This document summarizes a study examining the effects of wind conditions and seasonal stratification on heat and salt flux across the Mid-Atlantic Bight shelf-break using an ocean circulation model. The study analyzed model outputs of velocity, temperature, salinity, and calculated heat and salt flux. Results showed higher heat and salt transport in the winter mixed period compared to the stratified summer. Transport was also higher during periods of strong upwelling and downwelling winds compared to calm winds. The research aims to improve understanding of cross-shelf exchange processes in this region.
1. Exchanges of Heat and Salt Across
the Mid Atlantic Bight Shelf-Break
from ROMS ESPRESSO Model
James Brown
Bates College
Mentors: Daniel Wang, Dr. Donglai Gong
4. Research Questions
• “How can different wind conditions affect the
salt and heat flux across the MAB shelf-
break?”
• “How can seasonal stratification affect the salt
and heat flux across the MAB shelf-break?”
Well mixed Stratified Upwelling Downwelling
6. Methods
Heat flux = velocity * temperature * density of water * specific heat of water
Salt flux = velocity * [(Measured Salinity – Reference Salinity) / Reference Salinity]
7. Seasonal averages of velocity (m/s) distribution
v – negative values are in the offshore direction while positive values are in the onshore direction
Depth(m)
Distance along 100m isobath (km)
Winter
Spring
Summer
Fall
S-N
8. Seasonal averages of temperature (° C) distribution
Depth(m)
Distance along 100m isobath (km)
Winter
Spring
Summer
Fall
S-N
10. Seasonal average heat flux (J/m2*s) distribution
Positive values are in the onshore direction and negative values are in the offshore direction
Depth(m)
Distance along 100m isobath (km)
Winter
Spring
Summer
Fall
S-N
11. Seasonal averages of salt flux (kL/m2*s) distribution
Positive values are in the onshore direction and negative values are in the offshore direction
Depth(m)
Distance along 100m isobath (km)
Winter
Spring
Summer
Fall
S-N
12. Wind Distribution of middle Transect
Positive u values and negative v values correlate to periods of upwelling. Negative u values
and positive v values correlate to periods of downwelling.
13. Transport and Wind data
-4.00E+06
-2.00E+06
0.00E+00
2.00E+06
4.00E+06
6.00E+06
8.00E+06
1.00E+07
1.20E+07
Winter Spring Summer Fall
Salt Transport
StrongUpwelling (m^3/s)
StrongDownwelling
(m^3/s)
CalmWinds (m^3/s)
-2.00E+15
-1.50E+15
-1.00E+15
-5.00E+14
0.00E+00
5.00E+14
1.00E+15
1.50E+15
2.00E+15
Winter Spring Summer Fall
Heat Transport
StrongUpwelling (J/s)
StrongDownwelling (J/s)
CalmWinds (J/s)
14. Transport and seasons data
HeatTransport(J/s) SaltTransport (kL/s)
Winter(mixed) 1.83E+15 -6.20E+06
Summer(Stratified) -5.97E+15 -1.60E+07
16. Acknowledgements
• I want to express my deepest gratitude to Daniel Wang,
who guided me through my project throughout the
summer, and Dr. Donglai Gong, who gave me the
opportunity to work in his lab and provided great oversight.
• Thanks to Jenny Dreyer and Melissa Karp for coordinating
the REU program at VIMS.
• Program funding for the VIMS Research Experience for
Undergraduates was made available through a grant
awarded from the National Science Foundation (grant #
NSF OCE 1062882) to Drs. Linda Schaffner and Rochelle
Seitz.
Editor's Notes
The Mid Atlantic Bight aka MAB region, is a coastal region off the US eat coast extending from Nantucketshoals, NE to Cape Hatteras NC
Water is fresher and colder between the shelf break and coast and warmer and saltier between the shelf-break and open ocean
Studying the physics of this region is important bc of use as a food supply, rec. use, industrial use, and its effect on atmospheric weather and climate
For the big picture, this research is important because of its potential use in the study of fish seasonal distribution. For ex. The black sea bass commonly found in the MAB may tolerate temperatures as low as 6 C but are captured more frequently in waters 9 C and above.
