Dynamics of the Block Island Sound Estuarine Plume

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QIANQIAN LIU* AND LEWIS M. ROTHSTEIN Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island

YIYONG LUO Physical Oceanography Laboratory, Ocean University of China, Qingdao, China

(Manuscript received 2 June 2015, in ﬁnal form 16 February 2016)

ABSTRACT

Buoyant discharge of freshwater from Long Island Sound (LIS) forms a seasonal buoyant plume outside Block Island Sound (BIS) between the coast of Long Island and the denser shelf waters. The plume’s seasonal variability and its response to tides, winds, and surface heating are investigated through a series of process-oriented experiments using the Regional Ocean Modeling System (ROMS). Results show the importance of river discharge, wind directions, and surface heating in the seasonal variation of the BIS buoyant plume. In winter and spring, the plume is intermediate with a large surface offshore extension detached from the bottom. From winter to spring, the river discharge increases; meanwhile, upwelling- favorable winds keep dominating. They compete with the increase of surface heating and generate a broader buoyant plume in spring than in winter. In summer, the plume is bottom advected with most of its width in contact with the bottom and is featured with the steepest isopycnals and narrowest plume, which is driven by a combination of strong insolation, weak buoyant discharge from LIS, and feeble winds. In fall, although the river discharge is comparable to that in winter, the upwelling-favorable wind is relatively weaker, corresponding to a narrower intermediate plume.

1. Introduction isolated from any interaction with the bottom, 2) a bottom-advected plume that is controlled by advection Block Island Sound (BIS) is a strait on the inner in the bottom boundary layer (Lentz and Helfrich shelf of the southern New England shelf. It separates 2002), and 3) an intermediate plume whose dynamics Block Island (BI) from the coast of Rhode Island and are a combination of 1 and 2. The historical develop- connects Long Island Sound (LIS) and Rhode Island ment of estuary plume models are well summarized by Sound (RIS; Fig. 1), working as the most important Horner-Devine et al. (2015) and O’Donnell (2010). passage of freshwater that originates from the The Connecticut River, as the purest surface-advected Connecticut River. plume that can be found (Garvine 1974), takes ;70% According to Yankovsky and Chapman (1997), of the buoyancy (i.e., freshwater) source from LIS and buoyant plumes generated by relatively fresh waters entrains that out of BIS (Latimer et al. 2014; O’Donnell and offshore denser waters are characterized by ad- et al. 2014); during that process a downshelf buoyant vective processes that generate three dynamically coastal current is generated along the southern shore of distinctive plumes: 1) a surface-advected plume that is Long Island, and a bottom-advected plume front is generatedtothesouthofBIS. The BIS bottom-advected plume was observed by * Current afﬁliation: School of Marine Sciences, University of Maine, Orono, Maine. Kirincich and Hebert (2005) during a 2-day experiment in April 2002, when the river discharge was strongest (Fig. 2). It is accompanied by an along-shelf Corresponding author address: Qianqian Liu, Graduate School of Oceanography, University of Rhode Island, 215 South Ferry coastal jet, which is almost linearly sheared with re- Road, Narragansett, RI 02882. versed velocities at the bottom and is in thermal wind E-mail: [email protected] balance with the mean density ﬁled (Yankovsky and

DOI: 10.1175/JPO-D-15-0099.1

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FIG. 1. Bathymetry and geographic features around the Rhode Island Sound. Place names: Connecticut River (CR), Long Island Sound (LIS), Rhode Island Sound (RIS), Block Island Sound (BIS), Narragansett Bay (NB), Buzzards Bay (BB), and Vineyard Sound (VS). The thick red line is the section used to describe the buoyant plume to the southwest of BIS, and the thick black lines are the sections referenced in Table 2, as discussed further in section 5.

Chapman 1997). In light of the strong dependence of while the across-shelf direction is dominated by weak alongshore velocities on the plume and its front, onshore winds. Based on the analysis of ferry-based Codiga (2005) used the velocity field between De- current observations and numerical simulations, cember 1999 and August 2002 to identify the location Whitney and Codiga (2011) found that the along- of the front and found that the plume experiences estuary winds are important for the water transport at strong seasonal shifts, involving the shallowest at- the mouth of LIS. tachment depth in winter, deepest in spring, and in- It is very common for a plume to encounter an am- termediate in fall, as well as a wider plume width in bient current from upstream. The BIS estuarine plume is spring than in winter and fall. Based on a series of nu- affected by the ambient current from RIS. However, merical simulations, Edwards et al. (2004a,b) studied most of the available studies have ignored the influence the front in BIS and concluded the downshelf jet is a of ambient currents that have been proved important by combination of tide-induced flow (nearshore) and Chapman and Lentz (1994). Chapman and Lentz (1994) buoyancy-driven flow (offshore), whose position can be found that this kind of flow limits the offshore spread of a modified by the change of buoyancy forcing. plume and even pushes the front shoreward. Therefore, The studies by both Kirincich and Hebert (2005) we believe that the seasonal cyclonic circulation in RIS and Codiga (2005) pointed out the importance of the (Kincaidetal.2003; Luo et al. 2013) not only exchanges highly variable alongshelf (defined as the direction waters between RIS and BIS but also limits the offshore parallel to the southern coast of Long Island) winds spread of the BIS estuarine plume. Over Rhode Island (blue line in Fig. 2b) in the surface offshore extension coastal waters, this upstream current is closely con- of the plume due to Ekman dynamics, which is con- nected with a bottom thermal gradient, which is as- sistent with the available literature (Csanady 1978; sociated with surface heating (Luo et al. 2013). Hence, Whitney and Garvine 2006; Moffat and Lentz 2012). the surface heating may modulate the plume by means In addition, modeling studies by Tilburg (2003) sug- of both an upstream current and a bottom thermal gested the importance of winds in the across-shelf gradient. direction (the direction normal to Long Island), This study will examine the seasonal variability of pointing out that an offshore wind stress can generate the plume to the southwest of BIS and its response to surface offshore transport and an onshore returning seasonally varying winds, river discharge, and surface flow below it, while an onshore wind can generate heating by means of seasonal upstream current and onshore surface transport and offshore near-bottom deep thermohaline gradients. The numerical experi- transport. During fall and winter, BIS is subjected to ment design and model–observation comparisons are strong offshore and upwelling winds; in summer, the described in sections 2 and 3. Model results and dis- winds become weaker and there are episodic occur- cussion are presented in sections 4 and 5.Weconclude rences of weak downwelling winds in September with a summary in section 6.

