Alongshore Flow Research Articles

Climate change impact on Northwestern African offshore wind energy resources

Offshore wind is one of the most important sources of renewable energy. Therefore, it is crucial to assess how this resource will evolve over the 21st century in the context of a changing climate. The North African Coastal Low-Level Jet (CLLJ) region, which encompasses offshore areas from Northwest Morocco to Senegal, has an enormous wind-harvesting potential, as it provides a strong, persistent alongshore flow. In the current study, the present climate wind energy potential is featured for two heights (100 and 250 m). More importantly, the climate change impact on the wind energy density in the region is also depicted. For this purpose, the newest and highest-resolution regional climate simulations available are used, which include two ROM simulations (uncoupled and coupled) at 25 km resolution and 19 CORDEX-Africa runs at 50 km resolution. Historical and future (under the RCP4.5 and RCP8.5 scenarios) simulations are used for the periods 1976–2005 and 2070–2099, respectively. Overall, the results show that the annual wind energy density is projected to increase slightly in the northern offshore areas (<+10%) and decrease in the southern ones (>−10%). In close connection to the projected changes for the seasonal changes of the CLLJ system, in the further north regions (downwind Cap Ghir), the spring season shows the largest increases of wind energy, up to +20%, while in the offshore western Sahara, an increase of wind energy is projected in all seasons. A decrease of wind energy is expected for the southern areas.

Structure of alongshore mobilization of sediments, eastern part of gulf of Danzig

Systematization of analyzed schemes of alongshore mobilization of sediments off the east coast of the Gulf of Danzig was conducted. Significant differences in the structure and localization of the countercurrent flows in convergence zones were detected. The results of near-bottom steady-state measurements of currents in the vicinity of the Baltic Canal in the depth range 6–16 m demonstrated that &gt;6-m/s winds cause the alongshore currents to be reoriented in the direction of the alongshore wind component. The morphodynamic accumulative criterion (the filling of the re-entrant angle and abrasion in the obstacle-shadow zone) and “abrasive” criterion (the orientation of abrasion kettle holes at the ends of breakwalls) were examined. The abrasive criterion is apparently preferred over the accumulative one during transportation of sediments along segments of the coast where waves approach the coast at normal or near-normal angles. Peculiarities of the mechanism of the northward transportation of sediments, bypassing Baltiysk backwalls and changing from abrasive to accumulative criteria for different segments of the coast, are demonstrated. Two schema representing the opposite Vistula and Sambiysky alongshore flows of sediments are proposed: prior to construction of the backwalls facing Baltic Canal incoming waters with the vast area of migration at the north end of the Vistula bar, and the post-construction backwalls with the resulting narrowing of the area and displacement of its south boundary on the north side to the level of these backwalls.

Alongshore momentum balance over shoreface-connected ridges, Fire Island, NY,

Hydrodynamic and hydrographic data collected on the inner shelf of Fire Island, NY, over a region of shoreface-connected ridges (SFCRs), are used to describe wind-driven circulation over uneven topographies along relatively straight coastlines. The data reveal a predominantly alongshore flow, under westward wind forcing, with localized offshore current veering over the SFCR crests and an onshore veering over the adjacent troughs. Alongshore momentum balance analysis of the observations reveals that total acceleration (i.e., local and advective) and bottom stress are balanced by wind stress and pressure gradient. A numerical model is used to verify the experimentally derived results and to analyze the cross-shore momentum balance which is not resolved by the experimental data. Analysis of the numerical and field data reveal that the total alongshore pressure gradient force can be considered as the sum of a local and a regional component. The former is correlated with the alongshore advective acceleration that develops over the crest of the ridges resembling a Bernoulli-like pressure-flow relationship. The regional alongshore pressure gradient is related to wind stress with which it maintains a strong, negative relationship. Over the ridge crests the cross-shore veering of the flow is due to mass conservation, associated with a balance between cross-shore advective acceleration and local cross-shore pressure gradient. Under variable wind conditions the regional pressure gradients are more important, as compared to quasi steady-state conditions which increase the influence of the local scale pressure gradient. A time scale of 6 h is found as the temporal scale required for the local pressure gradients to make a significant contribution to the momentum balance.

