Multiscale simulations of Langmuir cells and submesoscale eddies using XSEDE resources
L. P. Van Roekel, P. E. Hamlington, and B. Fox-Kemper. Multiscale simulations of Langmuir cells and submesoscale eddies using XSEDE resources. Extreme Science and Engineering Discovery Environment Conference 2012 (XSEDE12), 2012.
A proper treatment of upper ocean mixing is an essential part of accurate climate modeling. This problem is diffi cult... more
A proper treatment of upper ocean mixing is an essential part of accurate climate modeling. This problem is diffi cult because the upper ocean is home to many competing processes. Vertical turbulent mixing acts to unstratify the water column, while lateral submesoscale eddies attempt to stratify the column. Langmuir turbulence, which often dominates the vertical mixing, is driven by an interaction of the wind stress and surface wave (Stokes) drift, while the submesoscale eddies are driven by lateral density and velocity changes. Taken together, these processes span a large range of spatial and temporal scales. They have been studied separately via theory and modeling. It has been demonstrated that the way these scales are represented in climate models has a nontrivial impact on the global climate system. The largest impact is on upper ocean processes, which lter air-sea interactions. This interaction is especially interesting, because it is the interface between nonhydrostatic and hydrostatic, quasigeostrophic and ageostrophic, and small-scale and large-scale ocean dynamics. Previous studies have resulted in parameterizations for Langmuir turbulence and submesoscale fluxes, but these parameterizations assume that there is no interaction between these important processes. In this work we have utilized a large XSEDE allocation (9 million SUs) to perform multiscale simulations that encompass the Langmuir scale (O(10-100m)) and submesoscale eddies (O(1-10km)). One simulation includes a Stokes drift, and hence Langmuir turbulence, while the other does not.
To adequately represent such disparate spatial scales is a challenge in numerous regards. Numerical prediction algorithms must balance eciency, scalability, and accuracy. These simulations also present a large challenge for data storage and transfer. However, the results of these simulations will influence climate modeling for decades.
Ocean Salinities Reveal Strong Global Water Cycle Intensification during 1950-2000
Science, 2012
Fundamental thermodynamics and climate models suggest that dry regions will become drier and wet regions will become... more Fundamental thermodynamics and climate models suggest that dry regions will become drier and wet regions will become wetter in response to warming. Efforts to detect this long-term response in sparse surface observations of rainfall and evaporation remain ambiguous. We show that ocean salinity patterns express an identifiable fingerprint of an intensifying water cycle. Our 50-year observed global surface salinity changes, combined with changes from global climate models, present robust evidence of an intensified global water cycle at a rate of 8 ± 5% per degree of surface warming. This rate is double the response projected by current-generation climate models and suggests that a substantial (16 to 24%) intensification of the global water cycle will occur in a future 2° to 3° warmer world.
The nonlinear evolution and approximate scaling of directionally spread wave groups on deep water
with Richard Gibbs and Paul Taylor.
The seasonal appearance of ice shelf water in coastal Antarctica and its effect on sea ice growth
co-authored with Andy Mahoney
In this paper we report measurements from the first year-round mooring underneath sea ice in McMurdo Sound,... more In this paper we report measurements from the first year-round mooring underneath sea ice in McMurdo Sound, Antarctica, which we combine with full-depth ocean profiles to identify the incremental appearance of potentially supercooled ice shelf water (ISW). We investigate the effects of ISW on sea ice using observations of sea ice growth and crystal structure together with under-ice photography. We show that the appearance of ISW at the surface leads to a disruption in the columnar texture of the sea ice, but that persistent growth enhancement occurs only once the entire water column has cooled to the surface freezing point. In doing so, we demonstrate the possibility of inferring the presence of ISW beneath sea ice through crystallographic analysis of cores. These findings will be useful for both modeling and observing the extent of ISW-enhanced ice growth. In addition, we found that the local growth of first-year landfast sea ice only accounted for half of the observed increase in salinity over the water column, which indicates that polynyas are responsible for approximately half of the salt flux into McMurdo Sound.
Download (.pdf) Variability of water masses through the Mernoo saddle, South Island, New Zealand
by Ross Vennell
New Zealand Journal of Marine and Freshwater Research, Volume 34, Issue 1, March 2000, Pages 103-116
The Mernoo Saddle is situated c. 100 km east of Banks Peninsula, Canterbury, New Zealand at 44°S 174°E. The Saddle... more The Mernoo Saddle is situated c. 100 km east of Banks Peninsula, Canterbury, New Zealand at 44°S 174°E. The Saddle separates the South Island of New Zealand from an underwater ridge known as the Chatham Rise. The Rise acts as a partial barrier to the flow of the subantarctic surface water mass (SAW) and the subtropical surface water mass (STW), which are part of the global Subtropical Front extending around the Southern Ocean. This study examined the variability of the water masses flowing through the Mernoo Saddle using a 3-year data set of Advanced Very High Resolution Radiometer (AVHRR) Sea Surface Temperature (SST) imagery. This investigation revealed that SAW extended north through the western edge of the Mernoo Saddle for most of the year, however in winter and early spring a southward extension of STW was observed.
