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Edward D. Zaron

Abstract

The spatially averaged frequency spectrum of sea level has been computed at 4 cycle-per-year resolution and a Nyquist frequency of 0.5 cycles per hour using dual-satellite crossover data from the Jason and CryoSat-2 satellite altimeter missions. The novelty of the analysis is that it reveals unambiguous peaks due to high-frequency tidal signals, even after removing the predicted barotropic tide, without the usual aliasing caused by altimeter sampling. The tidal continuum, that is, a tidal cusp, is present in the spectrum in the diurnal and semidiurnal tidal bands, and a Lorentzian model spectrum has been fit within each band to identify the properties of the non-phase-locked tidal variability. An interesting feature of the semidiurnal tidal continuum is the unambiguous presence of an inner and an outer band, characterized by different Lorentzian bandwidths of roughly (180 day)−1 and (30 day)−1. Considering different latitude ranges, it is clear that the tidal continuum is most prominent in the range from −30° to 30° latitude. Within this range, it is found that 1.05-cm2 variance is associated with the semidiurnal continuum, and slightly less than half of this variance, 0.41 cm2, is associated with the slower, (180 day)−1 bandwidth, variability. The ratio of non-phase-locked to total baroclinic variability is about 62% in this latitude band, a value that is consistent with previous model-based estimates for this quantity. Quantification of the properties of the tidal continuum poleward of 30° latitude is not possible with the present data, due to the small size of the tidal signal compared to the mesoscale variability and other sources of noise.

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Hamed D. Ibrahim
and
Yunfang Sun

Abstract

The Atlantic multidecadal variability (AMV) switched from a cool to a warm phase in 1995 and the mean euphotic zone (EZT) and sea surface temperature (SST) shifted upward by 0.57° and 0.69°C, respectively, between 1982–91 and 2006–15 in the Atlantic region off northwest Africa. This ocean margin has many marine fisheries, and water temperature fluctuations may cause fish there to switch their habitats. Net radiation flux did not significantly change between these two decades. So, we hypothesized that the key driver of the EZT and SST increase is wind, which controls turbulent (sensible and latent) heat exchange with the atmosphere as well as bulk vertical and horizontal heat transport. Using satellite-derived SST and atmospheric and oceanic reanalyses to analyze the ocean top-200-m heat budget, we compared the relative contributions of the heat budget components to the cyclical changes in EZT and SST between these two decades. Results showed that the dominant heat source is horizontal heat flux convergence: weaker northeasterly trades and stronger southerly winds and monsoon enabled the southerly winds to drive warm water northward that subsequently warmed the domain. The dominant heat sink is latent heat loss: onshore–offshore atmospheric pressure gradients caused a complex wind adjustment that enabled the Sahara wind to accelerate evaporation over large subregions. These results highlight the important roles of ocean heat transport and atmosphere–ocean coupling for the tropical branch of the AMV. The regional EZT and SST anomalies associated with this AMV phase switch are mainly a consequence of wind-driven processes occurring at larger spatial scales.

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Xiangcheng Li
,
Xiaoping Cheng
,
Jianfang Fei
, and
Xiaogang Huang

Abstract

With two groups of numerical experiments with and without the cold-core eddy (CCE), the impacts of the CCE on the upper-ocean responses to Typhoon Trami (2018) were investigated using a coupled atmosphere–ocean model. It is commonly accepted that the CCE promotes the sea surface cooling (SSC) primally through the enhanced vertical mixing, while the contributions from the wind-driven advection and the near-inertial advection to the differences in the sea surface temperature (dSST) were underestimated. This study found that the presence of CCE contributed to the stronger along-track cold advection, which dominated the increase in the SSC near the radius of maximum wind (RMW) to the right of Trami’s track, and the stronger cross-track warm advection was acting to prevent the cooling induced by the vertical mixing. During the relaxation stage, the stronger near-inertial advection within the CCE accounted largely for the amplification and the redistribution of the dSST. As for the dynamic responses, the enhanced upwelling and downwelling within the CCE explained the larger cooling and warming in the subsurface temperature oscillations. The wind-driven acceleration of the currents in the mixing layer was larger during the typhoon–eddy interaction so that the CCE became an efficient mixer, thus contributing to the rapid surfacing of the cold water and the ensuing stronger wind-driven advection. These results highlight the importance of the advection processes in the modulating effect of the CCE. Therefore, 3D ocean models are needed to incorporate the mesoscale features of the oceanic eddies for realistically reproducing the upper-ocean responses to tropical cyclones (TCs).

