On the factors driving upper-ocean salinity variability at the western edge of the Eastern Pacific Fresh Pool

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Farrar, J. T., & Plueddemann, A. J. On the factors driving upper-ocean salinity variability at the western edge of the Eastern Pacific Fresh Pool. Oceanography, 32(2), (2019):30-39, doi:10.5670/oceanog.2019.209.The tropical Eastern Pacific Fresh Pool (EPFP) has some of the highest precipitation rates and lowest sea surface salinities found in the open ocean. In addition, the sea surface salinity in the EPFP exhibits one of the strongest annual cycles in the world ocean. The region is strongly affected by the meridionally migrating Intertropical Convergence Zone and is also influenced by large-scale ocean currents and wind-driven Ekman currents. Recognizing the complexity of competing regional influences and the importance of sea surface salinity as an integrator of freshwater forcing, the Salinity Processes Upper-ocean Regional Study (SPURS) was undertaken to better understand how ocean processes and surface freshwater fluxes set surface salinity. Instrumentation on a surface mooring, deployed for 14 months near the western edge of the EPFP, allowed estimation of the surface fluxes of momentum, heat, and freshwater. Subsurface instrumentation on the mooring provided upper-ocean vertical structure and horizontal currents. These observations, along with horizontal gradients of surface salinity from the Soil Moisture Active Passive (SMAP) satellite instrument, were used to estimate the surface-layer salinity budget at the western edge of the EPFP. While the low salinity associated with the presence of the EPFP at the mooring site was sustained by heavy rainfall, it was found that seasonal variability in large-scale currents was important to controlling the transition between the “salty” and “fresh” seasons. Ekman advection was important to prolonging local high salinity as rainfall decreased. Although illuminating some key processes, the temporal variability of the surface-layer salinity budget also shows significant complexity, with processes such as surface freshwater fluxes and vertical mixing making notable contributions. The surface flux term and the terms involving mixing across the base of the surface layer oppose and nearly cancel each other throughout the deployment, such that the horizontal advection term effectively accounts for most of the variability in surface salinity at the site on monthly to seasonal timescales. Further investigation, taking advantage of additional observations during SPURS-2, will be needed to more thoroughly examine the relevant physical processes.We are grateful for helpful comments on the manuscript from guest editor Andrey Shcherbina and two anonymous reviewers. We thank the members of the WHOI Upper Ocean Processes Group (Ben Pietro, Emerson Hasbrouck, Raymond Graham, Nan Galbraith, Kelan Huang, Sebastien Bigorre, Ben Greenwood, Jason Smith, Geoff Allsup, and Bob Weller) for their contributions to preparation, deployment, and recovery of the SPURS-2 surface mooring. We thank the captains and crews of R/V Roger Revelle and R/V Thomas Thompson, and the chief scientists for the deployment and recovery cruises (Andy Jessup and Kyla Drushka). SMAP salinity data are produced by Remote Sensing Systems and sponsored by the NASA Ocean Salinity Science Team (data are available at http://www.remss.com). This work was supported by NASA grants NNX15AG20G and 80NSSC18K1494. The buoy and mooring data will soon be available from the NASA JPL PO.DAAC data center

Scientific rationale and conceptual design of a process-oriented shelfbreak observatory: the OOI pioneer array

