Vertical structure and seasonal variability of the shelf currents off Bahía Magdalena, Mexico in 2011–2012: ADCP measurements

Regional Studies in Marine Science 34 (2020) 101165

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Vertical structure and seasonal variability of the shelf currents off Bahía Magdalena, Mexico in 2011–2012: ADCP measurements ∗

Jean R. Linero-Cueto a , , Oleg Zaytsev b a b

Facultad de Ingeniería, Universidad del Magdalena, Carrera 32 # 22-08, Santa Marta, Colombia Instituto Politécnico Nacional, Centro Interdisciplinario de Ciencias Marinas Ave. IPN, s/n, Playa Palo de Santa Rita, La Paz, 23096, B.C.S., Mexico

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Article history: Received 13 April 2019 Received in revised form 25 October 2019 Accepted 10 February 2020 Available online 25 February 2020 Keywords: Acoustic profiler Vertical current structure Tides Spectral harmonic analysis Bahía Magdalena

a b s t r a c t Currents in the continental shelf off Bahía Magdalena were examined with an Acoustic Doppler Current Profiler (ADCP) anchored at a depth of 100 m from February 2011 to February 2012. Variability of the currents in the entire water column was modulated by tidal movements, which represented approximately 22% (15% diurnal, 7% semidiurnal) of the total kinetic energy. These movements had a steep vertical structure characterised by diurnal currents with average speed up to 60 cm s−1 at the surface layer of 20 m in depth. Below this depth, the tide harmonic amplitudes decreased rapidly to a relatively weak barotropic flux. Movements in the upper layer were dominated by a diurnal radiational S1 constituent, which was coherent with the local sea breeze. Significant sea currents of semidiurnal radiational S2 constituent were also associated with sea breezes. The components of the gravitational tide showed a barotropic characteristic in the entire water column, except for M2 harmonics that had intensification at the surface layer. The analysis of residual currents confirmed an important feature of the shelf circulation in the southern part of the Baja California peninsula, which consists in the formation of a northward barotropic flow in the entire water column in summer (July–beginning of August). This flow provides an intrusion of tropical water masses along the coast at least to the middle part of the peninsula. Similar manifestations are occasionally observed in the autumn months. © 2020 Published by Elsevier B.V.

1. Introduction The California Current System (CCS) is an eastern Pacific system of currents conditionally defined in three zones (Lynn and Simpson, 1987): oceanic zone (OZ), transition zone (TZ) and coastal zone (CZ). The OZ is dominated by an annual cycle that follows the solar radiation pattern. The TZ shows an approximate width from 200–300 km parallel to the coast where the nonseasonal events dominate: mesoscale meanders and eddies. The CZ includes the continental shelf, which is relatively narrow (30– 50 km off the coast) and also dominated by mesoscale eddies, shelf currents, narrow counter-currents and upwelling events (Zaytsev et al., 2003; Durazo, 2009). These dynamic processes jointly determine the hydrologic and hydrobiological conditions on the continental shelf of the Baja California Peninsula (BCP). The thermohaline and dynamic characteristics of the OZ and TZ, as well as their seasonal and interannual variability, were studied within the framework of the multidisciplinary programs CalCOFI (California Cooperative Oceanic Fisheries Investigations) and IMECOCAL (Mexican Investigations of the California Current); among those, some general studies are: Lynn and Simpson ∗ Corresponding author. E-mail address: [email protected] (J.R. Linero-Cueto). https://doi.org/10.1016/j.rsma.2020.101165 2352-4855/© 2020 Published by Elsevier B.V.

(1987), Strub and James (2000), Durazo and Baumgartner (2002), Bograd and Lynn (2003), Soto-Mardones et al. (2004), Durazo et al. (2005), Durazo (2009), Durazo et al. (2010) and McClatchie (2014). Seasonal variability in CZ is characterised by a wide range of direction and intensity of the currents, as well as by the presence of mesoscale eddies. The depth-averaged flow in the surface layer of 200 m is directed to the southeast along the continental slope. However, a sub-superficial (below 200 m) northward current, referred to as the Coastal Counter-current, has been observed along the continental slope of California (USA) and the state of Baja California (MX) in autumn and winter (Reid and Schwartzlose, 1962; Hickey, 1979, 1998; Lynn and Simpson, 1987; Gay and Chereskin, 2009; Todd et al., 2011). Song and Chao (2004) argued that this northward sub-superficial flow is due to positive rotor of wind stress, while the balance of Sverdrup is maintained. This counter-current toward the North Pole mainly occurs along the continental slope and occasionally over the shelf. In rare cases the manifestation of this flow was observed in the surface layer (Durazo, 2009). The geostrophic currents calculated, based on IMECOCAL data, showed the mesoscale cyclonic eddies clearly close to the coasts

