Seiche excitation in a highly stratified fjord of southern Chile: the Reloncaví fjord

Abstract

We describe a seiche process based on current, temperature, and sea-level data obtained from the Reloncaví fjord (41.6°S, 72.5°W) in southern Chile. We combined 4 months of acoustic Doppler current profiler (ADCP) data with sea-level, temperature, and wind time series to analyze the dynamics of low-frequency (periods >1 day) internal oscillations in the fjord. Additionally, seasonal conductivity, temperature, and depth (CTD) data from 19 along-fjord stations were used to characterize the seasonality of the density field. The density profiles were used to estimate the internal long-wave phase speed (c) using two approximations: (1) a simple reduced gravity model (RGM) and (2) a continuously stratified model (CSM). No major seasonal changes in c were observed using either approximation (e.g., the CSM yielded 0.73<c<0.87ms^-1 for mode 1). The natural internal periods (T_N) were estimated using Merian's formula for a simple fjord-like basin and the above phase speeds. Estimated values of T_N varied between 2.9 and 3.5 days and were highly consistent with spectral peaks observed in the along-fjord currents and temperature time series. We conclude that these oscillations were forced by the wind stress, despite the moderate wind energy. Wind conditions at the end of winter gave us an excellent opportunity to explore the damping process. The observed damping time (T_d) was relatively long (T_d = 9.1 days).

Ocean Sci., 13, 145160, 2017 www.ocean-sci.net/13/145/2017/ doi:10.5194/os-13-145-2017 Author(s) 2017. CC Attribution 3.0 License.

Seiche excitation in a highly stratied fjord of southern Chile: the Reloncav fjord

Manuel I. Castillo1,2, Oscar Pizarro2,3,4, Nadin Ramrez2,4, and Mario Cceres1

1Escuela de Biologa Marina, Facultad de Ciencias del Mar y de Recursos Naturales, Universidad de Valparaso, Valparaso, Chile

2COPAS-Sur Austral, Universidad de Concepcin, Concepcin, Chile

3Departamento de Geofsica, Universidad de Concepcin, Concepcin, Chile

4Instituto Milenio de Oceanografa, Universidad de Concepcin, Concepcin, Chile

Correspondence to: Manuel I. Castillo ([email protected])

Received: 7 June 2016 Discussion started: 7 July 2016Revised: 25 January 2017 Accepted: 27 January 2017 Published: 17 February 2017

Abstract. We describe a seiche process based on current, temperature, and sea-level data obtained from the Reloncav fjord (41.6 S, 72.5 W) in southern Chile. We combined 4 months of acoustic Doppler current proler (ADCP) data with sea-level, temperature, and wind time series to analyze the dynamics of low-frequency (periods > 1 day) internal oscillations in the fjord. Additionally, seasonal conductivity, temperature, and depth (CTD) data from 19 along-fjord stations were used to characterize the seasonality of the density eld. The density proles were used to estimate the internal long-wave phase speed (c) using two approximations:(1) a simple reduced gravity model (RGM) and (2) a continuously stratied model (CSM). No major seasonal changes in c were observed using either approximation (e.g., the CSM yielded 0.73 < c < 0.87 m s1 for mode 1). The natural internal periods (TN) were estimated using Merians formula for a simple fjord-like basin and the above phase speeds. Estimated values of TN varied between 2.9 and 3.5 days and were highly consistent with spectral peaks observed in the along-fjord currents and temperature time series. We conclude that these oscillations were forced by the wind stress, despite the moderate wind energy. Wind conditions at the end of winter gave us an excellent opportunity to explore the damping process. The observed damping time (Td) was relatively long (Td = 9.1 days).

1 Introduction

Internal seiche oscillation has long been known in closed basin geometries (e.g., Watson, 1904; Wedderburn, 1907;Wedderburn and Young, 1915). The rst detailed description thereof was presented by Mortimer (1952). In these systems, wind is the main force affecting the surface and isotherms (Wiegand and Chamberlain, 1987), which produces a set of periodic oscillations and circulation cells throughout the water column that may contribute to internal mixing of the basin (Thorpe, 1974; Monismith, 1985; Wiegand and Chamberlain, 1987; Mnnich et al., 1992; Mans et al., 2011; Simpson et al., 2011).

Although external (barotropic) seiches are ubiquitous in closed basin geometries (Mnnich et al., 1992), it is not theoretically evident that there are internal seiches (baro-clinic) in a linearly stratied uid (Maas and Lam, 1995).It is possible to nd resonant basin modes, but only in well-behaved geometries (Arneborg and Liljebladh, 2001a). However, studies of lakes have yielded good results using layered models (e.g., Lemmin, 1987), normal-mode approximations (e.g., Wiegand and Chamberlain, 1987; Mnnich et al., 1992), or numerical model simulations (e.g., Goudsmit et al., 2002). In fact, internal seiches have been observed in semi-enclosed systems such as fjords (e.g., Djurfeldt, 1987; Pasmar and Stigebrandt, 1997; Arneborg and Liljebladh, 2001a) with complex geometries and where linear stratication is rarely observed, and thus the only way to maintain consistency with the theory is that the oscillation in the pycnocline

Published by Copernicus Publications on behalf of the European Geosciences Union.

146 M. I. Castillo et al.: Seiche excitation in a highly stratied fjord

Table 1. Characteristics of the Reloncav fjord. The name, mean depth (H), and length (L) of each sub-basin and for the entire fjord are presented.

Sub-basin Description H (m) L (km)

I mouthMarimeli 440 14.0 II MarimeliPuelo 250 13.0 III PueloCochamo 200 17.5 IV Cochamohead 82 10.5

Total mouthhead 250 55

dominates the internal seiche oscillation (Arneborg and Liljebladh, 2001a). Early in the development of a seiche, its amplitude is related to the forcing intensity, and the standing oscillation then becomes free and requires no additional forcing. The frequencies are retained, but the amplitude decays (damping) exponentially due to friction until the system comes to rest (Rabinovich, 2010). The development of seiche oscillations depends on the forcing and damping mechanisms; with large damping, it is impossible to observe a seiche, whereas small damping of a seiche allows for several oscillations (Arneborg and Liljebladh, 2001a).

In fjords with shallow sills, the interaction between the sill and the barotropic tide generates internal tides that are more energetic than other internal oscillations and are the focus of most studies regarding mixing and internal oscillations based on internal tides (e.g., Stigebrandt, 1980; Stigebrandt and Aure, 1989; Inall and Rippeth, 2002; Ross et al., 2014). In the case of fjords with a deep sill and low tidal energy, the breaking of the internal seiche oscillations at the boundaries could be an important contributor to the internal mixing, promoting the spreading of properties within the fjord, particularly in deep waters (Stigebrandt and Aure, 1989; Mnnich et al., 1992; Arneborg and Liljebladh, 2001b). Additionally, there is evidence that vertical isopycnal displacements in fjords could be generated by similar displacements outside the fjord (e.g., Svensen, 1980; Djurfeldt, 1987). These remotely generated oscillations could enhance the mixing and ventilation in deep fjords.