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FIG. 2. Wind and Long Island Sound river input. (a) Daily winds from NARR averaged from 2004 to 2009. (b) The corresponding along-shelf (blue) and across-shelf (green) wind components. For the along-shelf component, positive value represents upwelling-favorable wind, while negative represents downwelling-favorable wind. For the across-shelf component, positive is for onshore wind and negative is for offshore wind. (c) Daily and monthly Long Island Sound river input, estimated from the Connecticut River discharge by dividing the Connecticut River by 0.7 (from USGS).

2. Design of numerical experiments discharge obtained from the U.S. Geological Survey (USGS; http://waterdata.usgs.gov/nwis) in Thompsonville, 2 The numerical model used for this study is the Re- Connecticut, is about 550 m3 s 1. To account for the gional Ocean Modeling System (ROMS), a widely ungauged discharge, we multiply the total gauged recognized regional and basin-scale ocean model flow by the ratio of total area to gauged area. For the using a high-resolution, free-surface, terrain-following Connecticut River, the total gauged area is ;11 224 coordinate (Shchepetkin and McWilliams 1998, 2003, square miles (at USGS site 01194796 in Old Lyme, 2005; Haidvogel et al. 2000). Our configuration, cov- Connecticut, a site at the mouth of the Connecticut ering the domain of RIS, BIS, LIS, and the adjacent River), which is 1.16 times of the drainage area inner shelf area, is one-way nested within a larger do- (;9660 square miles) at site 01184000 in Thompson- main covering the Gulf of Maine/Georges Bank and ville, Connecticut. This gives the freshwater dis- 2 the New England shelf (Fig. 3; Luo et al. 2013). It has a charge from the Connecticut River as 638 m3 s 1. horizontal resolution varying from 600 m over the RIS Therefore, the total freshwater discharge into LIS is 2 and BIS to 1 km along the boundaries (Fig. 3)withthe ;911 m3 s 1, close to the value used in the idealized 2 number of vertical layers increased from 15 in Luo et al. experiments (1000 m3 s 1). (2013) to 30 in order to better capture both surface and To look at the influence of tides, the second experi- bottom boundary layers. ment RivTides is carried out. It is driven by the five

A series of experiments are executed to examine the major regional tidal components (M2,N2,S2,O1, and response of the estuarine plume, sourced from the K1) as obtained from the Advanced Circulation Model Connecticut River, to tides, wind directions, and sur- for Oceanic, Coastal and Estuarine Waters (ADCIRC; face heating as well as their seasonal variability under Luettich et al. 1992) tidal simulation and the constant 2 realistic atmospheric forcings (Table 1). The ﬁrst ex- Connecticut River of 1000 m3 s 1. Similar to Buoy, this periment, named Buoy, isolates buoyant discharge experiment also starts from a resting ocean with salinity from other forcings. The model starts from a resting of 35 psu and temperature of 148C. ocean with uniform salinity of 35 psu and temperature The realistic experiment (Real) integrates all avail- of 148C and is driven by a constant Connecticut River able, real forcings and is used to understand the sea- 2 pointsourcewithaninputof1000m3 s 1. According sonal variations of the plume. In addition to tides, the to O’Donnell et al. (2014), the long-term mean following are used: daily averaged, climatological

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FIG. 3. Model configurations. The local-scale ROMS grid (plot every eight grid points) varies in horizontal resolution from 600 m over the RIS and BIS to ;1 km along the boundaries, and the regional-scale ROMS grid (plot every four grid points) is uniform with a resolution of 5 km. atmospheric forcings from the North American Re- used for spinup and the third-year results used to an- gional Reanalysis (NARR; Mesinger et al. 2006)from alyze the seasonal variability of the plume. For verifi- 2004 to 2009; daily averaged, climatological river dis- cation purpose, we have carried out another similar charges for the Taunton River, Blackstone River, experiment driven by original, daily NARR forcing Pawtuxet River, and Connecticut River as obtained from 2004 to 2009. Since there is minor difference in from the USGS; and open boundary conditions from monthly mean currents and plume between these two our ‘‘regional’’ ROMS domain (from Luo et al. 2013). experiments, and the dynamics behind it is the same, to The case Real is driven by climatological daily aver- save computation time, the experiment driven by cli- aged data from 2004 and 2009 for 3 yr with the first 2 yr matological mean of forcing is used.

TABLE 1. List of Experiments with ROMS.

2 Run Tides Wind River discharge (m3 s 1) Surface heating Buoy No No 1000 No RivTides Yes No 1000 No RivTides_500 Yes No 500 No WndOn Yes Onshore 1000 No WndOff Yes Offshore 1000 No WndUp Yes Upwelling 1000 No WndDown Yes Downwelling 1000 No HeatS Yes No 1000 Climatological August surface heating HeatW Yes No 1000 0.5 3 August surface heating Real Yes Daily Data from NARR Daily Data from USGS Daily Data from NARR NoWind Yes No Daily Data from USGS Daily Data from NARR

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FIG. 4. Comparison of depth-averaged currents in RIS in spring and summer between the model (blue arrows) and observations (red arrows). The ﬁeld data are from Ullman and Codiga (2010).