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Gawarkiewicz, G., & Plueddemann, A. J. Scientific rationale and conceptual design of a process-oriented shelfbreak observatory: The OOI pioneer array. Journal of Operational Oceanography, 13(1), (2019): 19-36, doi: 10.1080/1755876X.2019.1679609.The Ocean Observatories Initiative (OOI) of the National Science Foundation in the USA includes a coastal observatory called the OOI Pioneer Array, which is focused on understanding shelf/slope exchange processes. The OOI Pioneer Array has been designed and constructed and is currently in operation. In order to fully understand the design principles and constraints, we first describe the basic exchange processes and review prior experiments in the region. Emphasis is placed on the space and time scales of important exchange processes such as frontal meandering and warm core ring interactions with the Shelfbreak Front, the dominant sources of variability in the region. The three major components of the Pioneer Array are then described, including preliminary data from the underwater gliders and Autonomous Underwater Vehicle (AUV) deployments. The relevance of the Pioneer Array to important recent scientific issues in the area, including enhanced warming of the continental shelf and increasing frequency and spatial extent of Gulf Stream interactions with the continental shelf is discussed. Finally, similar observatories in Asia are briefly described, and general conclusions regarding principles that should guide the design of shelfbreak observatories in other geographic regions are presented.Financial support for this work was provided by the National Science Foundation under grant number OCE-1657853 (GG) and OCE-1026342 (AJP). GG was also supported by a Senior Scientist Chair from the Woods Hole Oceanographic Institution

ADCP measurements from the ICESHELF 94 experiment

The ICESHELF 94 Experiment was conducted during April of 1994 from an ice camp in the Lincoln Sea at approximately 84 degrees N, 63 degrees W. An Acoustic Doppler Current Profiler (ADCP) was operated at the camp from 8 to 24 April. This report describes the ADCP configuration and presents the raw data recorded by the ADCP. Processing steps involved in computing horizontal velocities in geographic coordinates from the raw data are described, and time series and spectra of the resulting data are presented. Horizontal velocities with precision of about 1 cm/s were obtained between 27.7 m and 137.0 m depth with 7.8 m resolution. Data were obtained at five minute intervals, but averaged to 1 hr during processing to suppress instrument noise. Spectra show the velocity field to be dominated by variance at semi-siurnal frequency, with a maximum in energy between 50 and 110m depth. Maximum amplitudes of 8 to 10 cm/s were seen near 80 m depth. Velocities from an InterOcean S4 current meter deployed at the same site were compared to those from the ADCP. The largest differences were associated with peaks in the semi-diurnal oscilations, with the S4 underspeeding relative to the ADCP.Funding was provided by the Office of Naval Research through Contract No. N00014-90-J-1359

The annual cycle of air-sea fluxes in the northwest tropical Atlantic

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Bigorre, S. P., & Plueddemann, A. J. The annual cycle of air-sea fluxes in the northwest tropical Atlantic. Frontiers in Marine Science, 7, (2021): 612842, https://doi.org/10.3389/fmars.2020.612842.In this article we analyze 11 years of near-surface meteorology using observations from an open-ocean surface mooring located in the Northwestern Tropical Atlantic (51°W, 15°N). Air-sea fluxes of heat, freshwater, and momentum are derived from these observations using the Coupled Ocean–Atmosphere Response Experiment (COARE) bulk parameterization. Using this dataset, we compute a climatology of the annual cycle of near-surface meteorological conditions and air-sea fluxes. These in situ data are then compared with three reanalyses: the National Centers for Environmental Prediction-Department of Energy [NCEP-DOE (hereafter referred to as NCEP-2)], the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim and the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) reanalyses. Products from the Clouds and the Earth’s Radiant Energy System (CERES) and the Tropical Rainfall Measuring Mission (TRMM) are also used for comparison. We identify the agreements and characterize the discrepancies in the annual cycles of meteorological variables and the different components of air-sea heat fluxes (latent, sensible, shortwave, and longwave radiation). Recomputing the reanalyses fluxes by applying the COARE algorithm to the reanalyses meteorological variables results in better agreement with the in situ fluxes than using the reanalyses fluxes directly. However, the radiative fluxes (longwave and shortwave) from some of the reanalyses show significant discrepancies when compared with the in situ measurements. Longwave radiation from MERRA-2 is biased high (too much oceanic heat loss), and NCEP-2 longwave does not correlate to in situ observations and other reanalyses. Shortwave radiation from NCEP-2 is biased low in winter and does not track the observed variability in summer. The discrepancies in radiative fluxes versus in situ fluxes are explored, and the potential regional implications are discussed using maps of satellite and reanalyses products, including radiation and cloud cover.The NTAS project was funded by the Global Ocean Monitoring and Observing Program of the National Oceanic and Atmospheric Administration (CPO FundRef number 100007298), through the Cooperative Institute for the North Atlantic Region (CINAR) under Cooperative Agreement NA14OAR4320158

