scholarly journals Filaments, Fronts and Eddies in the Cabo Frio Coastal Upwelling System, Brazil

Fluids ◽  
2021 ◽  
Vol 6 (2) ◽  
pp. 54
Author(s):  
Paulo H. R. Calil ◽  
Nobuhiro Suzuki ◽  
Burkard Baschek ◽  
Ilson C. A. da Silveira

We investigate the dynamics of meso- and submesoscale features of the northern South Brazil Bight shelf region with a 500-m horizontal resolution regional model. We focus on the Cabo Frio upwelling center, where nutrient-rich, coastal waters are transported into the mid- and outer shelf, because of its importance for local and remote productivity. The Cabo Frio upwelling center undergoes an upwelling phase, from late September to March, and a relaxation phase, from April to early September. During the upwelling phase, an intense front around 200 km long and 20 km wide with horizontal temperature gradients as large as 8 ∘C over less than 10 km develops. A surface-intensified frontal jet of 0.7 ms−1 in the upper 20 m and velocities of around 0.3 ms−1 reaching down to 65 m depth makes this front a preferential cross-shelf transport pathway. Large vertical mixing and vertical velocities are observed within the frontal region. The front is associated with strong cyclonic vorticity and strong variance in relative vorticity, frequently with O(1) Rossby numbers. The dynamical balance within the front is between the pressure gradient, Coriolis and vertical mixing terms, which are induced both by the winds, during the upwelling season, and by the geostrophic frontal jet. Therefore, the frontal dynamics may be largely described as sum of Ekman and turbulent thermal wind balances. During the upwelling phase, a mix of barotropic and baroclinic instabilities dominates in the upwelling center. However, these instabilities do not lead to the local formation of coherent eddies when the front is strong. In the relaxation phase, the front vanishes, and the water column becomes less stratified. The interaction between eastward coastal currents generated by sea level variability, coastal intrusions of the Brazil Current, and sporadic wind-driven, coastal upwelling events induce the formation of cyclonic eddies with diameters of, approximately, 20 km. They are in gradient-wind balance and propagate along the 100-m isobath on the shelf. During this phase baroclinic instability dominates. Cold filaments with widths of 2 km are formed due to straining and stretching of cold, coastal temperature anomalies. They last for a few days and are characterized by downwelling as large as 1 cms−1. The turbulent thermal wind balance provides a good first order estimate of the dynamical balance within the filament, but vertical and horizontal advection are shown to be important. To our knowledge, this is the first account of these smaller scale features in the region. Because these meso- and submesoscale features on the shelf heavily affect the water properties crucial to productivity of the South Brazil Bight, it is important to take these features into account for a better understanding of the functioning of this ecosystem and its resilience to both direct human activities as well as to climate change.

2007 ◽  
Vol 25 (8) ◽  
pp. 1735-1744 ◽  
Author(s):  
S. H. Franchito ◽  
V. Brahmananda Rao ◽  
T. O. Oda ◽  
J. C. Conforte

Abstract. The effect of coastal upwelling on the evolution of the atmospheric boundary layer (ABL) in Cabo Frio (Brazil) is investigated. For this purpose, radiosounding data collected in two experiments made during the austral summer (upwelling case) and austral winter (no upwelling case) are analysed. The results show that during the austral summer, cold waters that crop up near the Cabo Frio coast favour the formation of an atmospheric stable layer, which persists during the upwelling episode. Due to the low SSTs, the descending branch of the sea-breeze circulation is located close to the coast, inhibiting the development of a mixed layer mainly during the day. At night, with the reduction of the land-sea thermal contrast the descending motion is weaker, allowing a vertical mixing. The stable ABL favours the formation of a low level jet, which may also contribute to the development of a nocturnal atmospheric mixed layer. During the austral winter, due to the higher SSTs observed near the coast, the ABL is less stable compared with that in the austral summer. Due to warming, a mixed layer is observed during the day. The observed vertical profiles of the zonal winds show that the easterlies at low levels are stronger in the austral summer, indicating that the upwelling modulates the sea-breeze signal, thus confirming model simulations.


2017 ◽  
Vol 47 (4) ◽  
pp. 755-774 ◽  
Author(s):  
Lingling Xie ◽  
Enric Pallàs-Sanz ◽  
Quanan Zheng ◽  
Shuwen Zhang ◽  
Xiaolong Zong ◽  
...  

