Collaborative Research: Coupled Atmosphere-Ocean Model Study of Wintertime Air-Sea Interaction off the East Coast of North America

In this project, the PIs will continue their investigation of wintertime air-sea interaction processes off the east coast of North America, emphasizing the Gulf Stream (GS) region east of Cape Hatteras and the coastal region in the Middle Atlantic Bight (MAB). Analyses of model results will include temporal and spatial variation of the air-sea exchanges of heat, moisture and momentum, evolution of the oceanic mixed layer and the marine atmospheric boundary layer during storm passage, and momentum, heat and potential vorticity balances in the ocea

Modeling the Circulation in Casco Bay (2011 Casco Bay Circulation Modeling Workshop Presentation)

https://digitalcommons.usm.maine.edu/cbep-presentations/1033/thumbnail.jp

A Numerical Investigation of the Gulf Stream and Its Meanders in Response to Cold Air Outbreaks

The three-dimensional Princeton Ocean Model is used to examine the modification of the Gulf Stream and its meanders by cold air outbreaks. Two types of Gulf Stream meanders are found in the model. Meanders on the shoreward side of the Gulf Stream are baroclinically unstable. They are affected little by the atmospheric forcing because their energy source is stored at the permanent thermocline, well below the influence of the surface forcing. Meanders on the seaward side of the stream are both barotropically and baroclinically unstable. The energy feeding these meanders is stored at the surface front separating the Gulf Stream and the Sargasso Seal which is greatly reduced in case of cold air outbreaks. Thus, meanders there reduce strength and also seem to slow their downstream propagation due to the southward Ekman flow. Heat budget calculations suggest two almost separable processes. The oceanic heal released to the atmosphere during these severe cooling episodes comes almost exclusively from the upper water column. Transport of heat by meanders from the Gulf Stream to the shelf, though it is large, does not disrupt the principal balance. It is balanced nicely with the net heat transport in the downstream direction

A Model Study of the Seasonal Circulation in the Gulf of Maine

The Princeton Ocean Model is used to study the circulation in the Gulf of Maine and its seasonal transition in response to wind, surface heat flux, river discharge, and the M-2 tide. The model has an orthogonal-curvature linear grid in the horizontal with variable spacing from 3 km nearshore to 7 km offshore and 19 levels in the vertical. It is initialized and forced at the open boundary with model results from the East Coast Forecast System. The first experiment is forced by monthly climatological wind and heat flux from the Comprehensive Ocean Atmosphere Data Set; discharges from the Saint John, Penobscot, Kennebec, and Merrimack Rivers are added in the second experiment; the semidiurnal lunar tide (M-2) is included as part of the open boundary forcing in the third experiment. It is found that the surface heat flux plays an important role in regulating the annual cycle of the circulation in the Gulf of Maine. The spinup of the cyclonic circulation between April and June is likely caused by the differential heating between the interior gulf and the exterior shelf/slope region. From June to December the cyclonic circulation continues to strengthen, but gradually shrinks in size. When winter cooling erodes the stratification, the cyclonic circulation penetrates deeper into the water column. The circulation quickly spins down from December to February as most of the energy is consumed by bottom friction. While inclusion of river discharge changes details of the circulation pattern, the annual evolution of the circulation is largely unaffected. On the other hand, inclusion of the tide results in not only the anticyclonic circulation on Georges Bank but also modifications to the seasonal circulation

A 2D Coupled Atmosphere–Ocean Model Study of Air–Sea Interactions during a Cold Air Outbreak over the Gulf Stream

The two-dimensional, Advanced Regional Prediction System (ARPS) has been coupled with the Princeton Ocean Model to study air-sea interaction processes during an extreme cold air outbreak over the Gulf Stream off the southeastern United States, Emphases have been placed on the development of the mesoscale front and local winds in the lower atmosphere due to differential fluxes over the land, the cold shelf water, and the warm Gulf Stream, and on how the mesoscale front and the local winds feed back to the ocean and modify the upper-ocean temperature and current fields. Model results show that a shallow mesoscale atmospheric front is generated over the Gulf Stream and progresses eastward with the prevailing airflow. Behind the front, the wind intensifies by as much as 75% and a northerly low-level wind maximum with speeds near 5 m s-1appears. The low-level northerly winds remain relatively strong even after the front has progressed past the Gulf Stream. The total surface heat flux in the coupled experiment is about 10% less than the total surface heat flux in the experiment with fixed SST, suggesting that the oceanic feedback to the atmosphere might not be of leading importance. On the other hand, the response of the upper-ocean velocity field to the local winds is on the order of 20 cm s-1, dominating over the response to the synoptic winds. This suggests the modification in the atmosphere by air-sea fluxes, which induces the locally enhanced winds, has considerable impact on the ocean. That is, there is significant atmospheric feedback to the ocean through the heat-flux-enhanced surface winds