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of the BCP, for example, in the surrounding area of Punta Eugenia (28o N). These eddies vary seasonally, weakening or disappearing in winter and spring and intensifying in summer and autumn when the presence of sub-superficial equatorial water is sometimes observed (Durazo, 2009). The southern limit of the CCS is situated in the oceanic region off the western-southern coast of the BCP (Aguad et al., 2011; Durazo et al., 2010). This coastal area, southward from Cabo San Lázaro, is out of the domain of the IMECOCAL program, which does not include the continental shelf of the BCP. In this area, the Bahía Magdalena-Almejas (BMA) lagoon system, the largest of the Mexican Pacific, is well-known for its great primary productivity (Cervantes-Duarte et al., 2013). The oceanographic conditions of this area of study, including water masses, physical–chemical parameters, upwelling conditions and hydrodynamic processes, have been poorly understood. The results of some sporadic studies were described by Cervantes-Duarte and Hernández-Trujillo (1989), Martínez-López (1993) and Zaitsev et al. (2007, 2014). Among the few hydrobiological studies in the southern part of the BCP are those from Funes-Rodríguez et al. (2006), Avendaño-Ibarra et al. (2010) and Murillo-Murillo et al. (2013), who described variability of some regional oceanographic and biological conditions at this area. Salinas-González and Pinet (1991) described the geostrophic currents on the continental shelf off Bahía Magdalena. The seasonal pattern of sea surface temperature (SST) and the geostrophic currents show the intrusion of subtropical waters toward the pole along the southern coast of the BCP in autumn (Zaitsev et al., 2007). This effect intensified during El Niño years, and it could be a mechanism of superficial transport of sub-tropical and sometimes tropical waters off the BMA lagoon system. Seasonal variability of coastal upwelling in the adjacent region to the BMA system was described by Zaytsev et al. (2003). The maximum upwelling activity takes place during spring under the influence of dominant winds and the topographic gradient of the continental slope with an upwelling index variation range from 50 to 280 m3 s−1 per each 100 m of coastal line. During upwelling events, the SST shows negative anomalies up to 4.5 ◦ C in the zone adjacent to the coast, which correspond to an elevation of isotherms toward the surface from 60–70 m in depth (Linero-Cueto, 2014; Zaitsev et al., 2014). The thermohaline structure and geostrophic currents on the continental shelf of the southern BCP show two hydrodynamic scenarios: (a) winter–spring, when an average transport along the coast is directed southward; (b) summer–autumn, when northward fluxes of subtropical sub- and superficial waters are occasionally recorded, reaching at least the southern Gulf of Ulloa and sometimes Punta Eugenia (Durazo, 2009; GonzálezRodríguez et al., 2012; Zaitsev et al., 2014). Variability and intensity of these fluxes have not been sufficiently documented, especially on the continental shelf. Direct recordings of this flow using current meters have never been performed. Therefore, the main objective of this research was to study variability of the continental shelf current vertical pattern off the BMA system in a wide time scale from diurnal to seasonal variations. Current measurements were performed during 2011– 2012 in the framework of the project ‘‘Servicios ambientales del ecosistema costero de Baja California en relación al cambio climático’’ (Environmental services of the Baja California coastal ecosystem related to climate change). 2. Materials and methods The geographical location of the study area is shown in Fig. 1. In this region bathymetry has a topographic slope of approximately 0.01 (Zaitsev et al., 2007).

Fig. 1. Area of study. The asterisk indicates the deploy location of the Doppler acoustic profiler. Red and blue arrows show directions of the flow parallel to the coast, northward and southward, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Six oceanographic research cruises were conducted on board R/V Francisco de Ulloa off the BMA lagoon system in February, April, July and October 2011, February and April 2012 to cover a predetermined grid of 68 hydrographical stations separated by a distance of 5 nautical miles. Using the data obtained during these field observations, Zaitsev et al. (2014) described the variability of the thermohaline structure to a maximum depth of 1000 m. To measure the vertical current structure, the ADCP was deployed at a depth of 85 m in a 100 m deep area off BMA. The mooring consisted of an upward-looking SonTek 250 kHz acoustic profiler mounted on a model MSI-125 subsurface float. The sampling interval was 30 min with a 1-min averaging interval; the vertical resolution was 5 m. Current velocity measurements were made from 9 February 2011 to 17 February 2012 close to the mouth of Bahía Almejas, approximately 15 km off Isla Margarita (see Fig. 1). Current measurements were performed for 17 layers with a nominal water depth (corresponding to the centre of each layer) of 85, 80, . . . , 10 and 5 m. Unfortunately, while installing the equipment, the pressure sensor was damaged, so sea level variations could not be analysed. To analyse tidal currents, current velocity records were transformed to latitudinal and longitudinal components (u,v) and subjected to a harmonic analysis (Foreman, 2004; Pawlowicz et al., 2002). The tidal ellipse parameters were calculated for the major constituents from the u, v component amplitudes and phases (Pugh, 1987). Then, tides were eliminated from initial records to obtain residual velocity time-series, which were rotated to 41◦ (anticlockwise) to show parallel and perpendicular

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Table 1 Constant harmonics of the principal constituents of tidal currents at 20 m depth. Tidal constituents

Period (h)

Amplitude (cm s−1 )

Phase (o UTS)

O1 P1 S1 K1 N2 M2 S2 K2

25.8193 24.0659 24.0000 23.9345 12.6584 12.4206 12.0000 11.9672

5.9 4.7 11.2 8.6 3.3 9.9 5.4 2.4

244 248 181 282 176 186 168 204

Fig. 2. (a) Profiles of the acoustic signal intensity and (b) signal-noise ratio for three acoustic beams of the ADCP installed off Bahia Magdalena.

