Sea Surface Heat Fluxes and Fortnightly Modulation of the Surface ...

Journal of Coastal Research

29

6

1400–1412

Coconut Creek, Florida

November 2013

Sea Surface Heat Fluxes and Fortnightly Modulation of the Surface Temperature within the Ballenas Channel, Gulf of California ‡ ´ ´ †, Rubén Castro‡, E. Santamar´ıa-del-Angel A. Mart´ınez-D´ıaz-de-Leon , I. Pacheco-Ruiz†, and R. Blanco-Betancourt† † Instituto de Investigaciones Oceanológicas Universidad Autónoma de Baja California Carretera Tijuana-Ensenada, km 105 Ensenada, Baja California, México [email protected]

‡ Facultad de Ciencias Marinas Universidad Autónoma de Baja California Carretera Tijuana-Ensenada, km 105 Ensenada, Baja California, México

ABSTRACT ´ Mart´ınez-D´ıaz-de-León, A.; Castro, R.; Santamar´ıa-del-Angel, E.; Pacheco-Ruiz, I., and Blanco-Betancourt, R. 2013. Sea surface heat fluxes and fortnightly modulation of the surface temperature within Ballenas Channel, Gulf of California. Journal of Coastal Research, 29(6), 1400–1412. Coconut Creek (Florida), ISSN 0749-0208. Sea surface temperature (SST) and meteorological information from July 2004 to July 2005 for Alcatraz Bay, which is located within the Ballenas Channel (BC) in the Gulf of California, was used to investigate the occurrence of sudden and drastic drops of almost 78C in the SST. This occurrence was previously observed in the analysis of SST time series that were recorded simultaneously in six bays along the western coast of the BC. Contrary to previous observations, wind speed showed a weak seasonal variability, with a directionality that departed from the typical monsoonal behavior of wind throughout the Gulf. Although the presented ocean surface heat flux calculations were obtained from coastal data, the results are in agreement with previous estimates, as the net heat flux mainly went into the sea for the whole year, with an annual mean heat gain of 197 W m2. SST analysis showed that along with a marked seasonal cycle, there are also four main periods of variability: quarter-diurnal, semidiurnal, diurnal, and fortnightly, corresponding to the main timescales of variability induced by tides. The sudden and drastic drops observed in the SST are clearly related to the intensification of the vertical mixing that is induced by tides and are enhanced at a fortnightly tidal period. MODIS-Aqua SST imagery was used as a supporting, indirect evidence of the mesoscale representativeness of our results, which we suggest can be considered valid for the whole BC.

ADDITIONAL INDEX WORDS: Tidal modulation, surface mixing, MODIS-Aqua.

INTRODUCTION The Ballenas Channel (BC), located in the archipelago or midriff islands zone of the Gulf of California (GC) (Figure 1), is a very productive and dynamic zone, and because of its oceanographic and meteorological conditions, it has been identified as an oceanographic province of its own (Lav´ın, Beier, and Badan-Dangon, 1997). Its vertical structure is controlled mainly by strong tidal currents and tidal mixing that are induced by large energy dissipation, both by friction against the bottom and in the interior of the fluid (Argote et al., 1995). Mixing is modulated at fortnightly, diurnal, semidiurnal (Paden et al., 1991; Souza, 1991), and quarter-diurnal (Mart´ınez-D´ıaz-de-León et al., 2006) tidal frequencies and is strongest close to the North Ballenas and San Lorenzo sills, even during neap tides. The North Ballenas and San Lorenzo sills are located to the north and south extremes of the channel, respectively (Marinone and Lav´ın, 2003). DOI: 10.2112/JCOASTRES-D-12-00189.1 received 21 September 2012; accepted in revision 21 December 2012; corrected proofs received 19 February 2013. Published Pre-print online 19 March 2013. Ó Coastal Education & Research Foundation 2013

One of the most outstanding characteristics of the BC is its year-round low sea surface temperature (SST) compared with the whole Gulf’s surface water temperatures that are clearly observed in SST imagery (Badan-Dangon, Koblinsky, and Baumgartner, 1985; Bray and Robles, 1991; Paden, Abbott, and Winant, 1991; Soto-Mardones, Marinone, and Pares, 1999). The outcropping of low SSTs, which is mainly associated with intense tidal mixing, plays a major role in controlling the biology of the BC. In the BC, there are high chlorophyll (.2 mg m3) and nutrient (PO4, 1.0–2.0 lM; NO3, 12–16 lM; SiO4, 25– 32 lM) concentrations, thus making this water some of the ´ most productive of any ocean in the world (Alvarez-Borrego et al., 1978; Flores-de-Santiago et al., 2007; Gaxiola-Castro et al., ´ ´ 1995; Santamar´ıa-del-Angel, Alvarez-Borrego, and MullerKarger, 1994). Furthermore, the spatial and temporal distribution and concentration variability of chemical variables in this region, such as cadmium and total CO2, have been attributed to mixing that is induced by tides (Hernandez-Ayón et al., 2007). Another mechanism that has recently been proposed to explain the year-round low SST in the BC is that there is a deep inflow to the BC and an outflow in the upper levels through the San Lorenzo and North Ballenas sills, where the upwelling associated with the surface divergence results in

Fortnightly Modulation of the SST

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in Alcatraz Bay, which is located on the NW coast of the BC (Figure 1), just 10 km S of the North Ballenas sill, where it is known that strong tidal mixing occurs (Marinone and Lav´ın, 2003). Measurements began in July 2004 and ended in July 2005. This database also provides more insight into the impact that the tide-induced, fortnightly modulation can have on the SST and sea surface heat fluxes within the BC.

