Oscillations in the water column, or inter-nal waves, are common on island slopes in theSouthern California Bight (SCB) in both diur-nal and semidiurnal frequency bands. Gelpi andNorris (2005) found internal waves to be ubiq-uitous in a survey of temperature data takenaround Santa Catalina Island and in similar,though informal, analyses of temperature dataprovided by the Channel Islands National Park.The large specific heat capacity of water impliesthat large changes in temperature are due toadvection of temperature gradients rather thanlocal heating or cooling. Hence, temperature isa good tracer of water movements when thesegradients exist, and rapid oscillations in tem-perature often signify internal waves.

Internal waves may have a significant impacton water mixing, and understanding this impactbears on many problems in physical and bio-logical oceanography, as well as in island ma -rine ecology. Gelpi and Norris (2008) demon-strated that vertical mixing in the upper 30 min the inner SCB is much less than that in the

adjacent California Current. In the bight, ver-tical mixing is characterized by a vertical eddydiffusion coefficient of ~1 cm2

⋅ s–1; outsidethe bight in the California Current, diffusionis too rapid to measure with their techniques(i.e., the upper layers are well mixed). Under-standing the physics that produces this valuefor the diffusion coefficient will provide insightinto the water and nutrient flows of the bight.It may also help us discern mixing in the abyss,which has been inferred to have a similar value(e.g., Munk and Wunsch 1998).

Understanding island internal waves mayprovide insight into whether the forcing is atop-down mechanism (energy input at the sur-face that diffuses to depth) or a bottom-up one(energy input at depth that diffuses upward).Examples of the former are wind and surfacewaves that can mix the surface layers; an ex -ample of the latter are tidal interactions withseabed topography that may create mixing atthe bottom. Each mechanism has ramifica-tions to nutrient diffusion against the vertical

Monographs of the Western North American Naturalist 7, © 2014, pp. 21–34

DIURNAL AND SEMIDIURNAL INTERNAL WAVES NEAR TWO HARBORS, SANTA CATALINA, CALIFORNIA

Craig Gelpi1

ABSTRACT.—Internal waves at both diurnal and semidiurnal frequencies are common on island slopes in the South-ern California Bight (SCB). Amplitudes in both regimes are similar, although the phenomenology at the 2 frequencies isexpected to be different since the bight is north of the critical latitude (30°), making diurnal modulations evanescentthough semidiurnal waves can propagate. Understanding the driving forces for the internal waves may provide insightinto vertical mixing in the bight. We used long-duration time series of ocean temperatures measured near the WrigleyInstitute for Environmental Studies (WIES), Santa Catalina Island, to study highly resolved spectra and seasonal char-acteristics of internal waves. These characteristics were analyzed using available, sometimes nonconcurrent, ancillarydata sets that include local tide and wind measurements. We conclude that the semidiurnal waves are driven by tides,whereas the diurnal waves are due to meteorological forcing.

RESUMEN.—Las olas internas en frecuencias diurnas y semidiurnas son comunes en las pendientes de las islas de laBahía Southern California Bight (SCB). Las amplitudes de ambas frecuencias son similares, sin embargo se estima que lafenomenología en ambos casos debe ser diferente, debido a que la bahía se encuentra al norte de la latitud crítica (30°),lo cual determina que las modulaciones diurnas sean evanescentes y las olas semidiurnas se puedan propagar. El cono-cer cuáles son las fuerzas impulsoras de las olas internas ayudará a comprender la mezcla vertical que se produce en labahía. Las series de tiempo prolongado de las temperaturas oceánicas que se miden cerca del Wrigley Institute of Envi-ronmental Studies (WIES), Isla Santa Catalina, se utilizan para estudiar los espectros de alta resolución y las característi-cas estacionales de las olas internas. Dichas características se analizan utilizando conjuntos de datos auxiliares, a vecesno concurrentes, incluyendo las mediciones locales del viento y la marea. La conclusión es que las mareas impulsan lasolas semidiurnas y que las olas diurnas se producen debido al forzamiento meteorológico.

1Catalina Marine Society, 15954 Leadwell St., Lake Balboa, CA 91406. E-mail: craig@catalinamarinesociety.org

21

nutrient gradient. Each may also elucidate thecontribution of topography to a possible islandmass effect: that is, the increased productivityproduced by the presence of the island.

