Inertial Bands Research Articles

Fewer tropical cyclones yield more near-inertial wind work to the global ocean over the past four decades

In general, tropical cyclones (TCs) will inject energy into oceanic inertial motion‒a prevalent phenomenon in the ocean. Under global warming, the intensity of TCs is on the rise, while their frequency has exhibited a decline since 2000. However, the long-term trend of this energy infusion is an underexplored problem in this context. Using a damped-slab model, we computed the wind work exerted by TCs on the ocean’s mixed-layer inertial motions. Our results show that the global wind work has increased by approximately 50% from 1979 to 2023. The wind work increase of strong TCs (Saffir–Simpson levels 4–5) is the major contributor to the increasing trend of global wind work, primarily due to their increasing frequency and substantial wind stress. At basin scale, the wind work input of the North Atlantic TCs has increased by 2 times, owing to an increase in both their intensity and frequency. Specifically, in the South Indian and the eastern North Pacific basins, the rise in wind work is primarily attributed to the enhanced wind energy of TCs within the inertial bands.

Observed Near Inertial Waves in the Wake of Typhoon Linfa (2015) in the Northern South China Sea

This study examined the characteristics and vertical propagation of near inertial waves (NIWs) induced by Typhoon Linfa (2015), based on in situ observations conducted southeast of Dongsha Islands in the South China Sea. The results demonstrate that the near inertial currents induced by Linfa had velocities up to 35cm s−1 in the mixed layer and 20cm s−1 in the ocean interior. The near inertial currents were polarized with predominantly clockwise-rotating components, the magnitudes of which were about 10 times larger than the counter-clockwise rotating components. The energy density spectrum showed that the emergence of NIWs resulted in energy redistribution from the diurnal band to the near inertial band. The wavenumber spectrum and the downward/ upward current decomposition demonstrated that the NIWs and energy flux propagated mainly downward. The estimated vertical phase velocity and group velocity are 1.44 and 0.48m h−1, respectively, corresponding to a vertical wavelength of 49.7m. The e-folding time scale was 7.5 d based on the near inertial kinetic energy in the ocean interior. We found no obvious wave-wave interaction during the decay process of the NIWs. The frequency was blue-shifted, being 0.03f0 higher than the local inertial frequency, which was caused by the background vorticity. The normal mode analysis suggests that the higher mode plays a dominant role in the propagation stage of the NIWs.

On the Nature of Near-Inertial Oscillations in the Uppermost Part of the Ocean and a Possible Route toward HF Radar Probing of Stratification

AbstractInertial band response of the upper ocean to changing wind is studied both theoretically and by analysis of observations in the northwestern Mediterranean. On the nontraditional f plane, because of the horizontal component of the earth’s rotation for waves of inertial band with frequencies slightly below the local inertial frequency f, there is a waveguide in the mixed layer confined from below by the pycnocline. It is argued that when the stratification is shallow these waves are most easily and strongly excited by varying winds as near-inertial oscillations (NIOs). These motions have been overlooked in previous studies because they are absent under the traditional approximation. The observations that employed buoys with thermistors, ADCPs, and two 16.3-MHz Wellen Radar (WERA) HF radars were carried out in the Gulf of Lion in April–June 2006. The observations support the theoretical picture: a pronounced inertial band response occurs only in the presence of shallow stratification and is confined to the mixed layer, and the NIO penetration below the stratified layer is weak. NIO surface magnitude and vertical localization are strongly affected by the presence of even weak density stratification in the upper 10 m. The NIO surface signatures are easily captured by HF radars. Continuous 1.8-yr HF observations near the Porquerolles Island confirm that shallow stratification is indeed the precondition for a strong NIO response. The response sensitivity to stratification provides a foundation for developing HF radar probing of stratification and, indirectly, vertical mixing, including spotting dramatic mixing events and spikes of vertical heat, mass, and momentum exchange.

Surface circulation in the Gulf of Trieste (northern Adriatic Sea) from radar, model, and ADCP comparisons

[1] High resolution high frequency (HF) radar observations of surface currents in the Gulf of Trieste (GoT) are presented and compared to moored subsurface current measurements and to high-resolution simulations from the Northern Adriatic Princeton Ocean Model (NAPOM) oceanographic model. Dominant circulation modes of the GoT were resolved and numerical model capabilities in reconstructing them were assessed. The time frame covers March 2011 through October 2012. NAPOM reconstructs the dominant surface circulation features observed by the radar, such as the general basin-wide cyclonic circulation scheme and the coastal jet outflowing the GoT, but is in general less energetic than radar currents. Comparisons between radar, ADCP, and model currents suggest that the model underestimates originate within the low-frequency, diurnal, semidiurnal, and inertial bands, and that both radar and model currents do not reproduce the diurnal tidal ellipse structure observed by the moored current meter. However, radar-model results for the semidiurnal tides are spatially consistent. Using empirical orthogonal function (EOF) decomposition, the coherent spatial scales and corresponding temporal scales were extracted. Findings suggest that HF radar current observations complement model simulations in regions of enhanced topographic variability where variance of model currents at the surface is distorted by the effects of the sigma layer grid. On the contrary, model results complement radar observations in areas with poor radar coverage, and furthermore provide spatial and temporal continuity of ocean state forecasts.