DATA

Figure 1. Geographical location of Alcatraz and de los Angeles bays (black dots). The thick lines starting in the dots marking the position of the bays indicate the annual mean wind velocity vector at each bay and the ellipse represents the variability of mean wind velocity vector. A wind velocity scale is given at the top center of the figure. The segmented line between Angel de la Guarda Island and the Baja California Peninsula indicates the transect along which the image-derived SSTs were obtained to generate Figure 10. Elevation and bathymetric contours are given in meters.

the low SST characteristic of the area (López, Candela, and Argote, 2006). In studying the spatial and temporal variability of the SST within the BC, Mart´ınez-D´ıaz-de-León et al. (2006), hereinafter referred to as MDL-06, analyzed a year-long temperature time series that was recorded simultaneously in six bays distributed along the western coast of the BC. In addition to a marked seasonal cycle, MDL-06 observed drastic drops of almost 78C in water temperature during September and October and a predominant fortnightly modulation in the temperature variability for all bays. Although the coherent SST variability observed in all bays suggested that the spring–neap tide cycle plays an important role in inducing temporal SST variability in the BC, the drastic drops in the SST that were observed could not be attributed to specific physical processes, since they may be attributed to either a very intense loss of heat through the sea surface or to intense vertical mixing, or a combination of both processes. In this work, in contrast to MDL-06, we attempt to acquire a more complete oceanographic and meteorological database, which, in addition to the SST, allowed for the calculation of sea surface heat fluxes, which enables us to further investigate the possible causes of the drastic drops in the SST that were observed mainly during September and October. However, this only allowed for a single bay to be studied, not all six bays that were considered previously in MDL-06. Because this study is closely related to a research project that focused on determining the biomass variation of economically important seaweeds, and because previous studies showed that the strongest biomass variation, with almost complete disappearance by late summer, occurs in Alcatraz Bay (PachecoRuiz et al., 2002, 2003), we selected to carry out measurements

Meteorological information was acquired using an automatic weather station from Aanderaa Instruments (model 2700). This weather station was installed on the coast at a distance of 15 m from the coastline and was programmed to measure wind speed (m s1), wind direction (azimuth), atmospheric pressure (mbar), air temperature (8C), relative humidity (%), and net radiation (difference between incoming short-wave radiation and outgoing long-wave radiation, W m2) at 10 m above sea level every half hour. Oceanographic variables were obtained using a self-contained StowAway temperature data logger produced by Onset Computer Corporation, which was installed at an average depth of 5 m, and an Aanderaa Instruments’ pressure sensor, which was bottom-mounted at a depth of 25 m and provided information of sea level (m). As supporting, indirect evidence of the mesoscale representativeness of our in situ observations, daily SST (8C) and chlorophyll a (Chl a in mg m3) images, which were derived from MODIS-Aqua, for the archipelago or midriff islands zone of the GC were compiled for the same period (July 2004 to July 2005). Images correspond to the local area coverage product, with a spatial resolution of 1 km.

ESTIMATION OF SURFACE HEAT FLUX The net sea surface heat flux Qn (W m2) was calculated as the sum of three components: Qn ¼ Qt þ Qs þ Q1 ; where Qt is the net radiation, or the difference between the incoming and outgoing radiation at the earth’s surface, Qs is the sensible heat flux due to air–sea temperature differences, and Ql is the latent heat flux due to water vapor transport. In all cases, positive values indicate heat flux into the ocean. Net radiation was provided directly by the weather station and sensible and latent heat fluxes were estimated using the standard bulk formulae from the tropical ocean global atmosphere/coupled ocean atmosphere response experiment (Fairall et al., 1996): Qs ¼ qCp Ch UðTa SSTÞ; and Q1 ¼ qLe Ce Uðqa qs Þ; where q is the air density [kg m3], Cp is the specific heat of the air (J kg1 C1) at constant pressure, Ch is the sensible heat transfer coefficient (dimensionless), U is the wind speed [m s1], Ta is the air temperature [8C], SST is the sea surface temperature [8C], Le is the latent heat of evaporation of seawater [J kg1], Ce is a latent heat transfer coefficient, qa is

Journal of Coastal Research, Vol. 29, No. 6, 2013

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Mart´ınez-D´ıaz-de-León et al.

the specific humidity at air temperature (g kg1), and qs is the specific humidity at SST [g kg1].