Finally, understanding internal oscillationsat diurnal and semidiurnal frequencies shouldprove useful for interpreting biologically rele-vant oscillations found in ocean chemistry mea -surements. Recently Frieder et al. (2012) re -ported dissolved oxygen and pH measurementsfrom a kelp forest off La Jolla, California. Theyfound that at 7 m the diurnal variation of theseparameters was larger than the semidiurnalvariation, though the simultaneously measuredtemperature and pressure exhibited larger semi -diurnal changes than diurnal ones. These dif-ferences may indicate different driving mech-anisms or complicated gradients that must beinterpreted within the context of the physicaloceanography. Dissolved oxygen and pH arenow being measured at the same site wherethe temperature data reported in this paperwere taken, and the results obtained will beused in interpreting modulations found in thesequantities.

There have been many studies of internalwaves in the SCB and in the adjacent Califor-nia Current (Price et al. 1986, Rosenfeld 1988,1990, Lerczak et al. 2001, 2003, Pidgeon andWinant 2005, Beckenbach and Terrill 2008,Noble et al. 2009, Nam and Send 2011, 2013).Both diurnal and semidiurnal waves have beenobserved even though the phenomenology atthe 2 frequencies is expected to be differentbecause the bight is north of the critical latitudefor diurnal waves (30°). The critical latitude(Lerczak et al. 2001) is where the inertial fre-quency (Apel 1987) is equal to the diurnal fre-quency (1/24 h or 0.04167 h–1). For locationspoleward of the critical latitude, diurnal mod-ulations are damped by inertial responses,though semidiurnal waves can propagate. Theinertial frequency for the latitude of the studyarea is 0.04606 h–1.

Several explanations and models have beensuggested to account for diurnal temperaturemodulations poleward of the critical latitude.These include currents modifying the effectiveinertial frequency (Lerczak et al. 2001), baro -tropic tides driving nonlinear or localizedbehavior (Beckenbach and Terrill 2008, Pid-geon and Winant 2005), meteorological heat-ing and mixing (Price et al. 1986), and diurnalsea breezes pooling warm surface water

against the coast, as described by Kaplan et al.(2003).

Our goal is to understand the upper-oceaninternal-wave modulations at Santa Catalina Is -land in order to ascertain the contribution of themodulations to vertical mixing in the South-ern California Bight. This study differs from theothers mentioned above in that the measure-ments are made near the island and its associ-ated topographical and meteorological effects.

METHODS

Data Sets

Water-temperature data was collected bythe Catalina Conservancy Divers (CCD) atits WIES site (Wrigley Institute for Environ-mental Studies, 33.45° N, 118.48° W) and anearby National Oceanic and AtmosphericAdministration (NOAA) buoy (46025, 33.75°N,119.08° W). Mean-sea-level (MSL) data wascollected from Los Angeles Outer Harbor Sta-tion (9410660, 33.72° N, 118.27° W). All siteswere sampled hourly during the year 2000.WIES is located on the leeward side, near theisland isthmus (Fig. 1), where there is a near-sea-level pass from the windward side (CatalinaHarbor) to the leeward side (Two Harbors).Elsewhere, the 2 sides are separated by moun-tains approximately 300 m in height. Data col-lected at WIES for this study were measure-ments made on the bottom at 4 depths: 4.6 m,9.1 m, 18.3 m, and 30.5 m. The local and is -land shelf slope where these instruments weresited are both ~10°. The NOAA buoy recordednear-surface temperature at 0.5 m depth. (forgreater detail on these data, see Gelpi andNorris [2008] and references therein). The year2000 was chosen because few dropouts oc -curred in the CCD data during this intervalamong the 4 depths at WIES. Data gaps werefilled with values interpolated from the nearestgood samples. There were approximately 10such repairs, each consisting of one or 2 miss-ing measurements.

In addition to the data described above, weemployed meteorological data provided by aweather station (sampled every 0.5 h) at WIESduring the years 2006–2009 and by the NOAAbuoy for the years 2000 and 2006–2009. Theweather station was located within a few hun-dred meters of the thermographs. The data sets,their locations, data types, intervals of operation,and sampling are listed in Table 1. The locations

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are shown also on the map in Fig. 1. The dis-tances from WIES to the NOAA buoy and tidestation are 65 km and 36 km, respectively. Theseocean surface and tide measurements were thenearest available to the WIES study site.