Observations of mixing and transport on a steep beach

Surfzone mixing and transport on a sandy, steep (∼1/8 slope), reflective beach at Carmel River State Beach, California, are described for a range of wave and alongshore flow conditions. Depth-limited wave breaking occurred close to the shore due to the steepness of the beach, creating a narrow surf/swash zone (∼10 m wide). Fluorescent Rhodamine dye was released as a slug in the surfzone, and the temporal and spatial evolution was measured using in-situ dye sensors. Dye concentration measured as a function of time reveals sharp fronts that quickly decay resulting in narrow peaks near the dye release, which subsequently broaden and decrease in peak concentration with alongshore distance. The measurements indicate two stages of mixing and transport occur inside the surfzone on the steep beach. 1) In the near-field ( 50 m downstream from the dye release location), the mass transport was dominated by advection. The distance to the far-field is much shorter in the alongshore on a steep beach compared with a dissipative beach. Estimates of cross-shore and alongshore diffusion coefficients (κ x , κ y ) were found to be similar in magnitude within the surfzone. Outside the surfzone in the far-field, the results suggest that the mixing processes are independent of those inside the surfzone. The mixing and transport of material observed on this steep beach are found to be analogous to that previously measured on dissipative beaches, however the diffusion coefficients within and outside the surfzone were found to be smaller on this steep beach.

The Effect of the Coriolis Force and Wind on the Dynamics of the Fall Thermal Bar

The effect of the Coriolis force and wind stress on hydrodynamic processes in Kamloops Lake during the development of the fall thermal bar is investigated using numerical simulation methods and meteorological data for December 1–30, 2017. It has been found that Coriolis forces in combination with strong winds that generate large-scale circulation flows of water masses have a significant impact on the alongshore flow velocity. The analysis of the spatial distributions of physical parameters showed that strong westerly winds directed opposite to the horizontal movement of the thermal front lead to a shift of the cyclonic circulation induced by the Coriolis force into the lower layers of the water column.

Assessing the Use of Area- and Time-Averaging Based on Known De-correlation Scales to Provide Satellite Derived Sea Surface Temperatures in Coastal Areas

Satellite derived sea surface temperatures (SSTs) are often used as a proxy for in situ water temperatures, as they are readily available over large spatial and temporal scales. However, contamination of satellite images can prohibit their use in coastal areas. We compared in situ temperatures to SST foundation (~10 m depth) at 31 sites inshore of the East Australian Current (EAC), the dynamic western boundary current of the south Pacific gyre, using an area averaging approach to overcome coastal contamination. Varying across- and along-shelf distances were used to area average SST measurements and de-correlation time scales were used to gap fill data. As the EAC is typically anisotropic (dominant along-shore flow) the choice of across-shelf distances influenced the correlation with in situ temperatures more than along-shelf distances. However, the “optimal” distances for both measurements were within known de-correlation length scales. Incorporating both SST area and time averaging (based on de-correlation time scales) produced data for an average of 96% of days that in situ loggers were deployed, compared to 27% (52%) without (with) area averaging. Temperature differences between the in situ data and SSTs varied depending on time of year, with higher differences in the austral summer when daily in situ temperatures can range by up to 4.20°C. The differences between the in situ and SST measurements were, however, significant with or without area averaging (t-test: p-values < 0.05). Nevertheless, when using the area averaging approaches SSTs were only an average of ~1.05°C different from in situ temperatures and less than in situ temperature fluctuations. Linear mixed models revealed that latitude, distance to the coast and nearest estuary did not influence the difference between the in situ and satellite data as much as the water depth. This study shows that using de-correlation length and time scales to inform how to process satellite data can overcome contamination and missing data thereby greatly increasing the coverage and utility of SST data, particularly in coastal areas.

