1. Correlation between cross-valley winds and along-valley velocity
Freshwater flow along the Hudson Shelf Valley: Do fish in the Mid-Atlantic Bight really care?
PO45K-08
Josh Kohut1, John Manderson2, Matt Oliver3, Laura Palamara1, Donglai Gong1
1Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ
2Ecosystem Processes Division, NEFSC/NMFS/NOAA, James J. Howard Marine Lab, Highlands, NJ
3College of Earth Ocean and Environment, University of Delaware, Lewes, DE
Background
Seasonal Winds
Spring Season
Wind and Velocity Correlation
The water column response to along-shore wind is observed throughout the spring and summer season. In both seasons and through the transition there is a significant correlation between the along-shore winds and along-valley currents (below and left). During the spring, strong downwelling (upwelling) winds initially drive the surface waters inshore (offshore) followed by offshore (onshore) flow near the bottom. Within 24 hours, this offshore (onshore) flow extends throughout the entire water column. During the summer, the stratification appears to maintain two layer flow through the weaker wind events. Summer downwelling events are similar to that seen in the spring, except that two layer flow is maintained throughout the entire event. During upwelling favorable winds the shallow surface layer is always moving offshore with a much thicker interior layer moving inshore.
The Hudson Shelf Valley is the only shelf wide topographic feature of the Mid Atlantic Bight. It originates in the New York Bight apex and reaches offshore to the Hudson Canyon near the shelf break. It has been shown that this topographic feature can drive both offshore and onshore cross shelf transport throughout the water column. Long term averages of surface currents from a long range HF radar network show that there is a consistent jet of surface water that follows the axis south of the valley to at least the mid-shelf before turning right and heading down shelf (left). This jet is seen over time scales ranging from hours to years.
Spring
Summer
Downwelling
HSV
Correlation between cross-valley winds and along-valley velocity
Spring Summer
m/s
Mean Surface Current
( )
cm/s
This study will focus on the ocean response to local forcing during the transition from the spring to summer season in the central Mid Atlantic Bight. In addition to strengthening stratification driven by longer days and weaker surface winds, there is increased freshwater flow into the coastal ocean fed by heightened discharge from the Hudson River estuary (below). This spring freshet has been seen to advect both along the New Jersey coast and offshore along the south flank of the Hudson Shelf Valley (HSV). During our study period the freshwater input is consistent with the 27 year mean with slightly stronger discharge May through November (below).
The wind and ocean current data were binned into spring (April 6th through May 31st, 2009) and summer (June 1st through July 27th, 2009) seasons. The wind roses for the spring and summer season show that for both the winds are predominately cross-valley (along shore) with stronger winds in the spring.
Along Valley Velocity (cm/s)
Seasonal Mean Currents
In both the spring and summer seasons the positive correlation near surface and negative correlation near bottom at zero lag is consistent with an Ekman response to along-shore winds. The spring season is characterized by a deeper surface layer and the most negative correlation. This strongest negative correlation to the along-shore wind occurs at depth and is centered at a 12 hour lag. In the summer, the surface layer is shallower and more correlated with the surface winds. The interior is less correlated with the wind and shows less of a dependency on the lag. This flow on the south flank of the HSV is consistent with previous results from within the main axis of the valley in which the along shore winds drive surface waters toward (away from) the coast building a pressure gradient that drives the interior flow in the opposite direction. -5 -9
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Upwelling
Spring
Summer
Hudson River Discharge
Along-Valley (Cross-shore): The mean along-valley current is strongest near the surface with a second mid-water column peak near 25 meters. The summer current mean is larger throughout most of the water column except near the bottom, deeper than 30 meters.
Cross-Valley (Along-shore): The cross-valley flow is consistent with the well-documented along-shore flow toward the southwest seen in the MAB. While the entire summer mean is larger than that seen during the spring, the largest difference is in the surface layer.
Depth (m)
Study Period
Along-Valley Velocity (cm/s)
Objectives
Fisheries Implications -5 -9
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Spring
Summer
Recent work funded through the NOAA Fisheries And The Environment (FATE) program is developing ecological indicators based on available environmental data. Through Generalized Additive Models (GAMs), environmental variables are used to predict the distribution of target species. In the example shown below, squid distribution in the New York Bight was predicted based in part on surface divergence calculated from HF radar current maps. The initial results indicate that more squid are found in regions with more consistent upwelling (positive divergence trend). These results can then be mapped to the environmental data to show where squid are more likely to be
Use the remote sensed surface current fields to guide the deployment of a
bottom mounted ADCP to describe the subsurface velocity structure south of the main axis of the HSV.