There is still only limited understanding of the main oceanographic processes occurring in the fjord region of southern Chile, although there has been local research during the previous few decades. Since early studies of the hydrography by Pickard (1971), a systematic measurement program in the fjord region has been maintained since 1995 (Palma and Silva, 2004; Pantoja et al., 2011; Iriarte et al., 2014), although only a small number of studies have focused on the physical dynamics. Most studies have been conducted over short time spans (e.g., Cceres et al., 2002; Valle-Levinson et al., 2007), and only a few studies have been based on more than 1 month of data (e.g., Letelier et al., 2011; Castillo et al., 2012; Schneider et al., 2014), thereby limiting our understanding of sub-inertial variability. In the Reloncav fjord,

time series of approximately 4 months have shown evidence that 3-day oscillations of currents could be produced by internal seiche oscillations (Castillo et al., 2012), but lack a description of the forcing mechanism and the seasonal modulation.

This study presents the rst evidence of internal seiche oscillations in a fjord in southern Chile. The objective of this study was to address how these oscillations affect the temporal and spatial dynamics of currents and temperature, and how these oscillations are forced.

2 Study area

The Reloncav fjord (41.5 S, 72.5 W) is the northernmost fjord on the coast of Chile (Fig. 1). This J-shaped fjord is 55 km long and has a width that varies from 3 km near the mouth to 1 km near the head. There is a deep sill ( 200 m

depth) located 15 km inland, although it does not appear to be a barrier to the exchange of properties between the adjacent basins. Based on bathymetric features and the coastline morphology, this fjord can be separated into four sub-basins displaying the characteristics presented in Table 1 and Fig. 2.

The main river discharge is provided by the Puelo River (at the middle of the fjord), which produces a mean annual discharge of 650 m3 s1.The Petrohue River (at the head of the fjord) has an mean annual discharge of 255 m3 s1, and there are additional freshwater inputs of minor importance compared with the Cochamo River (mean annual discharge of 20 m3 s1) and the Canutillar hydroelectrical plant (mean annual discharge 75.5 m3 s1; Niemeyer and Cereceda, 1984). The freshwater input to the fjord due to direct precipitation is only approximately 2 % of the main river discharge (Len-Muoz, 2013), and its contribution may be in balance with evaporation (Castillo et al., 2016). The freshwater input creates a marked along-fjord pycnocline that is deeper at the head ( 8 m) and shallower at the mouth ( 3 m; Fig. 2).

During the winter, the mean wind stress () is low due to calm winds (< 103 N m2). During storm events in winter,

can reach values as high as 0.4 N m2 (winds of > 10 m s1), and the wind tends to blow out of the fjord, thereby reinforcing the upper outow of brackish water. In contrast, during the spring/summer, the winds exhibit a marked diurnal cycle, and can reach values as high as those observed in the winter, whereas the wind blows landward, i.e., toward the fjords head and against the upper ow. Tides in the Reloncav fjord are predominantly semi-diurnal, and during spring the tidal range never exceeds 6 m, whereas the neap tidal range is about 2 m. The tidal current is relatively weak in the upper layer, which is dominated by gravitational circulation (Valle-Levinson et al., 2007; Montero et al., 2011; Castillo et al., 2012).

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M. I. Castillo et al.: Seiche excitation in a highly stratied fjord 147

Figure 1. Study region and location of the measuring stations. The left panel shows the area of the Reloncav fjord (a). The location of the Reloncav sound (b) is also shown. The right panel shows the study area (close-up view of a) and the positions of all measurements. Numbers are CTD stations.

3 Data and methods

3.1 Field observations

Current measurements were obtained using Teledyne RD Instruments acoustic Doppler current proler (ADCPs) in three subsurface mooring systems. These subsurface systems were located near the fjord mouth, near the Puelo River and between the Cochamo and Petrohue rivers (Fig. 1). The longest time series spanned the period of August through November 2008 (Fig. 1 and Table 1). At the mouth, two upward-looking ADCPs were positioned at nominal depths of 10 (300 kHz) and 450 m (75 kHz). The Puelo mooring held two ADCPs, one facing up at a depth of 30 m (600 kHz) and one facing downward at a depth of 35 m (300 kHz). The Cochamo mooring held one facing-up ADCP at a depth of 11 m (300 kHz). Note that due to the large tidal range, the depths of the ADCPs signicantly changed with the tides. These effects along with small vertical deviations of the ADCPs related to the line movements were corrected using the ADCPs pressure sensors, and all of the bin depths were referenced to the water surface level. The mooring systems were designed to obtain the best vertical resolution available with emphasis on the upper layer. The ADCP cell sizes were 0.5 (600 kHz), 1 (300 kHz), and 4 m (75 kHz), and the data-acquisition time intervals were 10 min in most of the ADCPs, with the exception of the deepest ADCP, which was set to acquire data at an interval of 20 min. All the ADCP congurations maintain

a standard deviation of < 2 cm s1 (details in Supplement Ta

ble S2).

The morphology of the fjord exhibits a sharp bend in the middle, and thus the x and y components of the currents were rotated to the local orientation of the along-fjord axis (Fig. 1 and Table 1). A right-handed coordinate system with a positive-up z axis and an along-fjord y axis (positive toward the fjord head) was used. Consequently, the cross-fjord x component was positive toward the south (east) near the fjord mouth (head). To assess the contribution of the tides to the currents, the amplitudes and phases of several tidal components were calculated at all of the moored ADCPs using a standard harmonic analysis from Pawlowicz et al. (2002).

The vertical structure of the temperature was obtained from Onset HOBO-U22 temperature sensors installed in three mooring systems along the fjord (Fig. 1). These moorings held surface buoys supporting the thermistor chains with an anchor located at a 25 m depth to maintain their nominal depths (0, 1, 2, 3, 4, 5, 7, 9, 11, 13, 15, and 20 m) from the surface independent of tidal uctuations. Temperature data were collected every 10 min at all locations.