Temporal Variability in Thermally Driven Cross-Shore Exchange: The Role of Semidiurnal Tides

AbstractWe examine temporal variability of thermally driven baroclinic cross-shore exchange in the context of a tropical fringing reef system focusing on the role of tidally driven alongshore flow. Ensemble diurnal phase averaging of cross-shore flow at the Kilo Nalu Observatory (KNO) in Oahu, Hawaii, shows a robust diurnal signal associated with an unsteady buoyancy/diffusive dynamic balance, although significant variability is observed at subdiurnal time scales. In particular, persistent fortnightly variability in the cross-shore diurnal flow pattern is consistent with modulation by the semidiurnal alongshore tidal flow. The alongshore flow plays a direct role in the cross-shore exchange momentum balance via Coriolis acceleration but also affects the cross-shore circulation indirectly via its influence on vertical turbulent diffusion. An idealized linear theoretical model for thermally driven cross-shore flow is formulated using the long-term time-averaged diurnal dynamic balance at KNO as a baseline. The model is driven at leading order by the surface heat flux, with contributions from the alongshore flow and cross-shore wind appearing as linear perturbations. Superposition of the idealized solutions for Coriolis and time-varying eddy viscosity perturbations are able to reproduce key aspects of the fortnightly variability. Modifying the model to consider a more realistic alongshore flow and considering effects of nightly convection lead to further improvements in comparisons with KNO observations. The ability of the theoretical approach to reproduce the fortnightly patterns indicates that semidiurnal variations in the alongshore flow are effective in modulating the cross-shore flow via Coriolis and vertical turbulent transport mechanisms.

On extracting high-frequency tidal variability from HF radar data in the northwestern Bay of Bengal

ABSTRACTFirst results from a systematic harmonic analysis of HF radar (HFR) derived ocean surface current observations in the northwestern Bay of Bengal (BoB) during 2010 is presented. The daily-averaged HFR currents compared reasonably well to composite daily surface currents from multiple satellites with correlation coefficient of 0.90 (0.69) for zonal (meridional) component. A set of sequential daily currents demonstrated sustained northward (southward) alongshore flow during February–April (October–December) with peak magnitude of about 1.8 (1.2) m/s. On tidal scales, harmonic analyses of zonal and meridional components at nearshore and offshore locations indicated that among semi-diurnal tidal components, M2 dominates over S2 and N2; time-scales of which were verified from available coastal tide gauges nearby. Amplitudes of semi-major axis for M2 and K1 tidal ellipses are 7.16 (6.13) and 4.02 (3.30) cm/s in nearshore (offshore) location indicating relatively stronger tidal currents in nearshore location. Finally, significant shallow water constituents S4, MS4 and M3 (M4, 2SM6 and M6) at nearshore (offshore) location are identified, which are due to non-linear interaction of tidal currents with bathymetry. Both semi-diurnal and shallow water tidal currents show dominance along isobaths in offshore region, which turn progressively across-isobath as they move nearshore.

Lagrangian observations of the along-slope path of the California undercurrent

The flow of the California Undercurrent (CUC) along the continental slope and its interaction with deeper offshore waters is studied using trajectories of 53 isobaric RAFOS. The floats were launched off central California and data collected between 1993 and 2009. The mean pressure for the float measurements was 382 dbar. The trajectories were broken into 126 segments of continuous alongshore drift over the continental slope. Poleward drift occurred 88% of the time. The mean length of the poleward (equatorward) drift segments was 262 km (98 km) and the exit time for these distances was 32.3 ± 2.3 days (16.1 ± 2.2 days). Five poleward drift segments exceeded 500 km in length; the longest segment stretched from Central California to Central Oregon, a distance of 966 km.Locations where floats left or joined the along-slope flow occurred at most latitudes between 35.5°N to 42.5°N; here the mean rate at which floats left (joined) along-slope drift was 5.3 (3.5) per 50 km of coastline each year. Locations favored for leaving the Undercurrent included the northern edge of the Gulf of the Farallones (37.5°N) and between Cape Mendocino and Cape Blanco (41.5–42°N). A location favored for joining the Undercurrent was the northern edge of the Santa Lucia Bank (35.5°N). No floats were observed to join the alongshore flow immediately north of Cape Mendocino (40.5–41°N).Longer (> 290 day) float missions often included offshore excursions into the Northeastern Pacific Ocean. Thirty-one westward excursions of the Undercurrent were observed with a mean duration of 88 days and a mean offshore distance of 199 km. The character of the excursions varied but included offshore and onshore transport in jets, cyclonic and anticyclonic eddies, and interaction with cold filaments. The longest excursion lasted 587 days and extended 1007 km to the west.

