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Shoichiro Fukao
,
Toru Sato
,
Norikazu Yamasaki
,
Robert M. Harper
, and
Susumu Kato

Abstract

Vertical profiles of horizontal winds in the lower stratosphere and upper troposphere were measured by the UHF Doppler radar at Arecibo, Puerto Rico (18.35°N, 66.75°W) on 26 days in August and September 1977. On comparing these with horizontal winds measured by routine rawinsonde balloons launched some 80 km east of Arecibo, fairly good agreement between every wind profile can be seen. Most of the difference between the two sets of measurements in the lower stratosphere is shown to be caused by the experimental error of the rawinsonde, while the spatial and/or temporal variations in the wind field seem to dominate the difference in the upper troposphere.

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Shoichiro Fukao
,
Toru Sato
,
Norikazu Yamasaki
,
Robert M. Harper
, and
Susumu Kato

Abstract

Wind oscillations of tidal periods that showed a marked downward phase progression were detected at the lower stratosphere using the Arecibo radar. The amplitudes of 1–5 m s−1 were inferred for both diurnal and semidiurnal components, much larger than the values predicted by the classical tidal theory. The vertical wavelengths inferred were also less than the theoretical values; ∼5 km for the diurnal component and 2–9 km for the semidiurnal component.

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Jody M. Klymak
,
Harper L. Simmons
,
Dmitry Braznikov
,
Samuel Kelly
,
Jennifer A. MacKinnon
,
Matthew H. Alford
,
Robert Pinkel
, and
Jonathan D. Nash

Abstract

The reflection of a low-mode internal tide on the Tasman continental slope is investigated using simulations of realistic and simplified topographies. The slope is supercritical to the internal tide, which should predict a large fraction of the energy reflected. However, the response to the slope is complicated by a number of factors: the incoming beam is confined laterally, it impacts the slope at an angle, there is a roughly cylindrical rise directly offshore of the slope, and a leaky slope-mode wave is excited. These effects are isolated in simulations that simplify the topography. To separate the incident from the reflected signal, a response without the reflector is subtracted from the total response to arrive at a reflected signal. The real slope reflects approximately 65% of the mode-1 internal tide as mode 1, less than two-dimensional linear calculations predict, because of the three-dimensional concavity of the topography. It is also less than recent glider estimates, likely as a result of along-slope inhomogeneity. The inhomogeneity of the response comes from the Tasman Rise that diffracts the incoming tidal beam into two beams: one focused along beam and one diffracted to the north. Along-slope inhomogeneity is enhanced by a partially trapped, superinertial slope wave that propagates along the continental slope, locally removing energy from the deep-water internal tide and reradiating it into the deep water farther north. This wave is present even in a simplified, straight slope topography; its character can be predicted from linear resonance theory, and it represents up to 30% of the local energy budget.

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Olavo B. Marques
,
Matthew H. Alford
,
Robert Pinkel
,
Jennifer A. MacKinnon
,
Jody M. Klymak
,
Jonathan D. Nash
,
Amy F. Waterhouse
,
Samuel M. Kelly
,
Harper L. Simmons
, and
Dmitry Braznikov

Abstract

Mode-1 internal tides can propagate far away from their generation sites, but how and where their energy is dissipated is not well understood. One example is the semidiurnal internal tide generated south of New Zealand, which propagates over a thousand kilometers before impinging on the continental slope of Tasmania. In situ observations and model results from a recent process-study experiment are used to characterize the spatial and temporal variability of the internal tide on the southeastern Tasman slope, where previous studies have quantified large reflectivity. As expected, a standing wave pattern broadly explains the cross-slope and vertical structure of the observed internal tide. However, model and observations highlight several additional features of the internal tide on the continental slope. The standing wave pattern on the sloping bottom as well as small-scale bathymetric corrugations lead to bottom-enhanced tidal energy. Over the corrugations, larger tidal currents and isopycnal displacements are observed along the trough as opposed to the crest. Despite the long-range propagation of the internal tide, most of the variability in energy density on the slope is accounted by the spring–neap cycle. However, the timing of the semidiurnal spring tides is not consistent with a single remote wave and is instead explained by the complex interference between remote and local tides on the Tasman slope. These observations suggest that identifying the multiple waves in an interference pattern and their interaction with small-scale topography is an important step in modeling internal energy and dissipation.

