1,366 research outputs found
Interpreting wind-driven Southern Ocean variability in a stochastic framework
A stochastic model is derived from wind stress and bottom pressure gauge data to examine the response of the Antarctic Circumpolar Current (ACC) transport to wind stress forcing. A general method is used to estimate the drift and diffusion coefficients of a continuous stationary Markovian system. As a first approximation, the response of the ACC to wind stress forcing can be described by a multivariate Ornstein-Uhlenbeck process: Gaussian red noise wind stress drives the evolution of the ACC transport, which is damped by a linear drag term. The model indicates that about 30(±10)% of ACC variability is directly driven by the wind stress. This stochastic model can serve as a null hypothesis for studies of wind driven ACC variability. A more accurate stochastic description of the wind stress over the Southern Ocean requires a multiplicative noise component. The variability of the wind stress increases approximately linearly with increasing wind stress values. A multiplicative stochastic process generates a power-law distribution rather than a Gaussian distribution. A simple stochastic model shows that non-Gaussian forcing could have a significant impact on the velocity (or transport) probability density functions (PDFs) of the wind-driven circulation. The net oceanic damping determines whether the distribution of the oceanic flow is Gaussian (small damping) or resembles the distribution of the atmospheric forcing (large damping)
Energetics of wind-driven barotropic variability in the Southern Ocean
This study addresses the energetics of the Southern Ocean, in response to high-frequency wind forcing. A constant-density, multi-layer model is forced with a band of stochastically varying wind stress. The focus is on the interplay between the surface layer and the interior circulation.In line with previous examinations, it is concluded that the interior ocean is not directly energized by the wind work, but rather through the work done by the pressure field. The spatial and temporal characteristics of these terms differ substantially. Although the wind work may be negative in extensive regions of the World Ocean, the pressure work energizes the interior circulation almost everywhere. For low-frequency variability, the total work done by the wind and pressure on the barotropic flow is comparable, but discrepancies may arise for high-frequency variability. A mechanism is identified through which kinetic energy can leak from the wind-driven surface layer to the barotropic flow
Physical Drivers of Phytoplankton Bloom Initiation in the Southern Ocean's Scotia Sea
Abstract:
The Scotia Sea is the site of one of the largest spring phytoplankton blooms in the Southern Ocean. Past studies suggest that shelfâiron inputs are responsible for the high productivity in this region, but the physical mechanisms that initiate and sustain the bloom are not well understood. Analysis of profiling float data from 2002 to 2017 shows that the Scotia Sea has an unusually shallow mixedâlayer depth during the transition from winter to spring, allowing the region to support a bloom earlier in the season than elsewhere in the Antarctic Circumpolar Current. We compare these results to the mixedâlayer depth in the 1/6° dataâassimilating Southern Ocean State Estimate and then use the model output to assess the physical balances governing mixedâlayer variability in the region. Results indicate the importance of lateral advection of Weddell Sea surface waters in setting the stratification. A Lagrangian particle release experiment run backward in time suggests that Weddell outflow constitutes 10% of the waters in the upper 200 m of the water column in the bloom region. This dense Weddell water subducts below the surface waters in the Scotia Sea, establishing a sharp subsurface density contrast that cannot be overcome by wintertime convection. Profiling float trajectories are consistent with the formation of Taylor columns over the region's complex bathymetry, which may also contribute to the unique stratification. Furthermore, biogeochemical measurements from 2016 and 2017 bloom events suggest that vertical exchange associated with this Taylor column enhances productivity by delivering nutrients to the euphotic zone
High-resolution EPMA X-ray images of mother liquid inclusions in a Pd2Ga single crystal
During crystal growth from solution inclusions of different compositions were trapped at the rim of a Pd2Ga single crystal. Their fine-grained (< 5 mu m) internal structure demands special requirements for electron microprobe analysis, realized by low-voltage (5 keV) element mapping applying a step size of 0.138 mu m for each pixel. It can be shown, that these inclusions represent an isolated chemical system, and that crystallisation upon cooling follows the expected thermodynamic phase relations. Thus the final composition in the centre of the inclusion consists of a small-scale mixture of PdGa and Pd5Ga3 evolved out of a solid-solid decomposition of Pd5Ga4
Connections between ocean bottom topography and Earthâs climate
Author Posting. © Oceanography Society, 2004. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 17, 1 (2004): 65-74.The seafloor is one of the critical controls on the
oceanâs general circulation. Its influence comes through
a variety of mechanisms including the contribution of
mixing in the oceanâs interior through the generation of
internal waves created by currents flowing over rough
topography. The influence of topographic roughness on
the oceanâs general circulation occurs through a series
of connected processes. First, internal waves are generated
by currents and tides flowing over topographic
features in the presence of stratification. Some portion
of these waves is sufficiently nonlinear that they immediately
break creating locally enhanced vertical mixing.
The majority of the internal waves radiate away from
the source regions, and likely contribute to the background
mixing observed in the ocean interior. The
enhancement of vertical mixing over regions of rough
topography has important implications for the abyssal
stratification and circulation. These in turn have implications
for the storage and transport of energy in the climate
system, and ultimately the response of the climate
system to natural and anthropogenic forcing. Finally,
mixing of the stratified ocean leads to changes in sea
level; these changes need to be considered when predicting
future sea level.SRJ
was supported by the National Science Foundation
under grant OCE-0241061 and an Office of Naval
Research Young Investigator Award, LCS was supported
by the Office of Naval Research under grant
N00014-03-1-0307, and STG was supported by the
National Science Foundation under grant OCE-
9985203/OCE-0049066 and by the National
Aeronautics and Space Administration under JPL contract
1224031
Spatial and Temporal Patterns of Small-Scale Mixing in Drake Passage
Temperature and salinity profiles obtained with expendable CTD probes throughout Drake Passage between February 2002 and July 2005 are analyzed to estimate turbulent diapycnal eddy diffusivities to a depth of 1000 m. Diffusivity values are inferred from density/temperature inversions and internal wave vertical strain. Both methods reveal the same pattern of spatial variability across Drake Passage; diffusivity estimates from inversions exceed those from vertical strain by a factor of 3 over most of Drake Passage. The Polar Front (PF) separates two dynamically different regions. Strong thermohaline intrusions characterize profiles obtained north of the PF. South of the PF, stratification is determined largely by salinity, and temperature is typically unstably stratified between 100- and 600-m depth. In the upper 400 m, turbulent diapycnal diffusivities are O(10^(â3) m2 s^(â1)) north of the PF but decrease to O(10^(â4) m2 s^(â1)) or smaller south of the PF. Below 400 m diffusivities typically exceed 10^(â4) m^2 s^(â1). Diffusivities decay weakly with depth north of the PF, whereas diffusivities increase with depth and peak near the local temperature maximum south of the PF. The meridional pattern in near-surface mixing corresponds to local maxima and minima of both wind stress and wind stress variance. Near-surface diffusivities are also found to be larger during winter months north of the PF. Wind-driven near-inertial waves, strong mesoscale eddy activity, and double-diffusive convection are suggested as possible factors contributing to observed mixing pattern
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