Seasonal variations of hydrographic parameters off the Sudanese coast of the Red Sea, 2009–2015

The variations of temperature and salinity in the Sudanese coastal zone of the Red Sea are studied for the first time using measurements acquired from survey cruises during 2009–2013 and from a mooring during 2014–2015. The measurements show that temperature and salinity variability above the permanent pycnocline is dominated by seasonal signals, similar in character to seasonal temperature and salinity oscillations observed further north on the eastern side of the Red Sea. Using estimates of heat flux, circulation and horizontal temperature/salinity gradients derived from a number of sources, we determined that the observed seasonal signals of temperature and salinity are not the product of local heat and mass flux alone, but are also due to alongshore advection of waters with spatially varying temperature and salinity. As the temperature and salinity gradients, characterized by warmer and less saline water to the south, exhibit little seasonal variation, the seasonal salinity and temperature variations are closely linked to an observed seasonal oscillation in the along-shore flow, which also has a mean northward component. We find that the inclusion of the advection terms in the heat and mass balance has two principal effects on the computed temperature and salinity series. One is that the steady influx of warmer and less saline water from the south counteracts the long-term trend of declining temperatures and rising salinities computed with only the local surface flux terms, and produces a long-term steady state in temperature and salinity. The second effect is produced by the seasonal alongshore velocity oscillation and most profoundly affects the computed salinity, which shows no seasonal signal without the inclusion of the advective term. In both the observations and computed results, the seasonal salinity signal lags that of temperature by roughly 3 months.

Persistent Shoreline Shape Induced From Offshore Geologic Framework: Effects of Shoreface Connected Ridges

AbstractMechanisms relating offshore geologic framework to shoreline evolution are determined through geologic investigations, oceanographic deployments, and numerical modeling. Analysis of shoreline positions from the past 50 years along Fire Island, New York, a 50 km long barrier island, demonstrates a persistent undulating shape along the western half of the island. The shelf offshore of these persistent undulations is characterized with shoreface‐connected sand ridges (SFCR) of a similar alongshore length scale, leading to a hypothesis that the ridges control the shoreline shape through the modification of flow. To evaluate this, a hydrodynamic model was configured to start with the US East Coast and scale down to resolve the Fire Island nearshore. The model was validated using observations along western Fire Island and buoy data, and used to compute waves, currents and sediment fluxes. To isolate the influence of the SFCR on the generation of the persistent shoreline shape, simulations were performed with a linearized nearshore bathymetry to remove alongshore transport gradients associated with shoreline shape. The model accurately predicts the scale and variation of the alongshore transport that would generate the persistent shoreline undulations. In one location, however, the ridge crest connects to the nearshore and leads to an offshore‐directed transport that produces a difference in the shoreline shape. This qualitatively supports the hypothesized effect of cross‐shore fluxes on coastal evolution. Alongshore flows in the nearshore during a representative storm are driven by wave breaking, vortex force, advection and pressure gradient, all of which are affected by the SFCR.