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Amy F. Waterhouse
,
Tyler Hennon
,
Eric Kunze
,
Jennifer A. MacKinnon
,
Matthew H. Alford
,
Robert Pinkel
,
Harper Simmons
,
Caitlin B. Whalen
,
Elizabeth C. Fine
,
Jody Klymak
, and
Julia M. Hummon

Abstract

Internal waves are predominantly generated by winds, tide–topography interactions, and balanced flow–topography interactions. Observations of vertical shear of horizontal velocity (uz , υz ) from lowered acoustic Doppler current profilers (LADCP) profiles conducted during GO-SHIP hydrographic surveys, as well as vessel-mounted sonars, are used to interpret these signals. Vertical directionality of intermediate-wavenumber [ λ z O ( 100 )  m ] internal waves is inferred in this study from rotary-with-depth shears. Total shear variance and vertical asymmetry ratio (Ω), i.e., the normalized difference between downward- and upward-propagating intermediate wavenumber shear variance, where Ω > 0 (<0) indicates excess downgoing (upgoing) shear variance, are calculated for three depth ranges: 200–600 m, 600 m–1000 mab (meters above bottom), and below 1000 mab. Globally, downgoing (clockwise-with-depth in the Northern Hemisphere) exceeds upgoing (counterclockwise-with-depth in the Northern Hemisphere) shear variance by 30% in the upper 600 m of the water column (corresponding to the globally averaged asymmetry ratio of Ω ¯ = 0.13 ), with a near-equal distribution below 600-m depth ( Ω ¯ 0 ). Downgoing shear variance in the upper water column dominates at all latitudes. There is no statistically significant correlation between the global distribution of Ω and internal wave generation, pointing to an important role for processes that redistribute energy within the internal wave continuum on wavelengths of O ( 100 )  m .

Open access
Matthew H. Alford
,
Jennifer A. MacKinnon
,
Jonathan D. Nash
,
Harper Simmons
,
Andy Pickering
,
Jody M. Klymak
,
Robert Pinkel
,
Oliver Sun
,
Luc Rainville
,
Ruth Musgrave
,
Tamara Beitzel
,
Ke-Hsien Fu
, and
Chung-Wei Lu

Abstract

Internal tide generation, propagation, and dissipation are investigated in Luzon Strait, a system of two quasi-parallel ridges situated between Taiwan and the Philippines. Two profiling moorings deployed for about 20 days and a set of nineteen 36-h lowered ADCP–CTD time series stations allowed separate measurement of diurnal and semidiurnal internal tide signals. Measurements were concentrated on a northern line, where the ridge spacing was approximately equal to the mode-1 wavelength for semidiurnal motions, and a southern line, where the spacing was approximately two-thirds that. The authors contrast the two sites to emphasize the potential importance of resonance between generation sites. Throughout Luzon Strait, baroclinic energy, energy fluxes, and turbulent dissipation were some of the strongest ever measured. Peak-to-peak baroclinic velocity and vertical displacements often exceeded 2 m s−1 and 300 m, respectively. Energy fluxes exceeding 60 kW m−1 were measured at spring tide at the western end of the southern line. On the northern line, where the western ridge generates appreciable eastward-moving signals, net energy flux between the ridges was much smaller, exhibiting a nearly standing wave pattern. Overturns tens to hundreds of meters high were observed at almost all stations. Associated dissipation was elevated in the bottom 500–1000 m but was strongest by far atop the western ridge on the northern line, where >500-m overturns resulted in dissipation exceeding 2 × 10−6 W kg−1 (implying diapycnal diffusivity Kρ > 0.2 m2 s−1). Integrated dissipation at this location is comparable to conversion and flux divergence terms in the energy budget. The authors speculate that resonance between the two ridges may partly explain the energetic motions and heightened dissipation.

