SUB-TIDAL VARIABILITY IN THE HUDSON RIVER PLUME AS A RESULT OF HIGH FREQUENCY FORCING #543 Hunter, E.J., Rutgers University, Chant, R.J., Rutgers University, Wilkin, J., Rutgers University Abstract Observations of Sea/Land Breeze Variability Ocean Model Model cases During the course of Lagrangian Transport and Transformation Experiment (LaTTE) in 2004, 2005, and 2006, the structure of the Hudson River plume was highly variable. Although forcing due to variations in discharge, low-frequency winds, and ambient shelf circulation are important, high-frequency forcing and sub-tidal response of the plume is apparent in the observations. Tidal mixing in the estuary manifests as fortnightly variability in plume stratification in the 2005 and 2006 mooring records. Diurnal wind variability related to the sea-land breeze system (SLBS), while episodic, accounts for up to 50% of the energy in surface currents in the New York Bight Apex at times. Regional Ocean Modeling System (ROMS) simulations demonstrate a subtidal response to this forcing. Spring-neap variability in tidal mixing modifies the estuary outflow Rossby and Froude numbers, resulting in increased freshwater transport in the coastal current during spring tides with lower freshwater transport during neap tides. SLBS variability resulted in greater storage of fluid in the bulge region of the Hudson River plume and freshwater transport in the coastal current as low as 30 percent of the total river discharge. Fig. 5) Diurnal period rotary ellipses overlaid major axis magnitude derived from CODAR velocities. Note the increase in diurnal energy in b). a) February-March b) April-May Grid ~1 km horizontal resolution. 30 vertically stretched levels. Model setup Mellor and Yamada level 2.5 turbulence closure. Orlanski-type radiation Conditions. Tidal harmonics specified at the open boundary. Initial Conditions Constant temperature (5 degrees C). Constant Salinity (32 PPT). Quiescent fluid. Forcing No surface heat flux. Zero wind stress, idealized diurnal wind. Constant discharge (500 m^3/s,5 C,0 PPT) # M2 Tidal Range Wind Regime 1 Decreased No wind 2 Normal 3 Increased 4 NS SLBS 5 6 7 Mixed No Wind Fig. 9) ROMS output. Representative surface salinity and surface velocity for a) No wind, normal M2 tide and b) SLBS, normal M2 tide. The Regional Ocean Modeling System (ROMS; information online at http://www.myroms.org) is a three-dimensional, free-surface, hydrostatic, split-explicit, primitive-equation ocean model. Background Fig. 1) LaTTE study area. Fig. 2) Idealized Surfaced-Advected River Plume. Model Results Fig. 10) ROMS output. Along estuary velocity at the Hudson River mouth (fig. 1, line B) with salinity contours for model cases 1-6. Fig. 11) ROMS output. Along estuary velocity w/ salinity contours for model cases. a) Case 1 c) Case 3 e) Case 5 b) Case 2 d) Case 4 f) Case 6 There is a dramatic increase in diurnal energy near the mouth of the Hudson river beginning in April. The diurnal major axis doubles with the onset of the SLBS. (Hunter et al., 2007) Estuary Mouth Results Fig. 6) Time series of diurnal wind energy at Ambrose Tower and the percent of total energy in the diurnal band in the CODAR domain shown in fig. 5). Grey areas show sea-breeze days determined independently. The objective of the Lagrangian Transport and Transformation Experiment (LaTTE) is to characterize the evolution of physical, chemical and biological properties of the Hudson River Plume (Fig. 1). The Hudson River plume is typically considered to be a surfaced-advected plume such as that shown in Fig. 2). (Yankovsky and Chapman, 1997) The onset of the SLBS in April results in enhanced diurnal band energy in the coastal ocean near the Hudson river mouth. As much as 50% of the total kinetic energy (at times) in the surface water is the result of SLBS energy. (Hunter et al., 2007) Parameters of the Hudson River outflow vary with tidal range. Increased tidal range leads to increased outflow depth, decreased outflow Rossby number and increased cross channel asymmetry. With the exception of decreased tidal range cases, the SLBS has little effect on outflow. Fig 3. MODIS images of