A Davis Vantage Pro2 meteorological station was installed south of the Puelo River (see Fig. 1). This station held sensors for measuring the wind direction and velocity, solar radiation, rain, and air temperature. The wind magnitude and direction sensors were installed 10 m above sea level and were set to collect data every 10 min from 12 June 2008 to 30 March 2011. Gaps in the time series represented only 0.04 % of the total data. The wind stress () was calculated using a

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148 M. I. Castillo et al.: Seiche excitation in a highly stratied fjord

Table 2. Seasonal statistics of the descriptive parameters of the fjord. Here we present the mean depth of the upper layer (h1) and densities of the upper (1) and deep layers (2). In addition, the phase and modal velocities (c) and their periods (T ) estimated using the reduced gravity and continuously stratied models are shown.

Reduced gravity model (RGM)

h1 1 2 c T(m) (kg m3) (kg m3) (m s1) (days)

Winter 4.60 [notdef] 0.60 1009.72 [notdef] 4.32 1024.62 [notdef] 0.74 0.76 [notdef] 0.01 3.37 [notdef] 0.03

Spring 4.79 [notdef] 0.53 1007.63 [notdef] 5.32 1024.78 [notdef] 0.62 0.87 [notdef] 0.02 2.92 [notdef] 0.03

Summer 4.68 [notdef] 0.26 1008.77 [notdef] 3.26 1024.78 [notdef] 0.63 0.83 [notdef] 0.01 3.07 [notdef] 0.02

Autumn 4.05 [notdef] 0.41 1009.90 [notdef] 3.92 1024.95 [notdef] 0.48 0.75 [notdef] 0.01 3.38 [notdef] 0.03Continuous stratied model (CSM)

c1 c2 c3 T1 T2 T3 (m s1) (m s1) (m s1) (days) (days) (days)

Winter 0.73 [notdef] 0.11 1.46 [notdef] 0.21 2.18 [notdef] 0.32 3.50 [notdef] 0.25 1.75 [notdef] 0.13 1.17 [notdef] 0.08

Spring 0.87 [notdef] 0.10 1.73 [notdef] 0.21 2.59 [notdef] 0.31 2.94 [notdef] 0.18 1.47 [notdef] 0.09 0.98 [notdef] 0.06

Summer 0.84 [notdef] 0.07 1.68 [notdef] 0.13 2.52 [notdef] 0.20 3.03 [notdef] 0.12 1.51 [notdef] 0.06 1.01 [notdef] 0.04

Autumn 0.77 [notdef] 0.08 1.54 [notdef] 0.15 2.32 [notdef] 0.23 3.30 [notdef] 0.16 1.65 [notdef] 0.08 1.10 [notdef] 0.05

pump was set to a time interval of 1 min. After the start of the pumping, the instrument was maintained near the surface until the sensors stabilized. Then, the CTD was lowered to the maximum depth of the station (Table 2). The along-fjord transects typically required 12 to 24 h to complete, depending on local weather conditions. Due to technical limitations, the winter transect was performed to a maximum depth of 50 m.

The sea level was recorded every 10 min using two pressure sensors moored over the seabed. At Cochamo, the pressure sensor was an Onset HOBO-U20, whereas a SeaBird wave-tide gauge (SBE-26) was installed near the fjords mouth (Fig. 1). Subsurface pressure data were corrected for air pressure and converted to an adjusted sea level.

Discharge data were provided by Direccin General de Aguas, Chile (2016). These data are regularly collected at a station located 12 km upstream of the Puelo Rivers mouth (Fig. 1). The time series extended from January 2003 to December 2011, and data gaps represented only 2 % of the total.

3.2 Time series analysis

Previous ndings (Castillo et al., 2012) have shown an important oscillation with a period of approximately 3 days (72 h). To focus the study on these perturbations, the time series of currents and temperature were band-pass ltered using a cosine-Lanczos with half amplitudes at 60 and 100 h (see the results for the justication of the selected band). As part of the results, the band-passed time series of the current (Fig. 6) and temperature (Fig. 9) data are shown.

Spectral analyses of the current, wind stress, sea-level, and temperature time series were performed using Welchs modied average periodograms (Emery and Thomson, 1998). To

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Figure 2. Seasonal proles of density and bathymetry of the region. The upper panel shows the seasonal mean density proles in each sub-basin of the fjord (ad). In the panel below (e), the along-fjord bathymetry and sub-basin nomenclature are shown. The black line represents the mean pycnocline depth, and corresponding standard deviations are represented by the gray shading.

drag coefcient dependent on the magnitude (see Large and Pond, 1981) and a constant air density of 1.2 kg m3.

The salinity and temperature proles were obtained seasonally using a CTD SeaBird SBE 25 at 19 stations in the along-fjord transect shown in Fig. 1. The data were processed following the standard protocol suggested by the manufacturer and were averaged in vertical intervals of 0.5 m. Due to large salinity changes in the upper layer, the instrument

M. I. Castillo et al.: Seiche excitation in a highly stratied fjord 149

achieve statistical reliability of the spectral estimations, each time series was divided into non-overlapping segments to generate spectral estimates. In the case of the current time series, the spectra were (additionally) averaged among depth layers to obtain 12, 24, and 48 degrees of freedom, depending on the frequency (see Fig. 3). In addition, to evaluate the consistency of the periodicity between the time series, we calculate a Morlet cross-wavelet analysis following wavelet methods explained by Torrence and Compo (1998) and Grin-sted et al. (2004).

The phase velocity (c) was estimated using two models that took into account the fjord stratication: (1) a simple reduced-gravity model (RGM) and (2) a continuously stratied model (CSM).

The reduced-gravity model was developed using the typical density proles in each sub-basin. Here, the base of the upper layer was estimated from the pycnocline depth (Fig. 2), which in the Reloncav fjord is well represented by the depth of the 24 isohaline (h1; Castillo et al., 2016), considering that h1 is the pycnocline depth and H is the deepest CTD cast (mostly near to the sub-basins maximum depths). The mean density of the upper layer (1) was estimated from depths between the surface and h1, whereas the mean density for the deep layer (2) was estimated for depths between h1 and H.These estimations were made for all sub-basins and seasons (Table 2).

Using both densities, 1 and 2, the reduced gravity (g[prime] =

g(2 1)/2) was obtained; here g is the acceleration of

gravity. The internal phase velocity of each sub-basin, ci = g[prime] h1i

1/2, where i = 1 to 4 and h1i represents the mean depth of the upper layer in the sub-basin i, was used to estimate the effective phase speed in the entire fjord (Eq. 1),

c = L

n

The modal decomposition was used to obtain the contribution of each mode in the current variability (e.g., Emery and Thomson, 1998; Gill, 1982; van der Lee and Umlauf, 2011). The along- and cross-fjord band-pass currents [ubp, vbp] could be described by the vertical modes by Eq. (3),

[ubp,vbp[notdef](z,t) =

1 [upj,vpj[notdef](t) n(z). (3)

The along- and cross-fjord currents projected (upj, vbp) on the vertical modal structure ( n) were obtained by Eq. (4):

[upj,vpj[notdef](t) =

1 H

[integraldisplay]

H

[ubp,vbp[notdef](z,t) n(z)dz. (4)

0

Xi=1ci

Li , (1)

where Li is the i sub-basin length and L is the fjord length. This takes into account the changes in depths and lengths of the fjords sub-basins. Similarly, the effective period (T ) was obtained by T = cL1.