Time Series Analysis

The time series of each data set was analyzedusing both spectral and temporal methods.

Frequency decomposition via Fourier trans-form was performed to separate the effects ofvarious driving mechanisms, such as lunar tidesand meteorological effects, that operate at dif-ferent frequencies. Data variance in small, dis-crete frequency bands (i.e., power spectraldensities) were computed using conventionalFourier techniques as follows: the mean valueof the time series was subtracted, the transform

2014] INTERNAL WAVES AT SANTA CATALINA 23

Fig. 1. Map showing data collection sites and bathymetry of the Southern California Bight.

TABLE 1. Data sets used to analyze internal waves near Two Harbors, Santa Catalina, California.

Organization/location Instrument/Data Time interval Depth (m) Sampling

NOAA Buoy 46025 Wind Year 2000, 2006–2009 Surface Hourly33.75°N, 119.27°W

NOAA Buoy 46025 Ocean temperature Year 2000 0.5 Hourly33.75°N, 119.27°W

CCD Ocean temperature 01 Jan 2000: 4.6, 9.1, 18.3, 30.5 Hourly33.45°N, 118.48°W 31 Dec 2000

NOAA LA Station 9410660 Water level 01 Jan 2000: — Hourly33.72°N, 118.27°W 31 Dec 2000

WIES Wind 2006–2009 — 0.5 hour33.45°N, 118.48°W

computed and squared then scaled so the inte-gral over frequency of the density estimatesequals the variance of the original series. Theseries was not tapered so as to maintain thehighest spectral resolution. Frequency units arekept in inverse hours to facilitate comparison

with the insolation period of 24 h, and poweris displayed in decibels: that is, the base 10logarithm of the power multiplied by 10.Spectral density was computed at 2 resolu-tions: high-frequency resolution (0.000114 h–1),achieved using the entire 8760-h series; andlower resolution (0.001389 h–1), obtained using720-h snippets. The power spectral densitiescomputed from the snippets were concatenatedto form spectrograms (i.e., power spectral den-sity as a function of time of year). The latteranalysis is designed to investigate the seasonaldependence of the power spectral density.Phase was computed as a function of frequencyand time of year. Error bars to the phase valuewere computed using the coherence (Bendatand Piersol 1986) between the sinusoid andthe corresponding power spectral density aftersmoothing by 3 bins.

Temporal analysis consisted of inspecting theaverage intraday modulation. This modulationwas computed by subtracting the mean valuefor each 24-h interval and averaging the day-hour values over 90 days. The intraday modu-lation was computed for the winter and sum-mer seasons. The advantage of examining thedata using this technique is that diurnal, non-sinusoidal features are readily apparent, whereasa frequency analysis divides the features amongits natural frequency and its harmonics. Fre-quency analysis is effective for understandingnonsinusoidal forcing mechanisms. Althoughsimilar analyses have been performed by Beck-enbach and Terrill (2008) and Nam and Send(2011) with respect to the phase of diurnal sur-face tides, the analysis in this paper concernsthe solar day.

RESULTS

High-Resolution Spectral Analysis

The temperature time series obtained fromall the CCD thermographs at WIES for theyear 2000 are plotted in Fig. 2. The time seriesindicate the typical seasonal response of upper-ocean temperate seas, with a 7 °C warmingfrom winter to late summer. There are also sub-stantial modulations, particularly at depth wherethe modulation amplitude is as large as 6 °C.Modulations are the result of surface waterbeing transported to depth, as has been re -ported in Gelpi and Norris (2005). Note thatthe near-surface thermograph measured theleast high-frequency modulation (>1/24 h–1).

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Fig. 2. Time series of WIES temperature data for eachdepth: 4.6 m, 9.1 m, 18.3 m, and 30.5 m.

Figure 3 shows the power spectral densitiesobtained for the 4.6-m and 18.3-m data: thelatter exhibiting a pattern similar to the 9.1-mand 30.5-m data. The spectra indicate substan-tial power in both the diurnal (~1/24 h–1) andsemidiurnal (~1/12 h–1) bands. The peakpower spectral densities computed from the18.3-m depth data are larger than those ob -tained from the 4.6-m depth data. At 18.3-mdepth, the largest power peak is in the semidi-urnal regime; whereas, at 4.6 m, the largestpeak is in the diurnal regime. Both the 9.1-mand 30.5-m data (not shown) have spectraldensities similar in values to the 18.3-m data

and also have semidiurnal power greater thanthe diurnal power.

