We examine the role of different forcings on the subtidal circulation in a microtidal bay with freshwater inputs in the NW Mediterranean Sea: Alfacs Bay. Observations of subtidal flow in summer 2013 and winter 2014 reveal a two-layered, vertically sheared circulation. During the summer, there is a significant positive correlation between surface currents and winds along the main axis of the bay, while a negative correlation is observed between wind and the bottom layers. During the winter, the cross-shore response is correlated with the most energetic winds, showing a two-layered vertical structure inside the bay and a nearly depth-independent water motion caused by high wind speeds at the bay mouth. The vertical structure of the velocities, as determined through empirical orthogonal function analysis, confirms that surface layers are affected by winds and bottom currents correlated negatively with winds as a response of the wind set-up. Seasonal mean circulation reveals gravitational exchange at the bay mouth during the summer. However, mean circulation is unclear in the inner bay and close to the drainage channels. Observed flow patterns are supported by modelling results that confirm the persistence of averaged current in the low-frequency dynamics. Re-circulation areas in the inner bay indicate the rich spatial variability in flow at low-frequency time scales.
En esta contribución se examina el papel de distintos forzamientos en la circulación submareal de una bahía micromareal con aportes de agua dulce en el Mediterráneo noroccidental: la bahía de los
The dynamics and physical processes in estuaries can be investigated at different time scales. A common approach is to use the tidal period as a cut-off frequency and to consider the variability at periods longer than the tides. In most regions, the main tidal currents are semidiurnal (
The different time scales of winds and density gradients indicate the need to study subtidal flows at those scales. The importance of low frequency on the water environment is therefore great: residence times, as key parameters for evaluating the flow exchange between the open sea and the bay, and the consequent ecological status of the bay, are conditioned by the low-frequency water circulation, among other factors. For instance, the presence of gravitational circulation implies exchange between the estuary and the open sea, thus diminishing the residence time and affecting ecological and biological processes (e.g.
The main sources of flow variability in microtidal bays (NW Mediterranean Sea,
Alfacs Bay is defined as a bar-built estuary (
The synoptic winds on the Catalan coast are affected by orographic constraints, such as the blocking winds of the Pyrenees that promote tramontane (N) and mistral (NW) winds over the Ebro Delta area, and the wind channelling due to river valleys (
The bulk of the observational data came from two field campaigns: from July to mid-September 2013 and from February to April 2014. The data set consisted of water currents from two 2-MHz acoustic Doppler current profilers (ADCPs) moored at the mouth (A1) and the inner bay (A2). Instruments recorded ten-minute averaged data from ten pings per minute and with 25-cm vertical cells. Both devices were equipped with pressure and temperature sensors, and were mounted on the seabed at 6.5 m depth. Water level data were obtained from the Catalan Meteo-Oceanographic Observational Network (described in detail in
In order to analyse the dynamics at low frequencies, both currents and wind observations were filtered using a 30-h low-pass filter (Lanczos filter) (
The three-dimensional hydrodynamic model used in this study is the Regional Ocean Modelling System (ROMS). Numeric aspects are described in detail in
Spectral analysis (
During the summer, unfiltered data from M-A show a mean wind speed of 3.1 m s–1, with a standard deviation of 1.7 m s–1 and maximum hourly winds of 13.4 m s–1. The sea breeze pattern (diurnal cycle) is clearly observable during the entire summer (
The filtered depth-averaged current speeds at A1 and A2 (see locations in
Current rose plots for the filtered currents in the surface and bottom layers (1-m averaged) are shown in
Correlations between the surface and bottom layers for both A1 and A2 in both periods are shown in
Summer | Winter | |||
---|---|---|---|---|
A1 | A2 | A1 | A2 | |
R (eastward) | –0.40 | –0.39 | –0.49 | –0.32 |
R (northward) | –0.72 | –0.21 | –0.09 | –0.24 |
Low-frequency current observations show that the maximum variability of the water currents at A1 (
The variability explained by the new axis for each vertical layer is shown in
The linear correlations between the rotated currents and winds are plotted in
Considering the limitations of linear correlation coefficients, alongshore and cross-shore currents (both surface and bottom layers) are graphically compared with the corresponding wind components (rotated to the same axis as the currents) for the summer and winter periods in
In winter, two northerly wind events were observed on 3 to 5 March and 23-28 March (in orange in
Along-axis EOF analysis shows differences along both moorings and periods: the first EOF explains 42% in both periods, and reveals a clear two-layered structure (i.e.
