Mauro Cirano

Oceanographic characteristics of Baía de Todos os Santos, Brazil

Based on a vast set of in situ data, a first comprehensive overview of the oceanographic characteristics of Ba ´ia de Todos os Santos (BTS) is provided. BTS is the second largest coastal bay in Brazil (maximum area of 1223 km 2 and average depth of 9.8 m), and is located in the northeast Brazil, in the vicinity of Salvador city. The circulation inside the bay is mostly tidally driven and does not vary significantly throughout the year. On the other hand, the wet (winter) and dry (summer) seasons does alter significantly the distribution of water properties inside the BTS. During summer, the waters inside the bay have oceanic characteristics, with Tropical Water (TW) penetrating along the whole region, except for the mouth of Rio Paraguacu. The water temperature inside the bay is higher than in the coastal zone, and variations can be up to 3 ◦ C, reaching a maximum of nearly 30 ◦ C. During winter, with the increase of freshwater inflow, salinity variations of about 4 are observed between the innermost stations inside BTS and the adjacent coastal region. Salinity values inside the bay can be as low as 32.3, inhibiting the penetration of TW into the BTS, which is totally occupied by a locally formed Coastal Water (CW). An evaluation of the flushing time is also provided and shows that during summer, a 60-fold increase can be observed compared to winter (38 days). While the circulation does not vary seasonally inside the bay, the associated inner shelf is characterized by two different scenarios. During summer, the upwelling favorable easterlies drive a southwestward flow, while during winter the more frequent occurrence of cold fronts (southerly winds) tend to reverse the circulation.

Aspects of the Mean Wintertime Circulation along Australia's Southern Shelves: Numerical Studies

Abstract A high-resolution numerical model is used to study the mean wintertime shelf-slope circulation between the Gulfs of South Australia and the west coast of Tasmania. The mean downwelling-favorable winds for the region result in a continuous eastward coastal current (CC) extending from Cape Leeuwin to the eastern coast of Tasmania. The magnitude of the CC is generally largest over the shelf break, has speeds of more than 40 cm s−1 [transport 2.4 Sv (Sv ≡ 106 m3 s−1)], and is generally in good agreement with observations. The relative importance of the Leeuwin Current, zonal winds, and density gradients to the shelf circulation is established. Within Bass Strait, the eastward circulation is in good agreement with observations and simple models show that about 30% of the transport arises from local winds and the remainder from the CC. The total transport through the strait (0.95 Sv) is shown to be controlled by sea level on the east coast of Tasmania. A current of up to 10 cm s−1 is also found to flow...

Efficient tools for marine operational forecast and oil spill tracking

Ocean forecasting and oil spill modelling and tracking are complex activities requiring specialised institutions. In this work we present a lighter solution based on the Operational Ocean Forecast Python Engine (OOFε) and the oil spill model General NOAA Operational Modelling Environment (GNOME). These two are robust relocatable and simple to implement and maintain. Implementations of the operational engine in three different regions with distinct oceanic systems, using the ocean model Regional Ocean Modelling System (ROMS), are described, namely the Galician region, the southeastern Brazilian waters and the Texas-Louisiana shelf. GNOME was able to simulate the fate of the Prestige oil spill (Galicia) and compared well with observations of the Krimsk accident (Texas). Scenarios of hypothetical spills in Campos Basin (Brazil) are illustrated, evidencing the sensitiveness to the dynamical system. OOFε and GNOME are proved to be valuable, efficient and low cost tools and can be seen as an intermediate stage towards more complex operational implementations of ocean forecasting and oil spill modelling strategies.

Wind-Forced Downwelling Slope Currents: A Numerical Study

Abstract A study is made of the dynamics of slope currents that arise from a steady, constant alongshore wind over a uniform shelf. Over the first 10–20 days, the evolution of the downwelled system on an f plane is qualitatively described by linear coastal-trapped wave dynamics. After this time the thermal wind shear associated with the bottom mixed layer becomes important in the evolution of the undercurrent (UC), which is shown to be driven by the alongshore pressure gradient due to sea level. As the UC over the slope evolves, the bottom Ekman transport becomes small and negative, leading to the detachment of flow near the shelf break, localized spreading of isopycnals, and further intensification of the UC. In contrast to results obtained without bottom drag, bottom friction and boundary layer transport are shown to lead to a two- to threefold increase in cross-shelf interior transport, downwelling, and thermal wind shear. By day 60, the resultant UC has speeds of up to 15 cm s−1 and a net transport of...