The influence of diurnal winds on phytoplankton dynamics in a coastal upwelling system off southwestern Africa

At a coastal upwelling zone near 30°S latitude, diurnal wind variability forced energetic inertial current oscillations (>0.5ms−1) that materially influenced phytoplankton distribution and productivity. The diurnal–inertial band resonance found at this latitude in the Benguela upwelling system allowed rapid, efficient transfer of energy from counterclockwise rotating winds into anticyclonic currents upon the onset of the transition from relaxation to upwelling conditions. These inertial band oscillations caused regular pycnocline outcropping at the surface and the vertical advection of nutrient-rich waters in the coastal zone. Vertical pycnocline outcropping was coincident with the vertical redistribution of chlorophyll a fluorescence from a subsurface maximum to entrainment into the surface mixed layer, in effect turning vertical phytoplankton gradients into horizontal ones. The shear caused by the vertical structure of the inertial oscillations during (and after) the onset of wind forcing was intense enough to erode the strong stratification established during a prior relaxation period, according to Richardson number and strain analyses. This diapycnal mixing also had the consequence of mixing heat and chlorophyll downwards and nutrient-rich water upwards, such that the surface nitrate concentration became non-zero. Chlorophyll concentrations thereafter increased in what qualitatively appeared to be a phytoplankton bloom. This diurnal–inertial resonance-driven mechanism for mixing-driven nutrient flux, embedded within the low-frequency advective vertical flux forced by Ekman dynamics, enhanced the efficiency of wind forcing to produce high phytoplankton productivity, and is likely to be of first-order importance in bloom dynamics in the study area (including harmful algal blooms). Our results argue that, in general, high-frequency physical dynamics should be considered when studying the bottom-up forcing of algal blooms and red tide events.

Seasonal to tidal variability of currents and temperature in waters of the continental slope, southeastern Bay of Biscay

Since 2007, high temporal resolution time-series of currents, temperature and salinity, from two oceanic moorings, have provided information on the local water column dynamics and hydrography, over the slope of the southeastern part of the Bay of Biscay. In this study, these data are analysed to describe the variability observed in currents and stratification conditions, at different time scales. A well-defined seasonal cycle is the dominant signal in temperature and salinity, with a surface mixed layer that shows temperatures over 20°C, low salinities and depths between 30 and 50m from July to September. Likewise, currents present marked seasonal and mesoscale variability. The most intense currents, oriented to the E–SE, occur from November to January, when the Iberian Poleward Current is sampled by both moorings. Much weaker currents, mainly oriented to the SW, are observed during the stratified months. Vertically, the flow is markedly barotropic and shows low variability at that scales, although vertical coherence decreases for the stratified period and significant vertical variability linked to high frequency processes is observed. Since energy contents around the main tidal peaks are lower than in other areas of the Bay, the contribution of the inertial band to the high frequency variability appears to be especially important in this region. The results obtained suggest that inertial oscillations contribute significantly to the vertical shear during the stratified period and might have a significant role in favouring the vertical mixing over the area.

Subsynoptic scale spatial variability of sea ice deformation in the western Weddell Sea during early summer

Ice Station Polarstern (ISPOL), deployed in the western Weddell Sea from November 2004 to January 2005, included a study of subsynoptic scale variability in sea ice velocity and deformation using an array of 24 buoys. Upon deployment, the ISPOL buoy array measured 70 km in both zonal and meridional extent and consisted of subarrays that resolved sea ice deformation on scales from 10 to 70 km. Across the ISPOL array, divergence varied and did not show a distinct coherent length scale. Spectral analysis of divergence and shear of subarrays revealed that deformation did not vary smoothly across the array. This indicates variability of internal ice stress on the scale of 10 km. Ice conditions within the ISPOL array encompassed two distinct regimes separated by shear along the continental shelf break. Differences in spectral power of the tidal and inertial bands across the two regions do not mirror expected differences due to spatial variability of tide‐induced deformation on the shelf break. Instead, they indicate that the pack ice's internal stress behaved anisotropically on the scale of the shear zone (across the buoy array, 70 km).

