Geostrophic influence in the River Douro plume: a climatological study

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1 V Conferência Nacional de Mecânica dos Fluidos, Termodinâmica e Energia MEFTE 2014, Setembro 2014, Porto, Portugal APMTAC, 2014 Geostrophic influence in the River Douro plume: a climatological study I Iglesias 1,2, P Avilez-Valente 1,2,3, X Couvelard 4, RMA Caldeira 2 1 Centro Interdisciplinar de Investigação Marinha e Ambiental, Universidade do Porto, Rua dos Bragas 289, Porto, Portugal 2 Instituto de Hidráulica e Recursos Hídricos, Universidade do Porto, Rua Dr. Roberto Frias, Porto, Portugal 3 Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, Porto, Portugal 4 Centro de Ciências Matemáticas, Universidade da Madeira, Funchal, Portugal iiglesias@fe.up.pt, pvalente@fe.up.pt, Xavier.Couvelard.@ifremer.fr, rcaldeira@ciimar.up.pt ABSTRACT: The Douro is one of the most important rivers of the Iberian Peninsula. Its estuary is located in the northwest of Portugal and the flow is controlled by the Crestuma Dam, which induces a high variability of the river flow on the estuary. This variability produces a strong impact on the plumes formation and dispersion. On this work, the Regional Ocean Modelling System (ROMS) was selected to represent different plume scenarios. A multiyear climatological simulation was performed using climatological initial conditions. It was observed that the plume structure depends on the initial conditions imposed to the model. Bulge plus coastal current and southwest protruding jet-like plumes were observed depending on the season of the year. The wind conditions show an important influence on the plume dispersion, but this is also reactive to the offshore current. When the offshore current is at its highest strength it can force the plume to travel against the wind. On this work, a wide range of plume patterns were observed: jet-like features, bifurcations, jets protruding towards either north or south, patches, flapping filaments and mesoscale eddies. So we concluded that the Douro river plume response depends both on the wind and on the behaviour of the offshore geostrophic current system. Offshore eddies and filaments are also responsible for the cross-shore transport, through the horizontal advection of plume waters. KEY WORDS: Buoyant plumes; Cross-shore transport; NW Iberian Peninsula; Advection; Alongshore currents; Coastal processes. 1 INTRODUCTION River plumes are one of the most important mechanisms that transport terrestrial materials to the ocean. These might be pollutants, essential nutrients, which enhance the phytoplankton productivity, or sediments, which settle on the seabed producing modifications on the bathymetry and affecting the navigation channels. The mixing between the riverine and oceanic waters can induce instabilities, which might generate bulges, filaments, and buoyant currents over the continental shelf. Offshore, the buoyant riverine water can form a front with the oceanic waters often related with the occurrence of current-jets, eddies and strong mixing. The study and modelling of the river plumes is a key factor for the complete understanding of sediment transport mechanisms and patterns, and of coastal physical and dynamic processes. There are three main processes that govern plume dynamics: (i) the mixing induced by the turbulence; (ii) the alongshore current produced by the balance between the Coriolis force and the cross-shore pressure gradient and (iii) the acceleration produced by the balance between buoyancy and gravity forces [1]. Plumes are superficial structures that can be trapped in the inner shelves with a baroclinic boundary coastal current structure [2] but they occupy a small portion of the total water column [3] and their behaviour varies with several variables like the wind characteristics, tides, bottom friction on shallow waters, and offshore currents. 2 GEOGRAPHICAL SETTINGS The River Douro is located on the northwestern Iberian coast. The freshwater flow that reaches the estuary is controlled by the Crestuma Dam, located 21.6 km upstream. Its daily averaged freshwater discharge can range from 0 to m 3 /s, with a strong variability between dry and rainy months and years. This variability impacts the formation of the river plume and its dispersion along the continental shelf. MEFTE 2014, Set 2014, Porto, Portugal 75

