Coastal-trapped Wave Propagation Research Articles

Sea level variability in Gulf of Guinea from satellite altimetry

Coastal zones with dense populations, low elevations and/or inadequate adaptive capacity are on the frontline of unprecedented impacts from climate change. The Gulf of Guinea (GoG), stretching from Liberia to Gabon, is in particular vulnerable to coastal flooding caused by local and/or climate-induced sea level rise. In this region, interannual to decadal coastal sea level changes remain poorly understood, mainly due to a lack of tide gauge stations. Here we use nearly three decades (1993–2021) of satellite altimetry data to study the link between the Equatorial Atlantic and coastal GoG sea level variability. The rate of mean sea level rise increased from 3.47 to 3.89 ± 0.10 mm/yr from the Equatorial oceanic domain to the GoG coastal area, with an acceleration of 0.094 ± 0.050 mm/yr2. This corresponds to a mean sea level rise of about 8.9 cm over the entire altimetry period, 1993–2021. We focus on the (extreme) warm/cold events that occur in both the GoG during Atlantic Niños, and along the Angola-Namibia coast during Benguela Niños. Both events are driven by remote forcing via equatorial Kelvin waves and local forcing by local winds, freshwater fluxes and currents intensifications. Analysis of altimetry-based sea level, sea surface temperature anomalies, 20 °C isotherm based PIRATA moorings, and the Argo-based steric and thermometric sea level allows us to follow the coastal trapped waves (CTWs) along the GoG, and its link with major events observed along the strong Equatorial Atlantic warmings in 2010, 2012, 2019 and 2021. Both 2019 and 2021 warming have been identified as the warmest event ever reported in this region during the last 40 years. A lag of 1 month is observed between equatorial and West African coastal trapped wave propagation. This observation may help to better anticipate and manage the effects of extreme events on local ecosystems, fisheries, and socio-economic activities along the affected coastlines. In order to enable informed decision-making and guarantee the resilience of coastal communities in the face of climate change, it emphasises the significance of ongoing study in this field.

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.

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.

Numerical study of diurnal tidal currents on the Pacific shelf off the southern coast of Hokkaido, Japan

We developed a triply nested, high-resolution (1/50°) ocean model to reproduce tides and tidal currents under realistic oceanographic conditions, mainly on the Pacific shelf around Hokkaido, Japan. The model reproduced observed properties of tides and tidal currents reasonably well. Dominant diurnal tidal currents were simulated and accounted for mainly by propagation of coastal-trapped waves (CTWs) of the first mode. The first-mode CTWs were generated mainly around the Four Islands, which are located to the east of Hokkaido, where the massive energy of gravity waves was scattered to CTWs. After leaving the forcing area around the islands, diurnal CTWs propagated along the Hokkaido coast to Cape Erimo. Around the cape, CTWs were also excited via scattering of gravity waves and superposed with first-mode CTWs propagating from the Four Islands. The K1 (O1) currents associated with the CTWs weakened (strengthened) around and west of the cape because of out-of-phase (in-phase) superposition. The K1 currents therefore ended up being smaller than the O1 currents by one order of magnitude. Moreover, the first-mode CTWs generated around the Four Islands were attenuated along the propagation route through complicated processes. The unsteadiness of the K1 and O1 currents related to their 13.67-day beats therefore increased gradually along the propagation route. The unsteadiness on the shelf west of the cape was further enhanced by superposition with CTWs forced around the cape, which were occasionally unstable.

Comprehensive Observational Features for the Kuroshio Transport Decreasing Trend During a Recent Global Warming Hiatus

AbstractLinear trends in Kuroshio transport during a recent global warming hiatus (1998–2013) were evaluated using long‐term ferryboat ADCP (acoustic Doppler current profiler) data and tidal gauge data in the Tokara Strait south of Japan. The Kuroshio exhibited a remarkable weakening trend of approximately 0.05 Sv year−1 (1 Sv = 106 m3 s−1). The pycnocline in the weakened Kuroshio was relaxed and displayed shoaling at the offshore edge, which was attributed to vertical thermocline displacement rather than to water mass modification. Importantly, Kuroshio transport trends in the Tokara Strait were affected by sea surface height anomalies, which were driven by the combined effects of the clockwise baroclinic‐mode coastal trapped wave propagation along the southern coast of Japan and downstream Kuroshio advection in the East China Sea. Both features were induced by wind stress curl changes related to the global warming hiatus over the North Pacific.