RESULTS AND DISCUSSION Meteorological Information Time series of the observed meteorological variables are shown in Figure 2. The high variability of the diurnal fluctuations has been filtered using a low-pass Lanczos filter (Lanczos, 1956), with a cutoff period of 48 hours. For all variables except the wind velocity components, the seasonal trend is overplotted with a thick line and is calculated as an annual mean plus an annual harmonic. The explained variance (EV) was obtained by best fitting the seasonal trend to the data and is reported as a percentage in each plot (Ripa, 2002). Wind velocity components were rotated to the axis of maximum variance of the wind speed (Figure 2a). Mean wind speed and variability ellipse of the wind are shown in Figure 1. It can be observed that the ellipse is almost circular and that its major axis (maximum variance) does not greatly differ from the west. Negative values of the wind speed component flowing along the major axis of the ellipse (Figure 2a) can be assumed to represent wind that is approaching from the west and flowing offshore, practically perpendicular to the coast. Positive values of the wind speed component flowing along the minor axis of the ellipse represent wind approaching from the northwest. In contrast to the other meteorological variables, wind speed did not show a clear seasonal signal, which differs from the results of previous studies for the GC (Badan-Dangon et al., 1991; Merrifield and Winant, 1989; Paden, Winant, and Abbott, 1993) that report a high seasonal wind speed signal. This discrepancy can be attributed to the fact that we are analyzing coastal wind, which may be influenced by a high dominance of local forcing effects. The behavior of the wind direction was also interesting, as a dominance of the winds from the west for the whole year of data was observed (Figure 2a). This result is contrary to the typical polarization of the wind along the axis of the gulf (monsoonal behavior), which was observed in the analysis of annual-scale wind data, and establishes that in the GC, the wind blows from the southeast during summer and from the northwest during the rest of the year (Badan-Dangon et al., 1991). Although some authors (Merrifield and Winant, 1989; Paden, Abbott, and Winant, 1991; Zamudio, Metzer, and Hogan, 2011) have mentioned that the region between Angel de la Guarda Island and Baja California is often characterized by bursts of cross-channel winds, little attention has been given to the directional behavior of coastal winds in this zone. The observed dominance of the winds from the west in this region could be highly influenced by the topographic configuration of the mountain ridges that are present along the Baja California peninsula (Figure 1). These ridges shield most of the gulf from the cool environment of the lower layer of the atmosphere over the Pacific Ocean, thus producing a climate that is more Mediterranean than marine over the GC (Reyes and Lav´ın, 1997). However, the elevation of the mountains decreases around Alcatraz Bay from 800 m or more to less than 200 m. This gap could be channeling the winds from the

west, thus opening the BC to the oceanic influence from the Pacific. The observed behavior of the wind direction in this region of Baja California seems to be confirmed by the analysis of wind data acquired at de los Angeles Bay simultaneously with the wind data analyzed here. The de los Angeles Bay is 30 km south of Alcatraz Bay, and although the data express stronger wind speeds, a dominance of the winds from the W and SW can be observed in the mean wind direction and variability ellipse of the wind speed (Figure 1). The influence of the winds from the west in this region could create a different effect in the atmospheric–marine boundary layer (AMBL) from the effect produced by the monsoonal winds. However, we must not forget that winds blowing from the Pacific toward the BC could be modified by adiabatic effects, as the winds that cross the Peninsula toward the BC must ascend and descend the mountains. Though the mountains are not too high in this zone, this could still increase the wind temperature and reduce its humidity (Castro et al., 2006). On the other hand, the observed departure of the wind direction from the typical monsoonal behavior over the GC seems to influence the seasonal variability of the relative humidity (Figure 2d). Despite expressing a weak seasonal signal, humidity shows a phase difference of almost 6 months with regard to the phase of the seasonal variability previously reported for the humidity in the GC (Badan-Dangon et al., 1991; Paden, Winant, and Abbott, 1993). In other words, instead of observing maximum values of humidity during the summer when, in general, the monsoonal winds flow from the SE and advect tropical, warm, humid air over the Gulf, the maximum values of the humidity occur during the winter when we would expect winds to generally flow from the southwestern United States, thus bringing cold and dry air over the Gulf. This phase difference in the humidity can be explained if we consider that according to the observed dominant direction of the wind (Figures 1 and 2a), air during winter flows mainly from the W and NW. This air, despite being as cold as the typical monsoonal wind, is more humid, thus confirming perhaps the oceanic influence from the Pacific on this region. The annual atmospheric seasonal heating and cooling effect was evident for some of the meteorological variables, and is expressed in the large seasonal fluctuation of the air temperature (83% EV), atmospheric pressure (71% EV), and net radiation (82% EV) (Figure 2). Minimum and maximum monthly mean values for air temperature and net radiation occurred during winter and summer, respectively, whereas for air pressure, the minimum monthly mean was observed in the summer and the maximum was observed during the winter, when the pressure variability is also maximum (Table 1).

Tide-Induced Sea-Level Variability Sea level and the maximum daily tidal range are shown in Figure 3. Calculation of the form number (Carbajal and Backhaus, 1998), defined as F ¼ (O1 þ K1)/(S2 þ M2), where O1 and K1 are the amplitudes of the principal diurnal tides and M2 and S2 are the amplitudes of the semidiurnal tides, reveals that tides are of the mixed type (F ¼ 0.57) and that the semidiurnal tide is the dominating signal. This result is not surprising considering that the GC is almost resonant at this semidiurnal frequency (Carbajal and Backhaus, 1998; Marinone and Lav´ın,



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Figure 2. Meteorological observations for Alcatraz Bay, where high variability of the diurnal fluctuations has been filtered using a low-pass Lanczos filter. For variables b to e, the heavy line represents a seasonal trend, and EV (explained variance) expresses how much of the variance of the data is explained by the seasonal variability.

2005). The minimum tidal range occurred by the end of fall (0.7 m in November), and the maximum occurred during winter (3.9 m in January). The larger daily tidal range difference occurred in February (2.9 m), and the minimum occurred during September (0.9 m). Previous studies in this zone of the GC show that the near resonance of the semidiurnal tide, coupled with abrupt changes in bottom depth and cross-sectional area, causes very strong tidal currents (1–3 m s1). This leads to the dissipation of tidal energy by bottom friction and at the interior of the fluid, which induces vertical mixing and the formation of internal waves and high-frequency solitons, particularly in the vicinity of the sills located in the Ballenas and Salsipuedes channels (Argote et al., 1995; Filonov and Lav´ın, 2003; Fu and Holt, 1984; Marinone and Lav´ın, 2005).

