Manuel López

The mean and seasonal water exchange between the gulf of California and the Pacific Ocean (PO) are analyzed with a one-year global HYCOM simulation. At the mouth of the gulf there are six alternating layers of inflow and outflow of mean transport with inflow in the uppermost layer (0-68 m) and the largest outflow in the second layer (68-198 m). The three uppermost layers have the largest mean transports, and they can be identified more than two thirds along the length of the gulf with approximately the same thickness. The difference in the transport between the interior of the gulf and the transport at the mouth in the upper layer, shows that there must be an upward mean transport (upwelling) into the upper layer along almost the entire length of the gulf. The two deepest layers have smaller mean transports, but the deepest layer has an outflow that is about half of the inflow of the surface layer implying a very large vertical transport into that layer. We obtained the seasonal exchange by fitting annual and semiannual harmonics to the monthly mean transports in each layer. The maxima and minima of the seasonal exchange are larger than the mean and they occur in summer and autumn showing that most of the exchange with the PO occurs during those two seasons. The maximum inflow (~ 0.8 Sv, 1 Sv = 1 × 106 m3/s) in the upper layer occurs at the beginning of July and the maximum outflow at the beginning of November (~ 0.4 Sv). The transport in the second layer is out of the gulf all year round. The fourth layer (380-822 m) has the smallest mean transport of all layers but, together with the first layer, has the largest seasonal transport. The net outflowing transport is about 0.2972 Sv, which gives a turnover time of approximately 14 years for the gulf.

Because of the need to diversify the renewable energy matrix and because hydrokinetic tidal energy technologies are mature, many in-stream tidal energy resource studies are available globally. Still, there are many questions regarding the effect of seabed changes on tidal energy resources. For coastal regions in particular, where the seabed is generally more mobile than in deep waters, bathymetric evolution could significantly affect tidal energy production. Here, two models are used to analyse the potential effect of natural morphodynamic change on tidal energy resources at two macro-tidal sandy bays, Adaír Bay and San Jorge Bay, in the Upper Gulf of California, Mexico. One of the models is (purely) hydrodynamic, and the other is a morphodynamic model (with hydrodynamic–morphodynamic coupling). The models are validated against tidal current observations obtained with acoustic Doppler current profilers in the region of interest, using three different error statistics, which showed good agreement between models and observations. The results also showed that the most significant bed changes and the largest renewable energy resources are located near the shore. Moreover, there was a good correlation between (a) regions with the most significant depth changes and (b) the areas where the difference in annual energy production with and without depth change was largest. Finally, a two-year simulation with the morphodynamic model permitted to analyse the seabed evolution of a zonal profile off Punta Choya, the headland between the two bays. This profile evolved towards a featureless equilibrium, as expected from the morphological classification for macro-tidal sandy environments under a dominant tidal forcing. However, most importantly, this natural evolution would not be detrimental to tidal energy exploitation at the site.

While many in-steam tidal energy resource studies have been carried out globally, very few studies have assessed the effect of seabed changes on tidal energy resources. For coastal regions in particular, where the seabed is generally more mobile than in deep waters, bathymetric evolution could have a significant effect on tidal energy production. Here two high-resolution models, one purely hydrodynamic and one morphodynamic, are used to analyse the potential effect of natural morphodynamic evolution on tidal energy resources at two macro-tidal sandy bays, Adaír Bay and San Jorge Bay, in the Upper Gulf of California, Mexico. The high-resolution models are validated using a low-resolution model and ADCP observations to assess the agreement between model predictions and observations of tides at three ADCP moorings within the domain of interest. The models’ skill is evaluated using several error statistics such as the mean relative error, the root mean square error (RMSE), and the correlation coefficient. It was found that the regions with the largest bed changes, and also the largest renewable energy resources, were near the shore. Moreover, the results indicated a good correlation between a) regions with the most significant depth changes, and b) the regions where the difference in annual energy production with and without depth change was largest. Finally, the morphodynamic model was run for two years, and the evolution of a zonal profile (in the west-east direction) off the coast at the southeastern corner of Adaír Bay was inspected. This profile evolved towards a featureless equilibrium profile, in good agreement with the morphological classification for macro-tidal sandy environments and with the model assumptions. But most importantly, this natural evolution would not be detrimental to tidal energy exploitation at the site.

