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Lei Liu
and
Huijie Xue

Abstract

Observational surface data are utilized to reconstruct the subsurface density and geostrophic velocity fields via the “interior + surface quasigeostrophic” (isQG) method in a subdomain of the Antarctic Circumpolar Current (ACC). The input variables include the satellite-derived sea surface height (SSH), satellite-derived sea surface temperature (SST), satellite-derived or Argo-based sea surface salinity (SSS), and a monthly estimate of the stratification. The density reconstruction is assessed against a newly released high-resolution in situ dataset that is collected by a southern elephant seal. The results show that the observed mesoscale structures are reasonably reconstructed. In the Argo-SSS-based experiment, pattern correlations between the reconstructed and observed density mostly exceed 0.8 in the upper 300 m. Uncertainties in the SSS products notably influence the isQG performance, and the Argo-SSS-based experiment yields better density reconstruction than the satellite-SSS-based one. Through the two-dimensional (2D) omega equation, we further employ the isQG reconstructions to diagnose the upper-ocean vertical velocities (denoted w isQG2D), which are then compared against the seal-data-based 2D diagnosis of w seal. Notable discrepancies are found between w isQG2D and w seal, primarily because the density reconstruction does not capture the seal-observed smaller-scale signals. Within several subtransects, the Argo-SSS-based w isQG2D reasonably reproduce the spatial structures of w seal, but present smaller magnitude. We also apply the isQG reconstructions to the 3D omega equation, and the 3D diagnosis of w isQG3D is very different from w isQG2D, indicating the limitations of the 2D diagnostic equation. With reduced uncertainties in satellite-derived products in the future, we expect the isQG framework to achieve better subsurface estimations.

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Marco Larrañaga
,
Lionel Renault
, and
Julien Jouanno

Abstract

The surface oceanic current feedback (CFB) to the atmosphere has been shown to correct long-lasting biases in the representation of ocean dynamics by providing an unambiguous energy sink mechanism. However, its effects on the Gulf of Mexico (GoM) oceanic circulation are not known. Here, twin ocean–atmosphere eddy-rich coupled simulations, with and without CFB, are performed for the period 1993–2016 over the GoM to assess to which extent CFB modulates the GoM dynamics. CFB, through the eddy killing mechanism and the associated transfer of momentum from mesoscale currents to the atmosphere, damps the mesoscale activity by roughly 20% and alters eddy statistics. We furthermore show that the Loop Current (LC) extensions can be classified into three categories: a retracted LC, a canonical LC, and an elongated LC. CFB, by damping the mesoscale activity, enhance the occurrence of the elongated category (by about 7%). Finally, by increasing the LC extension, CFB plays a key role in determining LC eddy separations and statistics. Taking into account CFB improves the representation of the GoM dynamics, and it should be taken into account in ocean models.

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Xiaoting Yang
and
Eli Tziperman

Abstract

The middepth ocean temperature profile was found by Munk in 1966 to agree with an exponential profile and shown to be consistent with a vertical advective–diffusive balance. However, tracer release experiments show that vertical diffusivity in the middepth ocean is an order of magnitude too small to explain the observed 1-km exponential scale. Alternative mechanisms suggested that nearly all middepth water upwells adiabatically in the Southern Ocean (SO). In this picture, SO eddies and wind set SO isopycnal slopes and therefore determine a nonvanishing middepth interior stratification even in the adiabatic limit. The effect of SO eddies on SO isopycnal slopes can be understood via either a marginal criticality condition or a near-vanishing SO residual deep overturning condition in the adiabatic limit. We examine the interplay between SO dynamics and interior mixing in setting the exponential profiles of σ 2 and ∂ zσ 2. We use eddy-permitting numerical simulations, in which we artificially change the diapycnal mixing only away from the SO. We find that SO isopycnal slopes change in response to changes in the interior diapycnal mixing even when the wind forcing is constant, consistent with previous studies (that did not address these near-exponential profiles). However, in the limit of small interior mixing, the interior ∂ zσ 2 profile is not exponential, suggesting that SO processes alone, in an adiabatic limit, do not lead to the observed near-exponential structures of such profiles. The results suggest that while SO wind and eddies contribute to the nonvanishing middepth interior stratification, the exponential shape of the ∂ zσ 2 profiles must also involve interior diapycnal mixing.

