Statement of Research Interests: Dr. Robin Robertson
Investigating Mixing, Tides, and their Influences using Models and Observations
Dr. Robin Robertson
Mixing processes, particularly vertical mixing mechanisms, are the focus of my research.
Recently, several researchers have proposed that tides are an important mixing process [Munk
and Wunsch, 1998], particularly in the polar regions [Muench et al., 2002]. Vertical mixing has
been linked to tides both in the deep ocean [Polzin et al., 1997] and over ridges and the
continental slopes [Munk and Wunsch, 1998]. Much of my recent research has been on tides and
tidal mixing. I have done some of the pioneering research in modeling tides in the Weddell and
Ross Seas, including adding capability for ice shelves into the models.
Basic areas of research areas, which I am or plan to be involved in, are:
1. Internal tides and tidal mixing and their influence on AABW formation, upper-ocean heat
fluxes, and sea ice
2. Incorporation of tides into a global climate model
3. Relationship between tides and the continental slopes
4. Vertical mixing in the deep ocean
5. Vertical mixing in the polar oceans
6. Changes in the deep waters in the Weddell Sea, such as warming or freshening
7. Tidal mixing in rivers, such as the Hudson River
Below, I have described some of the specific research projects for these topics.
1. Internal tides and tidal mixing …:
Tidal mixing is believed to play an important role in the Antarctic Bottom Water (AABW)
formation, upper ocean heat transport, lead formation in the pack ice, and the ocean-atmosphere
heat transfer [Muench et al., 2001; Robertson, 2001]. I modeled the internal tides for the Ross
Sea using the Regional Ocean Modeling System (ROMS) as a NSF International Research Fellow
at the Alfred-Wegener-Institut in Bremerhaven, Germany. A paper on the baroclinic M2 tidal
dynamics for the Ross Sea will be published in a special issue of Antarctic Science. I determined
that ROMS was more suitable for tidal modeling than the Princeton Ocean Model (POM),
because of its more accurate representation of the density field and implementation of the
baroclinic pressure gradient. Use of ROMS gives me a distinct advantage over most of the other
tidal modelers, since nearly all are using the original version of POM, which is unsuitable for
modeling tides near the critical latitudes [Robertson, 1999; Robertson et al., 2001].
I presently have a three-year 6 mos./yr NSF OPP grant to investigate internal tides and tidal
mixing in the Weddell and Ross Seas. First, I plan to improve the topography and hydrography
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Statement of Research Interests: Dr. Robin Robertson
input fields and further investigate the effects of the different mixing schemes – Mellor-Yamada
2.5, Large-McWilliams-Doney, Pacanowski-Philander, etc. I will then model the internal tides
for the entire Ross and Weddell Seas separately on ~ 5-km resolution grids using ROMS. Both
single and multiple constituent simulations will be performed. With the results, I will investigate
the tidal effects on vertical mixing, upper ocean heat fluxes, AABW formation, Ice Shelf Water
formation, and divergence of the surface currents.
Tidal mixing may affect the available nutrients for biology, movement of the zoo- and
phytoplankton, and sequestration of carbon. A modeling study done at the Alfred-WegenerInstitut showed that tides played a significant role in the distribution of zoo- and phytoplankton
over Maud Rise [Beckmann, personal communication]. Other than this preliminary study, tides
have been ignored in the biological models. ROMS has the capability for modeling multiple
biological tracers using the Fasham biological algorithm. In the future, I plan to add biological
tracers and to collaborate with biologists to address the tidal effects on biology and carbon
sequestration.
Tides have been found to play a significant role in ice thickness and ice formation and
breakup along the continental shelf break in the Weddell Sea [Koentopp and Kottmeier, personal
communication]. At the time, only barotropic tidal velocities were available for their study. My
earlier research has shown that over the continental slope the divergence of baroclinic surface
velocities is roughly an order of magnitude greater than that of the barotropic velocities. I expect
the tidal effects will be more pronounced when the baroclinic velocities are used. A first step in
addressing the tidal effects on the ice pack would be to repeat their study using my baroclinic
velocities. We have discussed collaboration on this effort. A second step would be to couple
ROMS to an ice model. I am presently leading a group (B. Tremblay, E. Curchitser, and X.
Yuan), which is submitting a proposal to couple Tremblay’s ice model with ROMS (submitting
in June 2002 to NSF-OPP). This would provide LDEO with a unique capability, since it would
have the only coupled ice-ocean free surface model.
In the future, I plan on modeling internal tides for other regions, such as the Indonesian Sea,
the South China Sea, the Greenland Sea, and the Amery Sea. Strong baroclinic tides have been
observed in the Indonesian Seas and are believed to be an important mixing mechanism there [A.
Ffield, personal communication]. A. Ffield and I are collaborating on observational and
modeling efforts and have submitted a letter of intent to ONR to investigate internal tides in the
Indonesian Seas. We will collect observational data in order to analyze vertical mixing and the
baroclinic tides in this region and model the tides for this region using ROMS. The observational
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Statement of Research Interests: Dr. Robin Robertson
data will then be used to verify and evaluate the model results and the model results will be used
to indicate probable areas of high mixing for observational efforts.
Similarly I have submitted a letter of intent to ONR with T. Duda of WHOI to model internal
tides in the South China Sea in support of the AsiaEx program. This program has an extensive
observational program but no modeling component. Extremely large tides have been observed in
this region and they are interested in using model results to supplement the evaluation of the
observational data, particularly with respect to generation and propagation of internal tides.
Concerning the Amery Sea, an Australian group has contacted me about collaborating with
them. Originally, they offered me a year-long, fully funded position to help them implement tides
in their models and/or switch to using ROMS. I declined the offer in order to return to LDEO,
but expect to collaborate with them in the future, with them providing some funding for me.