components to the coast in the mooring site. To obtain additional information, the u and v time-series were then Fourier transformed to clockwise (CW) and counterclockwise (CCW) rotary components (Gonella, 1972; Mooers, 1973; Kulikov et al., 2004; Emery and Thomson, 1997; Rabinovich et al., 2013). Spectral estimates were used for each rotary component to examine the frequency content and physical properties of the current. To complete the current analysis, the geostrophic current fields were shown with respect to the reference level of 1000 dB. These fields were calculated on NOAA Atlantic Ocean Marine Laboratory website (www.aoml.noaa.gov/phod/dataphod/work/ trinanes/INTERFACE) that integrates TOPEX, ERS–2 and GFO altimetry data with a spatial resolution of 0.2◦ and the mean climatological dynamic heights estimated based on Levitus climatology for 1000 db. Moreover, the results of the HYCOM + NCODA Global 1/12◦ Analysis (GLBa0.08/expt_90.9), available on the website of the HYCOM model (https://www.hycom.org/data/ glba0pt08/expt-90pt9) were used for the analysis of the annual cycle of currents in the region. 3. Results Fig. 2 shows the average acoustic intensity and signal/noise ratio for all three ADCP beams; in the first 20 m in depth, the acoustic signal decreased from ∼180 to 60 dB to maintain it approximately constant in the rest of the ocean depths, rendering the good signal transmitted by the equipment. A similar behaviour was found in signal/noise ratio. These acoustic parameter values provided the statistical accuracy of the measurements. 3.1. Harmonic tide analysis To determine the harmonic constants of the principal tide constituents, a harmonic analysis of all the current time-series recorded by the ADCP was performed for the ‘‘north–south’’ and ‘‘east–west’’ components. The analysis of the calculations at the depth of 20 m (Table 1) showed that the constituents dominating at this depth were diurnal S1 and K1 , as well as the semidiurnal M2 with amplitudes 11.2, 8.6 and 9.4 cm s−1 , respectively. It was notable that the ‘‘radiational’’ S1 constituent, excited by solar radiation with a period of 24 h, exceeded the gravitational tidal constituents, which could be indicative of the diurnal wind (breeze) behaviour in this region. The K1 constituent value seemed relatively high, which was determined by the insufficient resolution with the adjacent S1 constituent; the M2 constituent dominated among the semidiurnal movements. To define the general tidal pattern, the tide factor was calculated as F = (K1 + O1 )/(M2 + S2 ), obtaining 0.96, which according

Fig. 3. Vertical distribution of the principal harmonic amplitudes of tidal currents on the continental shelf off Bahía Magdalena, Baja California Sur, Mexico; (a) diurnal harmonics, (b) semidiurnal harmonics.

to Bowden (1983) criteria indicated that tides in this region are mixed and mainly semidiurnal. Fig. 3 shows the vertical amplitude distribution of the principal tidal current constituents where the vertical structure showed two well-defined layers. The surface layer with an approximate thickness of 15 m had high kinetic energy and was strictly baroclinic. Below this depth, amplitudes of tidal constituents did not vary in depth, showing barotropic behaviour. The currents forced by radiational S1 constituent reached 35 cm s−1 when greater gravitational M2 and S2 constituents only reached 15 cm s−1 . This intensification of superficial currents was possibly generated by sea breeze. Below 20 m, the currents were vertically uniform for all the constituents. The S2 harmonic constituent consists of a trigonometric (vectorial) combination of gravitational and radiational contributions (Zetler, 1971). The theoretical amplitude of the S2 gravitational component is significant and is approximately 42% of the M2 amplitude (Pugh, 1987; Rabinovich and Medvedev, 2015). Zetler (1971) proposed a technique for separating the radiational and gravitational components of the S2 constituent. The performed calculations, according to this technique for sea level variations, showed that the typical ratio of the S2 radiational component is approximately 16% of the S2 gravitational component). It is clear that for the current harmonic analysis this relation does not have to be fulfilled. In this study, the S2 radiational component was about 25%. The harmonic current analysis for the principal gravitational diurnal harmonic (O1 , K1 , P1 ) and semidiurnal (N2 , M2 , K2 ), as well as radiational (S1 , S2 ) constituents, allowed calculating the tide ellipses shown in Fig. 4. In the surface layer, the S1 ellipse dominated the other tide components, and the maximum S1 variance was ∼3.8 times greater than that of P1 and K1 , and ∼8.8

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the surface layer where S2 was ∼1.5 times greater than M2 . As a result, the M2 constituent determined the tidal currents dynamics in a greater percentage instead of S2 with the exception of the surface layer (∼15 m) where S2 had greater predominance. The directions of the S1 and S2 ellipses at the surface layer indicated that these constituents were closely related. It is reasonable to suppose that gravitational tide forcing and stratification were the primary factors that determined the vertical structure of the CW movements at depth greater than 20 m while diurnal wind variability (sea breeze) was responsible for the predominance of the S1 substituent in the upper layer. Sea breeze did not affect the O1 , K1 and M2 constituents, so their ellipses were CW in the entire water column; thus, the ellipse directions and the major/minor axes ratios were permanent. The magnitude of the ellipses did not decrease gradually with depth. The maximum M2 currents occurred in the upper layer, but no dramatic change was observed in ellipse amplitude and direction in the rest of water column. 3.2. Spectral analysis of currents The rotational spectra of the currents for six representative layers are shown in Fig. 5. A characteristic feature in all spectra was the ‘‘radiational’’ S1 constituent, associated with solar radiation effects (Munk and Cartwright, 1966; Pugh, 1987), which was the most energetic of all the tide constituents. The spectral maximum in the semidiurnal band was relatively narrow and steep. The CW and CCW components in the semidiurnal band were approximately equal. On the surface (5 m), component S2 was greater than M2 , and the order was inverse starting from 10 m where M2 was more energetic. The influence of component O1 could be observed starting from 20 m in depth, and the influence of inertial frequencies started to be noticeable deeper than 30 m. In the diurnal band, the spectral peak of O1 splits with the S1 peak only in depths greater than 20 m with dominance of CW rotation.