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Jemima Rama
,
Callum J. Shakespeare
, and
Andrew McC. Hogg

Abstract

Wind-generated near-inertial internal waves (NIWs) are triggered in the mixed layer and propagate down into the ocean interior. Observational and numerical studies have shown the effects of background vorticity and high shear on propagating NIWs. However, the impacts of the background mean flow on NIWs as a function of the waves’ horizontal wavelength have yet to be fully investigated. Here, two distinct cases are analyzed, namely, the propagation of wind-generated, large-scale NIWs in negative vorticity and the behavior of small-scale NIWs in high shear. The propagation and energetics of the respective NIWs are investigated using a realistic eddy-resolving numerical simulation of the Kuroshio region. The large-scale NIWs display a rapid vertical propagation to depth in negative vorticity areas, while the small-scale NIWs are confined to shallower depths in high-shear regions. Furthermore, the dominant energy sources and sinks of near-inertial energy are estimated as the respective NIWs propagate into the ocean’s interior. The qualitative analysis of NIW energetics reveals that the wind triggers the generation of both the large-scale and small-scale NIWs, but the waves experience further amplification as they draw energy from the background mean flow upon propagation in negative vorticity and high-shear regions, respectively. In addition, the study demonstrates that small-scale NIWs can be induced independently by wind fluctuations and do not necessarily rely on straining nor refraction of large-scale NIWs by mesoscale motions.

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Mareike Körner
,
Martin Claus
,
Peter Brandt
, and
Franz Philip Tuchen

Abstract

In the equatorial Atlantic Ocean, meridional velocity variability exhibits a pronounced peak on intraseasonal time scales whereas zonal velocity dominantly varies on seasonal to interannual time scales. We focus on the intraseasonal meridional velocity variability away from the near-surface layer, its source regions, and its pathways into the deep ocean. This deep intraseasonal velocity variability plays a key role in equatorial dynamics as it is an important energy source for the deep equatorial circulation. The results are based on the output of a high-resolution ocean model revealing intraseasonal energy levels along the equator at all depths that are in good agreement with shipboard and moored velocity data. Spectral analyses reveal a pronounced signal of intraseasonal Yanai waves with westward phase velocities and zonal wavelengths longer than 450 km. Different sources and characteristics of these Yanai waves are identified: near the surface between 40° and 10°W, low-baroclinic-mode Yanai waves with periods of around 30 days are excited. These waves have a strong seasonal cycle with a maximum in August. High-frequency Yanai waves (10–20-day period) are excited at the surface east of 10°W. In the region between the North Brazil Current and the Equatorial Undercurrent, high-baroclinic-mode Yanai waves with periods between 30 and 40 days are generated. Yanai waves with longer periods (40–80 days) are shed from the deep western boundary current. The Yanai wave energy is carried along beams eastward and downward, thus explaining differences in strength, structure, and periodicity of the meridional intraseasonal variability in the equatorial Atlantic Ocean.

Significance Statement

Past studies show that intraseasonal meridional kinetic energy is important for the deep equatorial circulation (DEC). However, numerical studies use intraseasonal variability with varying characteristics to investigate the formation and maintenance of the DEC. This is partly because of sparse observations at depth that are limited to single locations. This study investigates intraseasonal meridional kinetic energy in the equatorial Atlantic in a high-resolution ocean model that is tested against available shipboard and moored observations. We analyze the spatial and temporal distribution and the baroclinic structure of intraseasonal variability. Using the model, we identify different sources and pathways of intraseasonal energy in the deep equatorial Atlantic. We offer groundwork for further studies on the formation and maintenance of the DEC.

Open access
Haihong Guo
,
Michael A. Spall
,
Joseph Pedlosky
, and
Zhaohui Chen

Abstract

A three-dimensional inertial model that conserves quasigeostrophic potential vorticity is proposed for wind-driven coastal upwelling along western boundaries. The dominant response to upwelling favorable winds is a surface-intensified baroclinic meridional boundary current with a subsurface countercurrent. The width of the current is not the baroclinic deformation radius but instead scales with the inertial boundary layer thickness while the depth scales as the ratio of the inertial boundary layer thickness to the baroclinic deformation radius. Thus, the boundary current scales depend on the stratification, wind stress, Coriolis parameter, and its meridional variation. In contrast to two-dimensional wind-driven coastal upwelling, the source waters that feed the Ekman upwelling are provided over the depth scale of this baroclinic current through a combination of onshore barotropic flow and from alongshore in the narrow boundary current. Topography forces an additional current whose characteristics depend on the topographic slope and width. For topography wider than the inertial boundary layer thickness the current is bottom intensified, while for narrow topography the current is wave-like in the vertical and trapped over the topography within the inertial boundary layer. An idealized primitive equation numerical model produces a similar baroclinic boundary current whose vertical length scale agrees with the theoretical scaling for both upwelling and downwelling favorable winds.

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Thilo Klenz
,
Harper L. Simmons
,
Luca Centurioni
,
Jonathan M. Lilly
,
Jeffrey J. Early
, and
Verena Hormann

Abstract

The Minimet is a Lagrangian surface drifter measuring near-surface winds in situ. Ten Minimets were deployed in the Iceland Basin over the course of two field seasons in 2018 and 2019. We compared Minimet wind measurements to coincident ship winds from the R/V Armstrong meteorology package and to hourly ERA5 reanalysis winds and found that the Minimets accurately captured wind variability across a variety of time scales. Comparisons between the ship, Minimets, and ERA5 winds point to significant discrepancies between the in situ wind measurements and ERA5, with the most reasonable explanation being related to spatial offsets of small-scale storm structures in the reanalysis model. After a general assessment of the Minimet performance, we compare estimates of wind power input in the near-inertial band using the Minimet winds and their measured drift to those using ERA5 winds and the Minimet drift. Minimet-derived near-inertial wind power estimates exceed those from Minimet drift combined with ERA5 winds by about 42%. The results highlight the importance of accurately capturing small-scale, high-frequency wind events and suggest that in situ Minimet measurements are beneficial for accurately quantifying near-inertial wind work on the ocean.