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Renjian Li
and
Ming Li

Abstract

Using an idealized channel representative of a coastal plain estuary, we conducted numerical simulations to investigate the generation of internal lee waves by lateral circulation. It is shown that the lee waves can be generated across all salinity regimes in an estuary. Since the lateral currents are usually subcritical with respect to the lowest mode, mode-2 lee waves are most prevalent but a hydraulic jump may develop during the transition to subcritical flows in the deep channel, producing high energy dissipation and strong mixing. Unlike flows over a sill, stratified water in the deep channel may become stagnant such that a mode-1 depression wave can form higher up in the water column. With the lee wave Froude number above 1 and the intrinsic wave frequency between the inertial and buoyancy frequency, the lee waves generated in coastal plain estuaries are nonlinear waves with the wave amplitude Δh scaling approximately with V / N ¯ , where V is the maximum lateral flow velocity and N ¯ is the buoyancy frequency. The model results are summarized using the estuarine classification diagram based on the freshwater Froude number Fr f and the mixing parameter M. The Δh decreases with increasing Fr f as stronger stratification suppresses waves, and no internal waves are generated at large Fr f . The Δh initially increases with increasing M as the lateral flows become stronger with stronger tidal currents, but decreases or saturates to a certain amplitude as M further increases. This modeling study suggests that lee waves can be generated over a wide range of estuarine conditions.

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Zhongxiang Zhao

Abstract

Previous satellite estimates of internal tides are usually based on 25 years of sea surface height (SSH) data from 1993 to 2017 measured by exact-repeat (ER) altimetry missions. In this study, new satellite estimates of internal tides are based on 8 years of SSH data from 2011 to 2018 measured mainly by nonrepeat (NR) altimetry missions. The two datasets are labeled ER25yr and NR8yr, respectively. NR8yr has advantages over ER25yr in observing internal tides because of its shorter time coverage and denser ground tracks. Mode-1 M2 internal tides are mapped from both datasets following the same procedure that consists of two rounds of plane wave analysis with a spatial bandpass filter in between. The denser ground tracks of NR8yr make it possible to examine the impact of window size in the first-round plane wave analysis. Internal tides mapped using six different windows ranging from 40 to 160 km have almost the same results on global average, but smaller windows can better resolve isolated generation sources. The impact of time coverage is studied by comparing NR8yr160km and ER25yr160km, which are mapped using 160-km windows in the first-round plane wave analysis. They are evaluated using independent satellite altimetry data in 2020. NR8yr160km has larger model variance and can cause larger variance reduction, suggesting that NR8yr160km is a better model than ER25yr160km. Their global energies are 43.6 and 33.6 PJ, respectively, with a difference of 10 PJ. Their energy difference is a function of location.

Significance Statement

Our understanding of internal tides is mainly limited by the scarcity of field measurements with sufficient spatiotemporal resolution. Satellite altimetry offers a unique technique for observing and predicting internal tides on a global scale. Previous satellite observations of internal tides are mainly based on 25 years of data from exact-repeat altimetry missions. This paper demonstrates that internal tides can be mapped using 8 years of data made by nonrepeat altimetry missions. The new dataset has shorter time coverage and denser ground tracks; therefore, one can examine the impact of window size and time coverage on mapping internal tides from satellite altimetry. A comparison of models mapped from the two datasets sheds new light on the spatiotemporal variability of internal tides.