Air-Sea Interactions During the Passage of a Winter Storm Over the Gulf Stream: A Three-Dimensional Coupled Atmosphere-Ocean Model Study

A three-dimensional, regional coupled atmosphere-ocean model with full physics is developed to study air-sea interactions during winter storms off the U. S. east coast. Because of the scarcity of open ocean observations, models such as this offer valuable opportunities to investigate how oceanic forcing drives atmospheric circulation and vice versa. The study presented here considers conditions of strong atmospheric forcing (high wind speeds) and strong oceanic forcing (significant sea surface temperature (SST) gradients). A simulated atmospheric cyclone evolves in a manner consistent with Eta reanalysis, and the simulated air-sea heat and momentum exchanges strongly affect the circulations in both the atmosphere and the ocean. For the simulated cyclone of 19-20 January 1998, maximum ocean-to-atmosphere heat fluxes first appear over the Gulf Stream in the South Atlantic Bight, and this results in rapid deepening of the cyclone off the Carolina coast. As the cyclone moves eastward, the heat flux maximum shifts into the region near Cape Hatteras and later northeast of Hatteras, where it enhances the wind locally. The oceanic response to the atmospheric forcing is closely related to the wind direction. Southerly and southwesterly winds tend to strengthen surface currents in the Gulf Stream, whereas northeasterly winds weaken the surface currents in the Gulf Stream and generate southwestward flows on the shelf. The oceanic feedback to the atmosphere moderates the cyclone strength. Compared with a simulation in which the oceanic model always passes the initial SST to the atmospheric model, the coupled simulation in which the oceanic model passes the evolving SST to the atmospheric model produces higher ocean-to-atmosphere heat flux near Gulf Stream meander troughs. This is due to wind-driven lateral shifts of the stream, which in turn enhance the local northeasterly winds. Away from the Gulf Stream the coupled simulation produces surface winds that are 5 similar to 10% weaker. Differences in the surface ocean currents between these two experiments are significant on the shelf and in the open ocean

Linear and Nonlinear Responses to Northeasters Coupled with Sea Level Rise: A Tale of Two Bays

This study aimed at dissecting the influence of sea level rise (SLR) on storm responses in two bays in the Gulf of Maine through high-resolution, three-dimensional, hydrodynamic modeling. Saco Bay, an open bay characterized by gentle coastal slopes, provided a contrast to Casco Bay that has steep shorelines and is sheltered by barrier islands and peninsulas. The Finite-Volume Coastal Ocean Model (FVCOM) was implemented for Saco Bay and Casco Bay to simulate the February 1978 northeaster and an April freshwater discharge event in 2007 following the Patriots Day storm. Both events were repeatedly simulated under SLR scenarios ranging from 0 to 7 ft. Modeled storm responses were identified from the 1978 Blizzard simulations and were tracked across SLR scenarios. By comparing changes in inundation, storm currents, and salinity distribution between the two bays, freshwater discharge and bathymetric structure were isolated as two determining factors in how storm responses change with the rising sea level. The steplike bottom relief at the shoreline of Casco Bay sets up nonlinear responses to SLR. In contrast, storm responses in Saco Bay varied significantly with SLR due to alterations in river dynamics attributed to SLR-induced flooding

