Coastal Trapped Waves Research Articles

High‐Frequency Variability Induced in the Southern California Bight by a Wind Event in Sebastián Vizcaíno Bay, Baja California

AbstractAlthough the California Current System extends as far south as the southern tip of the Baja California Peninsula (BCP), some past model domains have employed an open boundary near the Mexico‐US border, excluding Sebastián Vizcaíno Bay (SVB), a hook‐like coastal feature at mid‐length of the BCP. An earlier modeling study has shown the SVB to be a previously unknown forcing region that governs SSH variability in the Southern California Bight (SCB). Motivated by that evidence, our aim is to understand the variability that originates from the SVB and to identify what is lost in a numerical model of the SCB when the SVB is excluded. To do this we use an idealized configuration of the MITgcm, without tides, using the bathymetry with the bay and comparing it to model runs where the bay is removed. Our results show that a short wind event (≤4 hr) blowing over SVB can enhance variability everywhere in the domain. First, the event excites a seiche in the bay at frequency 5.2 cpd. Then, seiche energy leaks into the domain in the form of gravity waves that are topographically amplified in the SCB, coastal trapped waves (CTWs) that travel north along the coast, and internal waves, that enhance vertical velocity variance at particular depths by up to 27% compared to experiments without the SVB. Given the frequency of wind events like the one modeled, our results suggest that the SVB is a continuous source of gravity waves, internal waves, and CTWs to the SCB.

Drastic Fluctuation in Water Exchange Between the Yellow Sea and Bohai Sea Caused by Typhoon Lekima in August 2019: A Numerical Study

AbstractConnecting the Bohai Sea (BS) and Yellow Sea (YS), the Bohai Strait is vital in controlling physical‐biogeochemical conditions in the East Asian marginal seas. In August 2019, Typhoon Lekima moved northward inland into eastern China and caused drastic water exchange through the Bohai Strait. The specific variation and underlying dynamics involved were investigated with observations and high‐resolution numerical simulations. Volume transport featured an intense inflow followed by an intense outflow, with a fluctuation magnitude of 1.88 Sv, which was much greater than the seasonal‐interannual variations. The corresponding current patterns presented three phases: strait‐wide inflow, strait‐wide outflow, and outflow in the south and inflow in the north. The strait‐wide inflow was driven by strong east‐northeasterly winds through local Ekman dynamics. The strait‐wide outflow resulted from the dynamic adjustment of ocean currents accompanying the rapid weakening of easterly winds. The third current pattern was part of a large‐scale cyclonic circulation in the BS and North YS and was governed by different factors in the southern and northern straits. Further comparisons indicated that transport caused by winter storms and other typhoons in the year had opposite phase variations to that caused by Lekima. The underlying dynamics mainly involved local Ekman dynamics and/or remote wind forcing via the propagation of coastal trapped waves but could differ greatly from events. Finally, we found that to a large extent, geostrophic transport derived from the sea surface height difference across the Bohai Strait could be used as actual transport during synoptic weather events.

Cross‐Shelf Penetrating Fronts of Buoyant Coastal Currents Around the Headland

AbstractBuoyant coastal current induced by riverine outflow often propagates along the coast in the direction of coastal trapped wave. However, it is uncertain how the buoyant coastal current interacts with irregular coastal bathymetry. Measured surface salinity distributions indicate that the buoyant coastal current, which is induced by the Changjiang River plume, frequently penetrates offshore around the curved headlands in the Zhoushan Islands. This signal can also be seen in satellite chlorophyll data, reflecting its importance in cross‐shelf exchange both physically and ecologically. To understand the variation and underlying dynamic mechanisms of these penetrating fronts, realistic and idealized numerical experiments were conducted. The results indicated that the penetrated water salinity was about 30–32 psu, with a horizontal penetration scale of tens of kilometers. Curvature and self‐organizing map analyses suggested that it was a frequent event with an approximately 2‐week spring‐neap cycle from generation to decay. It more likely occurred in transition seasons such as spring and autumn, owing to the runoff accumulation and gentle wind conditions. Smoothing the topography would substantially eliminate the penetrating fronts. Idealized model experiments suggested that headland‐induced river plume penetration was a type of curved topography‐induced current separation. The criterion for this to occur is u/fRc > 1, in which u is the current velocity, f is the Coriolis parameter, and Rc is the curvature radius. In this case, the headland is too sharp that the Coriolis force could not balance the centrifugal force. Once penetrated, the buoyant water was further transported by the ambient Taiwan Warm Current.