Generation of Upwelling Circulation under Downwelling-Favorable Wind within Bottom-Attached, Buoyant Coastal Currents

AbstractA two-dimensional modeling study by Moffat and Lentz recently reported that downwelling-favorable wind can induce cross-shore upwelling circulation within a bottom-attached, buoyant coastal current. Here, we extend the problem to three dimensions. The driving mechanism and the sensitivity for the upwelling circulation are studied, using a primitive equation ocean model and an analytical model. After the initial downwelling adjustment that steepens the isopycnals and compresses the coastal current, the cross-shore flow can switch to steady upwelling circulation. This reverse circulation coincides with a vertically well-mixed water column and persists until interrupted by the arrival of river plume bulge from upstream. During the upwelling phase, the ageostrophic cross-shore flow follows the Ekman balance. The sense of cross-shore circulation is governed by a dimensionless parameter, the shear ratio, which measures the relative size of geostrophic shear and velocity shear supported by the wind in the shallow-water limit. Upwelling circulation occurs when the shear ratio is greater than one. This condition represents that, near the surface, the wind-intensified pressure gradient exceeds the maximum possible Coriolis force associated with the wind-forced, alongshore flow. The resulting upwelling circulation acts to slump the isopycnals to restore the geostrophic balance. Therefore, within a coastal current, decreasing wind stress in fact strengthens the upwelling circulation, as a weaker wind produces a weaker shear and thus increases the imbalance. This inverse relation holds until the wind is too weak to mix the water column. Based on the analytical model, a regime classification for the cross-shore circulation under downwelling-favorable wind is proposed. An observational example is given.

Rip currents and alongshore flows in single channels dredged in the surf zone

AbstractTo investigate the dynamics of flows near nonuniform bathymetry, single channels (on average 30 m wide and 1.5 m deep) were dredged across the surf zone at five different times, and the subsequent evolution of currents and morphology was observed for a range of wave and tidal conditions. In addition, circulation was simulated with the numerical modeling system COAWST, initialized with the observed incident waves and channel bathymetry, and with an extended set of wave conditions and channel geometries. The simulated flows are consistent with alongshore flows and rip‐current circulation patterns observed in the surf zone. Near the offshore‐directed flows that develop in the channel, the dominant terms in modeled momentum balances are wave‐breaking accelerations, pressure gradients, advection, and the vortex force. The balances vary spatially, and are sensitive to wave conditions and the channel geometry. The observed and modeled maximum offshore‐directed flow speeds are correlated with a parameter based on the alongshore gradient in breaking‐wave‐driven‐setup across the nonuniform bathymetry (a function of wave height and angle, water depths in the channel and on the sandbar, and a breaking threshold) and the breaking‐wave‐driven alongshore flow speed. The offshore‐directed flow speed increases with dissipation on the bar and reaches a maximum (when the surf zone is saturated) set by the vertical scale of the bathymetric variability.

Behavior of a wave‐driven buoyant surface jet on a coral reef

AbstractA wave‐driven surface‐buoyant jet exiting a coral reef was studied in order to quantify the amount of water reentrained over the reef crest. Both moored observations and Lagrangian drifters were used to study the fate of the buoyant jet. To investigate in detail the effects of buoyancy and alongshore flow variations, we developed an idealized numerical model of the system. Consistent with previous work, the ratio of alongshore velocity to jet velocity and the jet internal Froude number were found to be important determinants of the fate of the jet. In the absence of buoyancy, the entrainment of fluid at the reef crest creates a significant amount of retention, keeping 60% of water in the reef system. However, when the jet is lighter than the ambient ocean water, the net effect of buoyancy is to enhance the separation of the jet from shore, leading to a greater export of reef water. Matching observations, our modeling predicts that buoyancy limits retention to 30% of the jet flow for conditions existing on the Moorea reef. Overall, the combination of observations and modeling we present here shows that reef‐ocean temperature gradients can play an important role in reef‐ocean exchanges.