To investigate whether surface tides drivethe internal tides, we compared peak frequen-cies in the power spectral density of the MSLfound in Los Angeles Harbor and the 18.3-mWIES thermograph data (Fig. 4). To facilitatefrequency comparisons, the power spectraldensities are normalized by the highest powerpeak within the combined diurnal and semidi-urnal bands. The frequency of the normalizingpeak differed between the temperature andMSL spectra. An expanded view of the diurnalspectral regime is shown in Fig. 4A, where 3

2014] INTERNAL WAVES AT SANTA CATALINA 25

Fig. 3. High-resolution power spectral density for 4.6-m and 18.3-m depth data.

tidal frequencies are found in the MSL spec-trum: the principle lunar (O1) at 0.03873 h–1,the principle solar (P1) at 0.04155 h–1, and thelunisolar (K1) at 0.04178 h–1. All these powerpeaks extend many dB above the MSL noiselevel, which is substantially below the rangeplotted. The 2 lunar-related tides K1 and O1have the largest power in MSL. In contrast,the diurnal temperature spectrum exhibits onepeak above the background, which is exactlyat 0.04167 h–1 or 1/(24 h), the insolation fre-quency which lies between the P1 and K1 tidalpeaks. The peak is unambiguous, being ~10dB above the background. No temperaturepower is evident at the O1 frequency.

The semidiurnal regime is shown in Fig.4B, and 4 peaks in the MSL spectrum are evi-dent: lunar elliptic (N2), 0.07900 h–1; principlelunar (M2), 0.08051 h–1; principle solar (S2),0.08333 h–1; and lunisolar (K2), 0.08356 h–1.There are 2 complexes of peaks in the temper-ature spectrum centered at 0.08062 h–1 and at0.08333 h–1, roughly corresponding to the M2spectral peak (lunar tidal) and the S2 solar

tidal. Lower-magnitude tidal peaks are not ex -pected to be found in the temperature recordif their effects scale with MSL power (e.g., theN2 peak would be below the temperature powerspectral background, approximately –12 dB onthe relative scale).

Seasonal Spectra

The spectra in Figs. 3 and 4 are computedat the highest resolution in order to measurethe spectral peaks precisely. However, seasonaldynamics in the power spectra are lost whenall the data in the series are used in this man-ner. Seasonal dependence of the modulationswere investigated by computing the spectraover shorter time intervals to create spectro-grams. Spectrograms for the data gathered ateach depth are shown in Fig. 5. There are pro -minent diurnal and semidiurnal bands in eachpanel, as well as harmonics of these frequencies.The largest power values are observed duringthe summer months. However, there is con-siderable difference between the 4.6-m dataand the data for greater depths. Also note thatthe diurnal power is not large at the 9.1-m and18.3-m depth data during the first 90 daysof the year but is present in the 4.6-m and30.5-m depth data.

In addition to the power, the phase lag rela-tive to the beginning of the year in GMT (localmidnight has a phase of 120° in this conven-tion for the diurnal frequency) was computedfor the CCD and buoy temperature and MSLdata. The phases for the 1/24 h–1 bin are shownin Fig. 6A. The error bars are determined fromthe coherence and degrees of freedom. Thecoherence was high, averaging, for example,0.8 for the 30.5-m data. The temperature phasesare relatively constant throughout the year.The buoy and 4.6-m WIES temperatures arein phase during the winter, with an averagevalue near 0° which corresponds to 1600 localtime (LT). Temperatures at the 3 deeper depthsat WIES are in phase with a value of ~135°,corresponding to a time of maximum tempera-ture near local midnight.

