Microstructure mixing observations and finescale parameterizations in the Beaufort Sea

Author Posting. © American Meteorological Society, 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 51(1), (2021): 19-35, https://doi.org/10.1175/JPO-D-19-0233.1.In the Beaufort Sea in September of 2015, concurrent mooring and microstructure observations were used to assess dissipation rates in the vicinity of 72°35′N, 145°1′W. Microstructure measurements from a free-falling profiler survey showed very low [O(10−10) W kg−1] turbulent kinetic energy dissipation rates ε. A finescale parameterization based on both shear and strain measurements was applied to estimate the ratio of shear to strain Rω and ε at the mooring location, and a strain-based parameterization was applied to the microstructure survey (which occurred approximately 100 km away from the mooring site) for direct comparison with microstructure results. The finescale parameterization worked well, with discrepancies ranging from a factor of 1–2.5 depending on depth. The largest discrepancies occurred at depths with high shear. Mean Rω was 17, and Rω showed high variability with values ranging from 3 to 50 over 8 days. Observed ε was slightly elevated (factor of 2–3 compared with a later survey of 11 profiles taken over 3 h) from 25 to 125 m following a wind event which occurred at the beginning of the mooring deployment, reaching a maximum of ε= 6 × 10−10 W kg−1 at 30-m depth. Velocity signals associated with near-inertial waves (NIWs) were observed at depths greater than 200 m, where the Atlantic Water mass represents a reservoir of oceanic heat. However, no evidence of elevated ε or heat fluxes was observed in association with NIWs at these depths in either the microstructure survey or the finescale parameterization estimates.This work was supported by NSF Grants PLR 14-56705 and PLR-1303791 and by NSF Graduate Research Fellowship Grant DGE-1650112

Tracking icebergs with time-lapse photography and sparse optical flow, LeConte Bay, Alaska, 2016–2017

We present a workflow to track icebergs in proglacial fjords using oblique time-lapse photos and the Lucas-Kanade optical flow algorithm. We employ the workflow at LeConte Bay, Alaska, where we ran five time-lapse cameras between April 2016 and September 2017, capturing more than 400 000 photos at frame rates of 0.5–4.0 min−1. Hourly to daily average velocity fields in map coordinates illustrate dynamic currents in the bay, with dominant downfjord velocities (exceeding 0.5 m s−1 intermittently) and several eddies. Comparisons with simultaneous Acoustic Doppler Current Profiler (ADCP) measurements yield best agreement for the uppermost ADCP levels (∼ 12 m and above), in line with prevalent small icebergs that trace near-surface currents. Tracking results from multiple cameras compare favorably, although cameras with lower frame rates (0.5 min−1) tend to underestimate high flow speeds. Tests to determine requisite temporal and spatial image resolution confirm the importance of high image frame rates, while spatial resolution is of secondary importance. Application of our procedure to other fjords will be successful if iceberg concentrations are high enough and if the camera frame rates are sufficiently rapid (at least 1 min−1 for conditions similar to LeConte Bay).This work was funded by the U.S. National Science Foundation (OPP-1503910, OPP-1504288, OPP-1504521 and OPP-1504191).Ye

Double diffusion, shear instabilities, and heat impacts of a pacific summer water intrusion in the Beaufort Sea

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Fine, E., MacKinnon, J., Alford, M., Middleton, L., Taylor, J., Mickett, J., Cole, S., Couto, N., Boyer, A., & Peacock, T. Double diffusion, shear instabilities, and heat impacts of a pacific summer water intrusion in the Beaufort Sea. Journal of Physical Oceanography, 52(2), (2022): 189–203, https://doi.org/10.1175/jpo-d-21-0074.1.Pacific Summer Water eddies and intrusions transport heat and salt from boundary regions into the western Arctic basin. Here we examine concurrent effects of lateral stirring and vertical mixing using microstructure data collected within a Pacific Summer Water intrusion with a length scale of ∼20 km. This intrusion was characterized by complex thermohaline structure in which warm Pacific Summer Water interleaved in alternating layers of O(1) m thickness with cooler water, due to lateral stirring and intrusive processes. Along interfaces between warm/salty and cold/freshwater masses, the density ratio was favorable to double-diffusive processes. The rate of dissipation of turbulent kinetic energy (ε) was elevated along the interleaving surfaces, with values up to 3 × 10−8 W kg−1 compared to background ε of less than 10−9 W kg−1. Based on the distribution of ε as a function of density ratio Rρ, we conclude that double-diffusive convection is largely responsible for the elevated ε observed over the survey. The lateral processes that created the layered thermohaline structure resulted in vertical thermohaline gradients susceptible to double-diffusive convection, resulting in upward vertical heat fluxes. Bulk vertical heat fluxes above the intrusion are estimated in the range of 0.2–1 W m−2, with the localized flux above the uppermost warm layer elevated to 2–10 W m−2. Lateral fluxes are much larger, estimated between 1000 and 5000 W m−2, and set an overall decay rate for the intrusion of 1–5 years.This work was supported by ONR Grant N00014-16-1-2378 and NSF Grants PLR 14-56705 and PLR-1303791, NSF Graduate Research Fellowship Grant DGE-1650112, as well as by the Postdoctoral Scholar Program at Woods Hole Oceanographic Institution, with funding provided by the Weston Howland Jr. Postdoctoral Scholarship