Fig. 4. Tide ellipses for three gravitational tide harmonics of diurnal currents (K1 , O1 , P1 ), diurnal ‘‘radiational’’ (S1 ), semidiurnal gravitational tide (N2 , M2 , K2 ) and solar ‘‘radiational’’ (S2 ). Blue ellipses indicate CCW movement, red CW. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

times greater than O1 . The gravitational tide ellipses also showed different behaviour among themselves; for the diurnal components, movements were CW except for P1 that showed alternating CW and CCW movements. The ellipses of diurnal components, except for P1 at the upper layers, were directed perpendicular to the coast; K1 and O1 kept in general the magnitudes of maximum variance in entire water column, same as the spatial orientation. S1 decreased monotonously at depths greater than 15 m, and the directions were alternating. For the semidiurnal harmonic M2 and S2 , movements were mainly CCW although M2 showed this behaviour only to a depth of 65 m. Below 20 m, maximum variance remained constant for both M2 and S2 . The M2 ellipses were significantly larger than the S2 ones, with respect to magnitude (∼3 – 3.5 times), except for

The inertial frequency peaks with the period of 29.3 h were clearly observed in current spectra at depths over 50 m. Diurnal movements at the latitude of BMA were super-inertial, in such a way that internal waves could exist in diurnal and semidiurnal frequencies. The internal waves in the northern hemisphere were CW (cf. Gonella, 1972) and the peaks S1 and S2 also had CW rotation, which allowed us to consider a baroclinic tide, as a source of significant diurnal movements, especially on the surface layer of 20 m. In depths greater than 20 m, the spectral peaks of the gravitational constituents were typical for internal waves, especially in the diurnal band that apparently combined tide and inertial energy. The O1 peaks might have been influenced by breeze contributions (S1 ) and inertial movements. Guided by the maximum and minimum current spectra, spectral energy (σ 2 ) was divided in five frequency bands (∆ωj ), namely, (1) low frequency (LF), ω < 0.72 cpd; (2) inertial (f), ω ≈ 0.72 − 0.92 cpd; (3) diurnal (D), ω ≈ 0.92 − 1.5 cpd; (4) semidiurnal (SD), ω≈1.5 − 2.3 cpd; and (5) high frequencies (HF), ω > 2.3 cpd. For each jth frequency band (∆ωj ) and for each specific depth, variances CW, σj 2 (−), CCW, σj 2 (+) and total variance σj 2 (tot) of the ADCP currents were estimated, as: ∫ 2 σj (±) = Sj± (ω) dω ; σj2 (tot ) = σj2 (−) +σj2 (+) , ∆ωj

where Sj (ω) is spectral density and j = 1, 2, . . . , 5. Table 2 shows the summary of the σj 2 (tot) energy in (cm s−1 )2 . In accordance with these frequency ranges, at the surface layer (0–15 m) and lower (15–85 m) layers, the D and SD bands were responsible for 24% and 19.2% of the total kinetic energy,

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Fig. 6. Rotational wind spectra for the San Carlos, Baja California Sur meteorological surface station observations during 2011. Black dotted lines show ‘‘radiational’’ harmonics related with sea breeze. Table 2 Spectral energy (cm s−1 )2 of the currents recorded by the ADCP off Bahía Magdalena for different depths. Depth (m)

LF

f

D

SD

HF

Total

1055.7 2239.5 936.1 1410.4 59.0

77.7 153.9 55.6 95.7 4.0

265.7 679.3 229.4 391.5 16.4

134.8 239.2 168.0 180.7 7.6

313.4 391.7 224.9 310.0 13.0

1847.3 3703.6 1614 2388.3 100

381.0 469.8 180.8 384.1 320.9 243.8 170.9 131.3 111.4 266.0 59.8

19.2 21.7 18.9 19.9 17.6 15.6 17 15.3 15.8 17.9 4.0

47.2 69.1 57.7 55.9 47.1 44.5 44.4 45.0 63.7 52.7 11.8

43.9 51.1 32.6 41.8 29.2 22.1 21.7 23.6 27.2 32.6 7.4

94.5 96.0 93.7 81.1 73.0 65.2 60.3 58.3 57.3 75.5 17.0

602.2 707.7 383.7 582.8 487.9 391.3 314.4 273.4 258.9 444.7 100

37.3 4.0

137.4 14.8

69.6 7.5

134.1 14.4

930.6 100

Surface layer 0–5 5–10 10–15 Mean % Lower layer

Fig. 5. Rotational spectra of currents at different depths off Bahía Magdalena, Baja California Sur, Mexico from 9 February 2011 to 17 February 2012. Black dotted lines make reference to the principal tide harmonics. Red dotted lines denote inertial frequency at the ADCP deploy location. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

respectively. The LF band energy at the superficial layer and in the rest of the water column was almost equal, 59 and 59.8%, respectively. For all the bands, the greatest relative contribution of kinetic energy (64%) was found in the first 15 m. In the entire water column, tidal movements accumulated 22.3% (D with 14.8% and SD with 7.5%) in average. The intense diurnal surface currents were the most important feature that derived from time series recorded by the ADCP. The D currents were more energetic than those of the SD band. As it was expected, the surface current intensification depended on daily wind variability; to complete relevant information, the rotational spectrum of the winds recorded by the meteorological station located in San Carlos, BCS, Mexico (Fig. 1) were calculated from 26 May 2010 to 2 February 2012. The wind spectrum (Fig. 6) had narrow peaks in frequencies S1 , S2 , S3 and S4 of the ‘‘radiational’’ constituents. Diurnal warming forms a diurnal sea breeze (Ray and Egbert, 2004), which generally has a day–night asymmetry that is responsible for high-frequency solar harmonics, such as S2 , S3 and S4 (Kulikov and Rabinovich, 1983). These harmonics were observed clearly in the wind spectrum of San Carlos. The same harmonics were observed in the surface current spectra (Fig. 5), but they were almost absent in spectra from depths greater than 20 m. The CW components were dominant over the CCW ones, which agreed with the surface current spectra.

