The Influence of Winds and Tides in the Formation of Circulation Layers in a Bay, a Case Study: Concepción Bay, Chile

Estuarine, Coastal and Shelf Science (1997) 45, 729–736

The Influence of Winds and Tides in the Formation of Circulation Layers in a Bay, a Case Study: Concepcio´n Bay, Chile M. B. Sobarzoa, D. Figueroab and D. R. Arcosc a

Centro EULA-CHILE, Universidad de Concepcio´n, Casilla 156-C, Concepcio´n, Chile Departamento de Fı´sica de la Atmo´sfera y del Oce´ano (DEFAO), Universidad de Concepcio´n, Casilla 4009, Concepcio´n, Chile c Departamento of Oceanografı´a, Universidad of Concepcio´n, Casilla 2407-10, Concepcio´n, Chile b

Received 7 May 1996 and accepted in revised form 6 January 1997 Horizontal water velocities were measured using three current meters moored at the levels of 5, 18, and 30 m depth in the oriental side of Concepcio´n Bay (36)40*S; 73)02*W), an almost rectangular, shallow bay off central Chile, connected with the open sea through two mouths. The large momentum transfer from the wind to the water causes variations of the internal volume of the bay generating circulation layers having almost anti-parallel directions. Northerly winds carry superficial water into the bay, causing a compensating flow leaving the bay near its bottom. Winds from the SW, on the other hand, cause an outgoing circulation layer in the surface and a circulation layer entering into the bay near the bottom. During wind calm periods, longer than 12 h, the two layers disappear, leaving a current system comprising only one circulation layer. The response time necessary for the generation or dissipation of these layers, which fluctuates between 1 and 7 h, depends on the initial conditions of the bay and on the intensity, direction and persistence of the wind. ? 1997 Academic Press Limited Keywords: circulation layers; Concepcio´n Bay; Chile

Introduction The more clearly identifiable causes that regulate the movements of the coastal ocean are winds and tides, although coastal waters may be also affected by the climate, the topography and the stratification of the column of water (Winant, 1980). The importance of compensating flows in the internal volume of these sectors is also recognized (Heath, 1973; Petrie & Drinkwater, 1978). In the particular case of Concepcio´n Bay (36)40*S; 73)02*W), Chile, the studies have shown the importance of winds and tides in its circulation (Ahumada & Chuecas, 1979; Ahumada et al., 1983; Arcos & Wilson, 1984; Mesı´as & Salinas, 1986; Figueroa, 1990). However, due to the poor temporal and spatial resolution used in these studies, and to the absence of a time series relating winds and currents at different depths, the relation between circulation and its causes has not been appropriately quantified. The aim of the present study is to quantify and to compare the effect of the variability of winds and tides on the generation of circulation layers in Concepcio´n Bay. Concepcio´n Bay is an almost rectangular (about 10#16 km), shallow (maximal depth about 48 m) 0272–7714/97/060729+08 $25.00/0/ec970233

bay, having a gentle north–south slope between its head and its mouth. On its northern sector, the bay is connected with the open sea through the Boca Chica (‘ Small Mouth ’, 1·8 km wide and 15 m mean depth) and the Boca Grande (‘ Big Mouth ’, 5 km wide and 44 m mean depth), both separated by an island (Figure 1). The eastern coast of the bay consists of a chain of hills of 100–250 m of height, whereas the western coast (‘ Penı´nsula of Tumbes ’) is formed by hills of 100–200 m. To the south, the bay meets an alluvial plain. This topography favours the influence on the bay of winds from the SSW and NNW directions. Studies of the monthly superficial wind in the region show two patterns of dominant winds: (a) from September until March winds from SW prevail, and (b) from May until July northerly winds prevail. During April, the transition occurs from SW winds to northerlies, the opposite transition applies during August (Saavedra, 1980). A good weather condition (GWC) period is characterized by high atmospheric pressure (1015–1032 mb), a cloudless sky and a pronounced daily cycle of the superficial air temperature. During GWC winds flow from the SE, the S, or the SW directions, with daily fluctuations in their zonal component. A bad weather condition (BWC) ? 1997 Academic Press Limited

