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Christopher A. Edwards and Joseph Pedlosky

Abstract

The study of a simple, continuously stratified model of the abyssal ocean driven by upwelling out of the abyss into the main thermocline and by source fluid entering the basin through northern, western, and southern bound-aries is reported.

Our approach divides the deep mean into an inviscid interior and frictional boundary-layer regions. The interior circulation is fully determined by the horizontal distribution of upwelling and by the net, vertical distribution of sources entering the basin. Forcing by the local upwelling drives a barotropic velocity field, and remote forcing by both the upwelling and sources generates an underlying baroclinic flow, which can be considerably stronger and of opposite sign at some depths. The boundary current functions to redistribute around the perimeter fluid entering the boundary regions either through the basin walls or from the interior. In contrast to the interior flow, it depends also on the geographical location of sources. The boundary current is divided into three sublayers, one harotropic layer that is required to satisfy an overall mass balance and two baroclinic layers that close the baroclinic circulation. The outer baroclinic layer has a width that depends on the vertical scale of the flow and can extend far into the interior. Stratification induces the subdivision of both the interior and boundary layers into an upper region, dominantly driven by the upwelling, and a lower one, predominantly influenced by the sources.

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Christopher A. Edwards and Joseph Pedlosky

Abstract

A linear stability analysis of the shallow-water system in the tropical ocean examines the stability of the western boundary current and its latitudinal dependence. Despite a highly idealized formulation that assumes a purely meridional basic state and makes a local f-plane approximation, the stability analysis successfully predicts a length scale of the disturbance, a latitude for its origin, and a critical Reynolds number that agree well with accompanying numerical results. Realistic western boundary current profiles undergo a horizontal shear instability that is partially stabilized by viscosity. Calculations of the growth rate at several latitudes indicate that the instability is enhanced in the Tropics where the internal deformation radius is a maximum.

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Christopher A. Edwards and Joseph Pedlosky

Abstract

The transformation of potential vorticity within nonlinear deep western boundary currents in an idealized tropical ocean is studied using a shallow-water model. In a rectangular domain forced by a localized, Northern Hemisphere mass source and a distributed sink that require a net, cross-equatorial mass flux, a series of numerical experiments investigate how potential vorticity changes sign as fluid crosses the equator. Dissipation is included as momentum diffusion, and the Reynolds number, defined as the ratio of the mass source per unit depth to the viscosity, determines the nature of the flow. For Re less than a critical value (approximately 30) the flow is laminar and well described by linear theory. For Reynolds numbers just above this value, the system becomes time-dependent with eddies of one sign developing adjacent to the boundary and propagating steadily across the equator. For very large Re, an extensive and complicated network of both positive and negative anomalies emerges. Analysis of vorticity fluxes, decomposed into mean, eddy, and frictional elements, reveals the growth with Reynolds number of a turbulent boundary layer that exchanges vorticity between the inertial portion of the boundary current and a frictional sublayer where it is expelled from the basin. Thus, the eddy field is established as an essential mechanism for potential vorticity transformation in nonlinear cross-equatorial flow.

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Jann Paul Mattern, Christopher A. Edwards, and Andrew M. Moore

Abstract

A procedure to objectively adjust the error covariance matrices of a variational data assimilation system is presented. It is based on popular diagnostics that utilize differences between observations and prior and posterior model solutions at the observation locations. In the application to a data assimilation system that combines a three-dimensional, physical–biogeochemical ocean model with large datasets of physical and chlorophyll a observations, the tuning procedure leads to a decrease in the posterior model-observation misfit and small improvements in short-term forecasting skill. It also increases the consistency of the data assimilation system with respect to diagnostics, based on linear estimation theory, and reduces signs of overfitting. The tuning procedure is easy to implement and only relies on information that is either prescribed to the data assimilation system or can be obtained from a series of short data assimilation experiments. The implementation includes a lognormal representation for biogeochemical variables and associated modifications to the diagnostics. Furthermore, the effect of the length of the observation window (number and distribution of observations) used to compute the diagnostics and the effect of neglecting model dynamics in the tuning procedure are examined.

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Christopher A. Fiebrich, Kevin R. Brinson, Rezaul Mahmood, Stuart A. Foster, Megan Schargorodski, Nathan L. Edwards, Christopher A. Redmond, Jennie R. Atkins, Jeffrey A. Andresen, and Xiaomao Lin

Abstract

Although they share many common qualities in design and operation, mesonetworks across the United States were established independently and organically over the last several decades. In numerous instances, the unique ways each network matured and developed new protocols has led to important lessons learned. These experiences have been shared in informal ways among various network operators over the years to promote reliable operation. As existing networks begin to introduce new sensors and technologies, and as new networks come online, there is a common need for guidance on best practices. This paper aims to formally provide recommendations to improve and harmonize the various aspects of operating a “mesonet,” including siting, sensors, maintenance, quality assurance, and data processing.

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Nirnimesh Kumar, James A. Lerczak, Tongtong Xu, Amy F. Waterhouse, Jim Thomson, Eric J. Terrill, Christy Swann, Sutara H. Suanda, Matthew S. Spydell, Pieter B. Smit, Alexandra Simpson, Roland Romeiser, Stephen D. Pierce, Tony de Paolo, André Palóczy, Annika O’Dea, Lisa Nyman, James N. Moum, Melissa Moulton, Andrew M. Moore, Arthur J. Miller, Ryan S. Mieras, Sophia T. Merrifield, Kendall Melville, Jacqueline M. McSweeney, Jamie MacMahan, Jennifer A. MacKinnon, Björn Lund, Emanuele Di Lorenzo, Luc Lenain, Michael Kovatch, Tim T. Janssen, Sean R. Haney, Merrick C. Haller, Kevin Haas, Derek J. Grimes, Hans C. Graber, Matt K. Gough, David A. Fertitta, Falk Feddersen, Christopher A. Edwards, William Crawford, John Colosi, C. Chris Chickadel, Sean Celona, Joseph Calantoni, Edward F. Braithwaite III, Johannes Becherer, John A. Barth, and Seongho Ahn

Abstract

The inner shelf, the transition zone between the surfzone and the midshelf, is a dynamically complex region with the evolution of circulation and stratification driven by multiple physical processes. Cross-shelf exchange through the inner shelf has important implications for coastal water quality, ecological connectivity, and lateral movement of sediment and heat. The Inner-Shelf Dynamics Experiment (ISDE) was an intensive, coordinated, multi-institution field experiment from September–October 2017, conducted from the midshelf, through the inner shelf, and into the surfzone near Point Sal, California. Satellite, airborne, shore- and ship-based remote sensing, in-water moorings and ship-based sampling, and numerical ocean circulation models forced by winds, waves, and tides were used to investigate the dynamics governing the circulation and transport in the inner shelf and the role of coastline variability on regional circulation dynamics. Here, the following physical processes are highlighted: internal wave dynamics from the midshelf to the inner shelf; flow separation and eddy shedding off Point Sal; offshore ejection of surfzone waters from rip currents; and wind-driven subtidal circulation dynamics. The extensive dataset from ISDE allows for unprecedented investigations into the role of physical processes in creating spatial heterogeneity, and nonlinear interactions between various inner-shelf physical processes. Overall, the highly spatially and temporally resolved oceanographic measurements and numerical simulations of ISDE provide a central framework for studies exploring this complex and fascinating region of the ocean.

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