To better represent the salinity input from the of the plume’s response to surface heat flux, we con- Connecticut River, we rescale the discharges from struct another pair of experiments by adding constant the USGS to account for the ungauged portions of the surface heating from the sixth month of RivTides and watershed (Chantetal.2008; Zhang et al. 2010). run for another 2 months. An experiment, HeatS, Because the Connecticut River takes 70% of the driven by the climatological surface heat flux in August riverine input entering LIS, the total discharge into is carried out, and in the other experiment, HeatW, we LIS is estimated by dividing the Connecticut River by halve the heat flux to provide weaker surface heating, 0.7 (Fig. 2c). Please note, in this case, the Connecticut whereby the bottom thermal front still exists but River has salinity of 0 psu and the same temperature becomes weaker. with the closest water grid. Moreover, for simplicity, all rivers are idealized as point sources at the end of 3. Model–observation comparisons the long channels away from the water points. In addition to experiment Real, we perform another Luo et al. (2013) have shown that our realistically experiment to examine the winds’ contributions to the forced model of the region does an excellent job of seasonal variations of the BIS estuarine plume, that is, simulating tidal residual currents over the whole do- experiment NoWind omitting local winds from exper- main as well as temperature and salinity at the outer iment Real. To test the response of the plume to the shelf. A depth-averaged currents comparison between four typical winds in the studied region, that is, on- the model and observations (Fig. 4) demonstrates the shore wind (wind perpendicular to the south coast of model’s fidelity in RIS by capturing the cyclonic cir- Long Island and pointing onshore), offshore wind culation in RIS. The moored observations are a com- (similar to onshore wind but pointing offshore), bination of data from the Rhode Island Ocean Special upwelling-favorable wind (wind parallel to the south Area Management Plan project and the project funded coast of Long Island and pointing northeast), and by Rhode Island Sea Grant and Rhode Island En- downwelling-favorable wind (similar to upwelling- deavor Program (C. Wertman et al. 2016, unpublished favorable wind but pointing southwest), a series of manuscript) during 2009 and 2010. We average the ob- idealized experiments driven by buoyant discharge, servations in each season to compare with the modeling tides, and the corresponding winds are implemented. results. The average wind from 2004 to 2009 (Fig. 2)is Each case is restarted from the sixth month of RivTides generally southeastward in winter when it is strongest and then run for another 2 months (Table 1). These ex- and northeastward in summer. Its seasonal variation periments are referred to as WndOn, WndOff, WndUp, is consistent with the data at the deployments carried and WndDown, respectively, in which we set the winds out by the Ocean Special Area Management Plan 2 constant as 0.05 N m 2. The magnitudes represent the (OSAMP) in September 2009, December 2009, March typical wind stresses in the upwelling and offshore di- 2010, and June 2010 (Ullman and Codiga 2010). rections in winter and fall. Therefore, the model is under similar conditions with An experiment with halved, constant river discharge the observations. from the Connecticut River, referred to as RivTides_ In May, both the model and observations show rela- 2 500, is compared with RivTides to examine the river tively weak (;2cms 1) onshore velocities in RIS, with 2 discharge’s impact on the buoyant plume. For the study stronger currents (;8cms 1) located to the south of

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FIG. 5. Comparison of depth-averaged currents in four seasons between the model (blue arrows) and observations (red arrows) in Front-Resolving Observational Network with Telemetry project between 2000 and 2001 (Codiga 2005; Codiga and Houk 2002; red arrows).

Block Island in a region that connects RIS and BIS. In modeling results also agree well with the limited ob- July, the model captures the westward jet offshore of servations. In winter and fall, the model depth- Narragansett Bay as well as the southwestward current averaged currents are significantly diminished with leaving RIS that appears in the observations. The root- more variable directions, which may be due to the mean-square error (RMSE) is used as a measure of highly varying winds. The model results show a con- precision. In May, the RMSEs for u- and y-component sistency with the fields in amplitude but only capture 2 velocities are 1.7 and 1.1 cm s 1, respectively, and in about half of the stations in direction. In light of the 2 July, they are 2.1 and 1.1 cm s 1, respectively. Therefore, highly varying wind directions, the comparisons are the model well captures the cyclonic circulation at the acceptable. The success in tidal and realistic simula- center and south of RIS in spring and summer, which is tions as well as our previously published work (Luo proved important to the buoyant plume in BIS in the et al. 2013) indicate that our model successfully rep- following discussions. resents the essential physical dynamics of RIS and BIS. In addition, Fig. 5 shows that our Real experiment To validate the experiment in simulating the river simulates the circulation in BIS quite well by comparing discharge from LIS, we compare velocities across a its simulated depth-averaged currents in four seasons transect close to the Race (marked as section 1 in Fig. 1) with observations from the Front-Resolving Observa- between the case Real and the ADCP observations tional Network with Telemetry (FRONT) project collected between November 2002 and January 2005 by (Codiga 2005; Codiga and Houk 2002). Specifically, in the Cross Sound Ferry Services of New London, Con- spring, the depth-averaged flow features a robust necticut (Codiga and Aurin 2007). Both observations 2 (;15 cm s 1) southwestward current, which is consistent and model results show vigorous water exchange across with Codiga’s (2005) observations with an RMSE for the channel with a surface-intensified, fresher current, 2 2 the u-component velocity of 2.6 cm s 1 and for the ;15 cm s 1, moving out of the estuary along the south of 2 y-component velocity of 4.6 cm s 1; in summer, our the channel and bottom-intensified, saltier flow moving

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FIG. 6. Comparison of velocities across a transect close to the Race (from New London, Connecticut, to Orient Point, New York) between the (a) observation and (b) model. Color represents velocities across the Race with positive values for current moving out of LIS and negative value into LIS. Model-simulated salinity along the transect is shown by black contours on the right panel. into the estuary along the northern boundary (Fig. 6), as reasonable to treat the rivers in LIS as emanating from a result of a gravitational circulation. the Connecticut River region alone and shows that the Based on the available observations, mainly the advection by residual flow is important for the total better-sampled inward velocities, Codiga and Aurin exchange between LIS and coastal waters. (2007) estimated the annual-mean exchange transport To further verify the model in simulating the BIS es- 2 as 2.27 3 104 6 5000 m3 s 1.InthecaseReal,the tuarine plume, we compare the vertical currents in spring annual-mean inward and outward volume transports with the observations in the FRONT project (Codiga 2 2 are 2.51 3 104 m3 s 1 and 2.76 3 104 m3 s 1, respec- 2005). As shown in Fig. 7, consistent with Codiga’s (2005) tively, which is in good agreement with the estimation observations, the plume features southwestward currents by Codiga and Aurin (2007). In addition, consistent with robust, surface-intensified, along-shelf transport and with the observations, the exchange transport under- with the vertical currents veering clockwise with depth goes strong seasonal fluctuation with the bottom, in- for waters deeper than 10m. The vertical, clockwise- 2 ward transport of 2.9 3 104 m3 s 1 in summer and 2.4 3 veering structure is due to the decrease of downshelf 2 104 m3 s 1 in winter. The comparison suggests that it is and offshore velocities with depth. The model and

FIG. 7. Comparison of vertical currents in spring between (a) the observations in the FRONT project (Codiga 2005) and (b) model results. Color represents water depth with red representing currents at the top layers and blue at the deep, and arrows represent velocities at the corresponding depth.

