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Based Circulation Model for the Mid-Atlantic Bight and Gulf of Maine: Configuration and Comparison to Integrated Coastal Observing Network Observations

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Based Circulation Model for the Mid-Atlantic Bight and Gulf of Maine: Configuration and Comparison to Integrated Coastal Observing Network Observations Geosci. Model Dev., 13, 3709–3729, 2020 https://doi.org/10.5194/gmd-13-3709-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Doppio – a ROMS (v3.6)-based circulation model for the Mid-Atlantic Bight and Gulf of Maine: configuration and comparison to integrated coastal observing network observations Alexander G. López, John L. Wilkin, and Julia C. Levin Department of Marine and Coastal Sciences, Rutgers, The State University of New Jersey, New Brunswick, NJ 08901, United States of America Correspondence: Alexander G. López ([email protected]) Received: 20 December 2019 – Discussion started: 4 March 2020 Revised: 16 June 2020 – Accepted: 8 July 2020 – Published: 24 August 2020 Abstract. We describe “Doppio”, a ROMS-based (Regional sediments, or pollutants. The reduced geographic scope of Ocean Modeling System) model of the Mid-Atlantic Bight a regional model offers economies in computational effort and Gulf of Maine regions of the northwestern North Atlantic that allow much greater experimentation than would be pos- developed in anticipation of future applications to biogeo- sible with global models alone, such as by examining sen- chemical cycling, ecosystems, estuarine downscaling, and sitivity to resolution or parameterization of added physics, near-real-time forecasting. This free-running regional model and they present the opportunity to affordably explore nu- is introduced with circulation simulations covering 2007– merous application scenarios. Here we describe the develop- 2017. The ROMS configuration choices for the model are ment, evaluation, and application of a regional model of the detailed, and the forcing and boundary data choices are de- northeastern continental shelf of North America from Cape scribed and explained. A comprehensive observational data Hatteras, North Carolina, northward to near Halifax on the set is compiled for skill assessment from satellites and in Scotian Shelf of Canada. The model, intended principally for situ observations from regional associations of the U.S. In- studies of ocean physical circulation but conceived for future tegrated Ocean Observing Systems, including moorings, au- applications to biogeochemical cycles and ecosystems, uses tonomous gliders, profiling floats, surface-current-measuring the 3-D hydrostatic shelf circulation model ROMS (Regional coastal radar, and fishing fleet sensors. Doppio’s performance Ocean Modeling System; http://www.myroms.org, last ac- is evaluated with respect to these observations by represen- cess: 20 December 2019; Shchepetkin and McWilliams, tation of subregional temperature and salinity error statistics, 2005) as the underlying hydrodynamic model core. as well as velocity and sea level coherence spectra. Model The model configuration builds significantly on two ear- circulation for the Mid-Atlantic Bight and Gulf of Maine is lier regional modeling programs. A ROMS Northeast North visualized alongside the mean dynamic topography to con- American (NENA) shelf coupled circulation and biogeo- vey the model’s capabilities. chemical model encompassing the entire coastal ocean ex- tent from Florida to the Grand Banks of Newfoundland (Hof- mann et al., 2008) was used for numerous studies of nu- trient and carbon fluxes in this region (Fennel and Wilkin, 1 Introduction 2009; Fennel et al., 2006, 2008). The NENA biogeochemi- cal model performed well within the Mid-Atlantic Bight but Coastal ocean circulation models that downscale global less so for the Gulf of Maine (Hofmann et al., 2008), and ocean simulations are useful tools for exploring regional this lackluster performance in the Gulf of Maine was as- ocean dynamics and associated links to biogeochemistry, cribed to shortcomings of the modeled physical circulation ecosystems, geomorphology, and other applications, for ex- (Cahill et al., 2016). Accordingly, an emphasis in config- ample, by inferring transport pathways for nutrients, larvae, uring the model described here was to create an improved Published by Copernicus Publications on behalf of the European Geosciences Union. 3710 A. G. López et al.: Doppio – a ROMS (v3.6)-based circulation model The Doppio domain encompasses two very different dy- namical regimes in the Mid-Atlantic Bight (MAB) and the Gulf of Maine. The MAB (Cape Hatteras, North Carolina, to Cape Cod, Massachusetts; Fig. 1) is characterized by a broad ( ∼ 100 km wide) shelf with a permanent front at the shelf break that separates relatively cool and fresh shelf wa- ters from the warmer and more salty Slope Sea (Mountain, 2003). Instabilities in the shelf break front have wavelengths of typically 40 km that can evolve significantly in time over just a few days (Fratantoni and Pickart, 2003; Gawarkiewicz et al., 2004; Linder and Gawarkiewicz, 1998). The along- shelf currents generally reach the seafloor, exhibiting signif- icant flow–bathymetry interactions and establishing