Near-Resonant Ocean Response to Sea Breeze on a Stratified Continental Shelf

Abstract The spatial structure and temporal characteristics of sea breeze and the associated coastal ocean response in the northwest Gulf of Mexico are investigated using moored instruments, hydrographic stations, and wind measurements. Near the study area of 30°N, motions in the diurnal–inertial band (DIB) may be significantly enhanced by a near-resonant condition between local inertial and diurnal forcing frequencies. Wavelet analysis is used to quantify the results. Results indicate that diurnal sea-breeze variability peaks in summer and extends at least 300 km offshore with continuous seaward phase propagation. The maximum DIB oceanic response occurs in June when there is a shallow mixed layer, strong stratification, and an approximately 10-day period of continuous sea-breeze forcing. DIB current variance decreases in July and August as the consequence of the deepening of the mixed layer and a more variable phase relationship between the wind and current. River discharge varies interannually and can significantly alter the oceanic response during summer. The “great flood” of the Mississippi River in 1993 deepened the summer mixed layer and reduced the sea-breeze response during that year. Vertically, DIB currents are surface intensified, with a first baroclinic modal structure. The significance of these DIB motions on the shelf is that they can provide considerable vertical mixing in summer, as seen by the suppression of the bulk Richardson number (by a factor of 30) during strong DIB events. This provides a potential mechanism to ventilate seasonally occurring near-bottom hypoxic waters of the coastal ocean.

Variations of diurnal and inertial spectral peaks near the bi‐diurnal critical latitude

Three sets of ADCP data obtained from the upper ocean are used to examine possible influence of parametric subharmonic instability (PSI) at ϕc ≈ 13–15°N. Both kinetic‐energy and shear spectra at ϕc reveal that significant peaks appear at subharmonic frequencies of diurnal internal tides. Especially, the 0.5 K1 (subharmonic of K1 tides) can be trapped poleward of its critical latitude (14.52°N), significantly distinguished from wind generated near‐inertial internal waves. Moreover, the enhanced 0.5 K1 motions are more or less subject to a fortnightly spring‐neap circle. Relative to a higher latitude (say 18°N), diurnal motions at ϕc are greatly weakened. In contrast, small vertical‐scale motions in the inertial band and the shear induced by these motions are significantly enhanced. Likely, PSI mechanism plays an important role in these observations, whilst trapped sub‐inertial waves (0.5 K1) may be associated with the presence of negative sub‐inertial eddies.

Current oscillations in the diurnal–inertial band on the Catalonian shelf in spring

Measurements with acoustic Doppler current profilers and conventional current meters on the Catalonian shelf indicate that the energy spectrum is dominated by oscillations in the diurnal–inertial band. The oscillations take the form of strong clockwise near circular motions with near surface speeds of up to 15 cm s −1. Motion of comparable magnitude is found to penetrate down through the water column to the sea bed at all depths across the shelf and to the shelf edge (200 m). Analysis of the vertical structure reveals a persistent phase shift of close to 180° between surface and near bed velocities at both inertial and diurnal frequencies, which are observed to increase as one moves off-shore. The putative source of forcing for these motions is diurnal by on-shore–off-shore windstress with a strong response to the diurnal clockwise component of stress being favoured by the proximity of the site (latitude 40°N) to the critical latitude for diurnal–inertial resonance (30°N). A frictionless, two-layer, analytical model is used to demonstrate this feature of the flow and to explain both the depth penetration and phase reversal of the oscillations. The model, which is forced only by a diurnal on-shore–off-shore windstress, incorporates a condition from the vertically integrated solution of Craig (Continental Shelf Res. 9 (1989) 965) which relates the cross-slope pressure gradient to the forcing stress. This condition results in a component of forcing in the lower layers which is 180° out of phase with the surface stress. The model is extended in a numerical solution to a multilayer structure with frictional coupling between the layers via an eddy viscosity. The results confirm the central role of the pressure gradient forcing even when upper and lower layers are not de-coupled by an intervening pycnocline.

Satellite‐tracked drifter measurement of inertial and semidiurnal currents in the northeast Pacific