Acoustic Doppler Current Profiler measurements of the semi-diurnal internal tide on the west coast of New Zealand
by Ross Vennell
New Zealand Journal of Marine & Freshwater Research, Volume 27, Issue 1, 1993, Pages 31-38
Current measurements detail the vertical structure of the internal tide for a site at the continental shelf break. The... more Current measurements detail the vertical structure of the internal tide for a site at the continental shelf break. The measurements, made over 10 h in late winter, had an 8 m vertical resolution. Tidal analysis of the velocity data shows a 60° change in tidal phase and a 90° change in tide ellipse orientation across the base of the mixed layer. The profiles of tidal ellipse parameters are shown to be consistent with the sum of a barotropic tide and an internal tide in a two density-layer ocean. The barotropic tidal amplitude was 10 cm s-1 whereas the internal tide had a 6.3 cm s-1 amplitude. Both the barotropic and internal modes propagate in a direction approximately normal to the topography.
Resonance of long waves generated by storms obliquely crossing shelf topography in a rotating ocean
by Ross Vennell
Journal of Fluid Mechanics,Volume 682, 10 September 2011, Pages 261-288
http://dx.doi.org/10.1017/jfm.2011.221
The oceanic forced wave beneath a moving atmospheric disturbance is amplified by Proudman resonance. When modified by... more The oceanic forced wave beneath a moving atmospheric disturbance is amplified by Proudman resonance. When modified by the Earth's rotation this classical resonance only occurs if the disturbance time scale is smaller than the inertial period. With or without Coriolis effects, free transients generated by storm forced waves obliquely crossing step changes in water depth at particular angles are shown to resonate by exciting a range of long barotropic free waves. Rotationally influenced slow atmospherically forced waves crossing a vertical coast at a critical angle lead to a form of subcritical resonance, which occurs only when the component of the disturbances' phase velocities along the coast matches that of a free Kelvin wave (KW). In a rotating ocean, transients generated by disturbances crossing a step at a particular angle are shown to excite a free double Kelvin wave (DKW). This new type of resonance only occurs for sufficiently large steps and disturbances with time scale greater than the inertial period. A storm crossing a step shelf can result in the excitation of an infinite set of edge waves, a single KW, a unique DKW and a first-mode continental shelf wave, depending on the topography and the disturbance time scale, translation speed and incident angle. The study of resonances and wave mode excitations generated by storms crossing a coast or a continental shelf may contribute to understanding how a particular combination of the storm characteristics can result in destructive coastal events with time scales encompassing the typical meteotsunami period band (tens of minutes) and storm surges with periods of several hours or days
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ADCP measurements of momentum balance and dynamic topography in a constricted tidal channel
by Ross Vennell
Journal of Physical Oceanography, Volume 36, Issue 2, February 2006, Pages 177-188 http://dx.doi.org/10.1175/JPO2836.1
The dynamics of tidal flow through inlets are not fully understood; observations are scarce because of the small... more The dynamics of tidal flow through inlets are not fully understood; observations are scarce because of the small spatial scales over which the flow varies. This paper gives the first detailed measurements of the 2D structure of tidal currents and the dynamical terms of the momentum equation within a tidal inlet, leading to an improved understanding of the physics of tidal inlets. In the 180 cm s-1 peak flow the near-steady-state momentum balance is dominated by horizontal advection and the pressure gradient, with bottom friction playing a secondary role. At slack water, there is a balance between local acceleration and the pressure gradient. Numerical integration of the ADCP-measured terms in the momentum equation yields 60-m-resolution dynamic topography that shows a 7-cm variation at peak flood consistent with Bernoulli's equation. The surface topography because of friction forms a linear ramp with a peak irreversible head loss of 2 cm over 600 m. Tidal velocities were extracted from the ADCP measurements by extending an existing spline analysis technique. This technique is known to be sensitive to the number and location of the nodes where weights are applied to the spline. Simulations with artificial data representative of the tidally varying ADCP measurements show that, provided there are sufficient nodes to resolve the smallest spatial scale of interest, velocities predicted by the spline technique are insensitive to the number or locations of the nodes
Oscillating barotropic currents along short channels
by Ross Vennell
Journal of Physical Oceanography, Volume 28, Issue 8, August 1998, Pages 1561-1569
http://dx.doi.org/10.1175/2007JPO3687.1
Intuitively, the transport of an oscillating flow in a short channel should be constrained to be approximately the... more Intuitively, the transport of an oscillating flow in a short channel should be constrained to be approximately the same at all cross sections; that is, the transport is nondivergent. This intuitive constraint is quantified by developing a generalized definition of "shortness" given by the smallness of a nondimensional parameter ε. This parameter assesses the shortness of channels of variable cross-sectional area, with linear or nonlinear dynamical balances and, thus, is applicable to a wide range of channels, from tidal flow through the entrance to an estuary to subinertial flow through a strait. This analysis leads to a general result for a barotropic oscillating flow, which states that the variation in the amplitude of the transport along the channel is of order 2ε of the mean transport and the variation in the transport's phase is of order 2 tan-1ε. A diagnostic model for tidal flow within a "short" narrow channel of variable depth is developed. The model shows that the phase of the cross-sectional average velocity is dependent only on the relative amplitude and phase of the surface oscillation at the ends of the channel. Tidal measurements presented in a companion paper provide observational support for both the general result and diagnostic model.