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Fan Xu
,
Zhao Jing
,
Peiran Yang
, and
Shenghui Zhou

Abstract

Geostrophic stress caused by a strong horizontal density gradient embedded in the surface boundary layer plays an important role in generating vertical motion and associated tracer transport. However, dependence of this frictionally driven vertical velocity on the Ekman number (Ek), a key dimensionless parameter for frictional flows in a rotating reference frame, has not been systematically analyzed, especially for a finite Ek. In this study, we theoretically demonstrate that the geostrophic stress always induces an ageostrophic stress acting to offset itself, and such an offsetting effect becomes more evident with increasing Ek. When Ek approaches unity or larger, vertical motion driven by geostrophic stress is much weaker than that derived by Garrett and Loder (GL81), who neglect effects of ageostrophic stress and predict a vertical velocity magnitude scaled with curl of geostrophic stress. Although the cancellation tendency between geostrophic and ageostrophic stress is universal, its underlying dynamics depends on vertical structures of turbulent viscosity and geostrophic flows. A realistic simulation in the winter Kuroshio Extension is conducted to validate the theoretical results and examine which regime, a small versus finite Ek, is more relevant in this region. It is found that the characteristic vertical scale involved in the definition of Ek is primarily determined by the vertical structure of turbulent viscosity and evidently smaller than that of geostrophic flow. The value of Ek in the winter Kuroshio Extension is generally larger than unity. Correspondingly, the GL81 model results in severe overestimation of the geostrophic stress-driven vertical velocity and tracer transport.

Open access
A. Anutaliya
,
U. Send
,
J. L. McClean
,
J. Sprintall
,
M. Lankhorst
,
C. M. Lee
,
L. Rainville
,
W. N. C. Priyadarshani
, and
S. U. P. Jinadasa

Abstract

Boundary currents along the Sri Lankan eastern and southern coasts serve as a pathway for salt exchange between the Bay of Bengal and the Arabian Sea basins in the northern Indian Ocean, which are characterized by their contrasting salinities. Measurements from two pairs of pressure-sensing inverted echo sounders (PIES) deployed along the Sri Lankan eastern and southern coasts as well as satellite measurements are used to understand the variability of these boundary currents and the associated salt transport. The volume transport in the surface (0–200-m depth) layer exhibits a seasonal cycle associated with the monsoonal wind reversal and interannual variability associated with the Indian Ocean dipole (IOD). In this layer, the boundary currents transport low-salinity water out of the Bay of Bengal during the northeast monsoon and transport high-salinity water into the Bay of Bengal during the fall monsoon transition of some years (e.g., 2015 and 2018). The Bay of Bengal salt input increases during the 2016 negative IOD as the eastward flow of high-salinity water during the fall monsoon transition intensifies, whereas the effect of the 2015/16 El Niño on the Bay of Bengal salt input is still unclear. The time-mean eddy salt flux over the upper 200 m estimated for the April 2015–March 2019 period along the eastern coast accounts for 9% of the salt budget required to balance an estimated 0.13 Sv (1 Sv ≡ 106 m3 s−1) of annual freshwater input into the Bay of Bengal. The time-mean eddy salt flux over the upper 200 m estimated for the December 2015–November 2019 period along the southern coast accounts for 27% of that same salt budget.

Significance Statement

In the northern Indian Ocean, the highly saline Arabian Sea undergoes extreme evaporation while the Bay of Bengal (BoB) receives excess freshwater input. The focus of this study is the role of the observed time-variable circulation around Sri Lanka that permits the exchange between these basins to maintain their salinity distributions. The circulation fluctuates seasonally following the monsoon wind reversal and interannually in response to large-scale climate modes. The BoB freshwater export around Sri Lanka occurs during the northeast monsoon, whereas saline water import occurs during the fall monsoon transition of some years. However, rapid changes in both water volume transport and salt exchange can occur. The circulation over 0–200-m depth transports ∼9%–27% of the BoB salt budget.

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Houssam Yassin
and
Stephen M. Griffies

Abstract

Numerical and observational evidence indicates that, in regions where mixed layer instability is active, the surface geostrophic velocity is largely induced by surface buoyancy anomalies. Yet, in these regions, the observed surface kinetic energy spectrum is steeper than predicted by uniformly stratified surface quasigeostrophic theory. By generalizing surface quasigeostrophic theory to account for variable stratification, we show that surface buoyancy anomalies can generate a variety of dynamical regimes depending on the stratification’s vertical structure. Buoyancy anomalies generate longer-range velocity fields over decreasing stratification and shorter-range velocity fields over increasing stratification. As a result, the surface kinetic energy spectrum is steeper over decreasing stratification than over increasing stratification. An exception occurs if the near-surface stratification is much larger than the deep-ocean stratification. In this case, we find an extremely local turbulent regime with surface buoyancy homogenization and a steep surface kinetic energy spectrum, similar to equivalent barotropic turbulence. By applying the variable stratification theory to the wintertime North Atlantic, and assuming that mixed layer instability acts as a narrowband small-scale surface buoyancy forcing, we obtain a predicted surface kinetic energy spectrum between k −4/3 and k −7/3, which is consistent with the observed wintertime k −2 spectrum. We conclude by suggesting a method of measuring the buoyancy frequency’s vertical structure using satellite observations.