The Angola Current: Flow and hydrographic characteristics as observed at 11°S

The eastern boundary circulation off the coast of Angola has been described only sparsely to date, although it is a key element in the understanding of the highly productive tropical marine ecosystem off Angola. Here, we report for the first time direct velocity observations of the Angola Current (AC) at ∼11°S collected between July 2013 and October 2015 in the depth range from 45 to 450 m. The measurements reveal an alongshore flow that is dominated by intraseasonal to seasonal variability with periodically alternating southward and northward velocities in the range of ±40 cm/s. During the observation period, a weak southward mean flow of 5–8 cm/s at 50 m depth is observed, with the southward current extending down to about 200 m depth. Corresponding mean southward transport of the AC is estimated to be 0.32 ± 0.046 Sv. An extensive set of hydrographic measurements is used to investigate the thermal structure and seasonality in the hydrography of the eastern boundary circulation. Within the depth range of the AC, the superposition of annual and semiannual harmonics explains a significant part of the total variability, although salinity in the near surface layer appears to be also impacted by year-to-year variability and/or short-term freshening events. In the central water layer, temperature and salinity on isopycnals vary only weakly on seasonal to annual time scales. The available data set is further used to evaluate different reanalysis products particularly emphasizing the ocean's role in coupled climate model SST biases in the Eastern Tropical Atlantic.

Evaluation of Sediment Transport in the Naiband Gulf Area Using Mike21

Naiband Gulf is part of Naiband National Park known as Iran’s first marine national park. It is Iran’s most important habitats in the northern Persian Gulf due to mangrove forests, coral community, and community of marine animals. The study examines the sediment transport in the Naiband Gulf and around it, where to achieve this. First, the required data including coastline information, sediment profile, and bathymetry information and waves are obtained. Then by Mike21, 2011 Version models, it was attempted to simulate waves and currents due to it in the coastal zone to determine the rate and direction of sediment transport in the Naiband Gulf area. The results obtained in this study indicate that the majority of sediment transport directions on both sides of the gulf have been internal. The existence of structures in the north and south of the Gulf has reduced alongshore sediment flows and sedimentation, especially in the south of the gulf that is a serious threat to corals.

The recirculation of the intermediate western boundary current at the Tubarão Bight – Brazil

Lagrangian floats and current meter measurements from two moored arrays are analyzed, in combination with altimetry data, in order to investigate the recirculation of Antarctic Intermediate Waters (AAIW), and of the Intermediate Western Boundary Current (IWBC) at the Tubarão Bight, in the vicinity of the Vitória-Trindade Ridge (VTR), Brazil. Results from a high-resolution numerical simulation provide a complementary view of the flow at intermediate and surface levels. The data depict a topographically-induced cyclonic recirculation at intermediate levels, and five Argo floats are successively trapped inside the bight for two-and-a-half years, performing a total of 10 closed clockwise gyres during this period of time. In situ measurements at the western side of the bight show an intense alongshore flow at intermediate levels, with averaged velocities at 800m of ~30cm/s, and peak velocities exceeding 50cm/s, magnitudes comparable to the Brazil Current (BC) flow at surface levels. The recirculation extends from at least 1000m deep up to 370m, reaching sometimes depths as shallow as 150m, but is mostly uncoupled from the surface flow during the one-and-a-half year long current meter record. Three different flow patterns are observed, and simulated, at surface levels inside the bight during the time the recirculation is well established at intermediate levels: a shallow cyclonic circulation, somewhat akin to the Vitória Eddy; a recurrent anticyclonic flow that encompasses the entire bight; and a southwestward-oriented circulation, associated with the BC being reorganized in a coherent flow after negotiating its way through the VTR channels. A significant portion (about 50% according to the model) of the inflow of intermediate waters recirculates, enhancing the flow of the IWBC within the bight, and increasing the age of AAIW that will eventually cross the VTR on its way to lower latitudes. Although the data are not conclusive about a preferential pathway of the intermediate flow across the VTR, the simulation indicates that up to 70% will exit through the main channel located between the Besnard Bank and the Vitória Seamount, while 30% will follow eastward to the south of the VTR, in a region of intense mesoscale activity, and then cross the ridge farther east.