Coastal trapped waves propagation along the Southwestern Atlantic Continental Shelf

This study investigates the propagation of coastal trapped waves (CTWs) along the Brazilian continental shelf between 34°S and 11°S using in situ data combined with the outputs from a high-resolution ocean simulation with HYCOM. The CTWs generation area covers a wide region ranging from the Patagonian shelf to the southern Brazilian shelf. The spectral analysis of coastal sea level series between 54°S and 10.5°S shows three bands of high energy associated with periods from 5 to 12 days, 15–22 days, and 25–40 days. The energy of the CTWs decreases along their propagation for all frequency bands, showing a drastic reduction north of 22°S, due to abrupt variations in the width and depth of the continental shelf between Tubarão Bight and Abrolhos Bank. Their phase speed propagation varies along the coast, being faster (>25 m/s) in the southernmost region (between 42°S and 41°S), reaching ~11 m/s north of 41°S, and reducing to ~3 m/s further north (equatorward of 24°S). The free Continental Shelf Wave theory supports the notion that the intense deceleration north of 24°S can be explained by the narrowing of the continental shelf. The stratification parameter indicates that the Brazilian continental shelf has a barotropic response to wind-generated disturbances.

Circulation and Upwelling Induced by Coastal Trapped Waves Over a Submarine Canyon in an Idealized Eastern Boundary Margin

AbstractWind‐driven upwelling promotes the onshore transport of dense slope waters onto the continental shelf. New observational evidence suggests that the propagation of coastal trapped waves (CTWs) over a submarine canyon can also cause upwelling within the canyon, independent of the wind forcing. Here, we use idealized numerical experiments to assess the role of CTWs in promoting the onshore transport of deeper waters onto the continental shelf. The experiments are forced with a 7‐days period CTW. Overall, there is accumulated upwelled water in time with increased onshore transport during the low sea level phase of the CTWs. Most of the onshore flow is spread over the upwave side of the canyon during the low sea level phase, but it is advected further downwave during the subsequent high sea level phase. As a result, a dense pool of upwelled water is spread over the shelf extending primarily in the downwave direction.

The shelf sources of estuarine inflow

AbstractThe inflow to an estuary originates on the shelf. It flushes the estuary and can bring in nutrients, heat, salt, and hypoxic water, having consequences for estuarine ecosystems and fjordic glacial melt. However, the source of estuarine inflow has only been explored in simple models that do not resolve interactions between inflow and outflow outside of the estuarine channel. This study addressed the estuary inflow problem using variations on a three-dimensional primitive equation model of an idealized estuarine channel next to a sloping, unstratified shelf with mixing provided by a single frequency, 12-hour tide. Inflow was identified using particle tracking, momentum budgets, and Total Exchange Flow. Inflow sources were found in shelf water downstream of the estuary, river plume water, and shelf water upstream of the estuary. Downstream is defined here with respect to the direction of coastal trapped wave propagation, which is to the right for an observer looking seaward from the estuary mouth in the northern hemisphere. Downstream of the estuary and offshore of the plume, the dynamics were quasi-geostrophic, consistent with previous simple models. The effect of this inflowing current on the geometry of the river plume front was found to be small. Novel sources of inflow were identified which originated from within the plume and upstream of the estuary.

Coastal Trapped Wave Propagation along the Southwest African Shelf as Revealed by Moored Observations

AbstractCoastal trapped waves (CTWs) that propagate poleward along the southwest African shelf potentially leak energy from lower latitudes into the Benguela Upwelling System (BUS). Thus, in addition to local winds, these waves provide an important remote forcing mechanism for the upwelling region. The present study aims at elucidating the nature of CTWs in the northern BUS. To this end, we make use of multisite velocity observations from the Namibian shelf (18°, 20°, 23°S) and examine the alongshore velocity signal for signatures of CTWs by means of wavelet methods. We found that a substantial amount of energy is concentrated within a submonthly to subseasonal frequency band (10–50 days). Based on the coherence and phase spectra of the alongshelf currents, we provide evidence for a predominantly southward phase propagation and establish typical time and length scales of CTWs in the region. It turns out that their properties differ significantly within a few hundred kilometers along the coast. A comparison of the results with theoretical dispersion curves shows that this difference may be explained by variations in the bottom topography. Finally, we investigate the coupling of the alongshore currents with the coastal and equatorial wind stress and highlight regions of potential wave generation.