Turbulent entrainment fluxes within the eastern Pacific warm pool

Thesis (Ph. D.)--University of Washington, 2007.Mechanisms controlling turbulent entrainment fluxes, or vertical turbulent fluxes at the mixed layer base (z = -h), and the specific influence of heat entrainment on SST within the eastern Pacific warm pool (EPWP) are investigated using a 19-day timeseries of upper-ocean and atmospheric measurements collected in September 2001 at 10°N, 95°W, co-located buoy measurements, and a semi-empirical entrainment model. Buoyancy entrainment scaled with the cube of the friction velocity, u*, and the inverse finescale (8-m) gradient Richardson number at z = -h, Ri-1(-h), with the variance of the latter largely due to both near-inertial and sub-inertial shear. These two parameters explained more than half of buoyancy entrainment variance over the 19-day timeseries. Surface buoyancy flux also modulated entrainment with heavy rainfall and intense solar radiation suppressing and buoyant convection generating entrainment. Variability of vertical gradients of temperature and salinity at z = -h due to coincident large surface heat and freshwater (rainfall) fluxes and the comparable roles of temperature and salinity in determining the stratification at h were also found to modulate entrainment heat and salt fluxes produced by elevated turbulence at h.The 19-days of entrainment observations allowed the evaluation and modification of the Ni-iler and Kraus (1977) (N-K) entrainment parameterization, which was used with mooring buoy measurements to obtain entrainment estimates over 2001. The three empirically-motivated modifi cations reduced the base N-K model root-mean-square error by 35% and mean bias from +40% to less than +2%. Mixed layer temperature budget inferences and entrainment model results indicate that changes in h associated with Ekman pumping and Rossby waves strongly controls entrainment and, thus, SST. Large entrainment heat fluxes and large resultant drops in SST occur when Ekman pumping decreases h, exposing the concentrated temperature gradients at z = -h to stronger, surface-forced turbulence. With changes in depth-averaged mixed layer temperature due to entrainment inversely proportional to h, shoaling h also amplifies the effect of these fluxes on SST changes. Entrainment may account for most of the cooling needed to offset the net annual ∼23°C of warming that would result from the divergence of the surface and penetrative heat fluxes acting alone, and, thus, strongly controls the region's SST

Data to accompany the article "Destratification and Restratification of the Spring Surface Boundary-Layer in a Subtropical Front"

This archive contains the profile data from EM-APEX floats deployed at the third experimental site occupied during the 2017 SMILE cruise on the R/V Sikuliaq. These data were used for the analysis of restratification processes presented in the above-mentioned Journal of Physical Oceanography paper by Eric Kunze, John B. Mickett, and James B. Girton

Samoan Passage Poster @ RAPID Meeting Bristol 2015

Poster presented at the RAPID meeting in Bristol/UK in July 2015

Flow-topography interactions in the Samoan Passage

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Girton, J. B., Mickett, J. B., Zhao, Z., Alford, M. H., Voet, G., Cusack, J. M., Carter, G. S., Pearson-Potts, K. A., Pratt, L. J., Tan, S., & Klymak, J. M. Flow-topography interactions in the Samoan Passage. Oceanography, 32(4), (2019): 184-193, doi: 10.5670/oceanog.2019.424.Mixing in the Samoan Passage has implications for the abyssal water properties of the entire North Pacific—nearly 20% of the global ocean’s volume. Dense bottom water formed near Antarctica encounters the passage—a gap in a ridge extending from north of Samoa eastward across the Pacific at around 10°S—and forms an energetic cascade much like a river flowing through a canyon. The 2011–2014 Samoan Passage Abyssal Mixing Experiment explored the importance of topography to the dense water flow on a wide range of scales, including (1) constraints on transport due to the overall passage shape and the heights of its multiple sills, (2) rapid changes in water properties along particular pathways at localized mixing hotspots where there is extreme topographic roughness and/or downslope flow acceleration, and (3) diversion and disturbance of flow pathways and density surfaces by small-scale seamounts and ridges. The net result is a complex but fairly steady picture of interconnected pathways with a limited number of intense mixing locations that determine the net water mass transformation. The implication of this set of circumstances is that the dominant features of Samoan Passage flow and mixing (and their responses to variations in incoming or background properties) can be described by the dynamics of a single layer of dense water flowing beneath a less-dense one, combined with mixing and transformation that is determined by the small-scale topography encountered along flow pathways.We are grateful to Eric Boget, Andrew Cookson, Sam Fletcher, Trina Litchendorf, and Keith Magness for their assistance in the field program, and to the captains and crews of R/Vs Roger Revelle and Thomas G. Thompson for their excellent ship handling and assistance—without which this work would not have been possible. This work was supported by the National Science Foundation

A Tale of Two Spicy Seas

Upper-ocean turbulent heat fluxes in the Bay of Bengal and the Arctic Ocean drive regional monsoons and sea ice melt, respectively, important issues of societal interest. In both cases, accurate prediction of these heat transports depends on proper representation of the small-scale structure of vertical stratification, which in turn is created by a host of complex submesoscale processes. Though half a world apart and having dramatically different temperatures, there are surprising similarities between the two: both have (1) very fresh surface layers that are largely decoupled from the ocean below by a sharp halocline barrier, (2) evidence of interleaving lateral and vertical gradients that set upper-ocean stratification, and (3) vertical turbulent heat fluxes within the upper ocean that respond sensitively to these structures. However, there are clear differences in each ocean's horizontal scales of variability, suggesting that despite similar background states, the sharpening and evolution of mesoscale gradients at convergence zones plays out quite differently. Here, we conduct a qualitative and statistical comparison of these two seas, with the goal of bringing to light fundamental underlying dynamics that will hopefully improve the accuracy of forecast models in both parts of the world