Wind-driven modification of the Alaskan coastal current

Author Posting. © American Geophysical Union, 2012. 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 117 (2012): C03031, doi:10.1029/2011JC007650.Across-shelf transects over the eastern flank of Barrow Canyon were obtained in August 2005 with an autonomous underwater vehicle (AUV). Here, the shelf topography creates a “choke” point in which a substantial portion of Pacific inflow from the Bering Strait is concentrated within 30 km of the coast, providing an ideal setup for monitoring the flow with the AUV. Four transects, extending ∼10 km offshore of Barrow, Alaska, inshore of the ∼80 m isobath, were used in conjunction with a process-oriented numerical model to diagnose the wind-driven modification of the Alaskan coastal current. Poleward transports of 0.12 Sv were consistent among all sections, although the transport-weighted temperature was about 1°C colder in the transect obtained during peak winds. An idealized numerical model reproduces the observed hydrographic structure and across-shelf circulation reasonably well in that (1) winds were not sufficient to reverse the poleward flow, (2) upwelling was most pronounced in the nearshore, and (3) the onshore return flow occurred throughout the interior as opposed to the bottom boundary layer. The across-shelf circulation provides a possible mechanism for a meltwater intrusion observed on the offshore side of the AUV transect made during peak winds. Also of interest is that the observed anticyclonic shear was much stronger (∣∂u/∂y∣ > f) than previously measured in the region.Field work and analysis (A.J.P.) was supported by the Comer Science Education Foundation and the WHOI Ocean and Climate Change Institute. E.L.S. was supported as a WHOI Postdoctoral Scholar.2012-09-2

Northwest Tropical Atlantic Station (NTAS) : velocity data report for deployments 1 to 5

This report presents velocity data from the Northwest Tropical Atlantic Station (NTAS) deployments 1 through 5, from March 30, 2001, to February 28, 2006. The NTAS project has maintained a series of moorings near 14°50'N, 51°00'W in the northwest tropical Atlantic for air-sea flux measurement. The moorings include a surface buoy outfitted with Air- Sea Interaction Meteorology (ASIMET) systems for determination of bulk air-sea fluxes and oceanographic sensors along the upper 120 m of the mooring line. This report describes and presents the velocity data recovered from current meters and Acoustic Doppler Current Profilers (ADCPs) during the first five years of the NTAS project.Funding was provided by the National Oceanic and Atmospheric Administration under Grant No. NA09OAR432012

Atmospheric and offshore forcing of temperature variability at the shelf break

Author Posting. © The Oceanography Society, 2018. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 31, no. 1 (2018): 72–79, doi:10.5670/oceanog.2018.112.Knowledge of heat balance and associated temperature variability in the Northwest Atlantic coastal ocean is important for understanding impacts of climate change such as how ocean warming will affect the management of fisheries. Heat balances are particularly complicated near the edge of the continental shelf, where the cross-shelf temperature gradients within the shelf-break front complicate the competing influences of air-sea flux anomalies versus ocean advection. We review the atmospheric and oceanic processes associated with heat balance over the Northwest Atlantic continental shelf and slope, with an emphasis on the scale-dependent nature of the heat balance. We then use data from the Ocean Observatories Initiative (OOI) Pioneer Array to demonstrate heat balance scale dependence at the southern New England shelf break, and the capability of the array to capture multiscale ocean processes. Comparison of the cumulative effects of air-sea heat fluxes measured at the OOI Pioneer Array from May 2015 to April 2016 with the actual temperature change shows the importance of advective processes in overall heat balance near the shelf break.KC was partially supported by the National Science Foundation under grant OCE-1435602 and OCE- 1634094. GG was supported by the National Science Foundation under grant OCE-1657853. AP was supported by the National Science Foundation through the Cooperative Agreement (subaward) SA9-10 from the Consortium for Ocean Leadership to the Woods Hole Oceanographic Institution