Describe the correlation between local surface winds and the depth dependent along valley flow.
Identify potential fishery implications of the flow fields south of the HSV.
Depth (m)
Along Valley Velocity (cm/s)
Summer Season
Cross-Valley Velocity (cm/s)
Downwelling
Along-Valley Current Response
Data
Since the local wind forcing is predominately along shore through the spring to summer transition, we will concentrate on the ocean response to that component. Previous work has shown that within the valley there is a significant relationship between these along shore winds and flow along the valley. The surface current data shown in the background section of this poster identifies the region just south of the HSV characterized by cross-shelf flow. Using these fields as guidance, the moored ADCP was deployed within the main axis of this flow and used to observe its structure below the surface.
Surface Winds: Meteorological measurements were obtained from the NOAA National Data Buoy Center (NDBC). Buoy number is located 30 km northeast of the HSV (green circle below). Hourly surface winds were low-pass filtered with a 30 hour cutoff to match the ADCP measurements described below.
Sub-Surface Currents: Sub-surface currents were measured with a Teledyne RD 300 KHz Acoustic Doppler Current Profiler (ADCP) deployed in 42 m of water, 5 km south of the main axis of the HSV (red dot below). The bottom mounted instrument was configured to burst sample for 10 minutes each hour. The hourly velocity data collected between April 6 and July 27, 2009 were rotated into an along-valley coordinate system. The frequency spectrum of the raw along-valley current shows energy in the semi-diurnal, diurnal, near-inertial and sub-tidal frequency bands (inset below). The reminder of this analysis will focus on the sub-tidal band obtained with the same 30 hour lowpass filter applied to the surface winds.
found. The map above shows divergence trend during the spring season. Regions in which the surface ocean tends to be divergent (upwelling) are shown in red and regions in which the surface ocean tends to be convergent are shown in blue (downwelling). Putting the two plots together, the squid would likely avoid the region south of the HSV and concentrate further north and offshore. 2 1 -1 -2 -3 -4
Divergence Trend 15 10 5 -5
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Spring
Summer
Upwelling 41 40 39
Summary
Downwelling
Along Valley Velocity (cm/s)
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Along-shore winds through the late spring and early summer seasons on the MAB drive cross-shore currents along the south flank of the Hudson Shelf Valley.
The episodic wind events appear to drive a setup/setdown mechanism in which the surface flow directed toward or away from the coast is compensated with a return flow in the interior.
Ecological studies indicate that the flow associated with the Hudson Shelf Valley may define favorable or unfavorable habitat for certain species.
Upwelling
NDBC Buoy 44025
- Hourly surface winds
- April 6, 2009 to July 27, 2009
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Time (MM/DD)
300 KHz ADCP
- Hourly sub-surface currents
April 6, 2009 to July 27, 2009
2 m vertical resolution
cm/s
Time series of the alongshore component of the wind and the vertical time series of the sub-surface currents are shown above. The vertical black line separates the spring and summer seasons. The variability in both the winds and currents are clearly seen in both plots. There is a strong episodic forcing/response relationship between the along valley currents and the along shore winds. The winds show a frequent shift from upwelling (positive) and downwelling (negative) favorable directions. In order to show the variability in the ocean response to these events, the next column shows four different events in the time series, upwelling (green circles) and downwelling (red circles) events in both the spring and summer seasons. Each plot shows a three day window of the winds and ADCP currents. In all plots, positive velocities are directed offshore toward the shelf break and negative velocities are directed onshore toward the coast.
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Data presented in this study was supported in part by the National Science Foundation (NSF), the Integrated Ocean Observing System (IOOS), FATE (NOAA) and Rutgers, The State University of New Jersey. The authors would also like to thank the great crew of the R/V Hugh Sharp for their professionalism and help during the deployment of the ADCP.
Depth (m)
Frequency (cpd)
Along Valley Velocity (cm/s)
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