Observations of the Phase of Tidal Currents along a Strait
by Ross Vennell
Journal of Physical Oceanography, Volume 28, Issue 8, August 1998, Pages 1570-1577
The phase of the M2 surface elevation tide changes by 100° along a 40-km length of Cook Strait. New Zealand. Acoustic... more The phase of the M2 surface elevation tide changes by 100° along a 40-km length of Cook Strait. New Zealand. Acoustic Doppler Current Profiler measurements are presented that show a change of only 12° in the phase of depth-averaged semidiurnal tidal currents along this same length. This phase change supports the general result developed by Vennell, which states that the phase of the cross-sectional average velocity is approximately constant along a "short" strait of variable cross-sectional area. Vennell also developed a relationship between the constant phase of tidal currents along the strait and the amplitude and phase of the surface elevation tide at the ends of a short strait. The velocity phase given by this relationship is shown to agree well with the observed tidal velocity phase in Cook Strait and one other strait.
Coastal flows driven by alongshore density gradients
by Ross Vennell
JOURNAL OF PHYSICAL OCEANOGRAPHY Volume: 17 Issue: 6 Pages: 821-827 . JUN 1987 http://dx.doi.org/10.1175/1520-0485(1987)0172.0.CO;2
Moving vessel acoustic Doppler current profiler measurement of tidal stream function using radial basis functions
by Ross Vennell
Journal of Geophysical Research C: Oceans, Volume 111, Issue 9, 8 September 2006, Article number C09002, DOI: 10.1029/2005JC003321
Acoustic Doppler current profiler (ADCP) measurements from moving vessels have recently been used to obtain detailed... more Acoustic Doppler current profiler (ADCP) measurements from moving vessels have recently been used to obtain detailed observations of the spatial patterns of tidal flows, as well as patterns of vorticity and dynamical terms. Developments in radial basis function (RBF) interpolation theory are demonstrated to significantly improve the quality of the tidal velocity field extracted from the measurements using thin plate splines. These include placing centers at data locations, enforcing "side conditions" on the solution, and using higher-order splines. For tidal flows with a scale less than a few kilometers the differentiability of RBFs can be exploited to fit the stream function directly to the measurements. This ensures the observed tidal velocity field satisfies mass continuity. Enforcing mass continuity is demonstrated to significantly improve the ability of the splines to interpolate across wide gaps between vessel tracks. Extraordinarily detailed ADCP measurements of the tidal stream function within Bluff Harbour, New Zealand, reveal the stagnation streamline that separates a 200 m wide flood tidal jet from an associated 400 m diameter eddy. The stagnation streamline clearly shows that the eddy gains its fluid and vorticity from inshore of the jet's vorticity maximum. The eddy forms at 04:45 hours before high tide, grows in spiral fashion, and becomes isolated from the jet 02:30 hours before high tide; after which its vorticity rapidly decays. Copyright 2006 by the American Geophysical Union.
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High-resolution observations of the intensity of secondary circulation along a curved tidal channel
by Ross Vennell
Journal of Geophysical Research C: Oceans, Volume 112, Issue 11, 8 November 2007, Article number C11008,
http://dx.doi.org/10.1029/2006JC003764
High horizontal resolution moving vessel acoustic Doppler current profiler (ADCP) observations of the spatial pattern... more High horizontal resolution moving vessel acoustic Doppler current profiler (ADCP) observations of the spatial pattern of cross-stream velocities in a curved tidal channel show radially outward surface velocities up to 10 cm s-1 which are maximum midchannel, consistent with helical secondary flow in a vertical plane normal to the depth-averaged velocity. The 30-m-cross- and 150-m-along-channel resolution observations are from a 2700-m-long section of a 350-m-wide horizontally and vertically well mixed tidal channel with a radius of curvature 1-5 km. The along-channel resolution allows the intensity of the curvature induced secondary flow to be estimated from the linear correlation between the observed cross-channel component of vertical shear and the shear estimated from the streamwise velocity and its varying curvature using an existing analytic model. The two shears are highly correlated and the regression line slope demonstrates that the observed curvature induced secondary flow is 30% more intense than that predicted by the model for a typical bottom drag coefficient. The secondary flow is 50% more intense than that predicted using the drag coefficient which best fits the streamwise velocity profile. Numerical solutions demonstrate that the intensity of the secondary flow is sensitive to small changes in the shape of the eddy viscosity profile; hence intensity may be sensitive to the way turbulence is modeled. Lagged correlation of the observations showed that the secondary flow adapts to changes in curvature and primary flow over a 300-m length scale, or 20 water depths, consistent with the existing model and laboratory studies.