Significance Statement

In this study we introduce a novel, freely drifting wind measurement platform, the Minimet. After an initial validation of Minimet sea surface wind measurements against independent wind measurements from a nearby research vessel, we investigate their utility in context of the near-inertial work done by the wind on the ocean, which is important for the ocean’s energy budget. We find Minimet near-inertial wind work estimates exceed those estimated using winds from a state-of-the-art wind product by 42%. Our results indicate that capturing storm events happening on time scales less than 12 h is crucial for accurately quantifying near-inertial wind work on the ocean, making wind measurements from platforms such as the Minimet invaluable for these analyses.

Open access
Zhibin Yang
,
Zhao Jing
, and
Xiaoming Zhai

Abstract

Mesoscale eddies are ubiquitous dynamical features, accounting for over 90% of the total kinetic energy of the ocean. However, the pathway for eddy energy dissipation has not been fully understood. Here we investigate the effect of small-scale topography on eddy dissipation in the northern South China Sea by comparing high-resolution ocean simulations with smooth and synthetically generated rough topography. The presence of rough topography is found to 1) significantly enhance viscous dissipation and instabilities within a few hundred meters above the rough bottom, especially in the slope region, and 2) change the relative importance of energy dissipation by bottom frictional drag and interior viscosity. The role of lee wave generation in eddy energy dissipation is investigated using a Lagrangian filter method. About one-third of the enhanced viscous energy dissipation in the rough topography experiment is associated with lee wave energy dissipation, with the remaining two-thirds explained by nonwave energy dissipation, at least partly as a result of the nonpropagating form drag effect.

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Luc Rainville
,
Craig M. Lee
,
K. Arulananthan
,
S. U. P. Jinadasa
,
Harindra J. S. Fernando
,
W. N. C. Priyadarshani
, and
Hemantha Wijesekera

Abstract

We present high-resolution sustained, persistent observations of the ocean around Sri Lanka from autonomous gliders collected over several years, a region with complex, variable circulation patterns connecting the Bay of Bengal and the Arabian Sea to each other and the rest of the Indian Ocean. The Seaglider surveys resolve seasonal to interannual variability in vertical and horizontal structure, allowing quantification of volume, heat, and freshwater fluxes, as well as the transformations and transports of key water mass classes across sections normal to the east (2014–15) and south (2016–19) coasts of Sri Lanka. The resulting transports point to the importance of both surface and subsurface flows and show that the direct pathway along the Sri Lankan coast plays a significant role in the exchanges of waters between the Arabian Sea and the Bay of Bengal. Significant section-to-section variability highlights the need for sustained, long-term observations to quantify the circulation pathways and dynamics associated with exchange between the Bay of Bengal and Arabian Sea and provides context for interpreting observations collected as “snapshots” of more limited duration.

Significance Statement

The strong seasonal variations of the wind in the Indian Ocean create large and rapid changes in the ocean’s properties near Sri Lanka. This variable and poorly observed circulation is very important for how temperature and salinity are distributed across the northern Indian Ocean, both at the surface and at depths. Long-term and repeated surveys from autonomous Seagliders allow us to understand how freshwater inflow, atmospheric forcing, and underlying ocean variability act to produce observed contrasts (spatial and seasonal) in upper-ocean structure of the Bay of Bengal and Arabian Sea.

Open access
Martin Lazar
,
Maja Bubalo
, and
Josip Begić

Abstract

The paper investigates switches of circulation orientation in inland basins, either at the surface or near the bottom. The study is based on an analytical 2D model used to simulate thermohaline circulation in lakes and inland seas. The model allows different density profiles varying in both horizontal and vertical directions. By assuming some simplifications (such as steady state, vanishing of an alongshore variability, and flat bottom), we are able to obtain an explicit expression of the circulation in the central transverse section of an elongated basin. Starting from three typical density profiles (bottom dense water, surface light water, and a combination of the two), the model reveals different circulation types (cyclonic and anticyclonic surface circulation, either prevailing along the whole vertical column or accompanied by an opposite circulation in the bottom layer). In addition, we analyze the impact of friction coefficients and basin dimensions on the switch from one circulation type to another. The simplified assumptions turn out not to be limiting, as other studies have shown that they do not change the main flow characteristics. More importantly, the results obtained are in keeping with empirical findings, numerical simulations, and physical experiments studied elsewhere.

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