AbstractUsing the generalized omega equation and cruise observations in July 2012, this study analyzes the 3D vertical circulation in the upwelling region and frontal zone east of Hainan Island, China. The results show that there is a strong frontal zone in subsurface layer along the 100-m isobath, which is characterized by density gradient of O(10−4) kg m−4 and vertical eddy diffusivity of O(10−5–10−4) m2 s−1. The kinematic deformation term SDEF, ageostrophic advection term SADV, and vertical mixing forcing term SMIX are calculated from the observations. Their distribution patterns are featured by banded structure, that is, alternating positive–negative alongshore bands distributed in the cross-shelf direction. Correspondingly, alternating upwelling and downwelling bands appear from the coast to the deep waters. The maximum downward velocity reaches −5 × 10−5 m s−1 within the frontal zone, accompanied by the maximum upward velocity of 7 × 10−5 m s−1 on two sides. The dynamic diagnosis indicates that SADV contributes most to the coastal upwelling, while term SDEF, which is dominated by the ageostrophic component SDEFa, plays a dominant role in the frontal zone. The vertical mixing forcing term SMIX, which includes the momentum and buoyancy flux terms SMOM and SBUO, is comparable to SDEF and SADV in the upper ocean, but negligible below the thermocline. The effect of the vertical mixing on the vertical velocity is mainly concentrated at depths with relatively large eddy diffusivity and eddy diffusivity gradient in the frontal zone.


1998 ◽  
Vol 16 (7) ◽  
pp. 866-871 ◽  
Author(s):  
S. H. Franchito ◽  
V. B. Rao ◽  
J. L. Stech ◽  
J. A. Lorenzzetti

Abstract. The effect of coastal upwelling on sea-breeze circulation in Cabo Frio (Brazil) and the feedback of sea-breeze on the upwelling signal in this region are investigated. In order to study the effect of coastal upwelling on sea-breeze a non-linear, three-dimensional, primitive equation atmospheric model is employed. The model considers only dry air and employs boundary layer formulation. The surface temperature is determined by a forcing function applied to the Earth's surface. In order to investigate the seasonal variations of the circulation, numerical experiments considering three-month means are conducted: January-February-March (JFM), April-May-June (AMJ), July-August-September (JAS) and October-November-December (OND). The model results show that the sea-breeze is most intense near the coast at all the seasons. The sea-breeze is stronger in OND and JFM, when the upwelling occurs, and weaker in AMJ and JAS, when there is no upwelling. Numerical simulations also show that when the upwelling occurs the sea-breeze develops and attains maximum intensity earlier than when it does not occur. Observations show a similar behavior. In order to verify the effect of the sea-breeze surface wind on the upwelling, a two-layer finite element ocean model is also implemented. The results of simulations using this model, forced by the wind generated in the sea-breeze model, show that the sea-breeze effectively enhances the upwelling signal.Key words. Meteorology and atmospheric dynamics (mesoscale meteorology; ocean-atmosphere interactions) · Oceanography (numerical modeling)


2014 ◽  
Vol 44 (10) ◽  
pp. 2617-2643 ◽  
Author(s):  
Jonathan Gula ◽  
M. Jeroen Molemaker ◽  
James C. McWilliams

Abstract A set of realistic, very high-resolution simulations is made for the Gulf Stream region using the oceanic model Regional Oceanic Modeling System (ROMS) to study the life cycle of the intense submesoscale cold filaments that form on the subtropical gyre, interior wall of the Gulf Stream. The surface buoyancy gradients and ageostrophic secondary circulations intensify in response to the mesoscale strain field as predicted by the theory of filamentogenesis. It can be understood in terms of a dual frontogenetic process, along the lines understood for a single front. There is, however, a stronger secondary circulation due to the amplification at the center of a cold filament. Filament dynamics in the presence of a mixed layer are not adequately described by the classical thermal wind balance. The effect of vertical mixing of momentum due to turbulence in the surface layer is of the same order of magnitude as the pressure gradient and Coriolis force and contributes equally to a so-called turbulent thermal wind balance. Filamentogenesis is disrupted by vigorous submesoscale instabilities. The cause of the instability is the lateral shear as energy production by the horizontal Reynolds stress is the primary fluctuation source during the process; this contrasts with the usual baroclinic instability of submesoscale surface fronts. The filaments are lines of strong oceanic surface convergence as illustrated by the release of Lagrangian parcels in the Gulf Stream. Diabatic mixing is strong as parcels move across the filaments and downwell into the pycnocline. The life cycle of a filament is typically a few days in duration, from intensification to quasi stationarity to instability to dissipation.