Observational and model studies of synoptic current fluctuations in the Bohai Strait on the Chinese continental shelf

Three winter storms struck the Bohai and Yellow seas in succession during February 16–25, 2017. Periodic fluctuations of sea level, currents, temperature, and salinity were recorded at a moored station deployed in the Bohai Strait. Observations also captured significant synoptic fluctuations of inflow and outflow through the Bohai Strait with maximum magnitude exceeding 50 cm s−1. The sea level dropped by > 1 m and the bottom temperature decreased by 2.5 °C in < 30 h during these winter storms. A regional ocean model was used to investigate the prominent fluctuations of synoptic exchange flow through the Bohai Strait. The model results indicated that the Bohai and Yellow seas in their entirety responded strongly to the storm bursts and subsequent relaxation at the synoptic scale. The periodic winter storm bursts and relaxation excited the cyclonic propagation of coastal-trapped waves around the Bohai and Yellow seas. Strong periodic inflow and outflow during the passage of the three successive storms promoted water exchange between the Bohai Sea and the northern Yellow Sea. Successive winter storms might also affect the mean current structure through the Bohai Strait. The mean currents through the Bohai Strait during the studied storm period were characterized by inflow in both northern and southern parts of the strait and outflow through the middle of the strait. This arrangement is different from the traditional view of inflow via the northern channel and outflow through the southern channel.

Effect of coastal-trapped waves on the synoptic variations of the Yellow Sea Warm Current during winter

Significant short-term fluctuations of the Yellow Sea Warm Current (YSWC) at several days' time scale were observed in the Yellow Sea (YS) during winter of 2006–2007. Observations indicated that the synoptic fluctuations of the YSWC appear to be associated with low-frequency sea level variations. By analyzing direct current measurements, sea level elevations and results from a regional ocean model, this study characterizes the variation of the YSWC at synoptic time scale and examines its connection with propagation of coastal-trapped waves during winter. The model captures the observed short-term fluctuations of the YSWC at several days’ periods and suggests a close relation between coastal-trapped waves and the YSWC variability. The YSWC is intensified (weakened) when the westward sea level-related barotropic pressure gradient is enhanced (diminished) due to the propagation of coastal-trapped waves. Model experiments with idealized periodic surface forcing further identify the important role of the coastal-trapped waves on adjusting the strength of the YSWC. The model results also suggest that the propagation of coastal-trapped waves contribute to the intermittent northwestward intrusion of Cheju Warm Current (CWC) in the frontal region northwest of Cheju. Generation and propagation of the coastal-trapped waves give rise to the westward barotropic pressure gradient in the YS entrance, which periodically pumps the CWC water into the YS interior west of Cheju.

Wind-Driven Circulation in a Shelf Valley. Part II: Dynamics of the Along-Valley Velocity and Transport

AbstractThe dynamics controlling the along-valley (cross shelf) flow in idealized shallow shelf valleys with small to moderate Burger number are investigated, and analytical scales of the along-valley flows are derived. This paper follows Part I, which shows that along-shelf winds in the opposite direction to coastal-trapped wave propagation (upwelling regime) force a strong up-valley flow caused by the formation of a lee wave. In contrast, along-shelf winds in the other direction (downwelling regime) do not generate a lee wave and consequently force a relatively weak net down-valley flow. The valley flows in both regimes are cyclostrophic with O(1) Rossby number. A major difference between the two regimes is the along-shelf length scales of the along-valley flows Lx. In the upwelling regime Lx depends on the valley width Wc and the wavelength λlw of the coastal-trapped lee wave arrested by the along-shelf flow Us. In the downwelling regime Lx depends on the inertial length scale |Us|/f and Wc. The along-valley velocity scale in the upwelling regime, given byis based on potential vorticity (PV) conservation and lee-wave dynamics (Hs and Hc are the shelf and valley depth scales, respectively, and f is the Coriolis parameter). The velocity scale in the downwelling regime, given by is based on PV conservation. The velocity scales are validated by the numerical sensitivity simulations and can be useful for observational studies of along-valley transports. The work provides a framework for investigating cross-shelf transport induced by irregular shelf bathymetry and calls for future studies of this type under realistic environmental conditions and over a broader parameter space.