730 M. B. Sobarzo et al. 73° 10'

73° 00' W

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30 50 30 10 40 20

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F 1. Concepcio´n Bay (Chile). The locations of the mooring of current meters (C), the Carriel Sur meteorological station (S), the tidal gauge in Talcahuano (T), and the Meteorological Station Bellavista (B) are indicated.

period is characterized by cloudy skies; the daily cycle of the superficial air temperature disappears or weakens, and the wind flows mainly from the north without daily zonal fluctuations. During variable weather condition (VWC) periods these variables show strong changes. For example, on 10 September the pressure increased from 1015 to 1020 mb, the sky became cloudless, and the superficial air temperature dropped in the evening. On the other hand, 15 and 16 September were days of calm [Figure 2(b)]. Table 1 records the atmospheric variability during the period of study [see also Figure 2(b)]. Horizontal water velocities were measured using three current meters moored at the levels of 5, 18 and 30 m depth in the eastern side of Boca Grande (36 m depth) for a period of 17 days during September 1990. The Sensor-Data 2000 current meters were programmed for a 12-min sample frequency. The geographical positions of the mooring and sampling sites of tides and winds are shown in the Figure 1. The currents were analysed following Neshyba and Fonseca (1981). Currents and winds were identified with respect to the magnetic north, and tidal bands

were filtered using a symmetric Lanczos filter (Thompson, 1983). Results The occurrence of two frontal weather systems during the mooring period, separated by a 4-day high pressure period, produced strong variations in the speeds and directions of the wind and in the residual sea level. Figure 2(a) shows the north–south component of the wind stress and the residual sea level, this last variable calculated in the way utilized by Heathershaw (1982). The maximal wind speeds during the whole period were moderately strong (46·3 km h "1, by northerly winds, and 35·2 km h "1 by winds from the SW direction). On average, during the day northerly winds were slightly faster and more constant than southerly winds, which presented a daily variability in its intensity, which incremented after noon. Only during 7, 19 and 20 September was the alongshore wind stress greater than 1·5 dynes cm "2. During the analysed period, some days of calm were observed (15–17 September).

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Formation of circulation layers in Concepcio´n Bay, Chile 731

F 2. Hourly time series during the mooring period: (a) wind stress (——) and residual sea level (RSL) (· · ·); (b) atmospheric pressure (——), cloudiness (– · –) and air superficial temperature (· · ·); (c) alongshore component of the current at 5 m depth; (d) the same at 18 m depth; (e) the same at 30 m depth; (f) wind energy and shear between the alongshore currents at 5 and 18 m depth.

The progressive vector diagrams of Figure 3 show the similarity between the circulation at 18 and 30 m, and the opposite velocity direction in the superficial layer. It is also observed that the principal axis of the motion of the whole water column is approximately parallel to the eastern coast of the bay, being influenced by the bathymetry. That means that there is a

tendency to generate two horizontal circulation layers in the mouth of the bay, the direction of the superficial currents depending on the wind directions. Figure 2(f) shows a strong association between the shear of the v-component of the current velocity at the 5 and 18 m levels, and the mechanical energy of the wind. The larger mechanical energy of the wind

732 M. B. Sobarzo et al. T 1. Atmospheric conditions during the mooring period (September 1990) Atmospheric Condition Bad weather Variable weather Good weather Variable weather Bad weather

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F 3. Diagrams of progressive vector at the three levels of measured currents in Boca Grande of Concepcio´n Bay (September 1990).

associated with storms during northerly wind episodes increases the shear between these layers (7–12 and 18–20 September). Southerly winds, having less mechanical energy than those from the north, produce a shear of lesser intensity. During periods when the winds do not provide mechanical energy to the bay, the shear between these levels disappears, indicated by the absence of circulation layers in the mouth of the bay (15–17 September). In order to analyse relationships between the circulation in the Boca Grande and the weather, hourly profiles of the north–south current velocity are presented in Figures 4–6. It is observed that during GWC, and the associated SW winds, the water leaves the bay in the surface and enters into the bay at the levels of the current meters at 18 and 30 m. The 11 September shows a typical profile of this circulation pattern (Figure 4). During this day the upper circulation layer of water leaving the bay had a thickness varying between 8 and 13 m, according to graphic interpolation. When the wind from the SW ceases, at

