A numerical study of the circulation, ecology and biogeochemistry on the southern New England shelf

Shuwen Zhang, University of Rhode Island

Abstract

This Ph.D. dissertation presents a process-oriented study with two primary objectives: 1) to obtain a physical understanding of the impacts of local and remote forcing, including basin-scale natural climate variability, on the interannual variability of along-shelf transport and water properties of the Middle Atlantic Bight (Chapter 1); 2) to explore the impacts of physical processes on the seasonal variation of phytoplankton biomass in Rhode Island coastal waters (Chapter 2). To achieve the two objectives, we apply a numerical modeling approach employed by the Regional Ocean Modeling System (ROMS) together with substantive analyses of both in-situ and remote observations. ^ Chapter 1 focuses on the interannual-to-decadal variability of along-shelf transport and water properties of the Middle Atlantic Bight (MAB). A suite of process oriented numerical experiments is designed for separating local and remote forcing in order to explore their contributions to the interannual-to-decadal variability of along-shelf transport. Results show that the low-frequency variability is dominated by remote forcing from the open boundaries of the region. The penetration of the Labrador Current into the region contributes to a significant increase of along-shelf transport in the winters of 2009 and 2010. By contrast, the anticyclonic mesoscale eddies associated with the Gulf Stream have a negative impact on the along-shelf jet, and in certain cases even reverse the along-shelf transport. The along-shelf transport is also found to possess a decadal transition, i.e. weaker during 2004-2008 but stronger during 2009–2013. ^ Chapter 2 focuses on the mechanisms controlling the seasonal variation and spatial distribution of phytoplankton biomass in Rhode Island (RI) coastal waters. We first apply an Empirical Orthogonal Function (EOF) analysis to a nine-year monthly chlorophyll-a dataset in order to determine the spatial/temporal structure of the signal. The first EOF mode shows that the spatial variations of chlorophyll-a are in phase from the RI coast to the outer shelf, and the temporal variations are dominated by the strong seasonal cycle characterized by a broad peak during the fall-winter months (October to March). The spatial distribution of chlorophyll-a indicates that the phytoplankton biomass decreases with distance offshore as the water depth increases. To understand the mechanisms controlling the abovementioned features, we first set idealized one-dimensional (1-D) numerical experiments, using a physical-biological model (ROMS-NPZD), to isolate advective processes. Results show that the fall-winter bloom is initiated by enhanced vertical turbulent mixing, which results from the combined effects of the increased surface momentum forcing and surface cooling, bringing nutrients up into the euphotic zone. The extensive mixing also has a counteractive effect on the fall-winter bloom as phytoplankton are carried below the euphotic layer. A three-dimensional (3-D) experiment is then conducted to validate the key conclusions drawn from 1-D simulations from which we find that results from the 3-D experiment are qualitatively in agreement with those from the 1-D experiments.^

Subject Area

Physical oceanography|Biogeochemistry

Recommended Citation

Shuwen Zhang, "A numerical study of the circulation, ecology and biogeochemistry on the southern New England shelf" (2015). Dissertations and Master's Theses (Campus Access). Paper AAI3738386.
http://digitalcommons.uri.edu/dissertations/AAI3738386

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