Ocean Surface Heat Fluxes Sensible (Qs), latent (Ql), and net (Qn) heat fluxes are presented in Figure 4, where the high variability of the diurnal fluctuations has been filtered (cutoff period of 48 h). Monthly means and corresponding standard deviations for these variables are shown in Table 1. Although not directly comparable with previous results of heat fluxes for the GC because of differences in space and time resolutions of the data and formulae used, our results are in good agreement with previously reported results (Castro, Lavin, and Ripa, 1994; Lav´ın and Organista, 1988; Paden, Winant, and Abbott, 1993). A clear seasonal trend is observed in all heat fluxes, where the main contribution of heat to the ocean resulted from net radiation (Figure 2e). Because the meteorological station only provided net radiation, we were unable to distinguish between


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Table 1. Raw data monthly mean and standard deviation of meteorological variables (Ws ¼ wind speed; Wd ¼ wind direction; Ta ¼ air temperature; Pa ¼ atmospheric pressure; Rh ¼ relative humidity; NR ¼ net radiation), sea surface temperature (SST), and heat fluxes (Qs ¼ sensible; Ql ¼ latent; Qn ¼ net heat flux) for Alcatraz Bay from July 2004 to July 2005. Month 2004 July August September October November December 2005 January February March April May June July

Ws (m s1)

Wd (Az)

Ta (8C)

Pa (mbar)

NR (W m2)

Rh (%)

SST (8C)

Qs (W m2)

Ql (Wm2)

Qn (W m2)

4.4 4.3 4.2 4.0 3.8 3.7

6 6 6 6 6 6

1.5 2.1 2.4 2.0 2.3 2.2

225 228 228 260 292 295

6 6 6 6 6 6

83 88 108 64 61 65

29.5 30.7 27.7 23.0 18.5 16.7

6 6 6 6 6 6

3.2 3.2 3.5 3.5 2.7 2.9

1008 1010 1009 1011 1016 1018

6 6 6 6 6 6

1 1 2 3 4 3

33.8 38.0 52.0 48.9 54.3 52.8

6 6 6 6 6 6

14 20 18 10 13 14

380 328 275 239 180 158

6 6 6 6 6 6

437 402 371 336 278 245

25.2 25.9 26.4 23.6 20.6 16.6

6 6 6 6 6 6

0.5 0.9 1.9 1.7 1.6 0.8

21 20 6 5 13 6

6 6 6 6 6 6

23 21 20 20 16 18

175 111 128 147 129 90

6 6 6 6 6 6

98 92 98 83 91 65

226 238 154 87 37 61

6 6 6 6 6 6

401 420 385 363 293 256

2.3 3.0 4.1 4.0 4.0 4.7 3.9

6 6 6 6 6 6 6

1.9 1.8 2.3 2.5 2.2 2.3 1.8

288 266 274 263 252 248 219

6 6 6 6 6 6 6

87 78 76 86 104 78 92

16.3 16.2 18.2 20.0 22.7 26.0 29.7

6 6 6 6 6 6 6

2.5 2.7 2.9 3.3 3.7 5.0 3.4

1017 1015 1015 1013 1010 1007 1008

6 6 6 6 6 6 6

3 2 3 3 2 1 1

61.5 63.8 46.9 41.4 45.2 42.9 37.6

6 6 6 6 6 6 6

17 12 15 15 17 19 22

161 185 284 327 360 374 325

6 6 6 6 6 6 6

258 288 383 422 438 444 407

16.0 15.9 15.1 15.2 16.4 20.8 22.5

6 6 6 6 6 6 6

0.7 0.7 0.6 0.7 1.2 1.7 0.8

0 2 14 18 20 19 19

6 6 6 6 6 6 6

11 11 19 25 25 25 23

44 42 55 45 32 77 157

6 6 6 6 6 6 6

38 31 47 45 39 64 82

117 145 244 301 349 316 287

6 6 6 6 6 6 6

260 291 394 430 451 467 425

the magnitude differences of the short- and long-wave radiations. However, it is well-known for the GC that the short-wave radiation contribution is much higher than that of the long-wave radiation, indicating that solar radiation is the main source of heat to the ocean (Castro, Lavin, and Ripa, 1994). The evaporative, or latent, heat flux was the main cooling agent. It was negative during the whole year, with maximum losses occurring during summer and fall (402 W m2 in October) and minimum losses occurring from around the middle of winter to the end of spring. In contrast, Qs was negative from October to December, with maximum heat loss (47 W m2) occurring by the end of November. The rest of the year, when net radiation is high (Figure 2e) and air temperature is higher than the SST (Figures 2b and 5a respectively), the sensible heat flux was positive, with its maximum occurring in May (58 W m2). In congruence with previous results, the net heat flux was mainly positive for the whole year, with a mean value of 197 W m2 (Figure 4c). However, we observed some events during the fall that lasted a few days where the ocean loses net heat. The values were as strong as 200 W m2 in November and 190 W m2 in December, although the monthly means for these months were positive, 37 and 61 W m2, respectively (Table 1). The sea’s maximum net heat gain occurred in spring (389 W m2 in May), when net radiation (Figure 2e) and sensible heat flux (Figure 4a) were maximum and the loss of latent heat was minimum (Figure 4b). Perhaps the main differences with regard to previous results are: (1) our data are very local, gathered within a bay in the BC; (2) the higher annual mean net heat that we obtained (197 W m2) in comparison with the values reported by Paden, Winant, and Abbott (1993), Lavin and Organista (1988) and Castro, Lavin, and Ripa (1994), which were of lower magnitude of 110– 120 W m2, 69.3 W m2, and ~100 W m2, respectively; and (3) other authors obtain negative monthly mean heat values for November and December, whereas in this work, the net heat flux monthly means were positive for the whole year. These discrepancies could be due, in part, to differences in the bulk formulae that were used to calculate heat fluxes, the type of data acquisitions, which includes in this work the measure-

ment of net radiation instead of calculating the short- and longwave radiations, and the spatial and temporal resolution of the data set. In fact, Clark (1967) found that the net heat flux that is calculated from individual observations results in as large as a 20% increase over fluxes that are calculated from monthly mean meteorological variables. A large part of this increase comes from the sensible and latent heat fluxes, which are strongly dependent on the wind speed and air–sea temperature differences. These are variables that, in our case, fluctuated largely in time.