A note on the depth of sidelobe contamination in acoustic doppler current profiles

Author Posting. © American Meteorological Society, 2022. 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 the Atmospheric and Oceanic Technology 39(1), (2022): 31–35, https://doi.org/10.1175/JTECH-D-21-0075.1.Acoustic Doppler current profilers (ADCP) do not provide reliable water velocity measurements near the sea surface or bottom because acoustic sidelobe reflections from the boundary contaminate the Doppler velocity measurements. The apparent depth of the center of the sidelobe reflection is zsl = ha[1 − cos(θ)], where ha is the distance from the ADCP acoustic head to the sea surface and θ is the ADCP beam angle. However, sidelobe contamination extends one and a half ADCP bins below zsl as the range gating of the acoustic return causes overlap between adjacent ADCP bins. Consequently, the contaminated region z < zsl + 3Δz/2 is deeper than traditionally suggested, with a dependence on bin size Δz. Direct observations confirming both the center depth of the sidelobe reflection and the depth of contamination are presented for six bottom-mounted, upward-looking ADCPs. The sidelobe reflection is isolated by considering periods of weak wind stresses when the sea surface is smooth and there is nearly perfect reflection of the main beams away from the ADCP and hence little acoustic return from the main beams to the ADCP.This analysis was supported by NSF OCE 1558874 for Kirincich and Lentz. Plueddemann was supported by the Global Ocean Monitoring and Observing Program of the National Oceanic and Atmospheric Administration (CPO Fund Reference Number 100007298), through the Cooperative Institute for the North Atlantic Region (CINAR) under Cooperative Agreement NA14OAR4320158

Inhibited upper ocean restratification in nonequilibrium swell conditions

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 40 (2013): 3672–3676, doi:10.1002/grl.50708.Diurnal restratification of the ocean surface boundary layer (OSBL) represents a competition between mixing of the OSBL and solar heating. Langmuir turbulence (LT) is a mixing process in the OSBL, driven by wind and surface waves, that transfers momentum, heat, and mass. Observations in nonequilibrium swell conditions reveal that the OSBL does not restratify despite low winds and strong solar radiation. Motivated by observations, we use large-eddy simulations of the wave-averaged Navier-Stokes equations to show that LT is capable of inhibiting diurnal restratification of the OSBL. Incoming heat is redistributed vertically by LT, forming a warmer OSBL with a nearly uniform temperature. The inhibition of restratification is not reproduced by two common Reynolds-averaged Navier-Stokes equation models, highlighting the importance of properly representing sea-state dependent LT dynamics in OSBL models.This work was supported by the U.S. National Science Foundation (Grant OCE-1130678).2014-01-3

Design study for a moored surface-scanning sonar

This report contains the results of a design study for a surface scanning sonar instrument capable of long-term deployment on ocean moorings. The instrument is intended to sample the bubble field just below the ocean's surface and compute the backscattered intensity and Doppler velocity in small unit volumes. The principal motivation for the development of such an instrument is to enhance the study of upper ocean processes by utilizing the ability of the sonar to detect surface waves and Langmuir circulation. Important design parameters for the instrument are investigated and a detailed design proposed. Key technical issues such as the trade-offs among spatial resolution, temporal resolution, velocity precision, total range, and power are discussed. The azimuthal motion of the instrument on a mooring is considered as a potential problem, and possible solutions are discussed. Matlab functions used for the investigations are included in an appendix.Funding was provided by a grant from the Webster Foundation to the Woods Hole Oceanographic Institution