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Amy F. Waterhouse
,
Jennifer A. MacKinnon
,
Jonathan D. Nash
,
Matthew H. Alford
,
Eric Kunze
,
Harper L. Simmons
,
Kurt L. Polzin
,
Louis C. St. Laurent
,
Oliver M. Sun
,
Robert Pinkel
,
Lynne D. Talley
,
Caitlin B. Whalen
,
Tycho N. Huussen
,
Glenn S. Carter
,
Ilker Fer
,
Stephanie Waterman
,
Alberto C. Naveira Garabato
,
Thomas B. Sanford
, and
Craig M. Lee

Abstract

The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with indirect measurements of mixing obtained from (i) Thorpe-scale overturns from moored profilers, a finescale parameterization applied to (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strain from full-depth lowered acoustic Doppler current profilers (LADCP) and CTD profiles. Vertical profiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10−4) m2 s−1 and above 1000-m depth is O(10−5) m2 s−1. The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variability in the ratio between local internal wave generation and local dissipation. In some regions, the depth-integrated dissipation rate is comparable to the estimated power input into the local internal wave field. In a few cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However, at most locations the total power lost through turbulent dissipation is less than the input into the local internal wave field. This suggests dissipation elsewhere, such as continental margins.

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Jennifer A. MacKinnon
,
Zhongxiang Zhao
,
Caitlin B. Whalen
,
Amy F. Waterhouse
,
David S. Trossman
,
Oliver M. Sun
,
Louis C. St. Laurent
,
Harper L. Simmons
,
Kurt Polzin
,
Robert Pinkel
,
Andrew Pickering
,
Nancy J. Norton
,
Jonathan D. Nash
,
Ruth Musgrave
,
Lynne M. Merchant
,
Angelique V. Melet
,
Benjamin Mater
,
Sonya Legg
,
William G. Large
,
Eric Kunze
,
Jody M. Klymak
,
Markus Jochum
,
Steven R. Jayne
,
Robert W. Hallberg
,
Stephen M. Griffies
,
Steve Diggs
,
Gokhan Danabasoglu
,
Eric P. Chassignet
,
Maarten C. Buijsman
,
Frank O. Bryan
,
Bruce P. Briegleb
,
Andrew Barna
,
Brian K. Arbic
,
Joseph K. Ansong
, and
Matthew H. Alford

Abstract

Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatiotemporal patterns of mixing are largely driven by the geography of generation, propagation, and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last 5 years and under the auspices of U.S. Climate Variability and Predictability Program (CLIVAR), a National Science Foundation (NSF)- and National Oceanic and Atmospheric Administration (NOAA)-supported Climate Process Team has been engaged in developing, implementing, and testing dynamics-based parameterizations for internal wave–driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here, we review recent progress, describe the tools developed, and discuss future directions.

Open access
Hemantha W. Wijesekera
,
Emily Shroyer
,
Amit Tandon
,
M. Ravichandran
,
Debasis Sengupta
,
S. U. P. Jinadasa
,
Harindra J. S. Fernando
,
Neeraj Agrawal
,
K. Arulananthan
,
G. S. Bhat
,
Mark Baumgartner
,
Jared Buckley
,
Luca Centurioni
,
Patrick Conry
,
J. Thomas Farrar
,
Arnold L. Gordon
,
Verena Hormann
,
Ewa Jarosz
,
Tommy G. Jensen
,
Shaun Johnston
,
Matthias Lankhorst
,
Craig M. Lee
,
Laura S. Leo
,
Iossif Lozovatsky
,
Andrew J. Lucas
,
Jennifer Mackinnon
,
Amala Mahadevan
,
Jonathan Nash
,
Melissa M. Omand
,
Hieu Pham
,
Robert Pinkel
,
Luc Rainville
,
Sanjiv Ramachandran
,
Daniel L. Rudnick
,
Sutanu Sarkar
,
Uwe Send
,
Rashmi Sharma
,
Harper Simmons
,
Kathleen M. Stafford
,
Louis St. Laurent
,
Karan Venayagamoorthy
,
Ramasamy Venkatesan
,
William J. Teague
,
David W. Wang
,
Amy F. Waterhouse
,
Robert Weller
, and
Caitlin B. Whalen

Abstract

Air–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange along the periphery of Sri Lanka, which includes the 100-km-wide East India Coastal Current (EICC) carrying low-salinity water out of the BoB and an adjacent, broad northward flow (∼300 km wide) that carries high-salinity water into BoB during the northeast monsoon. Atmospheric boundary layer (ABL) observations during the decaying phase of the Madden–Julian oscillation (MJO) permit the study of multiscale atmospheric processes associated with non-MJO phenomena and their impacts on the marine boundary layer. Underway analyses that integrate observations and numerical simulations shed light on how air–sea interactions control the ABL and upper-ocean processes.

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