Chlorophyll concentration highlighting the Hudson River Plume in 2004,2005,and 2006. Drifter tracks from the three field seasons are in Magenta. Fig. 12) ROMS output. Along shelf velocity in the coastal current (fig. 1, line B) with salinity contours for model cases 1-6. Fig. 13) ROMS output. Freshwater flux in the coastal current as fraction of river discharge for the a) no wind cases and b) SLBS cases. a) Case 1 c) Case 3 e) Case 5 b) Case 2 d) Case 4 f) Case 6 Coastal Current Results River plumes in nature are rarely as well defined as Fig. 2). Fig. 3) illustrates this variability with three realizations of the Hudson river plume. It is well-established that low-frequency wind, river discharge, and ambient shelf circulation contribute to plume variability. The impact of forcing such as tides and high frequency winds on low-frequency plume variability is less understood. The coastal current is dramatically modified by the onset of the SLBS. Although the coastal current is wider and deeper during SLBS events (fig. 12), there is a decrease in freshwater transport (fig. 13) in the SLBS cases, suggesting enhanced storage of freshwater in the recirculating bulge region due to the SLBS. This is evident in the surface signature of the plume (fig. 14) and consistent with Chant et al. 2008. Observations of Fortnightly Variability Fig. 7) a) Time series of CODAR radial velocity along a single range cell (Fig. 1) positive is away from the Hudson river mouth. North-South wind (positive north) is overlayed. b) Sea level height at Sandy Hook. Fig. 14) The 31.5 ‰ salinity contour for model day 12 (a-c) and model day 40 (d-f). a) Day 12, Decreased M2 c) Day 12, Increased M2 e) Day 40, Normal M2 b) Day 12, Normal M2 d) Day 40, Decreased M2 f) Day 40, Increased M2 One net effect of the SLBS on the Hudson River outflow is the redirection of the outflow at each ebb tide. When on the northerly phase of the SLBS the ebb moves along long island and while on the southerly phases it moves into the coastal current. Conclusions a) b) SLBS variability and, to a lesser extent, spring-neap variability are clear signals in the LaTTE dataset. Changes in mixing over the spring-neap cycle in the estuary modify parameters of the outflow (depth, velocity, etc.) and hence the development of the plume. Spring tides tend to decreases stratification and increased freshwater transport in the coastal current and vice versa for neap tides. The SLBS acts directly on the Hudson river plume through both advective momentum flux and enhanced mixing . Freshwater transport in the coastal current decreases during SLBS events, with freshwater flux dropping to <40% of river transport. The SLBS combined with the geometry of the Hudson River mouth provides a secondary freshwater pathway along Long Island. Plume Results Fig. 4) Time series of low-pass filtered stratification at mooring locations in Fig. 1 along with tidal range at Sandy Hook. References: Hunter, E., R. Chant, L. Bowers, S. Glenn, and J. Kohut (2007), Spatial and temporal variability of diurnal wind forcing in the coastal ocean, Geophysical Research Letters, 34, L03607, doi:10.1029/2006GL028945. Yankovsky, A. E., and D. C. Chapman (1997), A simple theory for the fate of buoyant coastal discharges, Journal of Physical Oceanography, 27, 1386-1401. Chant, R. J., S. M. Glenn, E. Hunter, J. Kohut, R. F. Chen, R. W. Houghton, J. Bosch, and O. Schofield (2008), Bulge Formation of a Buoyant River Outflow, Journal Geophysical Research 113, C01017, doi:10.1029/2007JC004100. Fig. 8) Climatology of a) Hudson River discharge and SLBS frequency and b) sea surface temperature in April. The mooring deployments in 2005 and 2006 show spring-neap variability in the low frequency stratification time series at all mooring locations. Tidal currents outside of the estuary are small, suggesting fortnightly mixing variability in the estuary is related to stratification in the plume. The Spring/Neap variability in 2005 is somewhat obscured by a large discharge event in early April. Climatologies of river discharge, SLBS, and SST show the onset of the SLBS with increased river discharge and plume signature.