A circulação oceânica de larga-escala na região oeste do Atlântico Sul com base no modelo de circulação Global OCCAM

The OCCAM (Ocean Circulation and Climate Advanced Modeling Project) is one of the global ocean circulation models which has been commonly used by the Brazilian oceanographic community. In most cases it is associated to regional numerical modeling studies, where it provides initial and boundary conditions for higher resolution models. The aim of this work, based on the concept of water masses, is to compare on an annual basis, the OCCAM with the use of: i) climatological temperature and salinity data from the NODC (National Oceanographic Data Center) and ii) volume transports associated with the water masses and based on values available in the literature. The selected levels of comparison were chosen to represent the core of the main water masses of the South Atlantic and the associated currents, described based on their volume transports. The main results indicate that the model is capable of representing realistically the vertical structure of the main currents and the associated water masses for the study region. In the equatorial portion of the subtropical gyre the model shows, for example, the southward zonal migration of the South Equatorial Current bifurcation with the increase of depth. According to the model and for the Tropical Water level, the bifurcation occurs between 9oS-15oS, moving to 25oS at the level of the South Atlantic Central Water and between 25oS-30oS at the level of the Antarctic Intermediate Water. The North Atlantic Deep Water, which is part of the thermohaline circulation, is represented in the model with a net southward transport between 15 Sv and 20 Sv for the region of study and is in agreement with the literature values.

Wintertime circulation off southeast Australia: Strong forcing by the East Australian Current

Numerical results and observations are used to examine the sub-weather band wintertime circulation off the eastern shelf slope region between Tasmania and Cape Howe. The numerical model is forced using wintertime-averaged atmospheric fluxes of momentum, heat, and freshwater and using transports along open boundaries that are obtained from a global ocean model. The boundary to the north of Cape Howe has a prescribed transport of 17 Sv, which from available observations represents strong forcing by the East Australian Current (EAC). Nonetheless, the model results are in qualitative (and quantitative) agreement with several observational studies. In accord with observations, water within Bass Strait is found to be driven to the east and north with speeds of up to 30 cm s -1 off the southeast Victorian coast. The water is colder and denser than that found farther offshore, which is associated with the southward flowing (warm) EAC. In agreement with observations the density difference between the two water masses is about 0.6 kg m -3 and leads to a cascade of dense Bass Strait water to depths of 300 m and as a plume that extends 5-10 km in the cross-shelf direction. Typical vertical velocities of the model cascade are 20 m d -1 . Along-isobath currents become bottom intensified with speeds of 20 cm s -1 or so at depths of 200 m. The EAC is found to flow along isobaths and to the south and then west past Tasmania with speeds of up to 20 cm s -1 . Mesoscale eddies grow to diameters of 100 km (speeds <40 cm s -1 ), and in accord with observations a semipermanent anticyclonic eddy is found to the east of Cape Howe.

Development of a pilot Brazilian regional operational ocean forecast system, REMO-OOF

An oceanic forecast system was created for the southeastern Brazilian region. A 9.5 years spin up, from January 2000 to July 2009, was used to adjust the model Regional Ocean Modelling System (ROMS) forced by oceanic climatologies and National Centers for Environmental Prediction (NCEP) surface fluxes. After this period, the system became operational with the atmospheric fluxes from the Global Forecast System (GFS). The system produces daily oceanic analysis and five days forecasts in a fully automatic manner. In addition to the routine task of running the oceanic model, this operational implementation also collects data from global datasets (maps of sea surface elevations and temperatures, profiles of temperature and salinity), for validation and improvement of the model parameterisations. Daily outputs from the operational analysis of surface temperature and altimetry, averaged over the first operational year, compare reasonably well with satellite and in-situ observations and the main features of the southeastern Brazilian dynamics are reproduced by the operational model. In the near future the system aims to incorporate the usage of large scale assimilated ocean modelling products as lateral forcing and high resolution local area atmospheric forecasts to provide the surface fluxes for the ocean model. Operational outputs and other relevant data are made available online in real time.

