR corrects this term by removing the contribution of the geostrophic flow referenced to the bottom, which cannot generate topographic vortex stretching. In the evolution equation for the vorticity of the depth-integrated flow, a bottom-torque term is present. We show that the JFBAR term discussed above enters here as a contribution to the bottom torque, emphasizing the role of JEBAR as a forcing term. We also show that when a diagnostic calculation is formulated completely in terms of the transport str...

Wind‐driven motions at the mouth of the lower St Lawrence Estuary

Abstract The outflow from the lower St Lawrence Estuary (LSLE) is a major input to the Gulf of St Lawrence. The discharge of the St Lawrence River drives a pair of buoyant coastal jets in the estuary that combine to form the major part of the Gaspe Current, perhaps the dominant feature of the circulation in the Gulf. The dominant forcing agencies of the low‐frequency variability (aside from the seasonal freshwater discharge cycle) of motions in the LSLE and the Gaspe Current have not been definitively identified. Here we examine current data from the mouth of the estuary from two field programs (undertaken in 1962 and 1979) and use spectral and bulk correlation analyses to show that wind‐driven motions apparently exert a strong influence on the variability of the exchange between Gulf and estuarine waters. Meteorologically forced motions are shown to be most prominent in the 10‐ to 15‐d period range (corresponding to the typical interval between the passages of weather systems). The wind‐induced current f...

An Autumn Instability Event in the Gasp&aacute: Current

Abstract The Gaspe Current is a buoyant jet driven primarily by the freshwater discharge from the St. Lawrence estuary. Previously, many incidents of unstable wave development in the3 Gaspe Current have been reported under summer, high runoff, conditions. Here we present satellite infrared images and discuss the occurrence and evolution of an autumn episode of current instability; in the fall, runoff is lower. This event is notable in that the pattern observed differs considerably from that of summer events, apparently due to the offshore shift that of the core of the jet in early autumn. A stability analysis suggests that the Gaspe Current instabilities may grow slightly faster in autumn conditions. The possible role of wind forcing in triggering wave growth is discussed.

Properties of unstable waves in the lower St Lawrence Estuary

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Wind-Induced Eddy Motion in the Lower St. Lawrence Estuary

The lower St. Lawrence Estuary is a wide estuary, in the sense of being several internal Rossby radii in breadth. Thus, the Coriolis effects are significant and complex circulation patterns are to be expected. We analyse the response of this body to a strong up-channel pulse of wind, using data from an array of current meters and sea level gauges. Interesting features of the flow are found, such as coastal jets accelerating into the wind as the wind speed relaxes (apparently due to the establishment of an along-channel pressure gradient), and a pair of synoptic-scale eddies developing in the current pattern.

The Relationship of Pycnobathic Currents and Baroclinic Torques

Abstract Recent work has shown the importance of baroclinicity–bathymetry interactions in diagnostic calculations of the current field from hydrographic data. In particular, pycnobathic currents can be generated by the presence of an alongshore-bottom density gradient in a shelf-slope domain. We investigate the nature of the pycnobathic term, which is proportional to the product of the alongshore-bottom density gradient and the bottom slope (for a sea floor that varies in depth in the offshore direction only). Previous work involves formulations that calculate the vorticity of the alongshore-bottom velocity. It is shown that this vorticity is proportional to the magnitude of the relative vorticity vector perpendicular to the bottom. Therefore the offshore component of the baroclinic torque is introduced into the dynamics and accounts for the presence of the pycnobathic term.

A simple treatment of very low frequency wind-driven sea level variations

It is shown that very low frequency sea level variations can be calculated from the sum of the steric or density effect and the wind-driven setup derived from barotropic dynamics, provided that a weak slope criterion is met. This result is shown to be consistent with a recent two-layer formulation for wind-driven shelf motions.