A modeling study of the role that bottom topography plays in Gulf Stream dynamics and in influencing the tilt of mean sea level along the US East Coast

Two aspects of the interactions between the Gulf Stream (GS) and the bottom topography are investigated: 1. the spatial variations associated with the north-south tilt of mean sea level along the US East Coast and 2. the high-frequency temporal variations of coastal sea level (CSL) that are related to Gulf Stream dynamics. A regional ocean circulation model is used to assess the role of topography; this is done by conducting numerical simulations of the GS with two different topographies–one case with a realistic topography and another case with an idealized smooth topography that neglects the details of the coastline and the very deep ocean. High-frequency oscillations (with a 5-day period) in the zonal wind and in the GS transport are imposed on the model; the source of the GS variability is either the Florida Current (FC) in the south or the Slope Current (SC) in the north. The results demonstrate that the abrupt change of topography at Cape Hatteras, near the point where the GS separates from the coast, amplifies the northward downward mean sea level tilt along the coast there. The results suggest that idealized or coarse resolution models that do not resolve the details of the coastline may underestimate the difference between the higher mean sea level in the South Atlantic Bight (SAB) and the lower mean sea level in the Mid-Atlantic Bight (MAB). Imposed variations in the model’s GS transport can generate coherent sea level variability along the coast, similar to the observations. However, when the bottom topography in the model is modified (or not well resolved), the shape of the coastline and the continental shelf influence the propagation of coastal-trapped waves and impact the CSL variability. The results can explain the different characteristics of sea level variability in the SAB and in the MAB and help understand unexpected water level anomalies and flooding related to remote influence of the GS.

PRELIMINARY STUDIES ON THE IMPACT OF CLIMATE CHANGE ON THE UPWELLING PHENOMENON IN SOUTH CENTRAL VIETNAM WATERS IN SUMMER

Recent international studies have demonstrated the decadal variability in the circulation patterns of East Vietnam Sea in the summer with three phases in 1998, 2001 and 2010 respectively. From the analytical results of the sea water level in 20 years, the variation of the water level in the period from 3 - 7 years was considered. Uptrend in regional water levels can be separated into phases 1993-2001, 2007-2010, and the sea water level in the 2002-2005 period remained almost unchanged. Noted that the abnormal increases in the water level in 2001 and 2010 were within the period of La Niña active phase. Obviously, these changes will significantly affect the upwelling of South Central Vietnam. To seek an answer, we have analyzed the data sequence HYCOM + NCODA with 1/12.50 Global Reanalysis. This sequence is quite reliable to assess the scale and the characteristics of ocean eddies with the size of a few tens of kilometers. With this data series, the movement and formation of the eddies can be analyzed, including processes that control the surface mixed layer, the meanders of ocean currents and fronts, the generation and propagation of coastal trapped waves.

Shelfbreak upwelling induced by alongshore currents: analytical and numerical results

AbstractAlongshore flow in the direction of propagation of coastal trapped waves can result in upwelling at the shelfbreak. The intensity of this upwelling can be comparable in magnitude to wind-driven coastal upwelling, with its associated ecological features. Recent numerical experiments by Matano &amp; Palma indicate that this upwelling results from convergence of Ekman transport at the shelfbreak. The mechanism for this phenomenon can be understood in terms of steady solutions to the shallow water equations in the presence of Coriolis force and bottom drag. Matano &amp; Palma interpreted their numerical results in terms of the arrested topographic wave, but did not present direct comparisons. Here we present a family of analytical solutions to the equations of the arrested topographic wave that shows striking quantitative agreement with earlier numerical results.

Coastal Wind-Driven Circulation in the Vicinity of a Bank. Part I: Modeling Flow over Idealized Symmetric Banks

