Resumo em Inglês:

Abstract The energy sources for convection and the general circulation are revisited through an analysis of the compressible equations of motion, rather than the Boussinesq equations. We are motivated in this endeavor by a more straightforward connection in the compressible equations between thermodynamics and dynamics, and the continuing debate in the field regarding the suggestion, made first in the form of Sandström’s theorem, that surface buoyancy fluxes can not drive the overturning circulation. While ultimately supporting the Sandström position, the analysis leads to some new insights into ocean energetics and surface energy fluxes. We argue the ultimate role of buoyancy fluxes are to damp the circulation and that ocean energy cycles between internal and kinetic energy. Ocean heating due to the general circulation, geothermal heat flux and the biosphere are evaluated for their roles and we suggest the latter two provide energy to the overturning much more effectively than surface forcing. All three also contribute significantly to net ocean surface energy flux, an effect that influences the interpretation of ocean heat content imbalances.

Resumo em Inglês:

ABSTRACT We examine nonlinear topographic waves in barotropic, rigid lid models with a focus on how depth shapes can or cannot support solitary waves. Comparisons between the full equations and the quasigeostrophic system show that the conditions for vanishing nonlinearity are different and that the transition from cyclonic to anticyclonic solitary waves does not happen for the same topographic shape. For these dynamics, however, such waves only exist for a channel geometry when the long waves can become non-dispersive. We therefore examine wave groups since short waves can have an isolated cross-topographic structure. Group solitary waves are found and analyzed with a hyperbolic tangent topography representing the transition from the shelf to deep water.

Resumo em Inglês:

ABSTRACT Kinematics of seasonal exchanges of mass and heat between the Pacific Ocean and the Gulf of California are described. Results are based on 18 occupations between 1992 and 2013 of a hydrographic section across Pescadero Basin at the mouth of the Gulf and two and a half years (November 2003 to May 2006) of moored velocity and CTD measurements in 130 m of water on either side of the Pescadero Basin. Cyclonic conditions dominated in mid-winter and summer with inflow along Sinaloa and outflow along Baja California Sur (BCS). Advection of warm Gulf waters into the Pacific along BCS in late fall extended the warming along BCS by almost two months compared to Sinaloa; as a consequence, steric heights at BCS were higher, and resulted in near surface transport out of the Gulf of ~ 0.01 m s-1. During warming periods from May through November, coastal trapped waves transported heat into the Gulf along Sinaloa; the trapped wave motions along BCS were about a tenth of the amplitude of those at Sinaloa and contributed little to transport into the Pacific. Poleward monsoon winds were in phase with near-surface geostrophic flows into the Gulf which were about the same magnitude as out flow associated with much stronger equatorward wind. Next to BCS, higher salinity Gulf waters extended to 180 m depth (~26.2 kg m-3) and flow into the Pacific; these waters can be traced to subduction in the mid-Gulf region and, when they reach the Pacific, flow poleward in the undercurrent. If global warming intensifies the overturning Gulf circulation, lower oxygen and higher salinity waters may be advected northward along the continental shelf of North America.

Resumo em Inglês:

Abstract We investigated the variability of the oceanic heat storage on the western boundary of the South Atlantic Ocean. We aimed to understand the specific contribution of the Brazil Current as it travels southwards. The heat storage is evaluated near three latitudes, namely at 15◦S, 24◦S and 34.5◦S. Numerical outputs of the oceanic potential temperature, salinity, and meridional components of the current’s velocity resulting from the Estimating the Circulation and Climate of the Ocean (ECCO) model were used. We examined the time series of the heat storage from 1992 to 2015 for the three latitudes. The mean heat storage increased towards the south with a maximum at 24◦S. At 34.5◦S, there is a decrease in this property possibly due to its proximity to the Brazil-Malvinas Confluence. The Brazil Current’s volume transport time series was correlated with the corresponding heat storage. The latitude that presented the highest correlation at interannual scale was 15◦S with a value of 0.84 at a 95% of confidence level. The model outputs were also compared with in situ expendable bathythermograph measurements. Despite the presence of gaps and peaks indicating extreme events in the in situ data, both time series were statistically close and they showed similar mean annual cycles. No significant long-term HS trend was found at any of the three latitudes.

Resumo em Inglês:

Abstract Submesoscale eddies (1-10 km diameter) were identified using surface velocity observations obtained from a high-frequency radar system (HFR) operated in Todos Santos Bay (TSB), Baja California, Mexico. Eddies were detected through a special case of the Okubo-Weiss parameter for divergent flows in the form of eigenvalues of the Jacobian matrix. The detection method, applied for a surface velocity grid, shows encouraging results in the recognition and tracking of submesoscale features in TSB. The detection method is rapid and efficient. Results show the formation and persistence of an eddy structure inside the Bay in December 6, 2010, displaying a trajectory from NE to SW until disappearing at the center of the Bay. The eddy is approximately 4 km in diameter with a frequency of ~0.1f (f is the Coriolis parameter). The real part of the Okubo-Weiss parameter ranged between −4×10−9 to −1×10−9 s−2, and outlined the eddy for approximately 9 hours. Although it is difficult to identify the origin of the detected submesoscale eddy, its appearance coincided with a drop in relative atmospheric humidity suggesting land-ocean Santa Ana winds as a possible generating mechanism.