The continuously stratied model (CSM) was developed using the normal mode analysis, which introduced the stratication as N2 = (g/)(@/@z), which is the buoyancy fre

quency, in the SturmLiuoville expression

d dz

[parenleftbigg]

1 N2

4 Results

4.1 Density structure

As a result of abundant freshwater input to the fjord, there were marked differences in density between the upper and lower layers along the fjord and small changes in stratication among seasons, particularly near the mouth of the fjord (Fig. 2). One important characteristic of the upper layer is its high and persistent stratication from the surface to the base of the pycnocline (Fig. 2). Along the fjord, the pycnocline depth exhibited clear deepening from 2.3 [notdef] 0.1 at the mouth

to 6.1 [notdef] 0.3 m near the head. The pycnocline depth exhibited

greater seasonal variability near the head of the fjord (Fig. 2).

4.2 Winds, sea-level, and freshwater discharge

The along-fjord wind stress () displayed two patterns during the transition from winter to spring. During the winter, was generally directed out of the fjord (0.4 [notdef] 3 [notdef] 102 N m2)

and displayed oscillations with a period longer than 1 day.

There were also strong events (> 0.2 N m2) during the rst half of August 2008 that could be associated with the end of winter storms in the region. This winter pattern drastically changed during the early spring (rst week of September 2008) and was maintained throughout the rest of the season.Changes were evident in a marked daily cycle and in switches from down- to up-fjord (average of 1.6 [notdef] 3 [notdef] 102 N m2),

against the upper layer outow (Fig. 3a).

The sea level was measured at the mouth and near Cochamo (Fig. 1). At both stations, the form factor was 0.12, which indicates that semi-diurnal tides dominate in the region. In fact, the M2 amplitude was 1.89 [notdef] 0.06 at the mouth

and 1.91 [notdef] 0.06 m near Cochamo. The mouth-to-head phase

difference in this harmonic was negative (2.4 ), indicat

ing propagation toward the head with a lag of approximately 5 min. The maximum tidal range during spring tides was approximately 6 m and less than 1 m during neap tides (Fig. 3b).

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+1c2n

n = 0 (2)

where n(z) is the vertical structure of the horizontal velocity for the mode n. Here cn represents the n mode speed (see Gill, 1982) and differs signicantly from phase speed if rotation plays a role (van der Lee and Umlauf, 2011).

Independent of the model used to obtain the phase speed (RGM or CSM), the natural oscillation period (TN) was determined using Merians formula for a semi-enclosed basin, as suggested by Rabinovich (2010), TN = 4T .

d n dz

150 M. I. Castillo et al.: Seiche excitation in a highly stratied fjord

Figure 3. (a) Along-fjord wind stress, positive up to the fjord, (b) sea-level, and (c) Puelo River discharge, where the straight line represents the long-term mean. Contours of along-fjord currents at (d) Cochamo, (e) Puelo, and (f) the mouth; in the lled contours, the blue (red) colors indicate a net outow (inow).

Similar ranges have been observed outside the fjord in the Reloncav sound (Aiken, 2008).

Discharge was greatest (approximately 1413 m3 s1) at the end of August 2008 (winter) and lower (approximately 459 m3 s1) at the end of October (spring). In the winter, the historical mean of 650 m3 s1 (Niemeyer and Cereceda, 1984; Leon et al., 2013) was exceeded 86 % of the time, whereas during the spring, this exceedance occurred only 18 % of the time. In fact, only a small variability around the mean was observed during the spring (Fig. 3c).

4.3 Along-fjord currents

The along-fjord currents were 1 order of magnitude larger than the cross-fjord currents (in this study we focused on the along-fjord component). At the three measurement sites at Cochamo (Fig. 3d), Puelo (Fig. 3e), and the mouth (Fig. 3f), the along-fjord currents displayed certain common features:(1) semi-diurnal oscillations attributed to tidal effect, (2) a two-layered structure with persistent outow above the pycnocline and an intermittent lower inow layer beneath,

and (3) several low-frequency (period > 1 day) oscillations present in the time series.

Currents in the upper outow layer displayed a mean velocity of 66 cm s1 at the mouth and 45 cm s1 at Cochamo, indicating that the outow increased through the mouth. Additionally, the upper layer was deeper at Cochamo (Fig. 3d) than at the mouth (Fig. 3f), which is consistent with the along-fjord pycnocline depth (Fig. 2). Below the upper layer, a sub-surface layer displayed intermittent inow (see Fig. 3d, e, and f) with a maximum (> 20 cm s1) centered at the 6 m

depth.

This two-layered pattern was clearly observed in the upper 1015 m and is consistent with a gravitational circulation due to the along-fjord pressure gradient. This pressure gradient is also consistent with the observed along-fjord pycnocline tilt (Fig. 2). At depths > 20 m, the along-fjord currents at Puelo and at the mouth exhibited an important inuence (> 40 % of the variability) of a semi-diurnal component of the tide. In addition, in this layer, low-frequency (periods > 7 days) oscillations suggest a bottom-to-surface propagation that was more intense from the end of August to the beginning of September during a period of high discharge (> 650 m3 s1).

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3-day period. The band was wider (between 2 and 7 days) at the mouth and Puelo and narrower (between 1.5 and 4 days) at Cochamo. At the mouth, the maximum spectral density was in the 3-day band (> 105 cm2 s2 cph1) and was 1 order of magnitude greater than the maximum spectral density ob-served at Cochamo ( 104 cm2 s2 cph1). Another impor

tant accumulation of energy in the along-fjord currents was centered on the 15-day period. One characteristic of the 15-day band is the inuence on the entire water column at Puelo and the mouth (Fig. 4).

The sea levels at Cochamo ( C) and at the mouth ( m) were similar at frequencies of less than 0.165 cph (periods longer than 6 h). The spectra displayed an important accumulation in the synoptic band (10 days). Both locations exhibited the same energy at the diurnal (K1) semidiurnal (M2)

frequencies, although M2 was clearly the dominant harmonic in the fjord. The spectral energy was 1 order of magnitude higher than the diurnal (K1) harmonics and 3 orders of magnitude higher than the quarter-diurnal (M4) harmonics. The spectra exhibited no accumulation of energy in the 3-day band, although at high frequencies (> 0.5 cph), an important accumulation of energy was observed in the 1.3 h band (between 1.16 and 1.56 h) at C (Fig. 4).