Surface circulation in the Gulf of Thailand from remotely sensed observations: seasonal and interannual timescales

Abstract. The Gulf of Thailand (GoT), a shallow semi-enclosed basin located in the western equatorial Pacific, undergoes much wind variabilities on both seasonal and interannual timescales that produce complex surface circulation. The local Ekman pumping modifies sea level in the northern GoT, while remote wind forcing influences sea level variability at the GoT western boundary, potentially through the coastal trapped Kelvin waves. The importance of the Ekman current on ageostrophic current is also important; the stronger influence of the Ekman current is found toward the southern part of the GoT. The GoT circulation reverses its direction seasonally following the monsoon wind reversal which is well-captured by the most dominant complex empirical orthogonal function explaining 28 % of the total circulation variance. During the monsoon transition, a strong meridional current along the western boundary that connects to the flow at the GoT southeastern entrance is observed. This implies high exchange between the GoT and the South China Sea and thus modification of the GoT water. On the interannual timescale, the GoT circulation is directly impacted by both the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Interestingly, the two climate modes have different spatial influences on the GoT circulation. The IOD dominates the interannual current along the GoT western boundary and the southern boundary of the observing domain (8∘ N), while the ENSO correlates with that in the interior. The results highlight the complex circulation pattern as being contributed by different dynamics over each region of the GoT.

Modeling the Sea Level Response of the Northern East China Sea to Different Types of Extratropical Cyclones

AbstractSea level response to extratropical cyclones, one of the most disastrous weather systems, varies depending on cyclone types and tracks. This study presents a systematic modeling assessment of storm surge induced by extratropical cyclones in the northern East China Sea using an ocean‐wave coupled model, based on the Coupled Ocean‐Atmosphere‐Wave‐Sediment Transport (COAWST) model system. We identified the type of extratropical cyclone most likely to cause disasters and the areas likely to suffer the worst effects. We conducted sensitivity experiments to analyze the driving mechanisms (such as wind, air pressure, tide, and wave) and influencing mechanisms (such as local and remote meteorological effect and Ekman effect) of storm surge. During strong winds, positive and negative storm surges were consistent with the wind direction, and formed source of coastal trapped waves. When an abnormally high water level occurred in a semi‐enclosed sea area, it propagated mainly along the cyclone direction. The energy of coastal trapped waves was easy to dissipate in case of caps and other locations with large topographic changes. Tide‐surge‐wave interaction and propagation were also greatly affected by coastal trapped waves.

Subinertial Variability in Four Southeast Greenland Fjords in Realistic Numerical Simulations

AbstractWe identified the nature and driving mechanisms of subinertial variability (variability at a time scale of several days) in four fjords in Southeast Greenland, in three high‐resolution numerical simulations. We find two dominant frequency ranges in along‐fjord velocity, volume transport of Atlantic Water, and along‐fjord heat transport: one around 2–4 days and one around 10 days. The higher frequency is most prominent in the two smaller fjords (Sermilik Fjord and Kangerdlugssuaq Fjord), while the lower frequency peak dominates in the larger fjords (Scoresby Sund and King Oscar Fjord). The cross‐fjord structure of variability patterns is determined by the fjord's dynamic width, while the vertical structure is determined by the stratification in the fjord. The dominant frequency range is a function of stratification and fjord length, through the travel time of resonant internal Kelvin waves. We find that the subinertial variability is the imprint of Coastal Trapped Waves, which manifest as Rossby‐type waves on the continental shelf and as internal Kelvin‐type waves inside the fjords. Between 50% and 80% of the variability in the fjord is directly forced by Coastal Trapped Waves propagating in from the shelf, with an additional role played by alongshore wind forcing on the shelf.

