To investigate the coastal current in the Gulf of Maine (GoME) and its relation to forcing from outside of the gulf, a high‐resolution circulation model was developed and validated. Our model shows that the Eastern Maine Coastal Current (EMCC) possesses two cores, an offshore and a nearshore core that peak in summer and spring, respectively. The two cores can be traced back to outflows from the Bay of Fundy from opposite sides of Grand Manan Island, and both cores are deeper and slightly more onshore in summer and fall in response to tidal mixing, surface thermal stratification and wind. The two cores merge south of Pleasant Bay, then split into two branches again east of Mount Desert Rock, where the nearshore branch flows along the coast, while the offshore branch turns southward to recirculate in the eastern GoME. Subject to variations of Scotian Shelf Water and Slope Water (SW) inflows, the offshore veering occurs further upstream (northeastward) in late winter and summer, but gradually shifts downstream (southwestward) from summer to winter. Diagnosis of momentum balance demonstrates that the EMCC is primarily driven by the pressure gradient force (PG), of which the barotropic PG is dominant and offshore, while the baroclinic PG is onshore and increases with depth. The large baroclinic PG at depths, modulated by SW, that is, blended by tidal mixing, offsets the barotropic PG. Near the surface, the barotropic PG is nearly balanced by the Coriolis force, forming the geostrophic EMCC.
A 3D unstructured‐grid ocean circulation model covering the continental shelf and coastal seas around New England is used to investigate freshwater transport (FWT) on the Scotian Shelf (SS) and its impact on the salinity in the Gulf of Maine (GoME). The model was first validated using observed elevation, velocity, temperature, and salinity at multiple locations, demonstrating generally high model skills. Intraseasonal variabilities of freshwater fluxes in 2017 and 2018 were then analyzed across different transects around SS and Browns Bank (BB). These indicated that the flow pattern in SS during 2017 was consistent with previous understanding: low salinity water flows southwestward along the shelf and turns into the GoME around BB. However, in 2018, most of the low salinity water did not enter the GoME, but was transported to the open ocean. The most striking difference occurred early in the year when the FWT, normally at its maximum, was practically stopped by an anticyclonic eddy impinging upon the shelf break near the western end of SS. Then in March, in contrast to the prevailing eastward wind, two southwestward wind events occurred that induced an excessive amount of FWT in SS. We further showed that when anticyclonic eddies impinge on the shelf break, the typical geostrophic balance associated with southwestward flow is interrupted, and a new geostrophic balance is established with the directions of pressure gradient force and flow reversed. The influence from anticyclonic eddies explains the abnormally low FWT in SS and higher GoME salinity in 2018.
more » « less- NSF-PAR ID:
- 10374569
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 127
- Issue:
- 1
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
null (Ed.)Abstract The Loop Current (LC) system has long been assumed to be close to geostrophic balance despite its strong flow and the development of large meanders and strong frontal eddies during unstable phases. The region between the LC meanders and its frontal eddies was shown to have high Rossby numbers indicating nonlinearity; however, the effect of the nonlinear term on the flow has not been studied so far. In this study, the ageostrophy of the LC meanders is assessed using a high-resolution numerical model and geostrophic velocities from altimetry. A formula to compute the radius of curvature of the flow from the velocity field is also presented. The results indicate that during strong meandering, especially before and during LC shedding and in the presence of frontal eddies, the centrifugal force becomes as important as the Coriolis force and the pressure gradient force: LC meanders are in gradient-wind balance. The centrifugal force modulates the balance and modifies the flow speed, resulting in a subgeostrophic flow in the LC meander trough around the LC frontal eddies and supergeostrophic flow in the LC meander crest. The same pattern is found when correcting the geostrophic velocities from altimetry to account for the centrifugal force. The ageostrophic percentage in the cyclonic and anticyclonic meanders is 47% ± 1% and 78% ± 8% in the model and 31% ± 3% and 78% ± 29% in the altimetry dataset, respectively. Thus, the ageostrophic velocity is an important component of the LC flow and cannot be neglected when studying the LC system.more » « less
-
more » « less
Abstract The mechanisms that control the export of freshwater from the East Greenland Current, in both liquid and solid form, are explored using an idealized numerical model and scaling theory. A regional, coupled ocean–sea ice model is applied to a series of calculations in which key parameters are varied and the scaling theory is used to interpret the model results. The offshore ice flux, occurring in late winter, is driven primarily by internal stresses and is most sensitive to the thickness of sea ice on the shelf coming out of Fram Strait and the strength of alongshore winds over the shelf. The offshore liquid freshwater flux is achieved by eddy fluxes in late summer while there is an onshore liquid freshwater flux in winter due to the ice–ocean stress, resulting in only weak annual mean flux. The scaling theory identifies the key nondimensional parameters that control the behavior and reproduces the general parameter dependence found in the numerical model. Climate models predict that winds will increase and ice export from the Arctic will decrease in the future, both of which will lead to a decrease in the offshore flux of sea ice, while the influence on liquid freshwater may increase or decrease, depending on the relative changes in the onshore Ekman transport and offshore eddy fluxes. Additional processes that have not been considered here, such as more complex topography and synoptic wind events, may also contribute to cross-shelf exchange.