Fluids ◽  
2020 ◽  
Vol 5 (2) ◽  
pp. 87
Author(s):  
Matthew N. Crowe ◽  
John R. Taylor

Here we consider the effects of surface buoyancy flux and wind stress on a front in turbulent thermal wind (TTW) balance using the framework of Crowe and Taylor (2018). The changes in the velocity and density profiles induced by the wind stress and buoyancy flux interact with the TTW and can qualitatively change the evolution of the front. In the absence of surface-forcing, Crowe and Taylor (2018) found that shear dispersion associated with the TTW circulation causes the frontal width to increase. In many cases, the flow induced by the surface-forcing enhances the spreading rate. However, if the wind stress drives a cross-front flow which opposes the frontal buoyancy gradient or the buoyancy flux drives an unstable stratification, it is possible to obtain an up-gradient cross-front buoyancy flux, which can act to sharpen the front. In certain conditions, an equilibrium state develops where the tendency for the TTW circulation to spread the front is balanced by the frontogenetic tendency of the surface forces. We use numerical solutions to a nonlinear diffusion equation in order to test these predictions. Finally, we describe the connection between surface-forcing and vertical mixing and discuss typical parameters for mid-ocean fronts.


2011 ◽  
Vol 59 (2) ◽  
pp. 131-144 ◽  
Author(s):  
Flávia Noronha Dutra Ribeiro ◽  
Jacyra Soares ◽  
Amauri Pereira de Oliveira

A coupled atmospheric-oceanic model was used to investigate whether there is a positive feedback between the coastal upwelling and the sea breeze at Cabo Frio - RJ (Brazil). Two experiments were performed to ascertain the influence of the sea breeze on the coastal upwelling: the first one used the coupled model forced with synoptic NE winds of 8 m s-1 and the sign of the sea breeze circulation was set by the atmospheric model; the second experiment used only the oceanic model with constant 8 m s-1 NE winds. Then, to study the influence of the coastal upwelling on the sea breeze, two more experiments were performed: one using a coastal upwelling representative SST initial field and the other one using a constant and homogeneous SST field of 26°C. Finally, two more experiments were conducted to verify the influence of the topography and the spatial distribution of the sea surface temperature on the previous results. The results showed that the sea breeze can intensify the coastal upwelling, but the coastal upwelling does not intensify the sea breeze circulation, suggesting that there is no positive feedback between these two phenomena at Cabo Frio.


1998 ◽  
Vol 46 (1) ◽  
pp. 01-17 ◽  
Author(s):  
Carlos Carbonel

A 1 1/2 reduced-gravity model is proposed to study the hydrodynamic and thermodynamic features of the coastal upwelling area of Cabo Frio (Rio de Janeiro-Brazil). The vertical structure of the model is described by an active layer overlaying a deep inert layer where the pressure gradient is set to zero. For the upper layer, the model includes the turbulent version of the momentum. continuity and heat equations. The conservation of heat is represented by a transport equation to describe the thermodynamic changes of the sea surface temperature (SST). The solution domain includes open boundaries in which weakly-retlective conditions are prescribed. Solutions are found numerically on a uniform grid and the fundamental equations are approximated by the finite difference method. Numerical experiments are performed to evaluate the dynamic response of the coastal area of Cabo Frio forced by uniform and non-uniform wind fields. The solutions differ considerably depending on the orientation of the winds. East and northeast winds correlate with colder waters in the zonal coastline of this area and the presence of tlows toward Cabo Frio correlates with north wind components. The proposed model is validated with the numerical simulation of an observed event of upwelling, where a time-dependent and non-uniform wind ficld develops a SST pattern similar as the observations, particularly the extension of the cool water plume in south-west direction and the rapid time variation of the SST.


2011 ◽  
Vol 11 (6) ◽  
pp. 551-572 ◽  
Author(s):  
Flávia Noronha Dutra Ribeiro ◽  
Jacyra Soares ◽  
Amauri Pereira de Oliveira

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