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FIG. 8. Time-averaged surface salinity for the case (a) Buoy and (b) RivTides after 100 days’ evolution. observations have relatively larger discrepancies for month is the same with the structure at the fourth velocities above 10 m, which may be due to the ex- month. trapolation of measurements. Near the bottom, both The difference between Buoy and RivTides derives the model result and observations show a weak onshore from the deep penetration of freshwater in RivTides and transport even when winds are not strong enough to is consistent with the available literatures, for example, affect the bottom waters in spring. In brief, the above Simpson (1997), which emphasized the importance comparisons indicate our simulation captures the dy- of tidal mixing in the plume spread. At the same time, namics of the BIS estuarine plume. the case RivTides generates a constant upstream flow 2 of ;3cms 1 from RIS imposed on the plume to the southwest of BIS. According to a series of process- 4. Results oriented experiments and the theoretical analysis of the depth-averaged vorticity equation, we infer that the a. Plume response to tides upstream current in RivTides is due to the topographi- Figure 8 depicts the pattern of the surface buoyant cally induced tidal rectification. This current works to plumes after 100 days’ simulation for the idealized ex- turn the trajectories of the particles into RIS back to periments of Buoy and RivTides. The case Buoy, an BIS. Therefore, waters entering RIS through the gap idealized experiment free of impacts of tides and winds, between Block Island and Point Judith are isolated from generates a plume extending broadly to the outer shelf RIS waters. According to Chapman and Lentz’s (1994) region (Fig. 8a). The plume is steered to the right by the study, because of the limitation by the upstream current, Coriolis force and features a relatively large anticy- we infer that the plume in RivTides should be narrower clonic circulation or ‘‘bulge’’ after it moves out of LIS and has a larger horizontal density gradient than the and BIS, which was discussed by Chantetal.(2008), theoretical estimates of Yankovsky and Chapman Kourafalou et al. (1996),andFong and Geyer (2002). (1997). In the following discussions the effects of tides Freshwater coming from the Connecticut River is sur- will be implicitly included. face trapped, that is, without deep signatures of either Figures 9b and 9e show the cross-shelf sections of the salinity or velocity (Figs. 9a,d). Therefore, the hori- along-shelf and cross-shelf velocities and salinity along zontal extension of the buoyancy-influenced region the section outside BIS, normal to the southern shore of must significantly increase to maintain mass conserva- Long Island (section 4 in Fig. 1) after 8 months’ evolution, as illustrated in Fig. 8a. tion. We choose this section away from the mouth of BIS In contrast to the broad surface extension in Buoy, to avoid the headland eddy around BIS. The total width the plume in RivTides is limited to a much smaller area of the plume is Wp 5 Wb 1 Ws and is composed of the encompassing LIS, BIS, south of BIS, and east of RIS part in direct contact with the bottom Wb and the part with a sluggish, horizontal propagation speed, accord- away from the bottom (from the bottom offshore edge ing to the salinity field comparisons (Fig. 8). After of the plume to its surface offshore edge Ws). Since Wb , about 4 months’ simulation, the structure of the buoy- Ws, the plume generated by RivTides is classified as ant plume reaches a steady state with a confined ‘‘intermediate,’’ with isopycnals sloping from the bot- offshore extension. The plume structure in the eighth tom at about 20 km offshore of the coast to the surface

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FIG. 9. Cross-shelf sections of buoyant plume normal to the south shore of Long Island (see Fig. 1) for the cases of Buoy, RivTides, and RivTides_500. Along-shelf velocity for the cases (a) Buoy, (b) RivTides, and (c) RivTides_ 500; across-shelf velocity for the cases (d) Buoy, (e) RivTides, and (f) RivTides_500. The white lines represent salinity, and the thick gray lines represent the isolines of 0. at 20 km farther offshore. The trapping depth, also balance (Fig. 11a). During the thermal wind calcula- called the attachment depth, at which the bottom- tion, we approximately set the bottom, weak velocity 2 advected plume becomes trapped (Yankovsky and less than 1 cm s 1 as 0, and in the diagnostic analysis, we Chapman 1997) with the onshore and offshore veloci- cast the momentum equations into a right-hand co- ties reaching a balance, is about 20 m for the transect ordinate system with x directed upshelf along the shore chosen, much smaller than the estimation by Kirincich of Long Island and y directed onshore. Meanwhile, and Hebert (2005) and Codiga (2005) at a farther up- the offshore flow responsible for the offshore spread of stream region around the mouth of BIS. the inner-shelf fresher waters penetrates to the depth The alongshelf flow is surface intensified with al- where the along-shelf velocity reverses. Such deep on- most linear vertical shear, reversing within several shore flow has been observed in the southwest of BIS by meters of the bottom at the foot of the front. It is the moored profiling current meter records in spring geostrophic, by comparison of the full physics model when wind effects are excluded (Codiga 2005; Fig. 7). output with a thermal wind calculation (Fig. 10) and by Diagnostic analysis of the along-shelf momentum bal- the diagnostic analysis of the across-shelf momentum ance in Fig. 11a verifies Yankovsky and Chapman’s

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b. Seasonal variability of the plume in the case Real We deﬁne the average from January to March as winter, April to June as spring, July to September as summer, and October to December as fall. The comparison between the simulated depth-averaged currents and observations from the FRONT project (Ullman and Codiga 2004) indicates a strong seasonal cycle of circulation. Figure 12 shows the seasonal-mean surface salinity, and Fig. 13 shows the seasonal mean of the vertical structure for the transect perpendicular to Long Island. They demonstrate a strong seasonal variability of the BIS estuarine plume generated by the freshwater emanating from LIS. BIS is fresher in winter and spring than in summer

FIG. 10. Cross-shelf section of the downshelf velocities based on and fall, with the widest and strongest buoyant plume in (color) the direct model output and (isoline) the thermal wind re- spring when river discharge peaks and the narrowest lation with the bottom velocity approximately set as 0. The thick and weakest plume in summer when river discharge gray line represents the zero isoline of the direct model output. is smallest (Fig. 13). Compared with that in spring, freshwater in winter is limited in its downshelf pene- (1997) hypothesis that the deep, onshore transport is tration and has a saltier plume, though their offshore due to bottom Ekman dynamics with a balance reached spreading is comparable. by the pressure gradient, Coriolis force, and vertical The transect normal to Long Island reveals an in- viscosity terms. termediate plume in spring with the bottom offshore edge

FIG. 11. Momentum terms in the (a) along-shelf and (b) across-shelf directions at 15 km offshore for the cases of RivTides. The thick, solid lines represent pressure gradient; the thick, dashed lines represent Coriolis term; the thin, solid lines represent along-shelf, u-momentum advection; and the thin, dashed lines represent vertical viscosity.