Ocean currents and thermohaline properties over the continental slope off northern Baja California are analyzed with data collected between 2008 and 2012. The data set is composed of 3‐year‐long time series of two moorings with current meter profilers and one chain of conductivity‐temperature‐depths, as well as data collected during 13 synoptic hydrographic surveys. The upper layer (<100 m) was characterized by low salinity Subarctic Waters. Geostrophic velocities suggest that waters of the lighter layer are advected equatorward by near‐surface flows. Below 50 m, poleward flow dominates, so the denser Subarctic Waters recirculates back north along the slope. The combination of geostrophic velocities derived from the hydrographic surveys and the current meter data confirm the presence of the California Undercurrent (CUC), a narrow subsurface poleward flow with a high‐velocity core confined to the continental slope. Mean core speeds were 12–15 cm/s at an average depth of ~150 m. Below the CUC, the core of spicy North Pacific Equatorial Water (NPEW) was found at a mean depth of ~200 m. The analysis of the continuous time series provides strong evidence of the seasonal signal in speed, spiciness, and depth of the CUC and NPEW cores, with a time resolution unprecedented to date. Most of the variance of water mass and flow properties is explained by the annual cycle. The phase shift between the depth of the NPEW core and the strength of the CUC suggests that remotely generated Kelvin waves are responsible for the seasonal variability observed off northern Baja California.

The main properties of the San Lorenzo (SL) overflow are studied, using data from two nonsimultaneous ADCP moorings (located at the sill, and 5 km downstream), as well as CTD and LADCP profiles. Strong tidal currents at the sill modulate the overflow, which is not shut down during the neaps. At the downstream site, the largest flood currents are associated with colder water advected from the sill, flowing downslope, and creating an asymmetry in the semidiurnal tidal cycle. The overflow introduces a significant fortnightly harmonic at the downstream site, and delays the M₂ tidal currents for more than an hour with respect to the currents at the sill. The overflow mixes with the overlying water by entrainment during its supercritical stage, reaching near‐bottom velocities as high as 1.5 ms⁻¹ and an estimated mean transport of 0.11 Sv; almost twice that estimated at the sill for the same period of the year. Estimated Froude numbers during spring tides suggest the development of an internal hydraulic jump. After relaxation of the maximum downstream currents, high‐frequency temperature fluctuations, likely linked to upstream traveling waves, are consistently observed. Direct estimations of the turbulent dissipation rates were used to compute diapycnal diffusivity (K_ρ) profiles. Mean estimates of K_ρ, as high as 5.5 × 10⁻² m²s⁻¹, show that shear at the interface is the most significant source of cross‐isopycnal mixing along the SL overflow during ebb tides.

Two overflows, originating at sills in the northern Gulf of California, are marked by strong average downstream slopes of 17 and 4%. Near‐bottom stratification upstream of both sills is relatively strong, but the near‐bottom water is very well mixed downstream of the sills. The homogenization of a thick bottom layer downstream, albeit close to the sills, is indicative of very strong mixing. Waters in the downstream basins are well ventilated with relatively high oxygen concentrations. These hydrographic patterns were observed during three different seasons. Time series of temperature, salinity, and potential density show that water, flowing over the sills, is essentially subsurface, subtropical water from the Pacific Ocean. The time series also indicate that water is nearly always denser at the sills compared to water in the downstream basins, despite the two‐fold (∼800 m) and more than three‐fold (∼1500 m) increase in depth. The long‐term (more than a year) average current at both sills is bottom intensified with maximum average speeds of 33 and 17 cm/s. There are also strong tidal currents at the sills, although the subinertial currents near the bottom are quite persistent with nearly no flow reversals. Transports, estimated exclusively with the mean flow, are about 0.08 and 0.09 Sv (1 Sv = 1× 10⁶, m³/s), which agree well with previous estimates based on shorter time series and a lower vertical resolution. These results indicate that the deep water at both downstream basins are renewed by these overflows, which enter at sill depths ≤400 m and are capable of reaching the deepest part of both basins. The overflows appear to be responsible for the main mechanisms that transform subsurface, subtropical water into Gulf of California water.