Since gravitational circulation typically occurs over long time scales (weekly and higher), it is instructive to examine the deployment-long mean currents at the moorings (
Low-pass filtered velocities revealed a clear two-layered structure, which for 60% to 80% of the time featured opposite directions in the surface and bottom layers. Previous investigations (
At A2, the main axis of the bay coincided with the axis of greatest variability in currents in both seasons and was almost equal at all depths (the percentage of variability explained by the first—alongshore—axis was more than 80% for the entire water column). However, A1 showed greater variability in the cross-shore component in winter (mostly in the surface layers), which is related to wind influence. In fact, A1 is 1.5 km from the coast, while A2 is at 600 m. As observed by
In contrast to a previous EOF analysis with unfiltered flow (
Similar relevant correlations are observed when filtered winds and surface currents in the along-axis are compared at both locations in summer. However, in winter, the effects of winds in along-axis surface currents are relatively weak, and the only noteworthy hydrodynamic response is observed in the cross-shore component of A1 during the most intense northwesterly wind. The low-frequency response shows the strongest correlations between winds and bottom currents. Different factors could be responsible for this hydrodynamic response. For example, stratification of the bay modifies the response to wind forcing in the water column. Surface layers are directly affected by wind, while bottom layers respond to the pressure gradient established along the bay due to the wind set-up, as observed in a shallow stratified system by
Another possibility is related to the effects of remote forcing. Winds can induce low-frequency variability in estuaries through a combination of remote and local effects. Considering the remote effect, winds on the continental shelf adjacent to an estuary can produce sea level fluctuations at the estuary mouth (
At the monthly time scale, the average circulation shows considerable differences from A1 to A2 (
Depth-averaged density fields observed on 7 May 2014 (I-5 campaign) are shown in
Another factor to consider is the wind stress influence on the gravitational circulation. Several reversals of surface flows have been observed in previous studies (
with L being the basin length (around 16 km), g the gravity acceleration (9.81 m s–2), Δρ the density difference between two points, h the depth, and τs the surface wind stress. The wind stress τs is computed following
A2 is located in the middle of the bay and close to drainage channels. Thus, if the bay is divided into two areas from a cross-sectional axis at A2, freshwater inputs are distributed on both sides, diminishing (even cancelling) the possible gravitational circulation along the main axis of A2. This means that the average circulation in the along-axis direction at this point is likely to be more influenced by other factors, but mainly wind forcing.
The spatial variability of the mean circulation in the bay is illustrated with the results from a numerical model (
Moreover, numerical results show how the near-surface average circulation exhibits two different behaviours inside the bay. The inner area (from A2 to the head of the bay) describes a near-surface anticyclonic circulation that follows isobaths. This behaviour was more evident in winter and related to spatial variability of northwesterly winds and bathymetry (
In summer, flows perpendicular to the cross sections (
The summer results revealed that during that season the average behaviour of the bay seems to predominate over the variability at this time scale (low frequency, with a cut-off time of 30 h). This dominance is clearer at the bay mouth, and probably directly correlated with the gravitational circulation. In winter, the average circulation is not as evident as in summer, and the variability dominates over the mean behaviour in the shallower areas.
Analysis of the flow structures in the bay and their variability, through EOF, revealed a clear two-layer response at low-frequency scales (>30 h). Observations indicated a positive relationship between surface currents and winds along the main axis of the bay in summer and winter, while negative correlation is observed in the bottom layers. On the other hand, in winter the cross-shore flow responds to winds at the bay mouth, showing one-layer flow. In winter, significant correlation coefficients between alongshore winds and bottom alongshore currents (negative values) indicate that the near-bottom currents in the bay respond to a wind set-up.
At much longer time scales (monthly), averaged circulation reveals gravitational circulation at the bay mouth, only noticeable in summer. In the inner bay (A2), and close to the drainage channels, no clear averaged circulation is observed (low velocities) and variability induced by wind stress and proximity to the coast is greater. Observations show that density structure within the bay may be responsible for this behaviour. Observed patterns are supported by modelling results that make it possible to estimate the spatial variability of averaged currents in the low-frequency dynamics. Re-circulation areas in the inner bay suggest the need for further studies in order to understand the influence of spatial variability on spatial distribution of nutrients and sediments.
Finally, both wind-induced and gravitational circulation are evident at the low-frequency band. Although gravitational circulation is prevalent in summer due to the persistence of the density gradients, episodic onshore winds alter this pattern (i.e. there are reversals in the estuarine circulation due to sea breezes).
We want to thank Joan Puigdefàbregas, Jordi Cateura and Joaquim Sospedra from the Maritime Engineering Laboratory (LIM/UPC) for all their help with campaigns and data analysis, and the XIOM network for the information provided. The campaigns were carried out thanks to the MESTRAL project (CTM2011-30489-C02-01). The authors also acknowledge the funding and support received from the Direcció General de Pesca i Afers Marítims in the framework of the project "Anàlisi ambiental de les Badies del Delta de l’Ebre i el seu entorn. Cap al desenvolupament d’una eina per a la seva gestió integrada” and the ECOSISTEMA project (CTM2017-84275-R). We also want to thank the Secretariat for Universities and Research of the Ministry of Economy and Knowledge of the Generalitat of Catalonia (Ref 2014SGR1253), who supported our research group.