Significance Statement The purpose of this study is to provide a basic understanding of what controls the flux of sea ice and low-salinity water from the East Greenland shelf into the interior of the Greenland and Iceland Seas. This is a potentially important process since it has been shown that sufficient freshening of the surface waters in the interior of the Nordic seas can inhibit deep convection and the associated air–sea heat flux and water mass transformation. A combination of idealized computer models and basic theory indicates that the fluxes of liquid and solid freshwater are controlled by different mechanisms and occur at different times of the year. Accurate representation in climate models will require representation of small-scale processes such as mesoscale eddies and gradients of ice thickness across the shelf.
-
The Gulf of Mexico is a very productive and economically important system where riverine runoff acts as a linkage between the continental shelf and the open ocean, providing nutrients in addition to freshwater. This work investigates the three-dimensional transport and pathway structure of this river runoff offshore the continental shelf using ensembles of numerical simulations with different configurations regarding grid resolution (mesoscale resolving and submesoscale permitting) and river setup using suites of 5-months long integrations covering nearly 3 years. The riverine forcing is applied only at the surface over an area around the river mouth, a strategy often adopted in numerical studies, or as a meridional flux with a vertical extension. The simulated flow captures the southward offshore transport of river runoff driven by its interaction with the largest mesoscale circulations in the basin, the Loop Current and Loop Current eddies. This pathway is strong and well-document during summer but also active and relevant in winter, despite a less obvious surface signature. The most intense transport occurs primarily at the peripheries of the Loop Current and the detached eddies, and the freshwater is subducted as deep as 600 m around the mesoscale anticyclonic eddies. Submesoscale motions strengthen slightly the spread of freshwater plumes in summer but their contribution is negligible, if not negative, in winter. Differences in the freshwater distribution and transport volume among runs are small and generally less than 10% among ensembles, with overall slightly higher volume of freshwater transported off-shore and at depth in submesoscale permitting runs that include a velocity flux in their riverine input representation.more » « less
-
more » « less
Abstract The Beaufort Gyre (BG) is a large anticyclonic circulation in the Arctic Ocean. Its strength is directly related to the halocline depth, and therefore also to the storage of freshwater. It has recently been proposed that the equilibrium state of the BG is set by the Ice‐Ocean Governor, a negative feedback between surface currents and ice‐ocean stress, rather than a balance between lateral mesoscale eddy fluxes and surface Ekman pumping. However, mesoscale eddies are present in the Arctic Ocean; it is therefore important to extend the Ice‐Ocean Governor theory to include lateral fluxes due to mesoscale eddies. Here, a nonlinear ordinary differential equation is derived that represents the effects of wind stress, the Ice‐Ocean Governor, and eddy fluxes. Equilibrium and time‐varying solutions to this three‐way balance equation are obtained and shown to closely match the output from a hierarchy of numerical simulations, indicating that the analytical model represents the processes controlling BG equilibration. The equilibration timescale derived from this three‐way balance is faster than the eddy equilibration timescale and slower than the Ice‐Ocean Governor equilibration timescales for most values of eddy diffusivity. The sensitivity of the BG equilibrium depth to changes in eddy diffusivity and the presence of the Ice‐Ocean Governor is also explored. These results show that predicting the response of the BG to changing surface forcing and sea ice conditions requires faithfully capturing the three‐way balance between the Ice‐Ocean Governor, wind stress, and eddy fluxes.