The MSL phase varies monotonically be -cause the large tidal component K1 (0.04178h–1) is a discrete, narrowband feature that isoffset from the center of the 1/24 h–1 fre-quency bin, thereby advancing the phase bythe frequency offset (0.00011 h–1) multipliedby the 720-h time increment between spectralcalculations (30°). The lack of much variation in

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Fig. 4. Normalized 18.3-m and MSL spectra: A, diurnalregime; B, semidiurnal regime.

the phase of the diurnal temperature through-out the year indicates that the power is eitheruniformly spread over the spectral bin (con-trary to the high-resolution measurements de -scribed above) or that it corresponds exactly toa period of 24 h.

The absolute phase for the semidiurnal fre-quency bin (1/12 h–1) is plotted in Fig. 6B. TheMSL phase averages 139.8°, corresponding tolocal times of 3.4 h before noon and midnight.NOAA uses a value of 141.1° for tidal predic-tion of this component at this location (http://tidesandcurrents.noaa.gov). The difference betweenthe published value and that obtained fromour analysis of the tide data falls within theuncertainty of our phase measurement. Also,

the K2 component is within the semidiurnalfrequency bin, with an offset of 0.00023 h–1,which produces a semiannual modulation tothe MSL phase. The 9.1-m, 18.3-m, and 30.5-m temperatures are all roughly in phase andvary between –30° and 150° in a systematicfashion, though the K2 modulation is not evi-dent as it is in the MSL data. The 9.1-m and18.3-m data are considered to be temporallyaliased at day 75 and day 105, as that main-tains continuity with the other values (thephases of the 9.1-m and 18.3-m data are slightlygreater than 180� ° for these days, but they areshown as negative values because the phase isforced to lie between 180° and –180°). Thenear-surface temperature phase at the buoy is

2014] INTERNAL WAVES AT SANTA CATALINA 27

Fig. 5. Spectrograms computed from WIES data. Units are dB relative to °C2-h. The diurnal and semidiurnal fre-quencies positions are marked.

nearly constant at –60° (1400 LT). The 4.6-mtemperature is in phase with the buoy temper-ature during winter and spring and in phasewith the data from the deeper instrumentationfor the rest of the year.

The wind forcing for the WIES and NOAAlocations were very similar. Figure 7 depictsthe spectrogram of the buoy’s westerly windfor the year 2000. The power spectral densi-ties for the east and north components (notshown) indicate that the wind was narrowbandand originating predominantly from the westat both locations. Wind at the WIES site is

certainly influenced by the presence of the is -land, which is oriented in a northwest–southeastdirection, and especially the isthmus, which isoriented in a northeast–southwest direction.The narrowband diurnal wind occurred all year.Other interesting features include the largerlower-frequency (<1/24 h–1) wind events foundduring the winter when the marine tempera-ture gradient collapses (Fig. 2).

Temporal Modulations

The spectrograms illustrate the Fourier de -composition of the signals as a function of time.

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Fig. 6. Phases for the spectrograms shown in Fig. 5: A, frequency bin 1/24 h–1 ; B, frequency bin 1/12 h–1.

Examining the average intraday modulation isalso useful because dynamic behavior may bemore closely identified with forcing mecha-nisms. The average intraday modulation of theWIES and buoy temperature data for the first90 days of the year (winter) are shown in Fig.8A and for the middle 90 days (summer) inFig. 8B. These intervals are chosen to separatethe seasonal dependence of the modulationsfound in the spectrograms of Fig. 5. Duringthe winter (Fig. 8A), the 4.6-m and buoy tem-peratures vary almost identically, reaching amaximum at about 1500 LT with amplitude of0.2 °C. This time of temperature maximum doesnot agree exactly with the time computed fromthe phase (1600 LT) given in Fig. 7 becausethe intraday plot contains the sum of all har-monics. The 9.1-m and 18.3-m temperatureshave a very different daily behavior compared tothe 4.6-m temperature, changing little through-out the winter day. This behavior is consistentwith the small power spectral density valuesfound in Fig. 5 for these depths. Close exami-nation does indicate a semidiurnal variation

consistent with diminished power found in thewinter for these depths. In contrast, the 30.5-mtemperature trace has maximums at midnightand 0700 LT and a deep minimum at 1300 LTthat is approximately 0.6 °C below the nighttimeplateau. The 30.5-m minimum occurs near thetime of temperature maximum for the near-surface and 4.6-m data, indicating upwelling atdepth as the near surface is heated. The averagetemperature gradient for this time of year andat this depth is –0.04° m–1 (Gelpi and Norris2008), indicating that the isotherms were up -welled 15 m.