Sea Surface Temperature With regard to SST (Figure 5a), a marked seasonal trend (EV ¼ 91%) was observed. This behavior has been well documented for the GC (Paden, Abbott, and Winant, 1991; Ripa and Marinone, 1989; Soto-Mardones, Marinone, and Pares, 1999, and references therein) and some of the bays located along the western coast of the BC (Mart´ınez-D´ıaz-de-León et al., 2006). As expected, the highest temperatures were recorded during the summer (29.38C in September), and the lowest were recorded in late winter and early spring (13.58C in April). In addition to the seasonal trend, the SST also showed other important temporal scales of variability, which are clearly resolved when a spectral analysis of the SST is performed. The variance-preserving spectrum (Figure 5c) shows peaks at the quarter-diurnal, semidiurnal, diurnal, and fortnightly periods. Furthermore, the SST also shows a high variability, particularly during September and October, when water temperature can drop up to 78C in a period of a few hours. This high variability is more evident when variability higher than 15 days is removed (Figure 5b). In discussing the different SST temporal scales of variability, we first focus on the 15-day filtered signal (Figure 5a). This signal shows a high connection with the variability of the net radiation (Figure 2e) during the year, which suggests the dominant effects of seasonal atmospheric heating and cooling. This connection is more clearly observed in Figure 6, where the 15-day-filtered signals of the air temperature and net heat flux have been also included. In general, it can be observed that the net radiation attains its maximum in June and then starts to decrease until the beginning of December, when it reaches its



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Figure 3. Tide-induced sea level (continuous line) and maximum daily tidal range (segmented line) for Alcatraz Bay.

minimum. After reaching its minimum, the net radiation monthly mean remains more or less constant for about 3 months, until the end of January, and then starts to increase again. In contrast, maximum values for net heat flux, air temperature, and SST are attained in June, August, and September, respectively (Figure 6). It seems that although air temperature responds faster to changes in net radiation, the SST continues to increase for about another month after the net radiation reaches it maximum. On the other hand, although net radiation starts increasing in January, the SST remains stable until the beginning of March, when then suddenly the SST starts to increase until it reaches its monthly maximum in September. This behavior clearly reflects the large heat capacity differences between the air and ocean water, but can also be a consequence of the fact that during winter the ocean surface mixing layer is much deeper than during the summer (Argote et al., 1995), such that during the winter the transfer of heat between the ocean and the atmosphere, which is needed to start the increase of the SST, must occur over a longer timescale than during the summer. The variability of the sea surface and air temperatures (Figure 6) shows that the AMBL was more or less stable (Ta . SST) during most of the winter and all of the spring and summer. During autumn, the AMBL remained practically unstable (SST . Ta). Because of its dependence on the air–sea temperature differences, the variability of the AMBL is greatly associated with the trend of the sensible heat flux (Figure 4a). This trend was negative during fall, meaning that the ocean lost sensible heat, but during the rest of the year, the sensible heat went into the ocean. In other words, we can expect that the atmospheric boundary layer during fall would be more forced by the ocean, inducing vertical convection and perhaps an increase in evaporation, which results in increased (negative) latent heat flux (Figure 4b). However, for the rest of the year, the sensible heat flux is into the ocean, inducing a more stable water column at the ocean surface. With reference to the fortnightly, diurnal, semidiurnal, and quarter-diurnal modes of variability of the SST (Figure 5c), and despite that the diurnal mode could be influenced by the day– night solar radiation cycle, we mainly attribute all of these

modes of variability to the manifestation on the SST of the vertical mixing induced by tides. As we have mentioned, tides in the Gulf are mixed, and there are considerable differences between the diurnal and semidiurnal components. Whereas the diurnal tide behaves like a standing wave whose amplitude does not vary greatly from the mouth to the head of the Gulf, the semidiurnal tide behaves as a progressive wave whose amplitude increases rapidly from the mouth of the Gulf toward the midriff islands and to the head of the Gulf (Filloux, 1973; Merrifield and Winant, 1989). Furthermore, the length of the Gulf is shorter than the wavelength of the semidiurnal tide, which makes the Gulf almost resonant at that frequency (Argote et al., 1995). This contributes to the increase in tidal amplitude, which, coupled with the abrupt changes in bottom depths and cross-sectional areas in the midriff island zone, causes strong vertical mixing (Mateos, Marinone, and Lavin, 2006). Particularly along the BC, most of the energy released by the barotropic tide is dissipated by the interaction of strong currents (~3 m s1 in the BC, Alvarez-Sanchez, BadanDangon, and Robles, 1984) with the bathymetry, mainly by bottom friction, and at the North Ballenas and Salsipuedes sills, through the formation of internal waves of various frequencies, ranging from internal tides to high-frequency solitons (Argote et al., 1995; Filonov and Lav´ın, 2003; Fu and Holt, 1984). Some of the energy released by tides in this region is used to induce vertical mixing, thus breaking down the stratification. Vertical mixing has long been thought to be associated with the presence of a low SST around the midriff islands, but especially along the BC. The presence of this low SST waters is more evident during the summer, when in general the Gulf’s surface water temperatures are high and its stratification is stronger. Because of the relevant role that the semidiurnal and diurnal tides play in breaking the stratification (Argote et al., 1995), it is not surprising that there is variability in the SST at the tidal frequencies, which is clearly resolved in the spectral analysis (Figure 5c). However, the low SST within the BC has been also attributed to another mechanism that considers that there is a deep inflow to the BC and an outflow in the upper levels through the San Lorenzo and North Ballenas sills, where the upwelling associated with