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bottom Ekman layer. Compared with the other seasons, during spring the waters on the landside of the front are freshest; the front has the sharpest gradient and widest frontal width with a slope angle around 0.001. The slope for the isohalines at the offshore edge of the plume is even smaller. Though there are not enough observations in summer to delineate the buoyant plume, the fact that our modeling result agrees well with the limited current observation gives us confidence in presenting the summer features. During this season, a robust downshelf jet 2 larger than 16 cm s 1 moves through the salinity front. The summer plume is closer to bottom advected with a much narrower plume width at the surface (;22 km) and steeper frontal slope (;0.002 for the isohaline of 32 psu, doubling the slope in spring) than the other seasons. Also, its bottom upshelf layer almost disappears, and the onshore deep transport is weaker and confined to a narrower region. In winter and fall, the plume is also intermediate. As described by Codiga (2005), the along-shelf transport is significantly weakened and moved offshore with the shallowest penetration depth of the shallow transport, which is especially obvious for winter’s result. In winter, the freshwater patch of 31.4 psu is transported away from the coast; the center of the downshelf jet moves to a region farther than 30 km offshore, and the deep upshelf jet is much thicker than the other seasons. At the same time, the across-shelf flow shows a strong surface offshore transport, even on the landside of the front, corresponding to a broad surface extension and small slope angles. The bottom onshore transport is obviously strengthened, spanning across the bottom of the entire transect and extends even more offshore. Correspondingly, the detachment depth disappears without a balance between the offshore and the onshore transport at the bottom. In fall, the downshelf jet is also diverted offshore and a strong offshore transport appears, generating a slope angle smaller than summer but larger than spring with a value of about 0.018 for isohaline of 32 psu. In contrast to winter, in fall, at the bottom of ;8 km offshore, the offshore transport at the landside and the onshore FIG. 12. Seasonal variability of surface salinity for the experiment transport at the seaside reach a balance, generating a Real in (a) winter, (b) spring, (c) summer, and (d) fall. detachment depth of ;35 m. c. Response to winds (32.6 psu) around 15 km offshore and is accompanied by a robust, surface-intensified, downshelf transport. The experiment NoWind, which omits the effects of The downshelf jet centers around 10 km offshore. In the local winds, produces a bottom-advected plume in win- across-shelf direction, there is an offshore transport at the ter and fall with strengthened downshelf transport and surface and a returning, onshore transport below it. This weakened offshore transport at the surface layers, as structure can be explained by the idealized experiments shown in Fig. 14. Meanwhile, the deep upshelf trans- in the next section. At depth, there are currents oppo- port and onshore transport become weaker and thin- site the transport above it, which is associated with the ner; the center of the surface downshelf transport is

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FIG. 13. Seasonal variability of the buoyant plume’s vertical structure along section 4 in Fig. 1 for the case Real. White lines represent salinity, and colors represent (left) along-shelf velocity and (right) across-shelf velocity. (a),(e) winter, (b),(f) spring, (c),(g) summer, (d),(h) fall. located more toward the inner shelf than in the surface outcropping position moves from 32 to 18 km off- experiment Real. shore, while its contact with the bottom is kept intact. An- In spring, the plume width in NoWind is narrower than in other important feature in the NoWind experiment is that, Real. Taking the isohaline of 31.6 psu as an example, its water at the inner shelf becomes fresher, creating a larger

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FIG. 14. As in Fig. 13, but for the case without winds’ effect (NoWind). salinity gradient than in Real, especially in spring. This may Figure 15 shows the responses of surface salinity after be due to the change of freshwater delivery out of BIS 3 days of wind forcing and the Lagrangian trajectories of caused by omission of the winds. the surface drifters during the initial 5 days, in which To further examine the roles of wind, we analyze the both the buoyant plume and the trajectories vary with four idealized experiments driven by winds in different wind direction. In every case, upon leaving the Con- directions: WndOn, WndOff, WndUp, and WndDown. necticut River, most of freshwater is stored in LIS,

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while a smaller fraction is transported out of LIS and mixed with the outer-shelf saline waters. As shown by surface salinity and freshwater flux (Table 2), an upwelling-favorable wind is most effective in spreading the freshwater offshore (Fig. 15c), and a downwelling- favorable wind is most effective in squeezing the plume against the Long Island coast (Fig. 15d). The winds’ effects on freshwater dispersion are accompanied by the changes in the vertical current structure (Fig. 16). In the along-shelf direction, the onshore wind reduces the downshelf velocities at the surface via Ekman dynamics (Fig. 16a); in the across- shelf direction, comparing WndOn with RivTides, the onshore wind generates a surface onshore current 2 (larger than 3 cm s 1) and a returning flow directly below it (Fig. 16e), which is responsible for the sharp halocline around 10-m depth. Such a structure is in agreement with the study of Tilburg (2003), who found a significant surface transport and a returning flow below it, though the depth-integrated transport is feeble. Similarly, while the offshore wind produces an offshore transport at the surface and a deeper returning, onshore transport below it (Fig. 16b), it triggers stronger alongshelf velocities via strong surface and bottom Ekman dynamics (Fig. 16f). Except in a thin layer around 10 m and offshore of 10 km, an upwelling-favorable wind drastically alters the alongshelf flow by reversing the downshelf currents over the water column (Fig. 16c). In the cross- shelf direction, an upwelling wind drives the waters shallower than 10 m offshore and activates a stronger bottom returning current, bringing about an intru- sion of offshore saline waters (Fig. 16g), which de- taches the core of the buoyant plume from the coast and diverts the downshelf transport of freshwater offshore. This is consistent with the study by Codiga (2005). He found that in fall and winter, when the upwelling-favorable winds dominate, the fronts extend farther offshore at the surface, the bottom onshore flow becomes stronger, and the downshelf transport becomes weaker. Conversely, a downwelling- favorable wind creates a strong onshore transport in shallower layers and a deep, offshore flow at the bottom, at the same time accelerating the downshelf transport and squeezing the front into a much narrower band (Figs. 16d,h). FIG. 15. Plane view of surface salinity after 3 days of wind d. Response to river discharge forcing and the Lagrangian trajectories of the drifters released at the locations marked by stars at the first day of wind forcing and Besides wind effects, buoyant discharge variations simulated with 1-h interval for the cases driven by (a) onshore can result in changes in the plume and its front winds (WndOn), (b) offshore winds (WndOff), (c) upwelling- (Chapman and Lentz 1994). To examine this, we con- favorable winds (WndUp), and (d) downwelling-favorable winds (WndDown). figure two idealized experiments driven by constant river and tides only: RivTides with a constant river

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3 21 TABLE 2. Freshwater ﬂux (m s ) across the sections in Fig. 1 for the idealized experiments with different winds and the buoyancy of the 0 2 22 plume, that is, g h0 (m s ) along section 2.