15–20 20–25 25–30 30–35 40–45 50–55 60–65 70–75 80–85 Mean %

Total water column Total mean Total %

552.1 59.3

The most interesting result of the harmonic and spectral analysis was the presence of intense S1 currents in the 20-m upper layer with speeds observed up to 35 cm s−1 . Below the surface layer, the influence of the ‘‘radiational’’ constituent (sea breeze) was insignificant, and current variations were mainly determined by gravitational tide harmonics. 3.3. Analysis of residual currents The initial time-series were displayed by latitudinal (north– south) and longitudinal (east–west) components. After tide elimination, these series were recalculated in the new coordinate system: parallel and transversal to the coast. Fig. 7 shows the behaviour of the parallel current to the coast for different depths. As expected, greater currents were found at the surface layer, which decreased gradually with depth. In winter and spring, surface currents were of greater intensity and formed a flux toward the south. In this period, northward flows were observed eventually due to the mesoscale variability of local winds. In summer, from the end of June to the beginning of August, this

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Fig. 7. Daily averaged current parallel to the coast off Bahía Magdalena, Baja California Sur, Mexico from 10 to 80 m in depth; from 9 February 2011 to 17 February 2012. Northward flows are marked red, southward marked blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

northward flux was observed completely developed in the entire water column, reaching speed about 45 cm s−1 in the upper layer and 15–25 cm s−1 in deeper layers. From mid-August to November, the flux was southward with speed from 30 to 50 cm s−1 with only one exception of an event on the first days of September when the flux changed its direction northward. From November 2011 to January 2012, hydrodynamics of the area was very unstable. During this time period, the flow fluctuated from northward to southward, changing every one or two weeks. Hypothetically, this phenomenon may be associated with seasonal adjustment of the wind field or flow instability on the coastal shelf, associated with changes in bottom friction under different tidal regimes. This process requires additional research. In general, two features are important in Fig. 7: (a) a flow toward the north with speed up to 45 cm s−1 observed in summer and (2) a mesoscale variability of currents when current intensity and direction varied with a period from 3–5 days in spring and 1–2 weeks in autumn, which showed that wind is an important factor that has an important influence in water dynamics in this area.

4. Discussion The results of the harmonic and spectral analyses indicate that the diurnal and semidiurnal currents are generated by two main mechanisms. The first one is related to the effect of solar radiation (sea breeze), and the second one is due to gravitational (tidal) forcing. The first mechanism was responsible for intensification (up to 35 cm s−1 ) of diurnal S1 surface current in the 15 m upper layer. Turrent and Zaitsev (2014) showed that sea breezes in this region are quite strong, caused by both the Pacific Ocean-BCP land SST contrast and the Pacific Ocean-Gulf of California SST contrast, which is much stronger in June and July (Fig. 9). Intense surface currents relevant for the sea breeze forcing were observed at the same latitude in the Bay of La Paz, Gulf of California (Zaytsev et al., 2010). While the gravitational tide was responsible for relatively weak (∼3–10 cm s−1 ) tidal currents observed in the rest of the water column except for the M2 constituent that reached 16 cm s−1 . Fig. 3 shows the principal harmonic amplitudes, from which we may conclude that in the upper layer of 15 m, the

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Fig. 8. Geostrophic currents (vectors) and sea level anomaly (contour) for April, July, August and October 2011 in the southern Baja California Peninsula oceanic zone, Mexico. Geostrophic currents were calculated by the NOAA Atlantic Ocean Marine Laboratory from satellite altimetry data and the Levitus climatology for 1000 db.

‘‘radiational’’ S1 constituent was twice greater than the sum of the principal diurnal gravitational constituents K1 , P1 and O1 while below this layer, the S1 amplitude was about half the sum of the gravitational components. The diurnal S1 surface currents have been previously identified in the ocean. Rosenfeld (1988) detected them in the 35-m upper layer with amplitudes up to 20 cm s−1 off northern California. Pidgeon and Winant (2005) also identified an intensification of diurnal S1 currents that rapidly decreased with depth in the coastal zone of central and southern California; these currents were about 6–8 cm s−1 at the surface layer. Zaytsev et al. (2010) observed strong diurnal CCW rotational currents with speeds up to 50 cm s−1 within a surface layer of approximately 30 m thick in the Bay of La Paz. In this study, the intense diurnal currents were highly correlated with local sea breeze. Amplitude decrease and change of direction to the CW rotation with a depth for S1 were indicative of friction forces coming from higher layers (Rosenfeld et al., 2008). The results of this study also showed a similarity between current and wind spectra (Figs. 5 and 6). Radiational diurnal currents were directly forced by sea breeze; they were not from natural surface oscillations (Eigen frequency), so they had a rapid attenuation with depth. The rotation direction observed in the S1 currents was CW (Fig. 4) according to that reported by Pidgeon and Winant (2005) and Rosenfeld (1988). At the surface level, the S1 ellipse was directed almost perpendicularly to the coast, which differed from that reported by Ray and Egbert (2004) who pointed out that for the Mexican Pacific coast the S1 ellipses should be directed parallel to the coasts (direction NW-SE). Among the semi-diurnal constituents, the M2 was greater than the rest, reaching 16 cm s−1 in the surface layer and about 8 cm s−1 in the rest of the water column. The K1 and O1 harmonics prevailed among the diurnal gravitational tidal components determining the diurnal tidal currents deeper than 15 m depth. These harmonics did not show considerable changes either in magnitude of maximum variance axes or in ellipse direction