Maine Tidal Power Initiative: Environmental Impact Protocols for Tidal Power

As a result of ongoing climate change, the pressure for the development of new sources of renewable energy has increased. It is extremely likely that climate change is caused by anthropogenic activities. Thus even if dramatic gains are made in energy efficiency; the addition of novel renewable energy sources is critical to reducing fossil fuel emissions. Even current goals for a reduction in the growth of greenhouse gas emissions mean that all possible low-carbon or non-carbon emitting energy sources be considered. In the marine environment, energy in tidal currents, waves, and thermal structure may be extracted to produce electricity. These energy sources are a critical element in the overall renewable portfolio since, unlike wind and solar energy, both marine thermal and tidal energy are reliable additions to the overall electrical grid. In the case of tidal energy, the contribution of periodic but reliable sources of renewable energy becomes increasingly critical as wind and solar penetration in the grid increase. In a high renewable energy penetration grid, a resource like tidal energy does not provide the same base load capacity as, for example, a nuclear power plant. However, tidal energy can have the effect of reducing the size of either storage or peaking capacity that is required for grid stability by providing power for recovery of dispatchable loads. However, as an immature technology, significant questions remain regarding basic questions like the scale of the potential resource, the impact on sediment transport, the effects on fish populations and communities, and the ability to design a system which is acceptable by the people in the associated communities. The objectives of the funded project were to examine tidal power development in Maine from all perspectives: engineering, resource assessment, biological effects, and social dimensions. Resource and environmental research focused on data collection for the Cobscook Bay/Western Passage, possibly the most viable commercial tidal energy site in the US, tidal power sites along with initial evaluation of the suitability of the approach for at least two other tidal development sites in Maine. Concomitantly, alternative energy research is used as a basis of education for a number of graduate and undergraduate students at the University of Maine and Maine Maritime Academy. The Maine Tidal Power Initiative has developed resource and environmental assessment protocols in conjunction with the deployment of a specific marine hydrokinetic device. The protocols are transferrable throughout Maine and the US to evaluate tidal energy resources and better understand the potential impact of this development on the environment. Again, site-specific social science and environmental research focused on the Cobscook Bay/Western Passage area near Eastport Maine. The protocols and methods developed at these sites have also been used to perform initial scoping reviews of locations in Castine Harbor and Wiscasset, Maine that represent a more modest and more typical small scale energy resource. Specific barrier issues which have been addressed for the industry are technologies and protocols for measuring and modeling tidal flows, responses of fishes to those flows, and people interacting in these environments. Measuring tidal flows is critical to the key economic driver for this industry, the size of the potential resource. The second barrier issue is the need for methods for measuring the impact of marine hydrokinetic (MHK) devices on fish. Acoustic methods have been used with ground truth validation from trawls. The protocols developed in this project have already had a significant impact on the approach that has been taken at other sites. Finally the assessment of the human community response to these technologies and impact on community cohesion and participation is perhaps the largest single barrier to the acceptance of the projects. This work also has the potential to be replicated at other sites, although in both the case of the environmental effects and the social response to these projects, details of the species impacted and the economic and social environment are the ultimate determinants of impact and acceptance. The technology focus for most of this work has been the cross-flow turbine developed by Ocean Renewable Power Company. Testing in the University of Maine tow tank has allowed a large design space to be explored for the optimization of the commercial turbine design. The design code developed for the project was validated using this data set. Both the design code and the data will be placed in a public repository. The most important outcome of the turbine design portion of the work is some general design parameters that can be used to assist in the site assessment and for benchmarking of proprietary designs. The design as well as the data is available for resource assessment and design comparisons. The appeal of this turbine design is that the potential exists for a low solidity turbine with lower tip speed ratios, which will have good performance. The low solidity and tip speed ratio is likely to reduce the risk of fish impacts and thus reduce environmental impact and community resistance to these technologies. The need for low carbon energy sources is undeniable. Resistance to large-scale renewable energy development also continues to increase. The overall approach to this project, where the design of the system considers environmental impacts and social acceptance from the initial engineering design stages and continues with an adaptive management scheme, is the only option for addressing energy needs at the scale required

A Census of Eddy Activities in the South China Sea During 1993-2007

Numerous mesoscale eddies occur each year in the South China Sea (SCS), but their statistical characteristics are still not well documented. A Pacific basin-wide three dimensional physical-biogeochemical model has been developed and the result in the SCS subdomain is used to quantify the eddy activities during the period of 1993-2007. The modeled results are compared with a merged and gridded satellite product of sea level anomaly by using the same eddy identification and tracking method. On average, there are about 32.9 +/- 2.4 eddies predicted by the model and 32.8 +/- 3.4 eddies observed by satellite each year, and about 52% of them are cyclonic eddies. The radius of these eddies ranges from about 46.5 to 223.5 km, with a mean value of 87.4 km. More than 70% of the eddies have a radius smaller than 100 km. The mean area covered by these eddies each year is around 160,170 km(2), equivalent to 9.8% of the SCS area with water depths greater than 1000 m. Linear relationships are found between eddy lifetime and eddy magnitude and between eddy vertical extent and eddy magnitude, showing that strong eddies usually last longer and penetrate deeper than weak ones. Interannual variations in eddy numbers and the total eddy-occupied area indicate that eddy activities in the SCS do not directly correspond to the El Nino-Southern Oscillation events. The wind stress curls are thought to be an important but not the only mechanism of eddy genesis in the SCS