WndOn WndOff WndUp WndDown NoWnd (RivTides) Freshwater ﬂux for section 1 (out of LIS) 530 614 1041 331 630 Freshwater ﬂux for section 2 (out of BIS) 210 608 785 371 496 0 g h0 for section 2 0.15 0.20 0.23 0.16 0.21

2 discharge of 1000 m3 s 1 and RivTides_500 with half of deep thermal front and a stronger cyclonic circulation 2 that river discharge (500 m3 s 1). The resulting buoyant around RIS, and, therefore, a stronger upstream cur- plumes are shown in the middle (Figs. 9b,e) and bottom rent is imposed on the plume front. In addition, the (Figs. 9c,f) panels of Fig. 9, whereby we find the stronger comparison of surface salinity indicates that the in- river input broadens the plume’s width by more than crease in surface heating pushes the surface plume 10 km at the surface and increases the strength of the onshore (Fig. 17), which results from the increase of the front. The doubled river discharge doubles the salinity upstream current from RIS according to the theory by difference between the seaside edge and landside edge, Chapman and Lentz (1994). which is approximately 0.6 psu in RivTides_500 and In the vertical, compared with the nonstratified case larger than 1.2 psu in RivTides. Therefore, in light of the RivTides, the increase of surface heating in HeatW and pronounced seasonal variation in LIS river input, we can HeatS significantly increases the surface-intensified infer that the increase in river discharge from winter to downshelf current with a narrower and weaker upshelf spring as shown in Fig. 2 contributes significantly to the current (Fig. 18). For the cross-shelf structure, the ex- sharper and broader plume in spring, especially when periments with surface heating show slightly weaker and the river discharge peaks in April, and the abrupt de- thinner bottom onshore transport; at the same time, a crease from spring to summer should be responsible for surface offshore transport occurs, between which there the weaker and narrower plume in summer, when dis- is an offshore transport. charge subsides to the bottom. Figures 19 and 20 show the momentum balance in On the other hand, the significant change in river the along-shelf and across-shelf directions for the input only slightly changes the offshore extension of experiments HeatW and HeatS, respectively. Ac- 2 the plume and has a limited influence (within 1 cm s 1) cording to Fig. 11, for RivTides, the major balance in on the cross-shelf and alongshelf velocities. However, the alongshelf direction is geostrophic, while at the in the NoWind experiment, we notice much more bottom, the bottom Ekman dynamics dominates. obvious changes in the velocities and plume’s offshore However, for HeatS, the pressure gradient term is extension. This fact, together with the study of Chapman drastically reduced to the scale of advection and and Lentz (1994), prompt us to investigate the effects of vertical viscosity, and for waters shallower than 25 m, the upstream current from RIS and the deep thermal the pressure gradient direction changes, generating fronts. the surface onshore transport. The further addition of surface heating from HeatW to HeatS generates a e. Plume response to surface heating stronger, surface downshelf pressure gradient and Chapman and Lentz (1994) paid attention to the therefore a stronger surface onshore transport. For all upstream flow’s impacts on the evolution of the buoy- three experiments, at the bottom, the dominant bal- ant plume, which is highly seasonally variable in BIS ance is the bottom Ekman dynamics, which generates and worthy of note. According to current observations the bottom onshore transport. at a mooring site to the south of Block Island (referred In fact, the across-shelf velocity structure is highly to as MD-S), Ullman and Codiga (2010) found strong dependent on the location chosen. For a section closer to seasonal changes of the upstream current from RIS. the mouth of BIS, the surface onshore transport disap- During spring and summer, the monthly mean subtidal pears; compared with RivTides, a stronger offshore 2 current reaches to 25 cm s 1, while in fall and winter it transport is formed. The difference may be due to the 2 is 5–10 cm s 1. change of the along-shelf pressure gradient with its rel- The seasonal change of upstream current from RIS ative position to the buoyant bulge structure and with and the deep thermal fronts are both related to the the topography along the coast. variation in surface heating. By artificially changing In the across-shelf direction, the dominant balance is shortwave radiation in idealized experiments, we find geostrophic for RivTides (Fig. 11b), HeatW (Fig. 19b), that the increase in surface heating forms a stronger and HeatS (Fig. 20b). The increase of surface heating

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FIG. 16. Buoyant plume’s vertical structure along section 4 in Fig. 1. Alongshelf velocity for (a) WndOn, (b) WndOff, (c) WndUp, and (d) WndDown; across-shelf velocities for (e) WndOn, (f) WndOff, (g) WndUp, and (h) WndDown. from RivTides to HeatW to HeatS has strengthened the of currents and plume to surface heating. Following pressure gradient. Csanady (1978) and Ullman and Codiga (2004),the As discussed above, the changes in pressure gradients pressure gradients can be decomposed into the part

[2(1/r0)(›P/›x), 2(1/r0)(›P/›y)] dominate the response arising from density gradients (baroclinic component)