below 20 m in depth (Fig. 4), which showed jointly with Fig. 3 their barotropic behaviour. Diurnal variability may also cause sea surface gradients perpendicular to the coast, which can affect barotropic flow on the shelf. The baroclinic tidal currents were sensitive to bathymetry (Rosenfeld, 1988) and could be observed in the patterns of the semidiurnal harmonics. Several previous studies (see Introduction), analysing the annual cycle of thermohaline structure and geostrophic balance off the southern part of the BCP, have suggested the existence of more or less stable northward fluxes along the continental slope in summer–autumn that transported tropical and equatorial waters to at least the northern part of the Gulf of Ulloa (27◦ N). Fig. 8 shows the geostrophic currents in this region for spring (April), summer (July and August) and autumn (October) 2011. In April (Fig. 8a) the currents formed a southward flow with a meander of the CCS off Bahía Magdalena. In July (Fig. 8b) the dynamic situation was not stable with a weak northward flow up to 5 cm s−1 . In August this flux (Fig. 8c) intensified (up to 15 cm s−1 ) and reached the Gulf of Ulloa. In October (Fig. 8d) the flow lowered its intensity to 10 cm s−1 , but it broadened offshore. Thus, Fig. 8 shows stable currents northward during the summer months. The geostrophic currents were calculated with respect to a reference level of 1000 db, and they did not reflect dynamic processes on the continental shelf. The main shelf hydrodynamic processes are formed within the first 30–50 km offshore, generally under the direct forcing of local winds and associated upwelling. In the area of study, according to Ekman’s theory, a flux off the coast exists at the surface layers and a compensatory flux toward the coast forms coastal upwelling. The vertical current structure reflected these movements (Huyer, 1983; Ramp and Abbot, 1998). The measurements in this study were taken in the coastal zone 15 km off the coast at a depth of 100 m. Therefore, the influence of local processes, such as local wind and associated upwelling were expected to be strong. Fig. 7 shows the stable northward fluxes only in July and beginning of August. In these

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Fig. 9. Annual cycle of monthly mean currents in the upper 30-m layer in the oceanic zone of the southern Baja California Peninsula, Mexico from February 2011 to Jan 2012. Colour reflects sea surface temperature. Currents and SST data were calculated using the HYCOM model.

months, Zaitsev et al. (2014) observed the presence of tropical waters, exactly off Bahía Magdalena. In autumn the flow parallel to the coast varied its direction from southward to northward depending on the mesoscale variability of local winds. Fig. 9 shows the annual cycle of monthly mean currents averaged in the upper 30-m layer from Feb 2011 to Jan 2012. Currents and SST data were calculated using the HYCOM model. In the winter–spring period from December to May, the surface currents on the southern Pacific shelf of the Baja California Peninsula were southward with maximum speeds (up to 0.5 m s−1 ) in April. Favourable conditions for the development of coastal northward flow occur in the summer–autumn period from July to September when warm and salty subtropical waters invade along the coast of the peninsula to Punta Eugenia, filling the entire shelf of Ulloa Bay. The dynamics and spatial distribution of these subtropical waters are well traced by SST starting in July when they are still relatively cool off the BMA lagoon to their maximum values in August and September. The remaining months are transitional when the currents are unstable. It is important to note that the monthly mean current pattern does not quite accurately reflect the variability of the northdirectional flow, since it has mesoscale (synoptic) variability (Fig. 7). In particular, mesoscale eddies, mainly cyclonic, are sometimes observed at the study area in the summer–autumn period (Fig. 8c). The question arises: can these eddies be the cause

of northward shelf currents? Figs. 8 and 9 show that the observed cyclonic eddies cannot be the direct cause of northward flows. Firstly, there are many scenarios when northward flows develop in the absence of mesoscale eddies. And secondly, these eddies are located on the abyssal, far enough from the coast to affect the near coastal shelf. More or less stable cyclonic eddies capable of contributing to northward coastal currents were observed north of the study area, in the area of Punta Eugenia (Soto-Mardones et al., 2004). Conclusions The analyses of the currents recorded with an ADCP deployed off the Bahía Magdalena lagoon system indicated that tidal movements at the 85-m upper layer represented 22.3% of the total energy of currents; 14.8% was associated with diurnal movements and 7.5% with semidiurnal movements. The diurnal tidal currents that intensified at the 15-m surface layer contrasted notably with semidiurnal currents that gradually decreased with depth. Wind forcing (sea breeze) provided the significant intensification, not only for diurnal harmonic S1 up to 34 cm s−1 but also for the semidiurnal harmonic S2 up to 14 cm s−1 . These two ‘‘radiational’’ harmonic S1 and S2 determined a vertical speed gradient in the upper layer. On the contrary, all the gravitational tidal movements below the depth of 30 m, including diurnal currents,