Mean Circulation, Seasonal Cycle, and Eddy Interactions in the Eastern Brazilian Margin, a Nested ROMS Model

Abstract A nested configuration of the Regional Ocean Modeling System (ROMS) is used to study the seasonal circulation patterns and mesoscale activity of the Eastern Brazilian Margin (EBM). The EBM encompass an oligotrophic and bathymetrically complex zone in the NW South Atlantic from 8°S to 20°S. Sea-level anomaly data are used to validate the model. Analysis of the mean circulation reveals that the EBM is dominated by seasonal and spatial dynamics of the southward Brazil Current (BC) and the northward North Brazil Undercurrent (NBUC), as well as their connection to the South Equatorial Current (SEC) dynamics. The EBM can be divided in three dynamic provinces, which are seasonally connected either by the permanent main flow or by mesoscale process. In the northern province, from 8°S to 13°S, the NBUC is the major permanent feature and the BC is just a thin flow, confined to the top few meters. As it moves southward, the BC gets deeper and stronger. In the middle province, from 13°S to 16°S, the dominance of the top 0–100-m circulation is seasonally alternated between the southward BC flow and the northward NBUC flow. In the southern province, from 16°S to 20°S, the BC appears as a dominating surface feature. While on the top (0–100 m) the main current presents a pronounced seasonal and spatial variability, on subsurface waters (100–500 m) the NBUC connects the EBM continuously. Finally, analysis of the regional simulation reveals well-defined cyclonic and anticyclonic eddies. They detach from the main flow and translate along the domain throughout the year. The translation patterns are associated with the seasonal variability of the main EBM flow, with anticyclonic mesoscale features translating southward and mesoscale cyclonic features translating northward. On their pathway, these features may come very close to the margin, interacting with the near-shelf flow.

Physical forcing mechanisms controlling the variability of chlorophyll-a over the Royal-Charlotte and Abrolhos Banks-Eastern Brazilian Shelf.

The Abrolhos Bank is part of the so-called Eastern Brazilian Shelf and is an area of high ecological and economic importance. The bank supports the largest and richest coral reefs in the South Atlantic and the largest rhodolith bed in the world. The spatial and seasonal variation of phytoplankton concentration, however, and the dynamic processes controlling that variability have remained poorly known. The present study investigates the seasonal and spatial distributions of chlorophyll-a (Chl-a) and water conditions by analyzing nine years (2003–2011) of level-3 Moderate-resolution Imaging Spectroradiometer (MODIS) derived Chl-a, National Centers for Environmental Prediction (NCEP)/ETA model-derived winds, NCEP model-derived heat fluxes, thermohaline and velocity results from the Hybrid Circulation Ocean Model (HYCOM) 1/12o assimilated simulation. The results show that low/high concentrations occurred in austral spring-summer (wet season)/autumn-winter (dry season), with the highest values observed in the northern portion of the Abrolhos Bank. The typical meteorological and oceanographic conditions during austral summer favor the development of strong stratification. These conditions are 1) N-NE winds that favor an upwelling-type Ekman circulation; 2) coupling between the open ocean and the continental shelf through the western boundary current, which promotes cooler subsurface water to rise onto the shelf break; and 3) positive net heat flux. In contrast, the S-SE winds during autumn are in the opposite direction of the predominant current system over the Abrolhos Bank, thus reducing their speed and inducing an inverse shear. The warmer ocean and a somewhat cool and dry atmosphere promote the evaporative cooling of the surface layer. The above processes drive mixed layer cooling and deepening that reaches its maximum in winter. The blooming of phytoplankton in the Abrolhos Bank waters appears to be regulated by changes in the mixed layer depth, with Chl-a levels that start to increase during autumn and reach their peak in June-July.

Oil spill modeling off the Brazilian eastern coast: the effect of tidal currents on oil fate

This study presents a comparative analysis of several theoretical oil spill scenarios occurring in the vicinity of the Abrolhos Reefs, a preservation area located off the Brazilian east coast. The following petroleum blocks were considered in the study: BM-CUM-1, BM-CUM-2, J-M-259 and ES-M-418, to verify the effect of the tide on the oil spreading on the continental shelf in the eastern Brazilian. The probabilistic oil spill simulations were carried out for 30 days and considered an oil of intermediate API (American Petroleum Institute) type. The simulations were performed by the OSCAR (Oil Spill Contingency and Response) model which was fed by synoptic wind data and hydrodynamic data simulated by the POM (Princeton Ocean Model) model. Two distinct seasonal scenarios were studied: the austral winter and the austral summer of 1989 and the results were discussed in terms of seasonal variability caused by the winter/summer winds and differences in mesoscale instabilities. The effect caused by tidal currents was analyzed through the comparison between simulations with and without the tides. It was concluded that the tide strongly influenced the probabilistic results of fate of oil in the spill scenarios. Many factors have influenced this fate of the spreading of oil in the absence of supra-inertial processes, such as the location of the spill, the behavior of the prevailing winds and tides in the region. The tide absence in the hydrodynamic data did not found the worst case scenarios of the spill in simulations in blocks BM-CUM-1, BM-CUM-2 and J-M-259, mainly in summer with use of intermediate oil, underestimating the real scenarios of spill.