Slope and Shelf Flow Anomalies Off Oregon Influenced by the El Niño Remote Oceanic Mechanism in 2014–2016

AbstractOutputs of the regional ocean circulation model are analyzed to demonstrate the measurable impact of the El Niño remote oceanic forcing mechanism along the US West Coast during the major heat wave period of 2014–2016. The 2‐km horizontal resolution model, based on the Regional Ocean Modeling System (ROMS), was run for the period of 2009–2018. Though the model does not assimilate observations, it performs well by comparison with time series data explaining observed variability on temporal scales from several days to seasonal and interannual. The El Niño‐related oceanic anomalies provided by a global state estimate are introduced in the regional model at the southern boundary at 24N. These propagate alongshore with coastally trapped waves (CTWs) and influence the variability off Oregon (41°–46°N). In particular, CTWs are evident in the subsurface along‐slope current, vs, and in the depth of the 26.5 kg m−3 isopycnal surface over the slope, z26.5. In summer 2014 and 2015, vs anomalies are positive (northward) and z26.5 anomalies are negative (deeper) along the US West Coast. In addition to the CTW patterns, z26.5 anomalies also exhibit slow‐moving features associated with undercurrent widening, separation, and subsurface eddy variability. Over the Oregon shelf, El Niño conditions contributed to the sharp weakening of the southward alongshore current throughout the water column in July 2014 and 2015, despite the near‐average southward, upwelling‐favorable winds.

Impact of Gulf Stream Warm-Core Rings on Slope Water Intrusion into the Gulf of Maine

Abstract Intruding slope water is a major source of nutrients to sustain the high biological productivity in the Gulf of Maine (GoM). Slope water intrusion into the GoM is affected by Gulf Stream warm-core rings (WCRs) impinging onto the nearby shelf edge. This study combines long-term mooring measurements, satellite remote sensing data, an idealized numerical ocean model, and a linear coastal-trapped wave (CTW) model to examine the impact of WCRs on slope water intrusion into the GoM through the Northeast Channel. Analysis of satellite sea surface height and temperature data shows that the slope sea region off the GoM is a hotspot of ring activities. A significant linear relationship is found between interannual variations of ring activities in the slope sea region off the GoM and bottom salinity at the Northeast Channel, suggesting the importance of WCRs in modulating variability of intruding slope water. Analysis of the mooring data reveals enhanced slope water intrusion through bottom-intensified along-channel flow following impingements of WCRs on the nearby shelf edge. Numerical simulations qualitatively reproduce the observed WCR impingement processes and associated episodic enhancement of slope water intrusion in the Northeast Channel. Diagnosis of the model result indicates that baroclinic CTWs excited by the ring–topography interaction are responsible for the episodically intensified subsurface along-channel inflow, which carries more slope water into the GoM. A WCR that impinges onto the shelf edge to the northeast of the Northeast Channel tends to generate stronger CTWs and cause stronger enhancement of the slope water intrusion into the GoM.

Sea level variability along the Japanese coast forced by the Kuroshio and its extension

Sea level variability along the Japanese coast and its relation to the Kuroshio-Kuroshio Extension (KE) are investigated using ocean reanalysis data. The first mode of an empirical-orthogonal-function for the coastal sea-level represents a simultaneous sea-level change along the whole Japanese coast, which is synchronized with sea levels in the KE region , the Japan Sea and the East China Sea. The second mode is characterized by an east–west dipole pattern at the south coast. The first mode is correlated with the Kuroshio variations around the Izu–Ogasawara Ridge (IOR) and tends to be in a positive phase when the Kuroshio takes a nearshore path around IOR. The Kuroshio’s position around IOR is closely related to the KE dynamic state. When the KE jet is in a stable (unstable) state, a nearshore (meandering) Kuroshio path is formed around IOR. A composite analysis suggests that the sea level along the Japanese coast becomes high due to propagation of coastal trapped waves when the Kuroshio takes a nearshore path around IOR. That is why the first mode is synchronized with the KE decadal variability. The second mode has a close relation with the Kuroshio Large Meander (LM). The eastern positive anomaly at the coast between the Izu and Kii Peninsulas is formed by warm Kuroshio water brought by a westward branch flow along the coast. The western negative anomaly is attributed to a southward shift in the Kuroshio south of the Kii Peninsula associated with the LM.