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displayed the almost barotropic pattern with tide ellipses that varied little with depth. The diurnal tidal currents are determined by three principal components of gravitational tide (K1 , O1 and P1 ) and by the strong ‘‘radiational’’ S1 component. Semidiurnal currents are formed from three gravitational components (M2 , K2 and N2 ) and a ‘‘radiational’’ S2 component. The analyses of the residual currents showed that the water fluxes on the continental shelf were parallel to the coast and directed preferably southward in winter and spring due to the predominant winds. In summer (July–beginning of August), the northward flow was observed with intensities up to 45 cm s−1 at the 15-m surface layer and up to 15–25 cm s−1 in the rest of the water column in an almost barotropic mode. The residual currents showed mesoscale variability with time periods from three to five days in winter–spring, and from one to two weeks in summer–autumn. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research was supported by the project of SEMARNATCONACyT ‘‘Servicios ambientales del ecosistema costero de Baja California en relación al cambio climático’’ (No. 2008-C01-107267, directed by G. Gaxiola-Castro, unfortunately recently deceased) and the Oceanology Department of CICESE (Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California). The authors are grateful to SNI (National Researcher System in Mexico) for additional support for OZ and CONACyT (National Council for Science and Technology in Mexico) for a doctoral scholarship for JLC. We gratefully acknowledge A. B. Rabinovich, A. Trasviña Castro and Ju. Cepeda Morales for valuable discussions and support during this study. We also thank the Captain and crew of B/O Francisco de Ulloa for their help during research cruisers, D. Fischer for translation and editorial services and the anonymous reviewers for their valuable comments. References Aguad, G., Remiche, D., Gilson, J., 2011. The California current system in relation to the northeast pacific ocean circulation. Prog. Oceanogr 91 (4), 576–592. Avendaño-Ibarra, R., De Silva-Dávila, R., Ordóñez Guillén, F., Vásquez-López, G., 2010. Composición estacional de larvas de peces frente a Baja California Sur (primavera y otoño de 2003). In: DináMica Del Ecosistema PeláGico Frente a Baja. Instituto Nacional de Ecología, pp. 129–147. Bograd, S.J., Lynn, R.J., 2003. Long-term variability in the southern California current system. Deep Sea Res. II 50 (14–16), 2355–2370. Bowden, K.F., 1983. Physical Oceanography of Coastal Waters. In: Ellis Horwood series, Ed., John Wiley & Sons, NJ, p. 299. Cervantes-Duarte, R., Hernández-Trujillo, S., 1989. Características hidrográficas de la parte sur de la corriente de California y su relación con algunas especies de copépodos en 1983. Inv. Mar. CICIMAR 4 (2), 211–224. Cervantes-Duarte, R., Prego, R., López-López, S., Aguirre-Bahena, F., OspinaAlvarez, N., 2013. Annual patterns of nutrients and chlorophyll in a subtropical coastal lagoon under the upwelling influence (SW of baja California peninsula). Estuar. Coast. Shelf Sci. 120, 54–63. Durazo, R., 2009. Climate and upper ocean variability off Baja California, Mexico: 1997-2008. Prog. Oceanogr. 83 (1–4), 361–368. Durazo, R., Baumgartner, T.R., 2002. Evolution of oceanographic conditions off Baja California: 1997-1999. Prog. Oceanogr. 54 (1), 7–31. Durazo, R., Gaxiola-Castro, G., Lavaniegos, B., Castro-Valdés, R., Gómez-Valdés, J., Mascarenhas, A., 2005. Condiciones oceanográficas frente a la costa occidental de Baja California, 2002-2003: Influencia de un El Niño débil y del incremento de agua subártica. Cienc. Mar. 31 (3), 537–552. Durazo, R., Ramírez-Manguilar, A.M., Miranda, L.E., Soto-Mardones, L.A., 2010. Climatología de variables hidrográficas. In: Gaxiola-Castro, G., Durazo, R. (Eds.), DináMica Del Ecosistema PeláGico Frente a Baja California 1997–2007. Instituto Nacional de Ecología, Centro de Investigación Científica y Educación Superior de Ensenada, Universidad Autónoma de Baja California, pp. 25–57.