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Figure 4. Sensible (a), latent (b), and net (c) heat fluxes for Alcatraz Bay. Segmented line represents seasonal trend, and EV (explained variance) expresses how much of the variance of the data is explained by the seasonal variability.

the surface divergence results in the low SST characteristic of the area (López, Candela, and Argote, 2006). In terms of the quarter-diurnal mode, we assume that this mode is also associated with the semidiurnal tide and is the manifestation of the barotropic mixing induced by the M2, which works at the quarter-diurnal or M4 frequency. Because the induced turbulence is not affected by the direction of the M2, we should expect two maximums. The maximums should be at the flood and at the ebb of the tide, resulting in an M4 spectral peak even though the M2 is the responsible tide. This behavior has also been observed on the near shore of the Southern California Bight (Souza and Pineda, 2001). Particularly interesting is the significant role that the fortnightly mode plays in the total variance of the SST

Figure 5. (a) Raw, filtered, and seasonal trend time series of sea surface temperature (SST) for Alcatraz Bay. EV (explained variance) expresses how much of the variance of the data is explained by the seasonal variability, and r is the correlation coefficient between raw and filtered signals. (b) Comparison between high- and low-pass filtered SST. (c) Variancepreserving spectrum of SST, where a Hamming window was applied and the linear trend removed. Numbers indicate periods of relevant peaks.



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Figure 7. (Top) Tide-induced sea level in Alcatraz Bay from 7 September to 22 October 2004. Letters (a to f) indicate phases of the tide for which the sea surface temperature (SST) and surface chlorophyll a imagery shown in Figure 8 were acquired. (Bottom) High-pass filtered SST, cutoff period of 48 h. Figure 6. Seasonal variability of sea surface temperature (SST), air temperature (Ta), net radiation (NR), and net heat flux (NHF) for Alcatraz Bay. In all cases, the variability lower than 15 d has been filtered.

spectrum (Figure 5c). The variance of the fortnightly mode is as important as the variance of the semidiurnal mode, which is the most energetic tidal mode in this zone of the gulf. The relevance of the fortnightly modulation on the SST is also evident when the low-pass filtered signal, using a cutoff period of 15 days, is superimposed on the raw temperatures (Figure 5a) or, even further, when the low-pass filtered signal is plotted together with the SST after the seasonal variability has been filtered (high pass filtered using a 30d cutoff period, Figure 5b). In both cases, a fortnightly modulation of the SST is clearly observed, a modulation that occurs during the whole year, although it is more marked during the summer and beginning of fall. We attribute this scale of variability to the expression of the modulation induced by the spring and neap variability of the tides on the SST. Last, but not least important, we discuss the high SST variability observed from the beginning of summer (July), when the mean SST starts to increase, to the end of fall (November), just before the mean SST reaches its minimum values during the year (Figures 5a and b). However, the high variability of the SST is particularly evident and more extreme during September and October when the water temperature drops up to 78C in periods of time spanning a few hours. This behavior was previously noticed by MDL-06 in the analysis of a 1-year temperature time series that was recorded simultaneously in six bays along the 150-km western coast of the BC. As discussed by MDL-06, this result suggested that the forcing mechanism of this high SST variability has a regional effect and, consequently, could be affecting the entire BC. Nevertheless, in the analysis made by MDL-06, this high variability cannot be attributed to a particular forcing agent, such as a very intense loss of heat through the ocean surface, intense vertical mixing, or a combination of both processes. With regard to the first hypothesis that the observed high SST variability could be induced by an intense loss of heat through the ocean surface, our results are in congruence with

previous calculations that show that the net heat flux through the ocean surface during most of the year is mainly positive, into the ocean. However, there is a marked seasonal variability where the minimum net heat gain by the ocean occurs during the fall and the beginning of winter (Figure 4c). This behavior of the net heat flux results in a seasonal cooling of the ocean surface that modifies its thermohaline stratification. This modification increases the surface density, induces vertical convection, and, as a consequence, an outcropping of cold water to the surface is observed. This process is episodic and cannot be considered as responsible for the high SST variability observed particularly during September and October, especially if we consider that the variability observed in the SST occurs within periods of approximately 12 hours, or a semidiurnal tidal cycle. Other than ocean surface heat fluxes (sensible and latent heat flux, solar heat flux, and infrared radiation), SST anomalies may also result from variability in horizontal currents and vertical motions, diffusion, and vertical turbulent mixing. In particular, we found that the magnitude and time occurrence of the observed SST high variability clearly coincides with the spring cycle of the tides (Figure 7). This strongly suggests that this variability is also a manifestation of vertical mixing induced by tides, and its occurrence and intensity are modulated with a fortnightly periodicity, as is also the case for other tidal modes observed in the SST (Figure 5c). Vertical mixing induced by tides can be a complementary mechanism to the one proposed by López, Candela, and Argote (2006) that considers a persistent upwelling of deep waters within the BC to explain its low SST. Similar SST high variability in coincidence with spring tides has been reported for a southern bay of Taiwan (Lee et al., 1997), where sudden temperature drops ranging up to 98C occur in a few hours as the tide approaches lower low water. In typical spring periods these sudden drops in temperature are phase-locked with diurnal tides, occur daily, last several hours each, and generally diminish as the tide approaches the neap period. The authors consider that tidally induced vertical mixing resulting in sudden temperature drops of such a magnitude is quite rare, if not unique. However, our results