The variation becomes more complicatedduring the summer (Fig. 8B). Here, the near-surface buoy temperature has a larger modula-tion (~0.5 °C), and it occurs 2 h earlier than inthe winter data. The deeper thermograph data(9.1 m, 18.3 m, and 30.5 m) clearly show thesemidiurnal behavior, with much larger ampli-tude than found in Fig. 8A. The 4.6-m summerdata are a mix of the characteristics of the near-surface data and the deeper data, but overallhave much less modulation.

2014] INTERNAL WAVES AT SANTA CATALINA 29

Fig. 7. Spectrogram of buoy westerly wind for year 2000. Units are dB relative to (m/s)2-h. Diurnal (D) and semidiur-nal (S) frequency locations are marked.

Although WIES and the buoy are located 65km apart, the average intraday wind patternsare remarkably similar between them. Figure 9shows the average intraday variation measuredfor 3 years (2006–2009) at the WIES weatherstation and the NOAA buoy. The intraday modu-lation is strongest at WIES, reaching almost9 m ⋅ s–1 out of the west at 1400 LT. This strongwind speed in the afternoon is well known anec-dotally among scuba divers at WIES (includingthe author). The maximum wind at the buoy issmaller, with the peak occurring at 1600 LT. Thenorth–south modulation is very small, with aminimum–maximum difference of 2 m ⋅ s–1 andpeaking between 1200 and 1300 LT for bothsites. Thus the wind forcing is qualitatively thesame at the 2 locations, though it peaks earlier inthe day at WIES and is stronger there. Lerczaket al. (2001) reported on wind patterns measuredat the Scripps Institution of Oceanography pier(32.87°N, 117.26°W) during summer 1999. Ac -cording to their measurements, the onshorewesterly wind peaked at 1440 LT, with a valueof 1.7 m ⋅ s–1 (i.e., it had similar phase thoughmuch less velocity than the values applicableto WIES).

SUMMARY OF RESULTS

Temperature modulations are found at thesurface and all depths in both the diurnal andsemidiurnal regimes and exhibit a complex phe-nomenology. The 2 frequency regimes differ inseasonal response, signal bandwidth, and cor-respondence to the barotropic tide, with thediurnal band having a narrow bandwidth at theinsolation frequency and the semidiurnal bandbeing broad with some power peaks at tidalfrequencies. The 4.6-m data exhibit semidiur-nal power predominantly during the summermonths and diurnal power all year. The 18.3-mdata show significant power all year in the semi-diurnal band; but in the diurnal band, its poweris greatly diminished around March, day 60.The diurnal variations of the near-surface and4.6-m temperatures are in phase and exhibit amaximum value at 1600 LT. The diurnal varia-tions at the deeper thermographs are in phasewith each other and have a temperature maxi-mum at about local midnight. The semidiurnalsurface temperature peaks between 0300 and0400 LT and again between 1500 and 1600 LT.The semidiurnal phase at 4.6 m alternates

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Fig. 8. Average intraday temperature modulation forWIES and buoy: A, winter; B, summer.

Fig. 9. Average intraday wind modulation for years2006–2009: A, westerly wind; B, southerly wind.

between the surface temperature phase (sum-mer) and those found at depth (winter). Thewind is a narrowband frequency phenomenon;and at WIES, peaks at about 1400 LT. Althoughconcurrent wind measurements and oceantemperature measurements are not available,by using the buoy wind measurements as aproxy for the island measurements, one findswesterly narrowband wind existed throughoutthe year 2000. The significance of narrow band-width is that it allows correlation with relatedphenomena.

DISCUSSION

In this section, the phenomenology describedabove are organized and assigned probablephysical driving mechanisms. They are placedin context and contrasted with results fromother similar studies performed on the nearbymainland. Finally, the significance of the resultsis discussed.

The analysis results above are divided intoregimes of frequency, depth, and season. Thefrequency regimes are diurnal and semidiur-nal. A depth of 5 m is used as the dividingpoint between near-surface (including the buoytemperature and 4.6-m thermograph data) anddeeper thermograph data. Seasons are winterand summer, corresponding to relatively smalland large stratification, respectively. The appar-ent forces can be divided into meteorological

and astronomical, with the former includingdirect solar heating and wind, whereas the lat-ter is tidal. The significant forces for each re -gime are described below and are also orga-nized into Table 2.