Superposition of coastal-trapped waves and Kuroshio warm water intrusions caused unusually high sea levels around the southern coasts of Japan in early September 1971

Superposition of coastal-trapped waves and Kuroshio warm water intrusions caused unusually high sea levels around the southern coasts of Japan in early September 1971

Mechanism of generation and propagation characteristics of coastal trapped waves in the Black Sea

Abstract. Coastal trapped waves (CTW) are a major mechanism to distribute energy from the atmosphere in the ocean and play a significant role in large-scale, low-frequency sea-level and current variability in continental shelf and slope areas. Despite their significance for coastal dynamics, observational evidence of the influence of CTWs on the large-scale circulation is rather limited. In this study, mode-1 coastal trapped waves that were captured at sea-level stations at five locations along the southern coast of the Black Sea are examined together with sea surface height reanalysis from the Copernicus Marine Service to reveal their generation mechanisms and role in the coastal dynamics. It is found that the CTWs were formed when water accumulated on the western shelf after gale-force alongshore winds in the western Black Sea. Excited waves propagate along the Black Sea coast from west to east with a speed of 2.3–2.6 m s−1 and transport the atmospherically induced energy all over the Black Sea. The coastal current generated by the CTWs (on the order of 1 m s−1 in magnitude) and the main Black Sea current merge and flow eastward as a single structure, resulting in intensification of the Black Sea circulation during winter. Hence, we present evidence of the influence of CTWs on the large-scale circulation.

Diurnal Coastal Trapped Waves and Associated Deep‐Ocean Mixing Near the Head of Suruga Trough

AbstractCoastal submarine canyons are expected to be sites of enhanced turbulent mixing which create productive fishing areas in coastal waters. Recent studies suggested the potential role of diurnal coastal trapped waves (CTWs) in inducing significant turbulent mixing in coastal submarine canyons poleward of 30° where the diurnal tide becomes subinertial. Nevertheless, the detailed physical processes responsible for the generation and dissipation of CTWs in such submarine canyons have hardly been investigated so far. In the present study, to investigate the turbulent mixing processes associated with diurnal CTWs in submarine canyons, we carry out high‐resolution (Δx, Δy = 1/240°) non‐hydrostatic three‐dimensional numerical experiments focusing on Suruga Bay located at the north end of Suruga Trough, Japan. The numerical experiments show that diurnal (subinertial) tidal currents over the Izu Ridge generate CTWs which propagate cyclonically along the coast of Suruga Bay. In the middle of Suruga Bay, bottom‐intensified flow due to the diurnal CTWs interacts with rough seafloor topography to excite upward propagating internal lee waves, which create mixing hotspots extending high above the seafloor. The resulting diurnal (subinertial) tidal energy dissipation in Suruga Bay is found to be much larger than that associated with semidiurnal (superinertial) internal tides. Furthermore, it is found that baroclinic energy flux based on a simple barotropic‐baroclinic decomposition severely underestimates the wave energy flux in areas with high CTW activity. These results strongly indicate that the existence of CTWs might be a key factor to clarify the dissipation/mixing processes in submarine canyons.

Oceanic Rossby Waves Induced Two Types of Ocean–Atmosphere Response and Opposite Indian Ocean Dipole Phases

AbstractThis study analyzed the downwelling Rossby waves in the south Indian Ocean (IO)-induced spring asymmetric mode and the relationship with the Indian Ocean dipole (IOD) event based on observations and reanalysis datasets. The westward downwelling Rossby waves favor significant sea surface temperature (SST) warming in the Seychelles thermocline dome that triggers atmosphere response and the asymmetric mode in spring. The zonal sea level pressure gradient causes anomalous easterly winds in the central and eastern equatorial IO, cooling the SST off Sumatra–Java. Meanwhile, the remainder of the downwelling Rossby waves reach the west coast, transform to northward coastal-trapped waves, and then reflect as eastward downwelling Kelvin waves along the equator. The downwelling Kelvin waves reach the Sumatra–Java coast during late spring to early summer, favoring SST warming in the southeastern tropical Indian Ocean. Thus, there are two types of ocean–atmosphere response almost at the same time along the equator. The final SST status depends on which process is stronger and, as a consequence, triggers a negative or a positive phase of the IOD event in the fall season. The results show four positive and three negative IOD events related to the above processes from 1960 to 2019. The strong downwelling Rossby waves are easier to induce an intense asymmetric mode and negative IOD event, usually associated with preceding strong El Niño in the Pacific. In contrast, the weak downwelling Rossby waves tend to induce a weak asymmetric mode and positive IOD event, usually associated with preceding weak El Niño or anomalous anticyclonic atmospheric circulation in the southeastern IO.