2 There are prominent capes, submarine canyons and promontories on the northwestern Iberian coast that produce persistent hydrodynamic features. The local tides have a semidiurnal regime, and their amplitudes range from 1 m to 1.7 m [4]. 3 MODEL SET-UP The study was carried out using the Regional Oceanic Modelling System (ROMS) [5,6]. The atmospheric forcing, the initial and boundary conditions and the model grid were built using the ROMSTOOL package [7]. To construct the bathymetry of the grid, the GEBCO 30 arc-second grid resolution database was selected [8]. 35 vertical levels, based on the stretched S-coordinates, were used to ensure an acceptable resolution of the upper ocean layer. The model domain extends from 39.5 ºN to 42.5 ºN and from ºW to 8 ºW (cf. Figure 1), and a grid resolution of approximately 1 km x 1 km. The climatological salinity and temperature profiles used to force and initiate the simulations were extracted from the World Ocean Atlas climatology [9,10]. The tidal forcing was extracted from the OSU tidal data inversion [11], and the atmospheric fluxes from COADS [12]. Figure 1: Bathymetry (m) and model domain. A 3 year spin-up simulation was performed without river flux for the volume averaged kinetic energy to reach a steady-state, after which a 1 month-long simulation with tide-ramp was performed. The results of this simulation were used as initial conditions for the case study. The case study is a 3 year-long multiyear climatological simulation, with monthly prescribed river outflow and water temperature. The river flux was added as a point source positioned at the River Douro mouth location with null river water salinity and water temperature and flow rate changing according to the values shown in Table 1. The temperature values were extracted from previous works [13,14], while the river fluxes are climatic averages at the Crestuma dam extracted from historical records of Jan/1986 to Jan/2012 time period. Table 1: River Douro mean temperature and monthly averaged discharge. MONTH Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec T (ºC) Q (m 3 /s) The depth of the plume was calculated as the depth at which the maximum value of the vertical salinity gradient is reached. The water masses deeper than 100 m and with salinity above 35.7 were excluded from the gradient computation to avoid the deep water salinity gradient. The seasonal averaged values of the Equivalent Depth were calculated according to the Choi and Wilkin [15] definition, as 76 MEFTE 2014, Set 2014, Porto, Portugal

3 ED SW dz, (1) h S S z where is the sea level elevation, h is the bottom depth, S SW is a shelf water reference salinity, taken as S z is the water column salinity in this work, and 4 RESULTS In Figure 2, the monthly mean surface salinity was depicted for the most characteristic plume patterns in the simulation, showing the variation of the plume structure with the climatological conditions in the area. During the first months of the simulation, a southwest protruding jet-like plume can be seen, while during winter months, particularly for October, November and December, the bulge and the coastal current seem to evolve to the north, according with the climatic wind conditions. The climatic wind presents a northern component for most of the year except for October, November and December, when it veers westward and southward. The wind strength varied between 2 and 8 m/s and produced currents on the range of m/s within the plume bulge and m/s close to the coast. January May August December S SW Figure 2: Monthly mean salinity and current vectors for four months of the multi-year climatological simulation. The representation of the plume depth (see Figure 3) showed that the Douro plume suffers an important influence of the coastal current. Although the wind presents an important influence on the plume dispersion (cf. Figures 3a and 3b), there are times when the plume induced current propagates against the wind. This happens when the offshore current presents a high strength on the direction opposite to the wind (cf. Figure 3c). Sometimes, the surface offshore currents propagate in direction opposite to their coastal counterpart (cf. Figure 3d), but when a strong offshore current is present it is able to veer the inshore current to travel against the wind (cf. Figure 3c). These snapshots of the plume depth allow us to observe different plume scenarios: jetlike feature (cf. Figure 3b), flapping filaments that eventually break into patches (cf. Figure 3c), plume bifurcations or changes on the plume direction at the river mouth with jets protruding to the north and to the south simultaneously (cf. Figure 3a). The analysis of the seasonal averaged values of ED (see Figure 4) allow for the identification of two distinct scenarios according to the season of the year: a bulge and an alongshore current for autumn and winter conditions, and a jet-like offshore protrusion of the river plume for spring and summer conditions. In Figure 5, snapshots of the streamlines and surface salinity for a 9 day-long sequence show the formation of a mesoscale eddy on the southern part of the domain. This kind of structures interact with inshore waters and are able to advect the river plume fresh water across the shelf. They are a mechanism to transport plume waters to remote locations. The eddies interaction with the ocean bottom can result in their dissipation, causing the plume water contents, such as coastal materials, sediments, nutrients or even pollutants, to settle in offshore regions. MEFTE 2014, Set 2014, Porto, Portugal 77