across- shelf transport in the bottom Ekman layer. Eddy shelf interactions tied to Gulf Stream-induced warm core rings (Zhang and Gawarkiewicz, 2015) lead to cross- shelf exchange with surface and subsurface structure at scales of 10–30 km and days to weeks. Subsequent across- shelf fluxes of heat, freshwater, nutrients, and carbon control water mass characteristics and impact ecosystem processes throughout the MAB. Figure 1. Doppio bathymetry with markers for all rivers used to The Gulf of Maine (GOM) is a semi-enclosed marginal sea force the model and tide gauges and moorings used for statistical distinctive in the world for having the largest tidal amplitude, comparisons. Those in bold are referenced in Figs. 4, 5, 8, 9, and over 16 m, due to its shape and length that lead to near res- 13. onance of the lunar semi-diurnal M2 constituent of the tide (Garrett, 1972). There are two main oceanic inflows to the Gulf of Maine: Scotian Shelf water flowing southwestward Gulf of Maine circulation so that subsequent biogeochem- along the coastline from Halifax and originating from the ical simulations will have a more skillful physical founda- Labrador Current; and Slope Sea water entering through the tion. A second prior ROMS-based modeling effort, termed Northeast Channel that derives from subpolar North Atlantic ESPreSSO (Experimental System for Predicting Shelf and waters mixed with eddies of the Gulf Stream. Additional in- Slope Optics), had a more limited geographic scope cover- flows are river runoff from many sources along the coasts of ing only the Mid-Atlantic Bight (Zavala-Garay et al., 2014). New England, New Brunswick, and Nova Scotia and the net This model has been widely used for studies ranging from difference in precipitation and evaporation within the Gulf hurricane-induced cooling via mixing (Seroka et al., 2017) of Maine (Brown and Beardsley, 1978). The two main out- to shelf-wide ecosystems (Xu et al., 2013) and dissolved or- flows are water exiting through the Great South Channel be- ganic carbon fluxes (Mannino et al., 2016). An operational tween Cape Cod and Georges Bank toward Nantucket, and forecast version of ESPreSSO that used 4-D variational as- around Georges Bank (Brown and Beardsley, 1978). This similation (Levin et al., 2018; Zavala-Garay et al., 2014) per- exchange flow through the Northeast Channel can be influ- formed the best of seven real-time models covering the re- enced by Gulf Stream eddies, episodically delivering warm, gion (Wilkin and Hunter, 2013). saline waters (Bisagni and Smith, 1998) in such quantity as to The present modeling effort, which we have dubbed change the physical circulation of the Gulf of Maine (Brooks, Doppio, focuses on maintaining the skill shown by 1987). The circulation is predominantly counterclockwise ESPreSSO while expanding the domain to include the Gulf about the Gulf of Maine, from Nova Scotia into or across of Maine. To assess the Doppio skill, the observing network the Bay of Fundy, then in a strong coastal current south- used for ESPreSSO was expanded, adding new satellite al- ward along the New England coast. While some water exits timeters and SST (sea surface temperature) sensors and the via the Great South Channel, the majority of flow proceeds Gulf of Maine in situ observations. eastward along the northern flank of Georges Bank, finally The moniker Doppio reflects that the Doppio domain is wrapping around the underwater plateau and continuing to- approximately twice the extent of ESPreSSO. The model do- wards the MAB (Bigelow, 1927; Wiebe et al., 2002). Within main is indicated in Fig. 1, colored by bathymetry and with the gulf, strongly irregular bathymetry exerts significant con- positions marked for several time series observation loca- trol on the low-frequency flow variability above three deep tions used for either forcing or analyses that will be discussed basins, which can be challenging to model as previous stud- later in the paper. ies have shown (Hofmann et al., 2008). The gulf’s conflu- ence of glacially carved bathymetry and strong tidal forcing Geosci. Model Dev., 13, 3709–3729, 2020 https://doi.org/10.5194/gmd-13-3709-2020 A. G. López et al.: Doppio – a ROMS (v3.6)-based circulation model 3711 lends itself to equally dynamic currents, namely a significant side the 100 m isobath), and there is better than 3 m resolu- along-bank current jet that may be the prime driver of trans- tion at the seafloor and 1.5 m resolution at the sea surface. port through the region (Loder et al., 1992). Vertical mixing is parameterized using the generic length- Physical circulation processes influence the biogeochem- scale (GLS) (Umlauf and Burchard, 2003) implementation istry of the Gulf of Maine via a number of mechanisms. of the k-kl turbulence closure (Mellor and Yamada, 1982). A Wintertime circulation is especially dynamic, influenced by detailed listing of other configuration options and parameter winds on short timescales and partial mixing of three sep- choices is presented in Table 1. arate water masses (Vermersch et al., 1979). Mixed-layer The Doppio model configuration has been applied to sim- depth influences the onset of primary productivity via spring- ulations of the decade 2007–2017.
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