Diurnal coastal trapped waves and associated deep-ocean mixing near the head of Suruga Trough

Diurnal coastal trapped waves and associated deep-ocean mixing near the head of Suruga Trough

Adding baroclinicity to a global operational model for forecasting total water level: Approach and impact

Operational flood forecast systems are typically based on barotropic models in order to limit computational cost. In this study, a computationally efficient way of adding spatial and temporal variations of water density (henceforth “adding baroclinicity”) to a global barotropic forecast system is described and evaluated. The approach is illustrated using a global high-resolution (1/12°) model with an optimized vertical grid of nine ocean levels. Temperature (T) and salinity (S) for each layer are weakly nudged to forecast fields provided by a coarser resolution (1/4°), data-assimilative ocean model. This adds realistic baroclinic variability with periods exceeding about 15 d to the 1/12° model while allowing the higher frequency variability to evolve freely. The computational cost of running the new model is only 10% higher than the cost of running the two-dimensional barotropic version of the model that forms the basis of the present total water level (TWL) forecast system. The value of adding baroclinicity is demonstrated across the sea level frequency spectrum using hourly observations of TWL made between October 20, 2019 and February 28, 2021 by a global distribution of 211 tide gauges. Large improvements in the tidal residuals (TWL-tide) are shown for periods exceeding 15 d. This is consistent with the known importance of baroclinic processes to sea level variability in this frequency range. For periods between 10 h and 15 d, baroclinicity also improves the predictions of extreme residuals by up to 15 cm, with the largest improvements occurring on narrow shelves. In terms of extremes, adding baroclinicity improves predictions of the three largest positive tidal residuals for 2020 at 89% of the tide gauges with the largest improvements reaching 42 cm. Similar results were found for the three largest negative residuals for each tide gauge record. The importance of modeling baroclinically-modified coastal trapped waves is illustrated for the west coast of North and South America.

Formation of a subsurface anticyclonic eddy from the California Undercurrent off northwestern Baja California, Mexico

Hydrographic observations off northwestern Baja California in March 2017 revealed the presence of a subsurface anticyclonic eddy in a cross-shelf transect at ∼ 32 ° N. This eddy’s width was roughly 30 km and it was located about 15 km from the shelf, at depths between 100 m and 400 m. The eddy’s core had hydrographic characteristics consistent with those found in the California Undercurrent (CUC), which suggests that the eddy was originated in the CUC. Numerical-modeling simulations were used to elucidate the origin and evolution of the observed eddy. These simulations also showed a subsurface eddy which crossed the cross-shelf transect’s location with size, depth, and hydrographic characteristics similar to those of the observed eddy (but formed in November 2007, in a simulation for the 2004–2011 period). Thus, this numerical eddy is considered as analogous to the observed one, hence the life cycle is assumed to be the same in both cases. The eddy formation occurred roughly 40 km southeast of the cross-shelf transect, in a zone characterized by a submarine point, where the isobaths change their orientation abruptly and where an enhanced eddy-energy production occurs, which shows that the irregular bottom-topography is a factor that triggered such an eddy formation. The eddy formed in a period of weak along-shelf velocities, 4-10 days after an event of the most intense velocities of the year, associated with the passage of a wind-driven coastal trapped wave generated in remote locations south of the study area. The flow disturbances forced by this wave could be an additional physical factor which predisposes the water column to instability. However, at least in the cases herein analyzed, the eddy separation required an additional momentum/vorticity input by an adjacent mesoscale cyclonic gyre. Comparison with numerical and laboratory–experimental results support the notion that a baroclinic instability process is involved in the eddy generation. The path followed by the subsurface eddy described herein shows a link of the hydrographic characteristics between northwestern Baja California and the Southern California Bight. • Hydrographic observations show a subsurface anticyclonic eddy off Baja California. • An eddy detected in numerical simulations shows consistency with the observations. • Numerical results elucidate the formation and evolution of the observed eddy. • The eddy formed in a zone where isobaths change their direction abruptly. • The formation was linked to weak along-shelf flows and an adjacent cyclonic gyre.