Upwelling (when deep cold water flows towards the surface), downwelling (when denser water sinks)
The proposed study investigated heat and salt flux as well as variability in the circulation in the shelf break region of the middle MAB region from Dec, 1 2012 to Nov, 30 2013
Describe plot – Analyzing region between white line along 100m isobath and the northern and southern lines in the cross shelf direction(middle MAB region) In the ocean there 3 components of the velocity of the water, but the important two is the u component in the along shelf direction and the v component in the cross shelf direction (which is more important to me in my calculations)
The ROMS ESPRESSO model covers the entire MAB region from coast to continental slope.
It is initialized by climatology data and it assimilates best available observational data such as satellite data. The model then outputs hydrographical data such as temperature, velocities, salinity, and density.
The spatial coverage and the multi-parameter outputs enables the model to provide insight on hydrography, dynamics, heat and salt budgets on daily to seasonal time scales, which enables us to investigate and understand the circulation and shelf slope exchange of heat and salt in the MAB
Ultimately I analyzed heat and salt flux, but first I needed to understand the components. Heat flux is calculated from multiplying velocity, temperature, density of water, and specific heat of water. Velocity has great influence bc it’s the only quantity that can be negative so it solely determines direction. Salt flux is calculated by subtracting a reference salinity from a measured salinity (which is typically 34.5 psu in the MAB)and then dividing by a reference salinity all times velocity. Here salinity and velocity play a role in determining direction.
Some orientation details, these are cross sections along the 100m isobath, where the left side is a few hundred meters off the coast of the eastern shore and right side is a few hundred meters off the coast of NY. These hold throughout all the seasonal average plots
Negative values of velocity are in blue color and flow in the offshore direction, positive values are in red/yellow in the onshore direction
From the plots we can see the water is typically flowing in the offshore directions (especially on the surface layer) with some water flowing in the onshore direction in the subsurface layers
Red is warm and blue is toward zero. The winter is well mixed, spring is well mixed but cooler, summer the surface layers warms, water is stratified, and thermocline depth is around 50m, the fall average is stratified as well but the thermocline depth has deepened (Mention where black sea bass would be found)
Red is saltier, while blue is fresher. Fresher in the north is partially due to runoff from the hudson, and saltier water in the south is partially from the gulf stream. Along shelf gradients is observed in most season averages except summer where the water is well stratified and fresher water is on the surface layer. This is partially caused by less mixing and less wind power in the summer, and that the denser water is more saline
Speak about components again.
Distribution is very similar to velocity plots caused by heavy influence of velocity
Warmer color is flow onshore and colder color is flow offshore, so heat is primarily flowing off the shelf across the shelfbreak
Similar to velocity plots but sort of like a negative. Warmer colors are onto the shelf and colder colors are off the shelf, so we can see the salt primarily flows onshore (especially the surface layer) with some offshore flow in the subsurface layer. Also heavily effected by velocity but salinity plays a role in direction of flow
Wind distribution plots created from a cross section in the cross shelf direction of the middle MAB. The top plot is the u component and the bottom plot is the v component. Warmer colors represent positive values of the components and cooler colors represent negative values of the components. Positive u and negative v correlate or winds from the SW to periods of upwelling, negative u and positive v or wind from the NE correlate to periods of downwelling, and when u and v are close to zero or green correlate to periods of calm winds.
Not any meaningful correlation between salt transport and different wind conditions, except all periods of upwelling have a net flow of salt off the shelf
Again not any meaningful correlation between heat transport and different wind conditions, except all periods of calm wind have a net flow of heat onto the shelf.
A greater net flow of salt and heat is observed from during the stratified summer then the well mixed winter. From these results I concluded that water column structure effects flow more than different wind conditions. However to improve the results, perhaps monthly averages would be better than seasonal averages and also more periods of upwelling, downwelling, and calm winds.
My mentor Daniel is researching the shelf break region around the NC and WC which is south of the middle MAB. He deployed a glider there for a few weeks and the model greatly compensates the glider survey, which spatially and temporally has a much smaller coverage but higher resolution, roms provides necessary regional and seasonal background oceanographic conditions for analyzing results from the survey. Again big picture, hopefully Daniel’s research can be used in fisheries studies of the region he is studying
Previous research on areas of the MAB region shows thermal structure of the water is influenced by seasonal stratification and that shelf water cools over the winter and spring, the surface warms in the summer but the bottom remains cool, and in the fall the surface layer mixes deep into the water column (Houghton et al. 1987).
There’s also research on the 100m isobath that there is onshore flow of salt flux and offshore flow of heat flux (Lentz 2010).