4 (a) (b) (c) (d) Figure 3: Plume depth (m) snapshots from the multi-year climatological simulation. Values over the land (from north to south) are the wind velocity (m/s) and direction, and the river flow rate (m 3 /s).values over the sea (from west to east) represent offshore and inshore current velocity (cm/s) and direction. Spring Summer Autumn Winter Figure 4: Seasonal ED (m) of the plume. 15 Apr Apr Apr Apr 0001 Figure 5: Surface salinity and streamlines for a 9 day-long sequence. 78 MEFTE 2014, Set 2014, Porto, Portugal

5 5 CONCLUSIONS The River Douro plume structure varies with the climatology imposed to the model. Two distinct scenarios were identified: a bulge and an alongshore current for autumn and winter conditions and a jetlike offshore protrusion of the river plume for spring and summer conditions. It was also found that the wind plays a dominant role on the control of the alongshore distribution of the plume over the continental shelf, but that it also depends on the behaviour of the offshore geostrophic current system. The formation of offshore eddies, filaments and other structures was identified in the performed simulation. These structures transport the plume waters cross-shore by means of horizontal advection. ACKNOWLEDGMENTS RAIA (0313-RAIA-1-E) and RAIA.co (0520-RAIA-CO-1-E) projects provided postdoctoral funds for Isabel Iglesias. The RAIA Coastal Observatory has been funded by the Programa Operativo de Cooperación Transfronteriza España-Portugal (POCTEP ). Numerical models were run at CIIMAR s HPC unit, acquired and maintained by FCT pluriannual funds (PesTC/Mar/LA0015/2013). Rui Caldeira was supported by funds from the ECORISK project (NORTE FEDER ). REFERENCES [1] TA McClimans (1986). Estuarine fronts and river plumes. In J Dronkers, W van Leussen, Eds., Physical Processes in Estuaries, Springer-Verlag, New York, [2] S Chao, WC Boicourt (1986). Onset of estuarine plumes. Journal of Physical Oceanography 16: [3] A Münchow, RW Garvine (1993). Dynamical properties of a buoyancy driven coastal current. Journal of Geophysical Research: Oceans 98: [4] M Benavent, J Arnoso, FG Montesinos (2009). Regional ocean tide loading modelling around the Iberian Peninsula. Journal of Geodynamics 48: [5] AF Shchepetkin, JC McWilliams (2003). A method for computing horizontal pressure-gradient force in an oceanic model with a nonaligned vertical coordinate. Journal of Geophysical Research: Oceans 108:3090. [6] AF Shchepetkin, JC McWilliams (2005). The regional oceanic modelling system (ROMS): A split-explicit, freesurface, topography-following-coordinate oceanic model., Ocean Modelling 9: [7] P Penven, P Marchesiello, L Debreu, J Lefèvre (2007). Software tools for pre- and post-processing of oceanic regional simulations. Environmental Modelling & Software 23: [8] GEBCO (2010). The GEBCO-08 Grid, version [WWW]. Available from: [9] JL Antonov, RA Locarnini, TP Boyer, AV Mishonov, HE Garcia (2006). World Ocean Atlas Volume 2: Salinity, Volume 62 of NOAA Atlas NESDIS, U.S. Department of Commerce, NOAA, NESDIS, Washington, D.C. [10] RA Locarnini, AV Mishonov, JI Antonov, TP Boyer, HE Garcia (2006). World Ocean Atlas Volume 1: Temperature, Volume 61 of NOAA Atlas NESDIS, U.S. Department of Commerce, NOAA, NESDIS, Washington, D.C. [11] GD Egbert, SY Erofeeva (2002). Efficient inverse modeling of barotropic ocean tides. Journal of Atmospheric and Ocean Technology 19: [12] AM da Silva, CC Young-Molling, S Levitus (1994). Atlas of Surface Marine Data Volume 1: Algorithms and Procedures, Volume 6 of NOAA Atlas NESDIS, U.S. Department of Commerce, NOAA, NESDIS, Washington, D.C. [13] IC Azevedo, PM Duarte, AA Bordalo (2006). Pelagic metabolism of the Douro estuary (Portugal) Factors controlling primary production. Estuarine, Coastal and Shelf Science 69: [14] J Pinto (2007). Influência do Regime de Escoamento Fluvial na Hidrologia e Dinâmica do Estuário do Douro, Relatório Final de Estágio, Universidade de Évora, Portugal. [15] B Choi, JL Wilkin (2007). The effect of wind on the dispersal of the Hudson River plume. Journal of Physical Oceanography 37: MEFTE 2014, Set 2014, Porto, Portugal 79

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