North Atlantic Simulations with the Hybrid Coordinate Ocean Model (HYCOM): Impact of the Vertical Coordinate Choice, Reference Pressure, and Thermobaricity

Abstract

The viability of a generalized (Hybrid) Coordinate Ocean Model (HYCOM), together with the importance of thermobaricity and the choice of reference pressure, is demonstrated by analyzing simulations carried out using the World Ocean Circulation Experiment (WOCE) Community Modeling Experiment (CME) Atlantic basin configuration. The standard hybrid vertical coordinate configuration is designed to remain isopycnic throughout as much of the water column as possible while smoothly making a transition to level (pressure) coordinates in regions with weak vertical density gradients, such as the surface mixed layer, and to terrain-following coordinates in shallow-water regions. Single-coordinate (pressure or density) experiments illustrate the flexibility of the model but also bring forward some of the limitations associated with such a choice. Hybrid experiments with potential density referenced to the surface () and to 20 MPa (ϳ2000 m) (2) illustrate the increased influence of pressure errors with increasing distance from the reference pressure. The hybrid experiment does not properly reproduce the northward flow of Antarctic Bottom Water (AABW), and large errors in near-surface pressure gradients in the 2 experiment produce a wind-driven gyre circulation that is too strong, when compared with observations, and a North Atlantic Current that follows an unrealistic path. These near-surface and nearbottom pressure errors are removed when thermobaric effects are included, resulting in a more accurate representation of the upper-ocean gyre circulation, the northward AABW flow near the bottom, and the meridional overturning circulation and heat flux.

Analysis of ageostrophy in strong surface eddies in the Atlantic Ocean

“…This study uses output from a long simulation of HYCOM [Bleck, 2002;Chassignet et al, 2003;Halliwell, 2004]. The details of the model setup used here, along with comparisons of its results to certain regions such as the Indonesia Sea, have been documented in Metzger et al [2010].…”

An eddy resolving tidal-driven model of the South China Sea assimilating along-track SLA data using the EnOI

“…In HYCOM, the vertical coordinate is typically isopycnal in the open, stratified ocean, with a smooth transition towards z-coordinate in the mixed layer (e.g. Bleck, 2002;Chassignet et al, 2003Chassignet et al, , 2007. It makes HYCOM a suitable model for the SCS that combines open stratified ocean, a wide shelf, steep topography and a strong influence of the East Asian Monsoon (Metzger, 2003;Metzger et al, 2006).…”

Modelling seasonal circulation and thermohaline structure of the Caspian Sea

“…The HYCOM, a generalized (hybrid, isopycnal/terrainfollowing σ/z level) coordinate primitive-equation model, has been widely used for global and regional studies and tested for performance. To cite a few recent studies, the choices of vertical coordinates have been evaluated with regard to the Atlantic Ocean by Chassignet et al (2003); performance in the Kuroshio Extension region has been tested by Kelly et al (2007); and the sensitivity to atmospheric forcing in the Black Sea has been investigated by Kara et al (2005).…”