An investigation of sea level and circulation response during a coastal trapped wave event on the Eastern Agulhas Bank, South Africa

Coastal trapped waves (CTW) have the potential to capture and transport a significant amount energy from atmospheric wind systems, in turn inducing sudden changes to the coastal marine environment over large areas, even those remote to the propagating wind system. In this study, the effects of a large, eastward travelling CTW that was captured on in situ moored acoustic doppler current profiler (ADCP) (∼30 m bottom depth) and sea level recording instrumentation in Algoa Bay on the eastern Agulhas Bank off South Africa is examined using a high-resolution (dx∼1 km) Regional Oceanographic Modelling System (ROMS) ocean model. Alongshore current reversals to the full depth were seen in both the model and two ADCP moorings as the CTW traversed the study domain, with strong currents exceeding 1 m/s developing directly to the south of Cape St Francis and Cape Recife at the peak of the event within the model. In a special model run with surface wind stresses removed, it was shown that the CTW carries much of the energy into the domain from remote wind forcing to the west, contributing an estimated 82% of the eastward flux of shelf water. The total volume of water displacement during the CTW was estimated at 335.2 x 109 m3 which balanced (completion of a full wave cycle) over a period of 150h after the initial zero-crossing point of the transport flux.

Interannual Variability of the Congo River Plume-Induced Sea Surface Salinity

Based on satellite surface salinity (SSS) observations from the SMOS, Aquarius and SMAP missions, we investigate the interannual SSS variability during the period from 2010 to 2020 in the Gulf of Guinea, impacted by the Congo River run-off. Combined with in situ data, the available 11 years of satellite salinity data suggest that the plume of Congo run-off primarily spreads into western directions, leading to reduced SSS. A fraction of it also shows a coastal southward extent subject to interannual variability influenced by coastal trapped waves. The low-salinity water is associated with high values of net primary production, confirming the riverine origin of the nutrient rich plume. No correlation can be found between the plume patterns and the different upwelling strengths in the subsequent upwelling months, nor could a correlation be found with the occurrence of the Benguela Niños. Linking the occurrence of a barrier layer to the occurrence of low-salinity plumes remains difficult, mainly because of the sparseness of in situ data. However, the influence of the low-salinity layer is evident in its stronger stratification and an increased available potential energy.

Interannual Bottom-Intensified Current Thickening Observed on the Continental Slope Off the Southeastern Coast of Hokkaido, Japan

By rotary empirical orthogonal function and coastal-trapped wave mode analyses, we analyzed current velocity data, collected from 2001 to 2016. The data were obtained by an acoustic Doppler current profiler, deployed upward at a location of 41°39.909′ N, 144°20.695′ E, on a 2630-m deep continental slope seabed off the southeastern coast of Hokkaido, Japan. The results indicate that the current intensifies toward the bottom and is directed nearly toward the shore, reaching an average speed of ~2.5 cm s−1 just above the bottom. The thickness of the along-slope northward component of the bottom-intensified current varied within the range of 50–350 m. We found that the current thickness change was caused by oceanic barotropic disturbances, produced by the intensification of the Aleutian Low, largely related to the El Niño–Southern Oscillation and modified through the excitation of bottom-trapped modes of coastal-trapped waves. This finding improves the prediction accuracy of the the bottom-intensified current change, being beneficial for suspended sediment studies, construction and maintenance of marine structures, planning of deep drilling, and so on.

Coastal trapped waves in the southern Caspian Sea

Previous studies reported coastal trapped waves (CTWs) in the Caspian Sea (CS). This study deals with the generation mechanisms, the temporal and spatial variability of CTWs in this area, and their transformations during propagation from the origin to the destination using recent measurements and high-resolution numerical simulations. CTWs are observed at all stations with periods of 2–6 days after northerly storms. The Absheron Peninsula, old Sefidrud delta, and Nur coasts were identified as the CTWs prone regions. The generation of CTWs in these locations was confirmed using numerical experiments. The propagation away from the generating location of CTWs was analyzed using a representative real wind storm. In the west part of the CS, the generation mechanism of CTWs is mainly similar to the barotropic Kelvin waves; in contrast, it is similar to the continental shelf waves in the southern shelves. The results can be used to study the contribution of generated CTWs to the transport of sediment and biological materials in all large lakes.