Unauthenticated | Downloaded 09/24/21 01:10 PM UTC MAY 2016 L I U E T A L . 1649 and the part from sea surface elevation gradients (barotropic component): ð 1 ›P › z ›z 2 5 g « dz0 2 g , and (1) r ›x ›x ›x ð0 ›P › z ›z 21 5 « 0 2 r › g › dz g › , (2) y y 0 y where P is the pressure, z is the free-surface elevation, and r is the density relating to a reference density anomaly r0 and a dimensionless density anomaly « fr 5 r0[1 1 «(x, y, z, t)]g. The contributions from the barotropic and baroclinic parts for the along-shelf direction are shown in Fig. 21 as solid and dashed lines, respectively. In the along-shelf direction, the pressure gradient arising from the baroclinic part al- ways offsets the effects through the changes in sea surface height. Still, as discussed before, the change of the along-shelf pressure gradient is highly dependent on the location chosen. For a location closer to the mouth of BIS, the effects from barotropic and baroclinic parts change significantly. In the across-shelf direction, the pressure gradient FIG. 17. Surface salinity for the cases with (a) weak surface heating (HeatW) and (b) strong surface heating (HeatS) after also finds opposite roles played by the barotropic 2 months of simulation. and baroclinic components; the sea level variation produces a stronger onshore pressure gradient, while the density gradient offsets the effect and contrib- 5. Discussion utes to a stronger offshore pressure gradient that is Freshwater delivery is quantified by a freshwater flux responsible for the stronger downshelf current in the that is defined as experiments with stronger surface heating. Further ð ð examination finds that the density gradient is arising l z e S 2 S from temperature, more specifically, from the for- F 5 a udzdl, (3) l 2h S mation of a thermal front. Thus, surface heating s a controls the BIS plume by way of an upstream current and a bottom thermal front, with the effect by where Sa represents ambient salinity, S is salinity, and the bottom thermal front dominated in the across- u represents velocity across the section. The flux is shelf velocity. integrated over the whole depth from the bottom 2h to The changes in vertical velocity structures from the sea surface displacement z over the length of the

RivTides to HeatS are accompanied by a steeper and domain starting with ls and ending with le. narrower plume over the whole depth; the surface edge of Figure 22 shows the monthly mean river input the plume (isohaline of 34.4 psu) shoals from 42 km off- into LIS (black line), and the mean freshwater de- shore in RivTides to 22 km in HeatW to 18 km in HeatS. livery out of LIS (blue line) and out of BIS into the This can be explained by the increase in the along-shelf estuarine plume (red line) across sections 2 and 3 in freshwater transport [u(›S/›x)] because of the increasing Fig. 1 as well as into RIS through the gap between u, which suppresses the buoyancy’s offshore spreading. Block Island and Point Judith (green line) for the According to the above analysis, we can conclude that, in case Real. addition to the seasonally varying wind and buoyancy According to the model results, the seasonal vari- discharge, surface heating in spring and summer plays an ability of the freshwater delivery into the BIS estua- important role in the buoyant plume by steepening the rine plume is highly correlated with the river discharge isohalines and narrowing the plume, in agreement with (r 5 0.78). In April, when river discharge peaks, the the study of Ullman and Codiga (2004) who stressed the transport is largest, with most of the freshwater importance of horizontal density gradients through moving across the mouth of LIS to the ocean and a analysis of historical hydrographic data. relatively smaller part moving westward into the

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FIG. 18. Buoyant plume’s vertical structure along section 4 in Fig. 1. Along-shelf velocity for the experiment with (a) weak surface heating (HeatW) and (b) strong surface heating (HeatS); across-shelf velocity for the experiment with (c) weak surface heating (HeatW) and (d) strong surface heating (HeatS).

middle and west parts of LIS or mixing vertically. Turbulent mixing in LIS shows strong values in From May to September, consistent with the trend of winter and fall, while much weaker in spring and river discharge, the transport through LIS decreases summer. The strong mixing in winter and fall entrains significantly. However, the freshwater transport out of more freshwater inside LIS. In spring and summer, the LIS is larger than river discharge, and the minimum weak values favor the horizontal spread. Therefore, transport out of LIS is delayed by 1 month compared although the river discharge declines significantly with the discharge. This is related to the seasonal from April to August, the weakening of vertical mix- change of turbulent mixing. ing deaccelerates the change of delivery out of LIS. Meanwhile, freshwater coming out of the interior and a. Relation between freshwater delivery and turbulent west of LIS, which is originally from the Connecticut mixing River water that moves westward into LIS before The turbulent mixing can be represented by the flowing eastward into the open ocean, also contributes vertical turbulent buoyancy fluxes B,whicharerepre- to the freshwater transport out of LIS. This can ex- sented as B 5 Krg(›r/›z), where Kr is the vertical plain why the freshwater out of LIS is larger than the salinity diffusivity, g is gravitational acceleration, and river discharge from May to August. r is potential density. Figures 23a and 23b show the Most of the freshwater entrained out of LIS flows vertically integrated turbulent mixing in LIS and BIS into the open ocean through BIS, and only a small (blue lines), respectively. To examine its relationship fraction flows into RIS (Fig. 22). Similar to the change with horizontal freshwater delivery, we remove the of freshwater delivery out of LIS, the seasonal change seasonal trend of freshwater flux from the upstream through BIS is also negatively correlated with the (green lines in Figs. 23a,b). The horizontal freshwater change of turbulent mixing in BIS (Fig. 23). There is propagation is negatively correlated with turbulent more freshwater entering BIS through LIS in April than mixing. June. However, the stronger turbulent mixing in April,

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FIG. 19. As in Fig. 11, but for the case with weak surface heating (HeatW). about 2 times that in June, restricts the propagation of transports more freshwater out of LIS than the case freshwater. free of winds. In the studied region, a typical wind 2 Even though turbulent mixing is affected by the speed, about 5 m s 1, corresponds to the Ekman layer strength of winds (O’Donnell et al. 2014), the comparison depth of 47 m (Luo et al. 2013),whichislargerthanthe of turbulent mixing (not shown here) for the experiments water depth inside LIS. So the effect of bottom friction Real and NoWind indicates the major seasonal change of steers the wind-driven flow to the right-hand side of wind turbulent mixing is not due to winds. However, winds are with an angle smaller than 458 or even in wind direction important for the freshwater delivery by the change of when the water depth is much smaller than the Ekman directions. depth. Therefore, considering the geometry, the upwelling- favorable and offshore winds deliver more freshwater out b. Relation between freshwater delivery and wind of LIS, while the onshore and downwelling-favorable directions winds suppress the delivery. Another dominant contributor to the change of After exiting LIS, the upwelling wind rapidly spreads freshwater delivery is wind. For an open ocean, the the buoyant plume eastward to the broader outer-shelf 2 maximum Ekman transport is the same for onshore, region through BIS at a rate of 785 m3 s 1 and through offshore, upwelling-favorable, and downwelling-favorable the gap between Point Judith and Block Island at a rate 3 21 winds with the same amplitude, that is, Mek 5 t/r0f (t is of 256 m s . By contrast, the offshore wind constrains the surface momentum stress and f is the Coriolis pa- most of the freshwater from LIS to the outer-shelf re- rameter), perpendicular to the direction of winds. gion directly through BIS; the freshwater flux through 2 However, the complex geometry in the studied region BIS is 608 m3 s 1 when leaving LIS at the rate of 2 restricts the exchange of freshwater between the estu- 614 m3 s 1 (Table 2). ary and the ocean. Table 2 lists freshwater delivery out The application of onshore and downwelling- of LIS and BIS for different wind directions. The favorable winds retains more freshwater in LIS by de- 2 presence of upwelling-favorable and offshore winds creasing the freshwater flux leaving LIS to 530 m3 s 1 in

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FIG. 20. As in Fig. 11, but for the case with strong surface heating (HeatS).