22.00h, the thickness and the speed of the superficial circulation layer diminishes considerably, due partially to the flood phase of the tide [Figure 4(f)]. It is observed that when the bay has two circulation layers, speed variations of the southerly wind from 2–10 m s "1 only influence the speed of the superficial layer, with a time delay of 1–2 h. During BWC, northerly winds drag the superficial layer into the bay. In this case, subsuperficial and bottom compensation flows of waters leaving the bay are generated (see the case of 19 September, Figure 5). The response time of the bay to relaxation of weak northerly winds is smaller than 1 hour. On the other hand, constant northerly winds having speeds larger than 10 m s "1 can maintain a BWC profile. The superficial circulation layer responds to the effect of these intense winds with a time delay of 1–2 h. During VWC typical profiles like these of Figure 6 (16 September) are observed. When winds have low speeds and variable directions, the circulation is clearly dominated by the tides. In this case the layers of water entering and leaving weaken or even disappear, and the column of water acquires a coherent movement, whose speed and direction are modulated by tides. Under these conditions the circulation is unstable, slow, with small daily residual displacements, showing some tendency to horizontal rotations. For example, on 15 September, the circulation layers disappear about 7 h after the cessation of the SW wind. It can be concluded that the structure of two circulation layers described for the mouth of Concepcio´n Bay is an important mechanism of compensation of the internal volume of the bay. This mechanism consists in a compensatory reaction of the bottom layers in the mouth of the bay to the effect of the wind stress in the superficial layer. The compensation flows respond to the pile up of water in the head of the bay, caused by northerly winds, and to the drag of superficial waters away from the bay, caused by winds from the SW. The large momentum transfer from the wind to the water causes variations of the internal volume of the bay, increasing or diminishing the residual sea level [see Figure 2(a)], and generating circulation layers with almost antiparallel directions. Thus, northerly winds carry superficial water into the bay, causing a compensating leaving flow near the bottom. Winds from the SW, on the other hand, cause an outgoing circulation layer in the surface and a circulation layer entering near the bottom. In the case of long calm periods (longer than 12 h) the two layers disappear, resulting in a current system comprising only one circulation layer.

Formation of circulation layers in Concepcio´n Bay, Chile 733 Inflow –20 –15 –10 –5

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F 4. Temporal evolution of the alongshore component profile of the current in Boca Grande of Concepcio´n Bay during good weather condition (case of 11 September 1990). (a) , 0 h; , 1 h; , 2 h; , 3 h. (b) , 4 h; , 5 h; , 6 h; , 7 h. (c) , 8 h; , 9 h; , 10 h; , 11 h. (d) , 12 h; , 13 h; , 14 h; , 15 h. (e) , 16 h; , 17 h; , 18 h; , 19 h. (f) , 20 h; , 21 h; , 22 h; , 23 h.

Northerly winds generate layers that have larger water speeds, but at the same time are more unstable. During storm conditions it is possible to find up to three horizontal circulation layers, a situation that does not last for more than 3 h in the case analysed here [see Figure 5(e)]. Similar, almost antiparallel, wind-induced circulation layers have also been described for other bays (Heath, 1973). The time necessary for the generation or dissipation of these layers after variations in the atmospheric conditions depend on the initial state of the bay (ebb or flood condition, stratification of the water column, and so on) and on the intensity, direction and persistence of the wind. According to our results, these response times fluctuate between 1 and 7 h in Concepcio´n Bay. The wind, being aperiodic and of variable intensity and persistence, is not able to maintain a close

relationship with the circulation of waters, except during some hours after the change of the atmospheric conditions, when no rigorous internal volume constraint has to be satisfied. In this way, when the wind begins to blow, circulation layers are generated in the bay, and good correlation can be found between wind and water circulation. After a time interval, depending of the wind persistence, volume compensating currents are settled, which can be independent of the wind. Thereby the variance of the non-tidal currents, explained for by winds having periods greater than 24 h, fluctuates between 8 and 25%, being slightly larger in the case of the north–south component. The tidal cycle is more evident in the alongshore velocity component than in the east–west one. Unlike winds, that can generate sudden current changes but do not modulate them, tides do not generate drastic

