Oceanography | Vol. 37, No. 4
INTRODUCTION
Near-inertial internal waves (NIW)
constitute a dominant mode of high-
frequency variability in the ocean’s inte-
rior, comprising about half the kinetic
energy in the ocean at most sites (and
even more in the winter beneath storm
tracks; Alford et al., 2016). Over the last
decade there has been a signifcant focus
in the physical oceanographic commu-
nity on internal tides, which produce large
thermocline displacements, afect sound
propagation, and control some hotspots
of elevated turbulent mixing. Near-
inertial internal gravity waves, which are
primarily generated not by tides but by
winds, are of similar importance, provid-
ing comparable kinetic energy and the
vast majority of the shear variance, and
likely leading to a substantial amount of
turbulent mixing. Signifcant defciencies
remain in our understanding of the phys-
ical processes that determine their gener-
ation, evolution, and destruction.
No existing regional or global numer-
ical models fully account for the gener-
ation, radiation, and breaking of NIWs,
largely because of the need for high reso-
lution to resolve the high-mode structure
and because the physics is not sufciently
understood. Te NIW problem has been
difcult to address, partially due to the
episodic nature of wind generation and
the nonlinear physics involved. Te sem-
inal experimental study of NIWs was the
Ocean Storms Experiment (OSE), which
took place in the late 1980s (D’Asaro
et al., 1995). Te main focus of the OSE
was on the larger-scale lateral structure of
NIWs, which theory predicts is shaped by
Earth’s curvature through the so-called
beta efect (Gill, 1984). During the OSE,
the role of the beta efect in leading to the
initial growth of horizontal gradients in
the NIW feld was clearly demonstrated,
leading to a qualitative agreement with
theory. However, the theory could not
reproduce the observed “beam,” wherein
energy migrated quickly downward with
time from the mixed layer following
storm events. An important consequence
is that neither the decay of mixed-layer
motions nor the rate of energy transfer
into the deep ocean can adequately be
predicted for the best-documented storm
response on record. Tis conundrum
has remained for the past 35 years since
these data were collected, in part because
the OSE data lacked sufcient vertical
and horizontal resolution to quantify the
detailed structures of the NIWs and their
evolution. Moreover, the vital question of
the distribution of mixing by the NIWs
was unaddressed by the OSE.
Motivated by these questions, in 2016
the US Ofce of Naval Research spon-
sored the Near-Inertial Shear and Kinetic
Energy in the North Atlantic experiment
(NISKINe). Te objective was to exam-
ine how NIWs rapidly radiate out of the
mixed layer by developing smaller-scale
horizontal structures through interaction
with ocean eddies and how NIWs gener-
ate turbulence and mixing. Conducted
in the eddy-rich, stormy North Atlantic
during certain periods from 2018 to 2020,
NISKINe utilized conceptualized studies,
numerical modeling, and the latest tech-
nology to make direct, high-resolution
observations of the NIW feld to examine
the physics. Here, we describe some high-
lights of the multi-year study and intro-
duce a collection of articles that elaborate
on the fndings.
NISKINe
NISKINe combined observational, mod-
eling, and theoretical approaches to
underpin the at-sea science. Te program
integrated results from three feld years
in the Iceland Basin: a 2018 pilot study,
a 2019 full-scale deployment, and a mod-
est (pandemic impacted) efort in 2020.
Tese data collection eforts were central
to NISKINe, as they formed the basis for
theoretical and process-oriented model-
ing eforts. Process-oriented studies that
addressed NIW generation, NIW-eddy
interactions, and the role of surface waves
in afecting the energy input to NIWs
included those by Asselin and Young
(2020), Asselin et al. (2020), Barkan
et al. (2021), Skyllingstad et al. (2023),
and Stokes et al. (2024). Tese detailed
works were framed by studies utilizing
global ocean models for broader under-
standing of NIW signifcance including
Arbic et al. (2022), Raja et al. (2022), and
Yang et al. (2023).
For the 2018 pilot experiment, a
dipole in the Icelandic Basin identifed
INTRODUCTION
THE NEAR-INERTIAL SHEAR AND KINETIC ENERGY
IN THE NORTH ATLANTIC EXPERIMENT
By Harper L. Simmons, Louis St. Laurent, Luc Rainville, and Leif Thomas
INTRODUCTION TO THE
SPECIAL ISSUE ON NISKINe