We examine current variability in the open northeast Pacific using 3‐hourly sampled trajectories from satellite‐tracked drifters drogued at depths of 15 and 120 m within the wind‐mixed layer and permanent pycnocline, respectively. Findings show the following. (1) Motions were dominated by inertial currents which accounted for 58% (35%) of the total variance measured by the shallow (deep) drifters. Semidiurnal motions accounted for roughly 10% of the variance at both depths. There were detectable currents at twice the inertial frequency (2f ≈ 3 cpd) and negligible diurnal currents. (2) Inertial currents attained root‐mean‐square (rms) speeds of around 50 cm/s in the mixed layer and 10 cm/s in the pycnocline. Speeds in the semidiurnal band were similar (≈10 cm/s) at both depths. (3) Inertial and semidiurnal currents were strongly clockwise rotary, while low‐frequency currents (<1 cpd) were primarily rectilinear. (4) Peak inertial frequencies ωf were at the local inertial frequency ƒ in the mixed layer (ωf/f ≈ 1.0006 ± 0.0022; n =26 estimates) but slightly greater frequencies in the pycnocline (ωf/ƒ ≈ 1.0128 ± 0.0055; n =21). (5) Currents in the inertial and low‐frequency bands were correlated vertically. In the inertial band, maximum horizontal coherency [γ2(ω; Δr) > 0.95] occurred for drifter separations Δr < 75 km. Horizontal coherency diminished for 75 < Δr < 425 km and became marginally significant (γ2 ≈ 0.4) for Δr > 500 km. Semidiurnal currents were uncorrelated beyond several tens of kilometers. (6) Inertial currents appear to be linked to the passage of atmospheric lows, and semidiurnal currents to the passage of remotely generated baroclinic tides. The spectral peak near 2f is indicative of nonlinear inertial wave interactions.

Current variability across the New England continental shelf-break and slope

The year-long SEEP—I moored array of current meters across the continental shelf-break and slope south of Cape Cod indicates mean alongshore flow to the west across the array during both the winter and summer. The mean flow consists of a surface-intensified jet at the shelf-break, offshore flow across the shelf-break of slightly greater than 1 cm s −1, and the suggestion of weaker flow over the mid-slope than either seaward or shoreward of this region. There is an onshore increase of kinetic energy at diurnal and semidiurnal frequencies. In the inertial band there exists a winter-to-summer near-surface increase in variability, and a subsurface decrease. In the synoptic band there is a larger response over the shelf to the winter-to-summer decrease in wind stress. The spectra are generally red with enhanced near-bottom energy levels over the slope in summer, indicative of increased topographic Rossby wave activity. Spatially coherent wind-driven motion does not extend seaward of the shelf-slope frontal region. The current response to the alongshore wind is consistent with Ekman dynamics, and more significant during the winter, while during the summer the response is more coherent with the cross-shore wind. There is significant synoptic band coherence between the cross-shore component of the wind stress and the currents throughout the year. Only one (known) warm-core ring intersected the array throughout the year, however heightened and sustained topographic Rossby wave activity was evident in summer. These waves have a period of roughly 16 days, an offshore phase speed of ∼9km day −1 and a coherence scale of ∼160km. There is some evidence of refraction of the wave energy towards the along-isobath direction as the topography shoals, and no evidence of these waves shoreward of the mid-slope region.

Topographic and inertial waves on the continental rise of the Mid‐Atlantic Bight

Analysis of an array of four closely spaced moorings, 2800 m water depth, instrumented by near‐bottom current meters, has been performed with a view to isolating coherent wave motions in the low‐frequency and inertial bands. The low‐frequency motions are dominated by topographic Rossby waves similar to the results of previous studies of deep currents on the continental rise of the Mid‐Atlantic Bight. Estimated wave parameters show good agreement with the linear topographic Rossby wave theory of Rhines (1970). Inertial oscillations are shown to be coherent over the array (horizontal separations ∼15–20 km). The peak frequency and phase and group velocity estimates are consistent with surface layer generation at a site north of the array. The relative vorticity of the mean currents and the low‐frequency wave motions do not account for the observed shift of the near‐inertial peak to frequencies ∼7% above f. An event of strong near‐bottom inertial currents (amplitudes ∼10 cm s−1) appears to be attributable to the passage of Hurricane Belle (August 9–10, 1976) 24 days earlier, primarily because the vertical travel times are consistent with the estimates of vertical group velocity. Strong ringing inertial currents north of the array (amplitude ∼40 cm s−1), due to Hurricane Belle were observed at the shelf edge (Mayer et al., 1981) with peak frequencies comparable to the near‐inertial frequencies observed at the array on the rise.

TIDAL AND INERTIAL CURRENTS AROUND SOUTH AFRICA

Limited information is available on currents in the semi-diurnal and diurnal frequency bands for the coastal ocean areas around Southern Africa. However, recently mooring data have become available from sites on the east, south and west coasts, and this paper analyses these results in an attempt to assess the importance of tidal and inertial currents. It is clear that on the narrow shelf on the east coast the Agulhas Current dominates the energy spectrum, and tidal currents should be relatively unimportant at such sites. In the south on the Agulhas Bank the Current is still important, but comparable energy resides in inertial and tidal fluctuations. Modal analysis indicates the tides are primarily barotropic, with the inertial fluctuations mainly baroclinic. In the absence of a major current on the west coast, most of the current variance occurs in the tidal and inertial bands; a complex vertical structure is also found. It is therefore clear that there are regions where such currents cannot be ignored.