734 M. B. Sobarzo et al. Inflow –20 –15 –10 –5

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F 5. Temporal evolution of the alongshore component profile of the current in Boca Grande of Concepcio´n Bay during bad weather condition (case of 19 September 1990). (a) , 0 h; , 1 h; , 2 h; , 3 h. (b) , 4 h; , 5 h; , 6 h; , 7 h. (c) , 8 h; , 9 h; , 10 h; , 11 h. (d) , 12 h; , 13 h; , 14 h; , 15 h. (e) , 16 h; , 17 h; , 18 h; , 19 h. (f) , 20 h; , 21 h; , 22 h; , 23 h.

changes in the currents. Once produced however, they can be more effective than the winds in the modulation of currents, possessing a tendency to restore the initial conditions of the bay. When the wind ceases, the layers tend to disappear and the tides continue modulating the circulation. In this way, the tidal currents explains 25–76% of the total current variances at different depths, better correlations are found in the north–south (alongshore) components. On the other hand, the forces that generate tides have a periodic and permanent character, that affects the whole water column. During calm periods (see 16 and 17 September) the layers of circulation disappear and the speeds and residual displacements are small and without constancy, due to the rotation that tides caused on the flow. In these conditions, tides alone are not able to generate circulation layers, and the alongshore current speeds in the windless bay fluctuate between 1 and 5 cm s "1. The situation analysed by

Mesı´as and Salinas (1986) can be classified in this kind of circulation pattern. The mean speeds during the sampling period fluctuate between 5·5 and 7·9 km day "1. Using these values, and assuming a steady wind, a conservative material starting at the mooring position would require between 6·1 and 7·4 h to leave the bay, and 2·0 and 2·7 days to arrive at the head of the bay. Thus, an approximated residence time for the waters of this bay fluctuates between 2 and 5 days, depending on the wind conditions. Heath (1973), studying St. Margaret’s Bay, a bay of similar dimensions (5#14 km) but deeper than Concepcio´n Bay, whose circulation also shows two layers, found residence times of 5–10 days in the upper layer and 10–30 days in the lower layer. With regards to the direction of the currents, the circulation layers in the mouth of the bay show a strong bathymetric component, that determines the

Formation of circulation layers in Concepcio´n Bay, Chile 735 Inflow –20 –15 –10 –5

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F 6. Temporal evolution of the alongshore component profile of the current in Boca Grande of Concepcio´n Bay during variable weather condition (case of 16 September 1990). HT and LT are high and low tide, respectively. (a) , 0 h LT; , 1 h LT; , 2 h, LT; , 3 h LT. (b) , 4 h HT; , 5 h HT; , 6 h HT; , 7 h HT. (c) , 8 h HT; , 9 h HT; , 10 h LT; , 11 h LT. (d) , 12 h LT; , 13 h LT; , 14 h LT; , 15 h LT. (e) , 16 h HT; , 17 h HT; , 18 h HT; , 19 h HT. (f) , 20 h HT; , 21 h HT; , 22 h HT; , 23 h LT.

direction of the principal axis of movement. Near the bottom the circulation at the mooring position follows an isobath of 30 m. Thus, when deep water enters in the bay, it flows mainly in the north–south direction, whereas it tends to have a SE–NW direction when it leaves the bay. The concept of seasonality has often been used for Concepcio´n Bay, in order to characterize variations found in the meteorological and oceanographic conditions, utilizing temporal scales of several months (Ahumada & Chuecas, 1979; Ahumada, 1989). However, although in summer winds prevail from the SW as opposed to the northerly winter winds, this study shows that the prevalence of wind of the opposite direction for 1 or 2 days suffices to change the circulation in the mouth of the bay in whichever of the two seasons. In this way, when the temporal variability of the vertical profile of the alongshore current in the

mouth of the bay lies in the range from some hours to days, it appears inconvenient to superimpose a dynamic seasonal characterization to the bay. In this case, a daily characterization, associated with the fluctuations of the local wind, is more suitable.

Acknowledgements This work was financed by the grant FONDECYT 90-245, of CONICYT, Chile, and the EULA-CHILE Center of the University of Concepcio´n. The authors appreciate the help given by the Servicio Hidrogra´fico y Oceanogra´fico (SHOA) of the Chilean Navy, and by the Aeropuerto Carriel Sur of the Direccio´n Meteorolo´gica de Chile (DMC), providing tidal and meteorological data used in this research.

736 M. B. Sobarzo et al.

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