2. Incorporation of tides into a global climate model:
Presently, global climate models ignore tidal effects. In order to address this inadequacy, I
am collaborating on a proposal with Gary Russell of GISS and Jim Miller of Rutgers University
to incorporate tides into their global climate model. They are providing the expertise on the
global model and I am providing the expertise on tides. I plan to implement tides in a unique
manner by modifying the treatment of gravity for the two-dimensional portion of the model. This
proposal is in preparation and should be submitted to NSF-OCE in August 2002.
3. Relationship between tides and the continental slopes ….:
I am collaborating with Mike Steckler in evaluating the relationship between tides and the
steepness of the continental slopes. The basic premise is that the slope of the continental slope
often coincides with the angle of the internal wave rays for the tides and that internal tides
through inducing sediment transport or slumping affect the steepness of the continental slope.
We are looking at the Ross and Weddell Seas because of the presence of the M2 critical latitude
and the different relations it has with the slope (equatorward or poleward).
4. Vertical mixing in the deep ocean:
Recently, Polzin et al. [1997] have determined that much of the deep ocean mixing occurs
over rough topography and have postulated that it is linked to tides. My earlier investigations of
tidal dynamics showed that the critical latitude greatly influenced the baroclinic tides particularly
in areas of weak stratification, such as the deep ocean. I have submitted a proposal to investigate
baroclinic tide generation, propagation, and mixing over ridges and the continental shelf/slope
break and the effects of the critical latitude. The goal of this work would be to develop
parameterizations for the tidal effects on mid-water column mixing, benthic boundary layer
thickness, and benthic mixing similar to that developed by Jayne and St.Laurent [2001]. The new
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Statement of Research Interests: Dr. Robin Robertson
parameterization would be based partially on the existing, limited data set, but primarily on the
model results. The parameterization would be dependent on topographic roughness and proximity
to the critical latitude.
5. Vertical mixing in the polar oceans:
The Ross Sea is presently the focus of a large group effort focusing on investigating mixing
and the role of the Antarctic Slope front, the AnSlope project. I expect to participate in both the
research cruise and the data analysis, because of my previous involvement with this group and
discussions with A Gordon. As part of my PhD research, I investigated the contribution of
several mixing mechanisms, shear instabilities, double diffusion, salt fingering, intrusions, on
upper ocean heat fluxes in the Weddell Sea. I plan to apply these technique to this new
observational data on a fine vertical scale in order to investigate the relative importance of these
mechanisms. Furthermore, my tidal modeling results for the Ross Sea will provide information
on the locations of expected baroclinic tidal mixing. I will also use the new observations to
improve the hydrographic inputs into future modeling efforts and to evaluate whether the
baroclinic tidal model results are reflected in the observations.
6. Changes in the deep waters …:
As part of the Consortium for Oceans Role in Climate (CORC) program, I analyzed
observational data (CTD’s) to investigate changes in the Warm Deep Water and Weddell Sea
Deep Water and have an accepted paper in press [Robertson et al., 2001b]. I expect to participate
in the next field experiment [personal communication, M. Visbeck] and to quantitatively evaluate
modifications in character and outflow of these water types from the Weddell Sea using CORC
observational data (CTD”S and mooring) as it becomes available. This work is presently funded
at 3 mos./yr for the next 2 years.
7. Tidal mixing in river flows …:
I am interested in using ROMS to model the tidal effects on mixing in the Hudson River.
This should be easily implemented, since the Hudson River has already been modeled using
ROMS by Hetland [personal communication]. After development of the bathymetric and
hydrographic fields, a wide range of scenarios could be investigated. This would include
increased/decreased rainfall, blocking by bridges, etc. Once the model has been verified to be
sufficiently simulating the physical quantities, it could be used to help address some of the water
quality issues. The model is capable of simulating multiple tracers; thus, the fates/distributions of
the heavy metals, pesticides, dioxins, PCB’s and other pollutants could be investigated for various
scenarios. I expect to collaborate with the water quality group of the Hudson River Project to
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Statement of Research Interests: Dr. Robin Robertson
learn what questions need to be addressed, to determine which scenarios are important, to learn
what the behavior of the tracers is, and to compare the model results to the observations.
Jayne, S. R., and L. C. St. Laurent, Parameterizing tidal dissipation over rough topography, J.
Geophy. Res., 28, 811-814, 2001.
Muench, R. D., L. Padman, S. L. Howard, and E. Fahrbach, 2001. Upper ocean diapycnal mixing
in the northwestern Weddell Sea, Deep-Sea Res., in press.
Munk, W. and C. Wunsch, The moon and mixing: abyssal recipies II, Deep-Sea Res., 45, 19772010, 1998.
Polzin, K. L., J. M. Toole, J. R. Ledwell, and R. W. Schmidt, Spatial variability of turbulent
mixing in the abyssal ocean, Science, 276, 93-96, 1997.
Robertson, R., Mixing and heat transport mechanisms for the upper water column in the Weddell
Sea, Ph.D. Dissertation, College of Oceanic and Atmospheric Sciences, Oregon State
University, 1999.
Robertson, R., Internal tides and baroclinicity in the southern Weddell Sea: Part II: Effects of the
critical latitude and stratification, accepted J. Geophys. Res., 2001.
Robertson, R., L. Padman, and M. D. Levine, A correction to the baroclinic pressure gradient
term in the Princeton Ocean Model, J. Atmos. Ocean. Tech., 18, 1068-1075, 2001a.
Robertson, R., M. Visbeck, A. L. Gordon, and E. Fahrbach, Long-term variability of Warm Deep
Water and Weddell Sea Deep and Bottom Waters, accepted for a special issue of Deep-Sea
Research, 2001b.
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