2 WndOn and 331 m3 s 1 in WndDown. Even though the upwelling-favorable wind rapidly spreads the drifters re- flux in WndDown is small, the wind is much more ef- leased at the mouth of LIS eastward through both RIS and fective in delivering freshwater to the BIS estuarine BIS, while the offshore wind drives all the drifters, even plume, while the onshore wind delivers more than half those closest to the northern shore of BIS, southward. of the freshwater going eastward into RIS through the Moreover, the onshore wind pushes the drifters eastward gap between Point Judith and Block Island. Note that into RIS, and in the experiment with downwelling- most of the freshwater going into RIS is entrained back favorable wind, drifters move around the mouth of LIS to BIS through south of Block Island, which explains with little chance of leaving LIS. why the plume appears to extend farther offshore under The above analysis can explain the freshwater flux onshore winds than offshore winds. change from Real to NoWind (Fig. 22). During fall and According to Fong and Geyer (2002), for a given winter, the study region experiences prevailing upwelling- latitude and background salinity, the freshwater trans- favorable and offshore winds, both of which strengthen the 0 port is a function of only g h0, the buoyancy of the freshwater advection into the buoyant plume. Therefore, 0 0 plume, where g fg 5 [(r0 2 r)/r0]gg is the reduced omitting local winds results in a weaker freshwater flux gravity based on the depth-averaged density anomaly, leaving BIS. In spring, the upwelling-favorable wind and r0 is ambient water density, and r is the depth-averaged onshore wind exert opposite influences on the freshwater density. For each case, we calculate the potential en- flux; unlike an upwelling wind, an onshore wind suppresses ergy at the coast (Table 2). The change of the buoyancy the freshwater delivery, which seems more important to the of the plume is consistent with the freshwater delivery, buoyancy transport in spring by holding more freshwater that is, larger for WndUp and WndOff and smaller for inside the estuaries. The winds in summer act similarly, WndDown and WndOn. though the amplitude is much smaller. Outside BIS, Ekman The drifter trajectories shown in Fig. 15 agree with the dynamics becomes important by changing the vertical freshwater flux in Table 2, from which we find that the structure of the buoyant plume.

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FIG. 21. Pressure gradients contributed by (solid lines) sea level variation and (dashed lines) density gradients in the (a) along-shelf direction and (b) across-shelf direction for the cases of (black) RivTides, (blue) HeatW, and (Red) HeatS.

6. Summary and spring, when the upwelling-favorable winds dominate, an intermediate plume occurs; in sum- A seasonally varying buoyant plume has been ob- mer, when BIS is subjected to weak onshore and served southwest of BIS. This study uses a numerical downwelling-favorable winds, the bottom-advected model to investigate its seasonal variability and its response to tides, winds, and surface heating. Idealized model experiments indicate that tidal mixing is required in the formation of the bottom-advected plume southwest of BIS by isolating the freshwater coming out of LIS to a deeper depth and trapping the buoyant plume in a conﬁned region. The effects of winds on the buoyant plume are examined by two process-oriented experiments and a series of idealized experiments driven by the typical winds—onshore, offshore, upwelling-favorable, and downwelling-favorable winds. Analysis of the idealized experiments reveals that the upwelling- favorable and offshore winds are effective in spreading freshwater out of LIS and BIS in the offshore direction, while the downwelling-favorable and on- shorewindstendtoresistthedeliveryoffreshwater FIG. 22. River discharge from the Connecticut River (black line) and the freshwater delivery out of LIS (blue line), out of BIS (red out of BIS and traps the plume closer to the Long line), and into RIS (green line) for the case Real. The freshwater Island coast. This explains the plume’s seasonal vari- ﬂux out of LIS (dashed blue line) and BIS (dashed red line) for the ability shown in the realistic experiment: in winter case NoWind is compared with the case Real.

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FIG. 23. (a) Correlation between the vertically integrated turbulent mixing in LIS and the horizontal freshwater propagation out of LIS without the seasonal trend from the upstream. (b) As in (a), but for BIS.

plume with steepest front and narrowest offshore the seasonal plume. According to the idealized ex- extension is generated. periments driven by the different extent of surface In addition, the seasonal variability in river dis- heating, we find that the increase of surface heating charge, mainly from the Connecticut River, plays an eventually produces a plume with a narrower offshore important role in the plume’s width and strength. When extension both at the surface and bottom. Mean- river discharge peaks in spring, the plume is widest and while, downshelf transport across the front becomes has a very small frontal slope of about 0.001. The de- stronger, which is mainly due to variations in density crease in river discharge from spring to summer con- gradients. Thus, in spring, the growing surface heat- tributes to the narrower and weaker plume. However, ing increases the upstream current from RIS and the river discharge’s impact on the offshore edge at the forms a bottom thermal front, which tends to compete bottom and the along-shelf and across-shelf velocities with the spring discharge peak by shoaling the bottom across the salinity front is limited. Meanwhile, the offshore edge of the plume and limiting the surface study finds that the seasonal fluctuation of the fresh- spreading of buoyancy. In summer, the narrowest and water delivery out of LIS and BIS is controlled by the shallowest plume occurs because of a combination seasonal change of turbulent mixing. In winter and fall, of several physical processes. First, the smallest river when turbulent mixing is strong, more freshwater is discharge limits the freshwater transport into the retained by the increase of vertical mixing; in spring plume. Second, the appearance of weak onshore and and summer, the decrease of turbulent mixing favors downwelling-favorable winds tends to push the sur- the horizontal propagation of freshwater. face plume onshore. Third, the strong surface heating This study also demonstrates that the seasonal varia- pushes the bottom offshore edge of the plume to an tion of surface heating is a competitive contributor to even shallower depth and narrower band.

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