9

Emery, W., Thomson, R., 1997. Data Analysis Methods in Physical Oceanography). Pergamon, New York, p. 634. Foreman, M., 2004. Manual for Tidal Heigths Analysis and Prediction. Pacific Marine Science Report, 77-10, Institute of Ocean Sciences, Patricia Bay. Sidney, BC. Funes-Rodríguez, R., Hinojosa-Medina, A., Aceves-Medina, G., JiménezRosenberg, S.P., Bautista-Romero, J., 2006. Influences of El Niño on assemblages of mesopelagic fish larvae along the Pacific coast of Baja California Sur. Fish. Oceanogr. 15 (3), 244–255. http://dx.doi.org/10.1111/j. 1365-2419.2005.00388.x. Gay, P., Chereskin, T., 2009. Mean structure and seasonal variability of the poleward undercurrent off southern California. J. Geophys. Res: Oceans 114 (C2), 2156–2202. Gonella, J., 1972. A rotary-component method for analysing meteorological and oceanographic vector time series. Deep Sea Res. Oceanogr. Abstr. 19 (12), 833–846. González-Rodríguez, E., Trasviña Castro, A., Gaxiola-Castro, G., Zamudio, L., Cervantes-Duarte, R.y., 2012. Net primary productivity, upwelling and coastalcurrents in The Gulf of Ulloa, Baja California, México. OceanScience 8 (4), 703–711. Hickey, B.M., 1979. The California current system-hypotheses and facts. Prog. Oceanogr. 8 (4), 191–279. Hickey, B.M., 1998. Coastal oceanography of western north america from the tip of baja California to vancouver island. In: The Sea, the Global Coastal Ocean, Regional Studies and Synthesis, Vol. 11. John Wiley and Sons, Inc., pp. 345–393. Huyer, A., 1983. Coastalupwelling in the California currentsystem. Prog. Oceanogr. 12, 259–284. Kulikov, E.A., Rabinovich, A.B., 1983. Radiational tides in the ocean and atmosphere. Trans. (Doklady) USSR Acad. Sci., Earth Sci. Sect. 271 (5), 221–225. Kulikov, E.A., Rabinovich, A.B., Carmack, E.C., 2004. Barotropic and baroclinic tidal currents on the Mackenzie shelf break in the southeastern Beaufort Sea. J. Geophys. Res.: Oceans 109 (C5), C05020, 18 pp. Linero-Cueto, J.R., 2014. Procesos Hidrofísicos En la Plataforma Pacífica Del sur de la Península de Baja California (Thesis). Instituto Politécnico Nacional. Centro Interdisciplinario de Ciencias Marinas, La Paz, B.C.S. México, p. 172. Lynn, R., Simpson, J., 1987. The California current system: The seasonal variability of its physical characteristics. J. Geophis. Res. Oceans 92 (C12), 12947–12966. Martínez-López, A., 1993. Distribución espacial del fitoplancton asociada con frentes en la costa occidental de Baja California Sur. Investig. Mar. CICIMAR 8 (2), 71–86. McClatchie, S., 2014. Regional fisheries oceanography of the California currentsystem. In: The CalCOFI Program. Springer, p. 235. Mooers, C., 1973. A technique for the cross spectrum analysis of pairs of complex-valued time series, with emphasis on properties of polarized components and rotational invariants. Deep Sea Res. Oceanogr. Abstr. 20 (12), 1129–1141. Munk, W.H., Cartwright, D.E., 1966. Tidal spectroscopy and prediction. Philos. Trans. R. Soc. Lond. A 259, 533–581. Murillo-Murillo, I., Cervantes-Duarte, R., Gaxiola-Castro, G., López-López, S., Aguirre-Bahena, F., González-Rodríguez, E., Hernández-Sandoval, F., 2013. Variabilidad de la productividad primaria y de pigmentos fotosintéticos en una zona de surgencias de la región sur de la Corriente de California. CICIMAR Oceánides 28 (1), 23–36. Pawlowicz, R., Beardsley, B., Lentz, S., 2002. Classical tidal harmonic analysis with error analysis in MATLAB using T_TIDE. Comput. Geosci. 28 (8), 929–937. Pidgeon, E.J., Winant, C.D., 2005. Diurnal variability in currents and temperature on the continental shelf between central and southern California. J. Geophys. Res. V110 (C03024), http://dx.doi.org/10.1029/2004JC002321. Pugh, D.T., 1987. Tides, Surges and Mean Sea-Level. John Wiley, Hoboken, N.J, p. 472. Rabinovich, A.B., Candella, R.N., Thomson, R.E., 2013. The open ocean energy decay of three recent trans-Pacific tsunamis. Geophys. Res. Lett. http://dx. doi.org/10.1002/grl.50625. Rabinovich, A.B., Medvedev, I.P., 2015. Radiational tides at the southeastern coast of the Baltic Sea. Oceanology 55 (3), 357–365. http://dx.doi.org/10.1134/ S0001437015030133. Ramp, S.R., Abbot, C.L., 1998. The vertical structure of currents over the continental shelf off Point Sur, CA during spring 1990. Deep Sea Res. II 45, 1443–1470. Ray, R.D., Egbert, G.D., 2004. The global S1 tide. J. Phys. Oceanogr. 34 (8), 1922–1935. Reid, J.L., Schwartzlose, R.A., 1962. Direct measurements of davidson current off central California. J. Geophys. Res. 67 (6), 2491–2497. Rosenfeld, L.K., 1988. Diurnal period wind stress and current fluctuations over the continental shelf off Northern California. J. Geophys. Res. 93 (C3), 2257–2276. Rosenfeld, L.K., Shulman, I., Cook, M., Paduan, J., Shulman, L., 2008. Methodology for a regional tidal model evaluation, with application to central California. Deep-Sea Res. II http://dx.doi.org/10.1016/j.dsr2.2008.08.007.

10

J.R. Linero-Cueto and O. Zaytsev / Regional Studies in Marine Science 34 (2020) 101165

Salinas-González, F., Pinet, R., 1991. Corrientes geostróficas frente a Bahía Magdalena Baja California Sur, México. Investig. Mar. CICIMAR 6 (2), 251–257. Song, Y.T., Chao, Y., 2004. A theoretical study of topographic effects on coastal upwelling and cross-shore exchange. Ocean Model. 6 (2), 151–176. Soto-Mardones, L., Parés-Sierra, A., Garcia, J., Durazo, R., Hormazabal, S., 2004. Analysis of the mesoscale structure in the IMECOCAL region (off Baja California) from hydrographic, ADCP and altimetry data. Deep Sea Res. II 51 (6–9), 785–798. Strub, P.T., James, C., 2000. Altimeter-derived variability of surface velocities in the California Current System: 2, Seasonal circulation and eddy statistics. Deep Sea Res. II 47 (5–6), 831–870. Todd, R.E., Rudnick, D.L., Mazloff, M.R., Davis, R.E., Cornuelle, B., 2011. Poleward flows in the southern California current system: Glider observations and numerical simulation. J. Geophys. Res. 116 (C2), C02026. Turrent, C., Zaitsev, O., 2014. Seasonal cycle of near-surface diurnal winds over the Bay of La Paz (México). Bound.-Lay. Meteorol. 151 (2), 353–371. http://dx.doi.org/10.1007/s10546-014-9908-4.

Zaitsev, O., Trasviña Castro, A., Linero-Cueto, J.R., Gaxiola-Castro, G., CepedaMorales, J., 2014. Condiciones oceanográficas en la plataforma continental frente a bahía Magdalena (México) en 2011-2012. Cienc. Mar. 40 (2), 89–112. Zaitsev, O., Sánchez-Montante, O., Robinson, C., 2007. Características del ambiente hidrofísico de la plataforma continental y zona oceánica adyacente al sistema lagunar. In: Estudios Ecológicos En BahÍa Magdalena. IPN_CICIMAR, La Paz, Baja California Sur, pp. 29–43. Zaytsev, O., Cervantes-Duarte, R., Montante, O., Gallegos-Garcia, A., 2003. Coastal upwelling activity on the pacific shelf of the baja California peninsula. J. Oceanogr. 59 (4), 489–502. Zaytsev, O., Rabinovich, A., Thomson, R., Silverberg, N., 2010. Intense diurnal surface currents in the Bay of La Paz, Mexico. Contin. Shelf Res. 30 (6), 608–619. Zetler, B.D., 1971. Radiational ocean tides along the coasts of the United States. J. Phys. Oceanogr. 1 (1), 34–38. http://dx.doi.org/10.1175/1520-0485.