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Figure 4. Spectra of along-fjord currents (top) at (a) the mouth, (b) Puelo, and (c) Cochamo. Here the black lines indicate the averaged spectra for the upper layer (depths h1), whereas the gray lines show spectra for currents at depths > h1. (d) Sea-level spectra at the mouth

(black line) and at Cochamo (gray). (e) Wind stress spectra for their along-fjord (black) and cross-fjord (gray) components. At the bottom of each panel the 95 % condence intervals for 48, 24, and 12 degrees of freedom are shown.

This layer on average exhibited a weak outow ( 1 cm s1)

at the mouth, which in turn implies a three-layered pattern of the residual ow near the mouth.

4.4 Spectral characteristics of currents, temperature, sea level, and winds

To obtain better statistical reliability, the spectra of the along-fjord currents were depth-averaged. The upper layer was dened until the pycnocline depth (z h1), whereas the deep

layer contains z>h1 (Fig. 4).

All of the spectra displayed an energetic peak at the semi-diurnal frequency (M2), and this peak was greater in the deep layer (Fig. 4). In the diurnal band, the spectra at Puelo and at the mouth presented a clear (and highly energetic) peak in the surface layers. This diurnal peak is likely due to the inuence of wind stress (see Fig. S1 in the Supplement), which displayed a marked diurnal cycle during the late winter (end of August) and spring (Fig. 3a). An important peak (104 cm2 s2 cph1) was observed only at Cochamo in the 6 h band (M4), suggesting an increase in the importance of non-linear interaction between M2 and the bathymetry in this sub-basin. The spectra in the upper layer displayed an important accumulation of energy in the band centered on the

152 M. I. Castillo et al.: Seiche excitation in a highly stratied fjord

The wind stress () indicated that the along-fjord wind stress was signicantly higher than the cross-fjord component. The spectra displayed a marked peak (particularly in the along-fjord component) in the diurnal band, which is likely due to the sea-breeze phenomenon. Another interesting feature of the spectrum was the peak in the semi-diurnal frequency, which was observed in both components. At longer periods (> 1 day), the along-fjord wind stress displayed an important but not statistically signicant peak at 2.8 days, which is highly consistent with the currents (Fig. 4).

4.5 Seasonality of the internal oscillations

The density structure on the fjord does not show an upper mixing layer along the seasons; indeed, a continuously stratied upper layer is present along the seasons (Fig. 5). The along-fjord mean of the pycnocline depth (h1), which was estimated based on a salinity / density gradient, was used to estimate the internal phase velocity (c) and the internal period (TN). Seasonally, h1 does not change signicantly during winter, spring, and summer (between 4.6 and 4.8 m), but was shallower during autumn ( 4.1 m; Table 2).

In the case of the RGM approximation, internal phase velocities (c) were highest during spring and summer (> 0.83 m s1), whereas in winter and autumn the intensities were < 0.76 m s1; thus, we obtain internal periods between2.9 and 3.4 days (70 and 82 h; Table 2).

The horizontal velocity structure ( n) prole of the rst

three internal modes obtained from the CSM showed high consistency along the fjord (in each sub-basin) and through the seasons (Fig. 5). Mode 1 was highly baroclinic, changing sign at nearly 10 (sub-basin I) and 15 m (sub-basin IV).In the case of modes 2 and 3, relatively high variability along the seasons was observed, especially at sub-basins I and IV above 20 m in depth. For depths > 30 m (not shown in Fig. 5), the internal modes do not show signicant variability. The modal speeds for the rst three modes described above were relatively high during spring and summer (c1 was > 0.84 m s1) and lower during winter and autumn (here c1 was < 0.77 m s1). These results were highly consistent with the internal speeds obtained by the RGM (Table 2).

Like the internal speeds (c), the natural internal period (TN) obtained by the RGM with mode 1 of the CSM was highly consistent. For comparison, we take into account TN obtained from mode 1 of the CSM, which ranged between2.9 days (spring) and 3.5 days (winter). The estimations of TN with RGM showed speeds between 2.9 days (spring) and3.4 days (winter and autumn), indicating that oscillations between these periods are dominated by mode 1 internal seiche oscillation.

To focus on these internal seiche oscillations, we ltered the along-fjord currents with a 70 to 90 h cosine-Lanczos band-pass lter. Additionally, mode 1 of the internal seiche was associated with the pycnocline depth, which is restricted

to the upper 8 m (Fig. 2). Therefore, we describe the along-fjord currents in the upper 10 m (Fig. 6).

The vertical pattern at the three locations shows inow/outow intermittence along the whole time series; also most of these along-fjord structures seem to develop an inclination which indicates the baroclinic nature of this pattern. The band-pass along-fjord currents were intense at the mouth (> 15 cm s1), but diminish toward the head. Intense perturbation oscillations were observed near the surface between 10 and 20 August 2008 at the mouth and Cochamo; internal intensication (between 4 and 10 m in depth) of the inow/outow pattern was clear at Puelo and Cochamo at the end of September. To decide whether the nature of the along-fjord current pattern was baroclinic or barotropic, we used n(z) to project the band-pass currents (Eqs. 3 and 4), similarly to van der Lee and Umlauf (2011).

The agreement between the 3-day band pass and the projected along-fjord currents at the mouth is shown in Fig. 7.Using only the rst three modes, it was possible to explain more than 70 % of the band-pass variability, changes in the outow/inow were highly consistent, and the intensications at the surface were clearly shown by the projected modes. In addition, the vertical structures of the outow/inow were well dened by the projections. To make an approximation of the relative importance of the currents variability, we estimated kinetic energy (KE = (u2 + v2)/2)

of (i) the projected modes 13, (ii) the 3-day band pass, and (iii) the semi-diurnal (12 h) + diurnal band pass (1-day)

along-fjord currents at the mouth.

The vertically averaged KE obtained with a 3-day band pass was higher than that generated with the other components (modes 13); the maximum was observed in the period 918 August (Fig. 7), which is consistent with the wind-stress intensication shown in Fig. 3a. During that period, the modal KE was about one-third of the 3-day band-pass kinetic energy; this ratio was higher (i.e., ca. 50 %) during September. The importance of the tides at the mouth was estimated by summing up the KE of the diurnal and semi-diurnal currents. In terms of energy, the KE contribution of tides was similar to the modal currents (Fig. 7).