Abstract This study examines how coastal banks influence wind-driven circulation along stratified continental shelves. Numerical experiments are conducted for idealized symmetric banks; the standard bank (200 km long and 50 km wide) has dimensions similar to the Heceta Bank complex along the Oregon shelf. Model runs are forced with 10 days of steady winds (0.1 Pa); upwelling and downwelling cases are compared. The bank introduces significant alongshelf variability in the currents and density fields. Upwelling-favorable winds create an upwelling front and a baroclinic jet (flowing opposite coastal-trapped wave propagation) that bend around the standard bank, approximately centered on the 90-m isobath. The upwelling jet is strongest over the upstream bank half, where it advects a tongue of dense water over the bank. There is a current reversal shoreward of the main jet at the bank center. Upwelling is most intense over the upstream part of the bank, while there is reduced upwelling and even downwelling over other bank sections. Downwelling-favorable winds create a near-bottom density front and a baroclinic jet (flowing in the direction of coastal-trapped wave propagation) that bend around the standard bank; the jet core moves from the 150-m isobath to the 100-m isobath and back over the bank. The downwelling jet is slowest and widest over the bank; there are no current reversals. Results over the bank are more similar to 2D results (that preclude alongshelf variability) than in the upwelling case. Downwelling is weakened over the bank. The density field evolution over the bank is fundamentally different from the upwelling case. Most model results for banks with different dimensions are qualitatively similar to the standard run. The exceptions are banks having a radius of curvature smaller than the inertial radius; the main jet remains detached from the coast far downstream from these banks. The lowest-order across-stream momentum balance indicates that the depth-averaged flow is geostrophic. Advection, ageostrophic pressure gradients, wind stress, and bottom stress are all important in the depth-averaged alongstream momentum balance over the bank. There is considerable variability in alongstream momentum balances over different bank sections. Across-shelf and alongshelf advection both change the density field over the bank. Barotropic potential vorticity is not conserved, but the tendency for relative vorticity changes and depth changes to partially counter each other results in differences between the upwelling and downwelling jet paths over the bank. Only certain areas of the bank have significant vertical velocities. In these areas of active upwelling and downwelling, vertical velocities at the top of the bottom boundary layer are due to either the jet crossing isobaths or bottom Ekman pumping.

Oxygenation episodes on the continental shelf of central Peru: Remote forcing and benthic ecosystem response

The interplay between the oxygen minimum zone and remotely-forced oxygenation episodes determines the fate of the benthic subsystem off the Central Peruvian coast. We analyzed a 12 year monthly time-series of oceanographic and benthic parameters at 94 m depth off Callao, Central Peru (12°S), to analyze: (i) near-bottom oxygen level on the continental shelf in relation to dynamic height on the equator (095°W); and (ii) benthic ecosystem responses to oxygen change (macrobiotic infauna, meiofauna, and sulphide-oxidizing bacteria, Thioploca spp.). Shelf oxygenation episodes occurred after equatorial dynamic height increases one month before, consistent with the propagation of coastal trapped waves. Several but not all of these episodes occurred during El Niños. The benthic biota responded to oxygenation episodes by undergoing succession through three major ecological states. Under strong oxygen deficiency or anoxia, the sediments were nearly defaunated of macro-invertebrates and Thioploca was scarce, such that nematode biomass dominated the macro- and meiobiotas. When frequency of oxygenation events reduced the periods of anoxia, but the prevailing oxygen range was 10–20 μmol L −1, mats of Thioploca formed and dominated the biomass. Finally, with frequent and intense (>40 μmol L −1) oxygenation, the sediments were colonized by macrofauna, which then dominated biomass. The Thioploca state evolved during the 2002–2003 weak EN, while the macrofauna state was developed during the onset of the strong1997–1998 EN. Repeated episodes of strong oxygen deficiency during the summer of 2004, in parallel with the occurrence of red tides in surface waters, resulted in the collapse of Thioploca mats and development of the Nematode state. Ecological interactions may affect persistence or the transition between benthic ecosystem states.

Patos Lagoon outflow within the Río de la Plata plume using an airborne salinity mapper: Observing an embedded plume