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clearly show that the GC is another place in the world where this occurs. Vertical mixing can be highly influenced by prolonged or strengthened stratification, as the higher the stratification, the more powerful the boundary, making it more difficult for the nutrient-rich, colder, deeper waters to reach the surface. This potential reduction in upwelling and mixing can have negative impacts, resulting in local or widespread biomass loss and changes in species composition. In the GC, density changes are more dominated by changes in temperature than in salinity (Argote et al., 1995; Bray, 1988; Paden, Abbott, and Winant, 1991), and in general, within the BC, the stratification exhibits a marked seasonal variability. This variability is lowest during the summer when net radiation is high, thus minimizing surface temperature gradients and potentially suppressing tidal mixing (Paden, Abbott, and Winant, 1991). In contrast, minimum stratification occurs during the winter when seasonal cooling takes place and tidal currents increase, thus reestablishing temperature differences between the archipelago region and elsewhere (Bray, 1988). In this context, it is interesting to notice the relevance of the fortnightly modulation that is induced by tides on the variability of the SST, particularly during the summer (Figures 5a and b) when ocean surface stratification is maximum (Argote et al., 1995). If we rationalize the variability of the SST in terms of the intensity of the tidal mixing, then the magnitude of the cooling during the periods of high variability of the SST (as large as 78C) would indicate a very strong mixing intensity. This strong mixing intensity is capable of breaking the marked ocean surface stratification that is characteristic of the summer, even though it has been documented that the strong stratification observed in this region during summer is capable of suppressing tidal mixing (Paden, Abbott, and Winant, 1991). Without a doubt, tide-induced mixing plays a relevant role, particularly during the summer and fall, in producing the most outstanding characteristic of the BC, which is having the lowest SSTs all year round, compared with the Gulf’s surface water temperatures, whose manifestation has been clearly observed in SST imagery (Badan-Dangon, Koblinsky, and Baumgartner, 1985; Bray and Robles, 1991; Paden, Abbott, and Winant, 1991; Soto-Mardones, Marinone, and Pares, 1999). It is also interesting that from the end of fall to the beginning of spring, the fortnightly tides are capable of inducing a modulation of the same periodicity on the SST (Figures 5a and b). However, the magnitude of the induced SST variability is not greater than 28C. This seems to confirm that the intense vertical mixing induced by the fortnightly tide occurs not only during the summer but all year round, even during the winter when a weak stratification occurs and the mixed layer is deeper. This suggests that even during the winter, the water column is not completely mixed. To investigate the possible synoptic effect of the results here inferred from the analysis of SST data acquired within a single bay on the west coast of the BC, subimages for the archipelago region of the GC that were coincident in time with the in situ measurements were extracted from daily MODIS-Aqua SST and Chl a images. Figure 8 (SST) and Figure 9 (Chl a) show some selected subimages corresponding to different phases of

the tide during September and October, when the SST variability is more drastic. The corresponding phase of the tide at noon on the day of acquisition of the subimages has been marked with a circle and identified with the letters a to f in the sea surface elevation variability that is induced by the tide for the corresponding months (Figure 7). Despite the high SST (.258C) associated with these months, it is possible to clearly observe the difference in SSTs between the subimages corresponding to the spring and neap tides, which show the occurrence of colder waters during spring tides. It is also interesting to observe how after spring tides, the intense solar heating associated with this time of year rapidly eliminates most of the SST gradients that are formed as a consequence of the upwelling of deep, colder water to the surface due to vertical mixing. The fact that the high variability observed in the SST is most probably a manifestation of vertical mixing, induced by tides, and is not due to an intense loss of heat through the sea surface has important biological consequences. This fact definitely implies that there is an increase in the supply of nutrients to the upper layers of the water column. This supply of nutrients seems to be suggested by the corresponding Chl a images (Figure 9), which show the biological effect of the upwelling of deeper, colder, richer water to the surface. Furthermore, the Chl a images seem to show how the vertical mixing induced at spring tides is not an exclusive process of the BC, but rather, occurs in most of the archipelago island region. On the other hand, the Chl a images seem to confirm the occurrence of pulses of increased surface concentrations of Chl a with a fortnightly periodicity, supporting the idea of the existence of a dominant effect of the spring and neap tidal cycle in the chlorophyll surface concentration in this region. Similar pulses of high Chl a for the midriff island region have been reported previously (Badan-Dangon, Koblinsky, and Baumgartner, 1985; Kahru et al., 2004; Paden, Abbott, and Winant, 1991). However, these works only speculate that the pulses could be associated with the fortnightly variation of the tidal energy that results in enhanced mixing, whereas our results seem to confirm that increased Chl a concentrations in this region of the GC are indeed associated with the strengthening of the vertical mixing produced by the fortnightly tides. Finally, we investigated the representativeness of our results with regard to the regional variability of the SST within the whole BC, considering that in situ temperature data analyzed here were acquired within a single bay in the NW coast of the channel. As supporting, indirect information we analyze the spatial and temporal variability of the SST within the BC from MODIS-Aqua SST images acquired from August 2004 to July 2005, the same period of time of the in situ data here analyzed. SSTs were extracted from the images for a transect of pixels along the BC. The transect was centered between the Baja California Peninsula and Angel de la Guardia Island (see Figure 1 for length and orientation of the transect). Figure 10 shows the temporal variability of the image-derived SSTs along the transect, where also in situ SSTs have been overplotted (black line). Scatter plots between image-derived SSTs and daily averaged in situ SSTs are shown in Figure 11 for the first (farthest north) and last (farthest south) pixels in the transect. The coefficient of correlation is indicated in each plot.