The deeper semidiurnal temperature re -sponse appears to be a tidally driven internalwave of large seasonal amplitude (with a dis-placement sometimes nearly equal to the depthof the water column). This measure is sup-ported by the presence of lunar tidal frequen-cies in the response and the diminishing sea-sonal response when the temperature gradientis smallest, implying that these waves are ver-tical advections of the temperature gradient.Observations of these waves are common (e.g.,Baines 1986), and perhaps the amplitudes areenhanced by the seabed gradient, which isabout 10° both locally and out to the islandshelf (Fig. 1). These waves are not damped, asthe semidiurnal frequency is above the inertialfrequency.

The summer shallow semidiurnal fluctua-tions, at 4.6 m, appear to be a combination ofsolar heating and internal waves. The mixed-layer depth (i.e., the upper layer of small tem-perature gradient) may shoal during the sum-mer as indicated by the temperature model forthe inner SBC developed by Gelpi and Norris(2008). This situation would be conducive tointernal wave activity being evident near thesurface. Internal wave activity is supported by

2014] INTERNAL WAVES AT SANTA CATALINA 31

TABLE 2. Internal wave phenomenology, forcing, and rationale.

Winter Summer______________________________________ ______________________________________Diurnal Semidiurnal Diurnal Semidiurnal

SHALLOW, <5 M Solar heating and Solar heating and Solar heating and Conventional buoy-wind mixing wind mixing wind mixing ancy oscillations

forced by tidesIn-phase relationship Harmonic of diurnal In-phase relationship Frequency corre-

with near-surface waves with near-surface sponding to lunarbuoy temperatures and insolation tide and in-phase and insolation relationship with

deeper wavesDEEP, >5 M Wind-driven upwell- Conventional buoy- Wind-driven upwell- Conventional buoy-

ing at 30.5 m Up- ancy oscillations ing ancy oscillations welling and solar forced by tides forced by tidesheating cancella-tion at intermedi-ate depths

Correspondence Frequency corre- Correspondence Frequency corre-with diurnal wind sponding to lunar with diurnal wind sponding to lunar and anti-correlated tide and same absolute tidewith insolation at phase as found in 30.5 m winter

the 4.6-m temperature data, which exhibitedsemidiurnal fluctuations with the same phaseas the deeper thermograph data. These fluctu-ations are not expected at the buoy becauseinternal wave amplitudes decrease to zero atthe surface and the buoy is not adjacent to to -pography conducive to generating these waves.The summer and winter portions (1000 h each)of the year 2000 data set studied here wereincluded in the Gelpi and Norris (2005) study,and the results presented here are consistentwith the other 2 data sets taken during the sum-mers and winters of 1998 and 1999. All of thesedata sets exhibit dichotomy in phase relation-ships as a function of depth and season (i.e., thesemidiurnal modulation found at 4.6 m is inphase with the modulations at deeper depthsduring the summer and out of phase with themduring the winter).

The winter shallow diurnal fluctuations areconsistent with solar heating of the surface andsimultaneous wind mixing to 5 m. This effecthas been previously studied (Price et al. 1986)and is evidenced here by the near-surface buoyand 4.6-m thermograph temperature modula-tions being almost identical and having thesame phase during most of the year. The semi-diurnal shallow fluctuations are also narrow-band and are probably the result of harmonicdistortion, as the mean daily fluctuation is nota sinusoid (Fig. 8).

Finally, the winter deep fluctuations haveadditional depth dependence in that there islittle modulation at 9.1 m and 18.3 m, but asignal at 30.5 m. Interestingly, the deep-depthsignal is out of phase with both the shallow-depth signal, the wind, and the insolation. Thisdifference in phase suggests that the at-depthsignal may be the result of upwelling producedby the wind. The diurnal wind has approxi-mately the correct phase to transport surfacewater away from the island and to producelocal upwelling as measured. This effect is simi-lar to that described by Kaplan et al. (2003),the difference being that the diurnal breezeblows away from the shore at the WIES siteand flows toward the shore at the site of theKaplan et al. study. The effects of solar heatingdominate near the surface but appear to beapproximately balanced with upwelling at inter-mediate depths so that temperature signals aresmall at 9.1 m and 18.3 m. Another possibleexplanation for the small signal at intermediatedepths is that collapse of stratification during

the winter has reduced the average seasonaltemperature gradient there. Countering thisseasonal trend is the simultaneous surface heat-ing and upwelling and cooling at 30-m depththat adds a temperature gradient of –0.03° m–1

in the afternoon. Thus, competing effects arebalanced at the intermediate depths.