The impact of climate change on the Iberian low-level wind jet: EURO-CORDEX regional climate simulation

A sharp temperature contrast, observed mostly in summer, between high temperatures over land and lower temperatures over the ocean and the typical summer synoptic scale configuration (high-pressure system over the ocean and thermal low inland) are responsible for the development of a coastal low-level jet (CLLJ). The low-level horizontal pressure gradient induces, through geostrophic adjustment, a strong alongshore flow, which is also influenced by local orography and the high-pressure subsidence over the maritime boundary layer. In this study, the EURO-CORDEX hindcast forced by ERA-Interim (1989–2009), the historic reference (1960–2006) and the future (2006–2100; RCP8.5) simulations, forced by EC-Earth global model, are used to determine the climate change signal on the CLLJ off the Iberian Peninsula's western coast. Although the boundary conditions of the hindcast and historic reference simulations have different resolutions, both have similar distributions and features of CLLJ. In the summer, a clear rise in the occurrence of CLLJ is expected throughout the 21st century, with the highest increase off the northwest coast of Iberia (~14%). The CLLJ prevailing height is confined between 300 and 400 m and the most frequent maximum wind speed is 15 m s−1 both in present and future climate; nevertheless, a shift to higher values is expected. The predominant wind direction at jet height is north–northeast in all simulations. The temporal evolution of CLLJ occurrence during the 21st century shows that there is no significant trend in spring and autumn, although some decadal variability is observed.

Subthermocline anticyclonic gyre east of Mindanao and its relationship with the Mindanao Undercurrent

The quasi-permanent anticyclonic gyre (ACG) east of Mindanao is a dominant feature of the subthermocline circulation in the southern Philippine Sea, and it is believed closely associated with the continuous northward alongshore flow of the Mindanao Undercurrent (MUC). In this study, the structure and variability of this ACG were investigated using the 1950–2012 output of the Oceanic General Circulation Model for the Earth Simulator (OFES), which can reproduce well the structure of the climatological intermediate-layer circulation and satellite-observed sea level variations in the southern Philippine Sea. Between 26.8–27.3 σθ, the ACG covers a large area from the Mindanao coast to 131°E and from 3°N to 10°N. Its anticyclonic flow structure is unrelated to the surface Halmahera Eddy. The eddy-resolving simulation of the OFES revealed that the ACG consists of two components. The southern ACG (SACG) is centered at ∼6°N, while the northern ACG (NACG) is centered at ∼10°N. Seasonal and interannual variations of the ACG are linked to the variations of the northward MUC transport along the Mindanao coast, and the role of the SACG is more important than the NACG. Stronger (weaker) ACGs lead to greater (smaller) MUC transport. On the interannual timescale, the SACG shows a spectrum peak at 4–8 years, while the NACG has enhanced power within the 3–5-year band. A lead–lag correlation analysis indicates that interannual variations of the ACGs and the MUC transport are partly associated with the El Nino-Southern Oscillation. Possible causes for the ACG variability are discussed.

Pathways and hydrography in the Mesoamerican Barrier Reef System Part 2: Water masses and thermohaline structure

Hydrographic data from two oceanographic cruises conducted during March 2006 and January/February 2007 are used to investigate the thermohaline structure related to the observed circulation along the Mesoamerican Barrier Reef System (MBRS). From our observations we identify three water masses in the MBRS: the Caribbean Surface Water (CSW), North Atlantic Subtropical Underwater (SUW), and Tropical Atlantic Central Water (TACW). Little vertical structure in temperature is observed in the upper 100m of the water column, but important differences are observed in the salinity distribution both horizontally and with depth. Freshwater inputs to the system from the mainland can be traced in the surface layer, with two possible sources: one from surface rivers located along the southern portion of the MBRS, and the other originating from an underground river system located along the northern portion of the MBRS. The thermohaline structure in the MBRS reflects the dynamics of the observed circulation. Uplifted isopycnals along most of the central and northern coastline of the MBRS reflect the effects of the strong geostrophic circulation flowing northward, i.e. the Yucatan Current. To the south along the MBRS, much weaker velocities are observed, with the Honduras Gyre dominating the flow in this region as presented during January/February 2007. These two regions are separated by onshore and divergent alongshore flow associated with the impingement of the Cayman Current on the shore and the MBRS.