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Figure 8. Daily sea surface temperature (SST) imagery acquired for the archipelago or midriff islands zone of the Gulf of California during different phases of the tide from 7 September to 22 October 2004. Letters a to f identify the phase of the tide at 12 noon for the day of acquisition of the image, according to Figure 7. (Color for this figure is available in the online version of this paper.)

As observed from Figure 10, there is a good correspondence between the temporal variability of the SST measured within Alcatraz Bay and the temporal variability of the image-derived SSTs, although it is clear that this correlation is higher (r ¼ 0.91) for pixels near Alcatraz Bay (Figure 11a) than for pixels farther south from the bay, where the correlation decreases to r ¼ 0.79 at the south end of the transect (Figure 11b). Also interesting is the fact that the image-derived SSTs (Figure 10) seem to express a fortnightly modulation along the whole year, in a clear correspondence with the fortnightly modulation expressed in the in situ SSTs. These results suggest that the relevance of the modulation induced by the spring and neap variability of the tides on the SST might not be a particular situation occurring within Alcatraz Bay but to be an effect that extends to the whole BC, as previously suggested by MDL-06 from the analysis of SSTs that were acquired simultaneously in six bays along the west coast of the BC.

CONCLUSIONS We analyzed meteorological and SST information that were acquired within the Alcatraz Bay, on the west cost of the BC. This information was used to investigate the forcing mecha-

nism of sudden and drastic pulses that were previously observed in the SST, particularly during September and October when they are more evident and extreme. From our analysis, we can conclude the following: Meteorological information showed a clear seasonal variability, except the wind speed, which also showed monthly mean extreme values contrary to previous observations, reflecting the high dominance of local coastal effects. On the other hand, wind directionality departed from the typical monsoonal behavior observed for the whole Gulf, as it was persistently from the west. This seems to confirm the scarce evidence that this region, between Angel de la Guarda Island and Baja California, is often characterized by cross-channel winds, which open the BC to the influence of not only the drier and hotter winds over the peninsula, but also the oceanic influence from the Pacific. This situation could induce a different effect in the atmospheric–marine boundary layer in contrast to the one produced by the typical monsoonal winds. Ocean surface heat flux calculations were in very good agreement with previous results for the whole Gulf, despite being coastal and calculated from data with different spatial and temporal resolutions. The net heat flux showed a marked


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Figure 9. Similar to Figure 8, but for chlorophyll a. (Color for this figure is available in the online version of this paper.)

seasonal signal, with a positive annual mean value of 197 W m2. The evaporative, or latent, heat flux was the main cooling agent for the ocean surface and was negative for the whole year, with maximum losses occurring during the summer and fall (402 W m2 in October). The SST showed a marked seasonal variability, with extreme temperatures occurring in late summer (29.68C in September) and early spring (13.48C in April). The SST spectral analysis identified four main periods of variability: quarter-diurnal, semidiurnal, diurnal, and fortnightly. The last mode contributed significantly to the variance of the spectrum and was almost as important as the semidiurnal mode. We attribute all of these modes of variability to the dominant scales of variability induced on the SST by the tide. In addition, the SST also presented sudden and drastic drops, particularly during September and October when the surface temperature can drop up to 78C in a period of a few hours. The time occurrence of these temperature drops is well correlated with the occurrence of the spring tides, strongly suggesting that they are forced by the vertical mixing induced by tides and that their occurrence and intensity is modulated with a fortnightly periodicity. Without a doubt, vertical mixing induced by the tides plays a relevant role in this region, and also contributes to

Figure 10. Comparisonbetweentheseasurfacetemperature(SST)obtainedin situ (black line) inside Alcatraz Bay and the image-derived temperature (color) along a line centered within Ballenas Channel (see Figure 1) from August 2004 to July 2005. See Figure 11 for the SST correlation between in situ measurements and image-derivedtemperaturesatthefirst and lastpixelsalong the line. (Color for this figure is available in the online version of this paper.)



Figure 11. Scatter plots between the sea surface temperature (SST) obtained in situ inside Alcatraz Bay and image-derived temperature at (a) the first and (b) the last pixels of the line centered within Ballenas Channel (see Figure 1). Coefficients of determination are shown in both plots.

give to the BC its most outstanding characteristic of low SSTs all year round. The SST and Chl a imagery seem to indirectly support the representativeness of the data that we analyzed for the regional variability of the SST within the BC. Furthermore, the data show that the outcropping of colder, deeper waters to the surface, induced mainly by the fortnightly tide, is not a particular characteristic of the BC, but instead occurs in most of the archipelago island region. This has, without a doubt, very important biological consequences for this region.

ACKNOWLEDGMENTS This is a contribution to the projest ‘‘Biogeograf´ıa e historias de vida de dos especies de Porphyra (Rhodophyta), endémicas del Golfo de California, México’’, funded by CONACyT under the number CB-2005-25340-50173Q and the Universidad Autonoma de Baja California under Project UABC-0572.

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