Recent analyses of the diurnal internal tidefound on the continental shelf off HuntingtonBeach northeast of the WIES study were con-ducted by Nam and Send (2011, 2013). At thislocation, diurnal wind is also present and issimilar to, though smaller than, the wind at theWIES site. Additionally, the wind blows inshoreat this location, it blows offshore at WIES.However, the submarine topography is verydifferent, with a slope of approximately 0.5°(much smaller than that at WIES). Diurnalmodulations off Huntington Beach (Nam andSend 2011) appear to be phase locked with thebarotropic or surface tide, rather than withdiurnal meteorological drivers. Deep (cooler)water was found to intrude into the surf zoneand then dissipate as the tide ebbed. This be -havior was not noticed at WIES.

In subsequent work in the same area (Namand Send 2013), diurnal oscillations driven bya sea/land breeze were reported. This report ispartially consistent with our observations ofwater at depth (20 m) flowing opposite thebreeze. Nam and Send attribute the diurnaloscillations to a resonant phenomenon wheresubinertial current shears can lower the effec-tive inertial frequency to below the diurnalfrequency (Lerczak et al. 2001). This mecha-nism does not appear to be present at WIES.Although current shear measurements are notavailable to confirm the model, diurnal modu-lations at depth occur throughout the year.However, the large-scale currents that wouldproduce the shear are known to change withthe season (Hickey 1993). Also, the phenome-non is not seen at intermediate depths—a factnot explained by shifts in the effective inertialfrequency.

The diurnal upwelling found in the 30-mWIES data attributed to the diurnal wind is incontrast to the island’s sheltering effects studiedby Caldeira and Marchesiello (2002). They notedthat the wind shadowing produced warmersea surfaces which probably resulted in lessmixing and nutrient availability. In contrast,the WIES site is not sheltered from the pre-vailing winds due to the near-sea-level pass

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connecting Catalina Harbor and Two Harbors.Indeed, the pass may amplify the wind and sub-ject the local area to higher winds than are foundelsewhere, as noted above. The result is up -welling found in the deeper-depth temperaturedata and its implied transport of nutrients toshallower depths. This effect is probably localto WIES due to the vicinity of the pass.

CONCLUSION

Internal waves deduced from temperaturedata measured near Two Harbors, Santa CatalinaIsland, exhibited a complex behavior that in -cludes variation at both diurnal and semidiur-nal frequencies and a pronounced seasonal anddepth dependence. During the winter, diurnalvariations in surface and at-depth temperaturewere out of phase. Diurnal variations weredriven by insolation near the surface (5 m) andby upwelling at 30-m depth—the latter proba-bly produced by the diurnal wind directed off-shore and lifting the isotherms by 15 m. Hence,both effects are driven by meteorological forces.During the winter, these effects appear to beapproximately balanced at intermediate depthsso that temperature signals are small at 9.1 mand 18.3 m.

During the summer, coherent water-columnsemidiurnal variations dominate, including nearthe surface, indicating that stratification wassignificant throughout the column. This strati-fication supported large amplitude waves thatappear to be typical internal waves occurringover steep topography and driven by semidi-urnal tides. Amplitudes were occasionally largeenough to transport near-surface water to depth.Hence, there is significant vertical movementin the upper 30 m of the water column nearWIES throughout the year.

ACKNOWLEDGMENTS

Karl Huggins of the University of SouthernCalifornia provided the wind velocity data forWIES. The Santa Catalina Island marine-tem-perature data were provided by the CatalinaConservancy Divers, a group that supports theCatalina Island Conservancy; and Thomas Tur-ney and John Turney were generous in secur-ing and preparing the data sets. Sea-level dataand buoy data were obtained from NOAA.This work profited from discussions with KarenNorris and was supported by the Catalina Ma -rine Society.

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Received 9 February 2013Accepted 6 November 2013

Early online ????

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