Along-currents were highly coherent at a 3-day band, which is the period of the rst mode of the internal seiche (Table 2). To describe the temporal variability of this high coherence, along the time, we selected 3 m depth ADCP bins (on the upper layer) from the mouth, Puelo, and Cochamo to make a Morlet cross-wavelet analysis and to estimate the squared coherence (only referred to as coherence hereafter) and phase spectra for the relations mouthPuelo (MP;Fig. 8b, c) and PueloCochamo (PC; Fig. 8d, e). Both relations showed high coherence in the semi-diurnal and diurnal bands, especially during spring tides.

A low coherence (< 0.6) was observed during the down-fjord winds (Fig. 8a and b). Similarly, the coherence for the PC relation was high along the 3-day band, except during the change of the wind direction described above (Fig. 8d). The

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M. I. Castillo et al.: Seiche excitation in a highly stratied fjord 153

Figure 5. The left panel shows mean density (t) within the sub-basins. The panels to the right of these show the rst three baroclinic n(z) modes and modal speeds obtained from the CSM analysis (normalized). Note that phase velocity is in m s1.

associated phase spectra (only the signicant coherence) at the 3-day band was 0 , indicating that the oscillation is in

phase along the fjord (Fig. 8c and e).

At the beginning of the time series, intense uctuations were observed at Cochamo and at the mouth (Fig. 6). To explore their relationship with the wind forcing, a detailed view of the period between 8 and 31 August 2008 is presented in Fig. 9. During this period, the along-fjord wind stress (not ltered) displayed three different states: (a) strong (> 0.2 N m2) up-fjord winds, (b) weak (< 0.1 N m2) or nearly calm winds, and (c) moderate ( 0.1 N m2) down-

fjord winds. During (c), the winds displayed an apparent diurnal cycle (e.g., Fig. 3a).

Although density is dominated by salinity, changes in the surface heat exchange may seasonally play a role in the upper column. The rivers in the region are colder in winter, producing a clear thermal inversion (Castillo et al., 2016), while in summer the surface waters reach 18 C by the heat gained by solar radiation. But the persistent pycnocline depth along the seasons is consistent, with the freshwater input suggesting that the variability of the density in the upper layer is dominated by the freshwater input instead of the surface heating/cooling variability. We used temperature moorings to emphasize that the internal oscillation reported here had an expression in other properties of the water within the fjord. In addition, the band-pass temperature time series and the along-fjord currents show consistent oscillation patterns

(Fig. 9). During (a), the upper outows weakened due to the opposing winds at the surface. This change reached depths down to the pycnocline (Fig. 2), causing a disruption and subsequently forcing of the internal oscillations observed in the currents and temperature elds (Fig. 9). Here, intense perturbations were observed that weakened the surface outow and introduced the colder water of the upper layer to depths > 2 m at Cochamo and Puelo. During (b), the upper outow displayed minimum perturbations in both the currents and temperature. In (c), perturbations in the currents and temperature were evident at Cochamo and at the mouth with no major oscillations at Puelo (Fig. 9). In addition, 3-day bandpass vertical velocities (w) were included as arrows on the contours of the along-fjord currents in Fig. 9. The maximum w were 1 cm s1 at the mouth; outow (inow) was related to downward (upward) circulation in the entire fjord. This implies that the oscillation observed on the along-fjord currents was also consistent with the vertical velocity patterns.

5 Discussion

We used data collected in one of the most extensive studies ever conducted in a Chilean fjord. The data included currents (ADCPs) and temperatures from moored instruments, seasonal CTD information, and times series of winds and sea

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154 M. I. Castillo et al.: Seiche excitation in a highly stratied fjord

Figure 6. Band-passed along-fjord currents. Contours of band-passed (70090 h) along-fjord currents. Negative (positive) currents in blue (in red) imply an outow (inow). Note the dotted square at the middle of August: it is zooming on Fig. 9.

level to study the dynamics of the internal seiche oscillations in the Reloncav fjord.

In fjords with shallow sills, such as the Gullmar fjord in

Sweden (Arneborg and Liljebladh, 2001a), the Knight Inlet in Canada (Farmer and Freeland, 1983) and the Aysen fjord in Chile (Cceres et al., 2002), internal tide oscillations may play a major role in the internal mixing (e.g., Stige-brandt, 1976; Farmer and Smith, 1980). In lakes, large internal seiche oscillations signicantly contribute to the mixing of the entire basin (Cossu and Wells, 2013), and these oscillations could also be important in fjords where the relative importance of internal tides may be less than the internal seiche oscillations (Arneborg and Liljebladh, 2001b). The semi-diurnal signal in the spectra of the along-fjord currents (Fig. 4) suggests the relative importance of internal tides for the region which is similar to other fjord regions (e.g., Stige-brandt, 1976; Allen and Simpson, 1998; Valle-Levinson et al., 2007). The tidal interaction with the bathymetry is not the only mechanism to produce internal oscillations. Recently, Ross et al. (2014, 2015) showed the forcing by glacier lake outburst oods (GLOFs) and by low-frequency changes in barometric pressure. The importance of the internal tides for the southern Patagonian fjords is unknown and future research should be conducted to determine its contribution to the dynamics of currents and mixing.

In this study, we demonstrate the presence (and persistence) of seiches in a Chilean fjord based on the sea-level slope (barotropic seiche), currents, and temperatures (internal seiche). We also studied the main processes forcing the natural oscillation of the pycnocline.

The basic dynamics of a barotropic seiche in a fjord originate from winds tilting the along-fjord surface and piling up water at the head of the fjord. The entire fjord basin begins to oscillate after the cessation of the wind. The maximum amplitude of the seiche is located at the head, whereas a node (zero amplitude) is located at the mouth of the fjord (Dyer, 1997; Rabinovich, 2010). During a baroclinic seiche, winds events perturb the pycnocline to induce oscillations with a period commensurate with the fjord stratication (Djurfeldt, 1987). The horizontal structure of currents associated with the seiche dynamics is related to the standing wave nature of the seiche oscillation where the maximum currents occur in a node (the mouth) and minimum currents are present in an anti-node (the head) in both closed and semi-closed basins (Dyer, 1997; Rabinovich, 2010).

At high frequencies, the tidal spectrum (Fig. 4) displayed a clear accumulation of energy centered at a period of 1.3 h. This frequency is not related to any tidal harmonic interaction (Pawlowicz et al., 2002), and the shape of the spectrum (not a peak) suggests resonance in this frequency band. We

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M. I. Castillo et al.: Seiche excitation in a highly stratied fjord 155

During the spring, the daily periodicity of winds was strong (Castillo et al., 2016), with intensities capable of perturbing the pycnocline and inducing the internal seiching process.