Major river systems discharging into continental shelf waters frequently form buoyant coastal currents that propagate along the continental shelf in the direction of coastal trapped wave propagation (with the coast on the right/left, in the northern/southern hemisphere). The combined flow of the Uruguay and Paraná Rivers, which discharges freshwater into the Río de la Plata estuary (Lat. ∼36°S), often gives rise to a buoyant coastal current (the ‘Plata plume’) that extends northward along the continental shelf off Uruguay and Southern Brazil. Depending upon the prevailing rainfall, wind and tidal conditions, the Patos/Mirim Lagoon complex (Lat. ∼32°S) may also produce a freshwater outflow plume that expands across the inner continental shelf. Under these circumstances the Patos outflow plume can be embedded in temperature, salinity and current fields that are strongly influenced by the larger Plata plume. The purpose of this paper is to present observations of such an embedded plume structure and to determine the dynamical characteristics of the ambient and embedded plumes. We describe selected results of coincident airborne remote sensing and shipboard in-situ surveys of the salinity distribution and extent of the Plata and Patos/Mirim Lagoon plumes conducted under contrasting winter (2003) and summer (2004) conditions. The surveys were carried out in the context of a comprehensive multi-disciplinary study of the Plata plume and its response to prevailing seasonal weather conditions. The objective was to map the surface salinity distribution of the Plata plume at synoptic scales under representative winter and summer conditions. Additionally, the airborne survey included finer-scale mapping of specific features including the Río de Plata estuarine front and the Patos Lagoon plume, with the objective of determining the distribution and behavior of the plumes in the estuaries and on the continental shelf. The airborne survey was conducted with an aircraft carrying an infrared and microwave radiometer system, the Naval Research Laboratory's (NRL) Salinity, Temperature and Roughness Remote Scanner (STARRS). A series of broad-scale flights over the continental shelf off Argentina, Uruguay and Brazil were made using STARRS to determine the spatial extent of the Plata plume, and detailed mapping flights were undertaken in the vicinity of Río de la Plata Estuary and Patos/Mirim outflows to observe associated frontal features. The results of the airborne surveys were compared with shipboard hydrographic data obtained from a conductivity, temperature and depth (CTD) profiler. The combined ship and aircraft data were used to estimate parameters of dynamical classification schemes. These schemes were used to characterize the gross behavior and dynamics of the ambient Plata plume and embedded Patos plume. The Plata plume was highly asymmetric with along-shelf development towards the north and it behaved dynamically like a buoyant coastal boundary current, with an approximately geostrophic across-shelf momentum balance. The Patos plume, on the other hand, maintained its integrity as a relatively symmetric, ageostrophic, frictionally dominated plume with significant across-shelf, and modest along-shelf, development. The dynamical implications of the embedding of the smaller scale Patos plume within the larger-scale ambient Plata plume were explored, and it was concluded that the ambient plume could exert a significant dynamical influence on the behavior of the embedded plume.

A Modeling Study of Coastal-Trapped Wave Propagation in the Gulf of California. Part II: Response to Idealized Forcing

The propagation of incident coastal-trapped waves in the Gulf of California is investigated using a hydrostatic primitive equation model. The behavior of idealized incident wave disturbances with different amplitudes and time scales is examined. The incident wave disturbances propagate northward up-gulf along the east side with no significant change. At the sill, which is 600 km north of the entrance, the wave splits and most of the energy is steered to the west side where it propagates southward down-gulf with decreased amplitude (50%). A small fraction (10%–20%) of the incident energy enters the north where it is dissipated. Sea level at the entrance of the gulf is well correlated with sea level everywhere inside the gulf. In contrast, correlations of depth-averaged velocity between Topolobampo (close to the entrance of the gulf) and locations around the gulf decrease along the propagation path of the wave. Most of the dissipation of wave energy in the gulf takes place through bottom friction in the vicinity of the sill. Incident waves with large, but realistic, sea level displacement magnitudes exhibit nonlinear properties. Phase speeds increase as the sea level displacements of the incident waves increase from −30 to +30 cm. Waves of sea level elevation steepen. On the east side, large-amplitude elevation waves produce a down-gulf current adjacent to the coast such that the up-gulf currents associated with the wave separate from the coast. The separation process seems to be connected with subsequent downslope propagation of energy. Energetic anticyclonic eddies with spatial scales of 50–80 km can be generated by long-time-scale or large-amplitude elevation waves.

A Modeling Study of Coastal-Trapped Wave Propagation in the Gulf of California. Part I: Response to Remote Forcing

The evolution of remotely forced coastal-trapped waves in the Gulf of California is studied using a hydrostatic primitive equation model. The sea level time variability at a remote station south of the gulf is assumed to propagate northward into the gulf as a mode-1 coastal-trapped wave (CTW). To validate this assumption, observations and model results are compared. In general, sea level fluctuations are reasonably well represented by the model, with model–data correlations decreasing from 0.76 at Topolobampo, close to the entrance of the gulf, to 0.52 at Santa Rosalia in the central gulf. Model–data correlations of velocity are lower (<0.6). In the gulf, CTWs propagate northward along the east side with no significant changes south of the sill, which is 600 km north of the entrance. When incident waves propagating northward in the gulf along the east side arrive at the sill, a small fraction of the wave energy enters the northern gulf and is dissipated. Most of the wave energy is steered at the sill to the west side of the gulf where it propagates southward with decreased sea level amplitude. The weakened waves leave the gulf at the southwest boundary approximately 6–7 days after entering. Some of the incident wave energy is lost into downslope propagating disturbances generated as the CTWs pass, resulting in relatively intense bottom currents. Wave disturbances exhibit nonlinear characteristics while propagating. For example, isopycnal displacements in the wave fronts steepen. This occurs primarily for waves of sea level elevation. The contribution of remotely forced CTWs in the Gulf of California to the total kinetic energy is comparable to that produced by the wind.