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 shelf‐iron 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 mixed‐layer 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 mixed‐layer depth in the 1/6° data‐assimilating Southern Ocean State Estimate and then use the model output to assess the physical balances governing mixed‐layer 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

Connections between ocean bottom topography and Earth’s 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 ocean’s general circulation. Its influence comes through a variety of mechanisms including the contribution of mixing in the ocean’s interior through the generation of internal waves created by currents flowing over rough topography. The influence of topographic roughness on the ocean’s 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^(−3) m2 s^(−1)) north of the PF but decrease to O(10^(−4) m2 s^(−1)) or smaller south of the PF. Below 400 m diffusivities typically exceed 10^(−4) m^2 s^(−1). 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

Characterizing the transition from balanced to unbalanced motions in the Southern California Current

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 124(3), (2019): 2088-2109, doi:10.1029/2018JC014583.As observations and models improve their resolution of oceanic motions at ever finer horizontal scales, interest has grown in characterizing the transition from the geostrophically balanced flows that dominate at large‐scale to submesoscale turbulence and waves that dominate at small scales. In this study we examine the mesoscale‐to‐submesoscale (100 to 10 km) transition in an eastern boundary current, the southern California Current System (CCS), using repeated acoustic Doppler current profiler transects, sea surface height from high‐resolution nadir altimetry and output from a (1/48)° global model simulation. In the CCS, the submesoscale is as energetic as in western boundary current regions, but the mesoscale is much weaker, and as a result the transition lacks the change in kinetic energy (KE) spectral slope observed for western boundary currents. Helmholtz and vortex‐wave decompositions of the KE spectra are used to identify balanced and unbalanced contributions. At horizontal scales greater than 70 km, we find that observed KE is dominated by balanced geostrophic motions. At scales from 40 to 10 km, unbalanced contributions such as inertia‐gravity waves contribute as much as balanced motions. The model KE transition occurs at longer scales, around 125 km. The altimeter spectra are consistent with acoustic Doppler current profiler/model spectra at scales longer than 70/125 km, respectively. Observed seasonality is weak. Taken together, our results suggest that geostrophic velocities can be diagnosed from sea surface height on scales larger than about 70 km in the southern CCS.This research was funded by NASA (NNX13AE44G, NNX13AE85G, NNX16AH67G, NNX16AO5OH, and NNX17AH53G). We thank Sung Yong Kim for providing the high‐frequency radar spectral estimates and the two anonymous reviewers for providing useful comments and suggestions that greatly improved the manuscript. High‐frequency ALES data for Jason‐1 and Jason‐2 altimeters are available upon request (https://openadb.dgfi.tum.de/en/contact/ALES). Both AltiKa and Sentinel‐3 altimeter products were produced and distributed by the Copernicus Marine and Environment Monitoring Service (CMEMS; http://www.marine.copernicus.eu). D. M. worked on the modeling component of this study at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA). High‐end computing resources were provided by the NASA Advanced Supercomputing (NAS) Division of the Ames Research Center. The LLC output can be obtained from the ECCO project (ftp://ecco.jpl.nasa.gov/ECCO2/LLC4320/). The ADCP data are available at the Joint Archive for Shipboard ADCP data (JASADCP; http://ilikai.soest.hawaii.edu/sadcp).2019-08-2

Characteristics of colliding sea breeze gravity current fronts : a laboratory study

Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Quarterly Journal of the Royal Meteorological Society 143 (2017): 1434–1441, doi:10.1002/qj.3015.Sea and land breeze circulations driven by surface temperature differences between land and sea often evolve into gravity currents with sharp fronts. Along narrow peninsulas, islands and enclosed seas, sea/land breeze fronts from opposing shorelines may converge and collide and may initiate deep convection and heavy precipitation. Here we investigate the collision of two sea breeze gravity current fronts in an analogue laboratory setting. We examine these collisions by means of ‘lock-exchange’ experiments in a rectangular channel. The effects of differences in gravity current density and height are studied. Upon collision, a sharp front separating the two currents develops. For symmetric collisions (the same current densities and heights) this front is vertical and stationary. For asymmetric collisions (density differences, similar heights) the front is tilted, changes shape in time and propagates in the same direction as the heavier current before the collision. Both symmetric and asymmetric collisions lead to upward displacement of fluid from the gravity currents and mixing along the plane of contact. The amount of mixing along the collision front decreases with asymmetry. Height differences impact post-collision horizontal propagation: there is significant propagation in the same direction as the higher current before collision, independent of density differences. Collisions of two gravity current fronts force sustained ascending motions which increase the potential for deep convection. From our experiments we conclude that this potential is larger in stationary collision fronts from symmetric sea breeze collisions than in propagating collision fronts from asymmetric sea breeze collisions.National Science Foundation Grant Number: OCE-0824636; Office of Naval Research Grant Number: N00014-09-1-0844; National Aeronautics and Space Administration Grant Number: NASA NNX14A078