The surface slope indicates that the sea level at Cochamo was 0.07 m higher than at the mouth, and this value can be taken as the amplitude of the surface seiche. According to the RGM, the pycnocline deviation ( 1) is related to the surface elevation ( 0) in the form 1 = (/[Delta1]) 0, which implies

that for a mean surface perturbation of 0.07 m and a typical [Delta1] of 15 kg m3, we obtain a mean 1 of 4.8 m. This

nding indicates that the water piles up at the head of the fjord, likely due to the predominant fjord winds in the region (Fig. 3a), and produces a pycnocline deepening of about 5 m (Fig. 2).

At low frequencies (periods > 1 day), the along-fjord current spectra displayed a marked peak in energy centered at 3 days. To explore the origin of this variability, we analyzed the density proles along the fjord (Fig. 2) and applied two methods, the RGM and the CSM. The internal phase velocities (c) obtained from both methods were similar, and ranged between 0.73 and 0.87 m s1 (taking into account mode 1 of the CSM for comparison). The high c value was obtained during the spring (November 2008), when the upper layer presented the lowest densities of the seasons, likely due to high discharge (> 1000 m3 s1). Remarkably, the stratication is linked to the freshwater input despite no major ob-

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Figure 7. (a) Reconstruction of the along-fjord band-passed currents at the mouth using modes 13, (b) band-passed along-fjord currents at the mouth, (c) kinetic energy (KE) estimated using reconstructed currents (black), the 3-day band-pass currents (red), and the diurnal and semi-diurnal band-pass currents (blue).

explored the effect of the natural oscillation of the basin in this pattern using the barotropic phase velocity (c) for a shallow water wave c = (gh)1/2, where h is the mean depth of

the fjord. If one assumes a mean fjord depth of h = 250 m

(Table 1), then c = 49.5 m s1 and the natural period TN =

4 L c1 = 1.24 h. This period is lower than the observed pe

riod in Fig. 5 (1.3 h) because the mean depth takes into account the entire fjord bottom prole (Fig. 1), and thus the effective depth (up to Cochamo) was 233 m, and it is closer to the 226 m necessary to obtain the observed period in Fig. 5. Winds in the region are moderate (see Fig. 3), but their intensity is sufcient to tilt the surface slope at Cochamo (Castillo et al., 2012), and thus the surface of the fjord oscillates with the natural period of the basin. Further evidence of this pattern is provided by the clear differences in amplitude of the sea-level spectrum at Cochamo (near the fjords head) and at the mouth. This association is attributed to the dynamics of seiches in fjords, which tend to produce a node at the mouth and an anti-node at the head (Dyer, 1997). At the node, the sea-level amplitude must be zero, whereas near the head, it must be a maximum. This pattern is highly consistent with the observed spectra at 1.3 h (Fig. 5). Based on all of these results, we suggest that oscillations close to 1.3 h will resonate with the natural period along the fjord.

Daily winds were highly coherent with surface along-fjord currents, especially in the brackish water layer (Fig. S1).

156 M. I. Castillo et al.: Seiche excitation in a highly stratied fjord

Figure 8. Coherence and phase wavelet spectra. (a) Time series of along-fjord wind stress, and (b, c, d, e) coherence and phase wavelet spectra, for the relations mouthPuelo (b, c) and PueloCochamo (d, e). In the contours, the thick black line indicates squared coherence

0.6; only the associated phases were present on the phase wavelet. The thick black curve is the inuence cone for the wavelet estimations.

served changes in c (Fig. 6eh). The high consistency between the CSM (mode 1) modal speeds and the phase speed obtained by the RGM suggest that rotation does not play a signicant role in the along-fjord dynamics of these oscillations (van der Lee and Umlauf, 2011). But cross-fjord, the dynamics has been nearly geostrophic, especially at the fjords mouth (Castillo et al., 2012).

For longer periods (> 10 days), there is evidence of baro-clinic oscillations clearly observed in the along-fjord time series (Fig. 3) and in the averaged spectra (Fig. 4). Recently, Ross et al. (2015) described a similar periodicity in currents of a southern Patagonian fjord of Chile associated with baro-clinic annular variability, a regional feature in the air pressure in the region. This mechanism of generation for the 10-day oscillations in the Reloncav fjord needs to be veried in future studies.

The internal TN of the entire fjord displayed periods between 2.9 and 3.5 days. These results suggest that the accumulation of energy observed in the along-fjord currents is due to the rst mode of an internal seiche oscillation in the fjord. This result could be explained by the presence of a node at the mouth, where the sea-level amplitude is minimum

(Fig. 5) but the currents are maxima (Figs. 3 and 6). This difference was also observed in the projected currents (upj,vpj), supporting the idea of the presence stationary wave along the fjord. Additionally, the currents were highly coherent and in phase (Fig. 8), as we expected from a basin-scale wave-like seiche. As a way to estimate the contribution of the internal seiche to the internal mixing the KE was enhanced during the fjord winds (Figs. 3 and 7), which were periods when the internal seiche band (3 days) was highly coherent along the fjord (Fig. 8).

The winds exhibited high coherence with the along-fjord currents until the pycnocline depths, at frequencies centered at 1 and 3 days (see Fig. S1). To study the extent to which the wind stress perturbs the pycnocline, we used the Wedder-burn number, which is given by the equation W = (h1/L)Ri

(Thompson and Imberger, 1980; Monismith, 1986), where Ri = g[prime](h1/u2 ) represents the bulk Richardson number, an

index of the stability of the upper layer (h1). The frictional velocity (u ) is obtained from the surface wind stress using

the equation u2 = /0, which results in the equation

W =

h21[Delta1]g

L . (5)

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M. I. Castillo et al.: Seiche excitation in a highly stratied fjord 157

Figure 9. Time series of along-fjord wind stress () and contours of along-fjord currents and temperatures at Cochamo, Puelo, and the mouth. There are three states of wind stress based on the Wedderburn number (W), with (a) strong W < 1, (b) weak W > 1, and(c) moderate W 1 winds. Note that contours of the currents and

temperature for a given location are plotted together. The arrows represent the 3-day band-pass vertical velocities where the maximum was 1 cm s1.