Open access
Nadine Steiger
,
Elin Darelius
,
Satoshi Kimura
,
Ryan D. Patmore
, and
Anna K. Wåhlin

Abstract

The vertical front of ice shelves represents a topographic barrier for barotropic currents that transport a considerable amount of heat toward the ice shelves. The blocking effect of the ice front on barotropic currents has recently been observed to substantially reduce the heat transport into the cavity beneath the Getz Ice Shelf in West Antarctica. We use an idealized numerical model to study the vorticity dynamics of an externally forced barotropic current at an ice front and the impact of ice shelf thickness, ice front steepness, and ocean stratification on the volume flux entering the cavity. Our simulations show that thicker ice shelves block a larger volume of the barotropic flow, in agreement with geostrophic theory. However, geostrophy breaks locally at the ice front, where relative vorticity and friction become essential for the flow to cross the discontinuity in water column thickness. The flow entering the cavity accelerates and induces high basal melt rates in the frontal region. Tilting the ice front, as undertaken in sigma-coordinate models, reduces this acceleration because the flow is more geostrophic. Viscous processes—typically exaggerated in low-resolution models—break the potential vorticity constraint and bring the flow deeper into the ice shelf cavity. The externally forced barotropic current can only enter the cavity if the stratification is weak, as strong vertical velocities are needed at the ice front to squeeze the water column beneath the ice shelf. If the stratification is strong, vertical velocities are suppressed and the barotropic flow is almost entirely blocked by the ice front.

Significance Statement

Ice shelves in West Antarctica are thinning, mostly from basal melting through oceanic heat entering the underlying ice shelf cavities. Thinning of ice shelves reduces their ability to buttress the grounded ice resting upstream, leading to sea level rise. To model the ice sheet’s contribution to sea level rise more accurately, the processes governing the oceanic heat flux into the ice shelf cavity must be articulated. This modeling study investigates the dynamics of a depth-independent current approaching the ice shelf; it corroborates previous findings on the blocking of such a current at the ice front. The amount of water that enters the cavity strongly depends on ice shelf thickness and ocean stratification. For a well-mixed ocean, the upper part of the flow can dive underneath the ice shelf and increase basal melting near the ice front. In a stratified ocean, the approaching depth-independent current is almost entirely blocked by the ice front.

Open access
Arthur Mouragues
,
Kévin Martins
,
Philippe Bonneton
, and
Bruno Castelle

Abstract

A wave-group-resolving model is used to investigate the driving mechanisms and the spatiotemporal variability of very low frequency (VLF) fluctuations of a headland deflection rip, measured during a 4-m oblique wave event. Surfzone eddies (SZE) occurring in the presence of a strongly sheared longshore current V at a longshore-uniform beach are first modeled. The spectral signature and the variability of SZE are displayed and compared with the literature. The model is then used to explore the dynamics of vorticity in the surf zone and against a headland under energetic oblique wave conditions. The resulting weakly sheared V is found to host large-scale SZE propagating toward the headland at a speed decreasing seaward. Vorticity animations and spectral diagrams indicate that VLF fluctuations of the deflection rip are driven by the deflection of the upstream SZE. In line with measurements, periods from 40 min to 1 h dominate the spectrum hundreds of meters from the headland at low tide. At high tide, vorticity spectra in the rip are much narrower than in the surf zone, suggesting that the headland enforces the merging of SZE. This mechanism is further analyzed using idealized simulations with varying headland lengths, aiming at extending traditional deflection patterns at the VLF scale. Finally, we discuss the existence of a continuum in SZE driving mechanisms, going from fully wave-group-driven to both wave-group- and shear-instability-driven SZE for weakly and strongly sheared V, respectively. This continuum suggests the importance of wave groups to produce SZE under energetic wave conditions.

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