Resumo em Inglês:

Abstract This study presents the implementation of a portable module designed for autonomous analysis of sea-surface inorganic nutrients onboard vessels of opportunity, as an additional tool for assessing the levels of eutrophication risk. The study was carried out during August-mid-September 2019 in the Ria de Arousa and outer shelf area (NW-Iberian coastal upwelling system). During this period, the distributions of the measured sea-surface concentrations of nitrate and phosphate were compared according to three Oceanographic Environments (OEs). The OEs were defined according to the interplay between upwelling/downwelling events and river discharge on the coastal system. The nutrient measurements agreed well with the OEs, showing that the portable module is a useful tool for opportune measurements of sea-surface nutrients and can serve as a complement for the available monitoring networks. An initial evaluation of the eutrophication risk in this area indicated low risk levels (following the Environmental European Agency criteria) for most of the measured points in summer, except for some vulnerable areas under certain OEs. Nutrient concentrations are sensitive to periods of Sustained Upwelling events, reaching medium risk levels (7.14 - 9.05 µmol L-1 for nitrate and 0.39 - 0.64 µmol L-1 for phosphate) in inner parts of the Ria de Arousa. These areas are characterized by abrupt bathymetric changes that channel and intensify the upwelling processes, increasing sea-surface nutrient concentrations. High eutrophication risk levels of phosphate (1.53 µmol L-1) were detected close to the coastline during Upwelling Relaxation periods. Under these conditions, continental flows, previously retained by the upwelling, are able to expand. The location of these samples and the difference in concentration between phosphate and nitrate indicate a most likely source in wastewater outflows. Our results highlight the need for deeper studies on the synergy between upwelling/downwelling processes and the continental water discharges and its modulation of sea-surface nutrients.

Resumo em Inglês:

Abstract The SOund Fixing And Ranging (RAFOS) floats were deployed by the Naval Postgraduate School (NPS) near California coast from 1992 to 2004 at depth between 150 and 600 m (http://www.oc.nps.edu/npsRAFOS/). Each drifter trajectory is adaptively decomposed using the empirical mode decomposition (EMD) into low-frequency (non-diffusive, i.e., current) and high-frequency (diffusive, i.e., eddies) components. The identified eddies are mostly anticyclonic with total 203 anticyclonic and 36 cyclonic spirals. Eddy characteristics are determined from the time series of individual RAFOS float trajectory. They are the current velocity scale, eddy radial scale, eddy velocity scale, eddy Rossby number, and eddy-current kinetic energy ratio. The California Current System is found an eddy-rich system with the overall eddy-current kinetic energy ratio of 1.78. It contains submesoscale and mesoscale eddies. The horizontal length scale of 10 km is a good threshold of the eddy radial scale (Leddy) to separate the two kinds of eddies. The mean eddy Rossby number is 0.72 for the submesoscale eddies and 0.06 for the mesoscale eddies. The current-eddy kinetic energy ratio is similar between submesoscale and mesoscale eddies. This may imply similar current-eddy kinetic energy transfer for submesoscale and mesoscale eddies. Statistical characteristics and interannual variability of current velocity scale and eddy characteristic parameters are also presented.

Resumo em Inglês:

Abstract The Brazil Current (BC) is a western boundary current, with an average vertical extension of 500 m, which occupies the southwestern margin of Atlantic Ocean. Recently, the South Brazil Bight - SBB (23º S - 28º S) has been intensively explored by the oil and gas industries, which brought a high demand in operational oceanography and ocean state monitoring. In this study we present a new technique based on the vertical dynamic modes of oscillation, to extrapolate velocity profiles in the SBB, with potential applications to data acquired by Vessel Mounted Acoustic Doppler Current Profilers (VMADCP). Using four quasi-synoptic crosssectional velocity data, acquired by Lowered Acoustic Doppler Current Profilers (LADCP), we observed that the mean circulation of this region can be characterized as a 2-dynamical mode system. This mean circulation is mainly dominated at surface by the BC and at intermediate levels by the Intermediate Western Boundary Current (IWBC), which flows northward. Furthermore, observed mesoscale vortical features in the SBB, such as eddies and meanders, are shown to be primarily barotropic-mode dominated. In our extrapolation methodology, the use of an independent set of velocity time series obtained by four moorings has shown to be a valid way for obtaining the relative contribution of each dynamical mode in the velocity field in the SBB. Normalized mean squared errors below 10% and radar diagrams show that the presented methodology was able to reproduce the main dynamic features observed by LADCP transects. Also, with this technique, we extrapolated three VMADCP sections and the results agreed with previous observations.