According to Thompson and Imberger (1980), this value indicates the effect of the wind stress on local upwelling in a stratied uid (i.e., perturbing the pycnocline). Under weak conditions (W 1), the wind energy is insufcient

to tilt the interface. Under strong conditions (W 1), how

ever, upwelling conditions dominate, there by tilting the interface, which produces conditions favorable to forcing of the internal seiche. The critical conditions (W 1) indicate

the beginning of upwelling (Thompson and Imberger, 1980; Stevens and Imberger, 1996), although the ideal transition

Figure 10. Damping signal in currents during a period of weak winds (W > 1) at Cochamo (16 to 24 August 2008). The band-pass currents at 3 m depth (black line) were compared with a damping oscillatory curve x(t) = Ae

(kt) cos(!t+) (gray line). The damping

time (Td) was 3.6 times longer than the fundamental internal period (TN).

point occurs at W = 0.5 (Monismith, 1986). All of these con

ditions were observed during the period of August 2008, as is shown in Fig. 9. During strong ( 0.3 N m2) condi

tions, W = 0.27 produced intense perturbation of the pycn

ocline (Fig. 9a). In contrast, during weak ( 0.01 N m2)

conditions, a value of W = 8 indicates that the wind was

too weak to perturb the pycnocline, favoring a seiche damping process (Fig. 9b). Transition conditions occurred when d 0.1 N m2 and W = 0.8, indicating that the winds were

strong enough to perturb the pycnocline and stop the damping process (Fig. 9c).

5.1 Internal seiche damping

The wind stress changed from a state where was strong enough to actively disturb the pycnocline (W < 1) to a period of nearly calm winds (W > 1) between 16 and 24 August 2008 (Fig. 9). During this period, both the along-fjord currents and temperatures tended to decay, which is clearly evident in the isolines of these properties at the three sites (Fig. 9).

To study the damping process in detail, we selected the time series of the along-fjord currents at a depth of 3 m at Cochamo during the above period in August to span the period of forcing, damping, and re-enforcing of the internal oscillation.

Typically, any real oscillations undergo damping, which is given by the equation

x(t) = Ae(kt) cos(!t + ) (6)

where t is time and A is the initial amplitude, k is the damping coefcient which has units of s1, ! = 2/TN, and is

the phase. In the case studied here, = 0, A = 8 cm s1, and

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158 M. I. Castillo et al.: Seiche excitation in a highly stratied fjord

TN = 2.5 days, which was the internal period at Cochamo

(Fig. 4). The best t occurred when k = 1/3 (Fig. 10).

The time for the initial amplitude A to decay to A 0 is

the damping time (Td). There was a good t (Fig. 10) between the observed current and the curve adjusted with the damping effect. Here, Td = 9.1 days, which is more than 3

times longer than the natural oscillation (TN); more precisely, Td = 3.6TN at this site. The observed internal oscillations of

the currents were not completely damped because the winds increased from nearly calm (W > 1) to moderate conditions, which disturbed the pycnocline (W 1) and induced the in

tense oscillations during the spring (Fig. 6). In the spring, the winds displayed a marked diurnal cycle that remained during the spring and summer (Castillo et al., 2012). This nding suggests that the internal seiche (mode 1) process is active without damping because it is forced daily (Fig. 3).Our ndings indicated that the internal seiche process is an active contributor to the mixing in the Reloncav fjord; the magnitude of this contribution might be similar to the tidal forcing. The maximum amplitude of the tidal currents in the Reloncav fjord is 10 cm s1 (Valle-Levinson et al., 2007;

Castillo et al., 2012); using KE to estimate the maximum contribution of the tide obtains 5 [notdef] 103 m2 s2, which is

similar to the observed KE at the mouth (Fig. 7). One example of the dissipation of the energy through this process was observed prior to 19 August 2008 (Fig. 10): then the maximum currents were 0.7 m s1 and, through Eq. (7), we obtain KE = 7 [notdef] 103 m2 s2, meaning that a great part of

this energy might be dissipated within the Reloncav fjord in 9 days.

6 Conclusions

The along-fjord seasonal density structure of the Reloncav fjord showed small changes in the stratication. The upper layer shows a persistent stratication from the surface to the pycnocline base, the latter of which has a mean depth of 2 m near the mouth and 6 m near the head of the fjord.

The along-fjord sea-level signal showed a 1.3 h energetic peak not related to any tidal harmonics; additionally, at this period the sea-level amplitude at the mouth was signicantly higher than the sea level at the head of the fjord. This pattern was consistent with the presence of a barotropic seiche in the Reloncav fjord.

Local wind stress was able to perturb the along-fjord pycnocline and produce internal seiche oscillations. The period centered on 3 days was consistent with the rst baroclinic oscillation mode. This mode explained 44 % of the variability of the 3-day band. The oscillation was highly coherent along the fjord and with a phase close to 0 , consistent with a standing wave, like an internal seiche, within the Reloncav fjord.

The internal seiche could be a strong contributor to the internal mixing within the fjord; in fact, the kinetic energy

(KE) associated with the internal seiche was similar to the maximum contribution of the tides in the along-fjord currents. During winter, the internal oscillations were present a relatively long period of time with nearly calm winds, which permitted the estimation of the damping time of the internal seiche to be 9 days; otherwise, during the spring daily winds continuously forced the pycnocline.

Future studies should focus on evaluating more precisely the available energy for the mixing process within the fjord and their effects on other water properties such as the salinity, oxygen, or nutrients.

7 Data availability

The installation of the moorings for measuring the current, temperature, and sea level in the region was approved by the Chilean Navy through permit DS711. No specic permits were required to install the meteorological station because the location is a publicly controlled site. This study also did not involve any endangerment to species in the region. The authors indicated that all data are available to download from a COPAS-SUR Austral (2012) website (http://www.reloncavi.udec.cl/

Web End =http://www.reloncavi.udec.cl/ ). The discharge data from the rivers of Chile are available from the Direccin General del Aguas de Chile website (http://dgasatel.mop.cl/

Web End =http://dgasatel.mop.cl/ ). Also, all data sets can be requested from the corresponding author (Manuel I. Castillo).

The Supplement related to this article is available online at http://dx.doi.org/10.5194/os-13-145-2017-supplement

Web End =doi:10.5194/os-13-145-2017-supplement .

Competing interests. The authors declare that they have no conict of interest.

Acknowledgements. The authors thank the students (from Chile and Sweden) and technicians of the Physical Oceanography group of the Universidad de Concepcin who collaborated in performing the eld measurements. This study was funded by the COPAS-Sur Austral CONICYT PIA PFB31 and Centro de Investigacin en Ecosistemas de la Patagonia by FIP2007-21. Manuel I. Castillo was supported by CONICYT-PAI no. 791220005 and by FONDECYT no. 11160500. Finally, we want to thank the three anonymous reviewers and the editor Mario Hoppema for their comments which helped to improve the present paper.

Edited by: M. HoppemaReviewed by: three anonymous referees

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M. I. Castillo et al.: Seiche excitation in a highly stratied fjord 159

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