DOI: 10.5061/dryad.00000008t
title: Relative phase between tidal and solar cycles influences the heating of a coral atoll lagoon

description: The residence time of water within reef-lagoon systems is of first-order
 importance to biogeochemical cycles and reef ecosystem functions.
 Circulation, mixing, and water renewal within these systems is highly
 dependent upon the geometry of the reef as well as wave, tide, and wind
 forcing. We are motivated to understand the physical driving a recent
 warming event as well as observed hypoxia-related fish kills and seagrass
 dieoff events in Dongsha Atoll lagoon, a 28 km wide coral atoll in the
 South China Sea. Though extensive work has been done on hypoxia events in
 temperate coastal systems, there remains a lack of understanding regarding
 the dynamics of these events in shallow tropical waters. Hydrographic
 observations were collected in summer 2019 to examine circulation and
 ocean-lagoon exchange in Dongsha Atoll lagoon. We construct a heat budget
 for the Dongsha Atoll lagoon to understand the physical processes
 governing lagoon circulation and ocean-lagoon exchange. We examine the
 influence of tides, winds, waves, advective exchange, and surface heating
 on lagoon heating and flushing time. Results show that surface heat fluxes
 drive rapid heating of the lagoon by approximately 1.5°C over the 22 day
 study period. Advective transport over the reef flat from tide- and
 wave-driven flows is an important component of oceanlagoon exchange that
 is balanced by a net outflow through the channels on subtidal timescales.
 Specifically, a near phase-locked interaction between diurnal solar
 forcing and near-diurnal tidal exchange drives persistent advective
 cooling over the reef flat. Estimated flushing times for the Dongsha Atoll
 lagoon range from 2.7 to 3.2 days for spring and neap tide, respectively.
description: The presented observations are part of a larger study of internal
 wave propagation around Dongsha Atoll Sinnett et al. 2022; Ramp et al.
 2022). We used measurements of currents, pressure, and temperature in and
 surrounding the lagoon (channels, reef flat, forereef), as well as
 meteorological measurements taken on Pratas Island collected from 20 May
 2019 - 09 June 2019. Water temperature in the lagoon
 was measured on two fixed vertical moorings in the north and south lagoon,
 two vertically profiling moorings (WireWalkers, Rainville and Pinkel
 (2001)) in the east and west lagoon, and on bottom moored thermistors at
 various locations around the lagoon. Water temperature was measured in
 both the north channel (NCH) and south channel (SCH) on fixed vertical
 moorings and on the reef flat by a bottom moored thermistor. determined
 from a pre-deployment calibration bath. Corrected temperatures are within
 0.055 $\degC$. Additionally, water temperature and salinity were measured
 with depth approximately every 2 days throughout the study period by a
 SonTek YSI Castaway CTD at three stations at the west, center, and east
 lagoon. Water velocities were measured by three 1200kHz
 Teledyne RDI Workhorse Acoustic Doppler Current Profilers (ADCPs), one in
 the north channel, one in the south channel, and one on the east reef
 flat. Pressure in the lagoon was recorded by a SBE-37 on the north lagoon
 mooring, reef flat pressure was recorded by the east reef flat ADCP and
 offshore pressure by a bottom mounted ADCP located on the 10 m mooring on
 the east forereef, outside of the lagoon. The
 meteorological data used in this study was measured at the airport located
 on Pratas Island. Measurements include precipitation and shortwave
 radiation sampled at 1-minute intervals. Offshore conditions including
 wind speed, wind direction, significant wave height, and direction were
 obtained from the European Center for Medium-Range Weather Forecasts
 (ECMWF) ERA5 1-hour data on a 30-km grid (Hersbach et al. 2020)
description: # Relative phase between tidal and solar cycles influences the heating of
 a coral atoll lagoon. The
 presented observations are part of a larger study of internal wave
 propagation around Dongsha Atoll Sinnett et al. 2022; Ramp et al. 2022).
 We used measurements of currents, pressure, and temperature in and
 surrounding the lagoon (channels, reef flat, forereef), as well as
 meteorological measurements taken on Pratas Island collected from 20 May
 2019 - 09 June 2019. Water temperature in the lagoon was measured on two
 fixed vertical moorings in the north and south lagoon, two vertically
 profiling moorings (WireWalkers, Rainville and Pinkel (2001)) in the east
 and west lagoon, and on bottom moored thermistors at various locations
 around the lagoon. Water temperature was measured in both the north
 channel (NCH) and south channel (SCH) on fixed vertical moorings and on
 the reef flat by a bottom moored thermistor. determined from a
 pre-deployment calibration bath. Corrected temperatures are within 0.055
 $\degC$. Additionally, water temperature and salinity were measured with
 depth approximately every 2 days throughout the study period by a SonTek
 YSI Castaway CTD at three stations at the west, center, and east lagoon.
 Water velocities were measured by three 1200kHz Teledyne RDI Workhorse
 Acoustic Doppler Current Profilers (ADCPs), one in the north channel, one
 in the south channel, and one on the east reef flat. Pressure in the
 lagoon was recorded by a SBE-37 on the north lagoon mooring, reef flat
 pressure was recorded by the east reef flat ADCP and offshore pressure by
 a bottom mounted ADCP located on the 10 m mooring on the east forereef,
 outside of the lagoon. The meteorological data used in this study was
 measured at the airport located on Pratas Island. Measurements include
 precipitation and shortwave radiation sampled at 1-minute intervals.
 Offshore conditions including wind speed, wind direction, significant wave
 height, and direction were obtained from the European Center for
 Medium-Range Weather Forecasts (ECMWF) ERA5 1-hour data on a 30-km grid
 (Hersbach et al. 2020) \**Instrumentation and mooring names below refer to
 Table 1 in the manuscript text. ** \** File names, instrument locations,
 instrument types, and variable names with units are listed below** *
 **10madcp.nc:** 20o 41.987, 116o 55.421             Teledyne RDI Workhorse
 ADCP             measurements included: pressure (dbar), time  ( Gregorian
 time) * \**    20madcp.nc:**        20o 41.963, 116o 55.536            
 Nortek AWAC             measurements included: pressure (dbar),
 Significant wave height (m), time (Gregorian time) *
 **Dongsha_T_bottom_mounted.txt:** Temp sensors: various depths as listed
 in file             locations: north channel: 20.75o  116.73o            
 north west: 20.72o  116.78o             north: 20.75o  116.84o            
 east: 20.70o  116.89o             east reef: 20.70o  116.89o            
 south east: 20.65o  116.86o             south: 20.63o  116.80o            
 south reef: 20.61o  116.80o             south west: 20.65o  116.75o
             center: 20.68o  116.82o measuring: time (gregorian),
 temperature (C) * **NCH_vel.nc:** 20o 43.676, 116o 45.338            
 Current meter: Teledyne RDI Workhorse ADCP             measuring: E:
 East/West velocity per depth bin  (m/s)             N: North/South
 velocity per depth bin  (m/s)             temp: temperature per depth bin 
 (C)             pressure: pressure at instrument (dbar)             bin
 size: height of depth bins (m)             time: timestep ( Gregorian
 time) * **RBRcon_065801_WWW_data.txt:** 20° 41.065, 116° 47.012 RBR
 Concerto (CTD), profiling whole water column measuring: Time
 (UTC),Conductivity(S/m),Temperature(C),Pressure(dbar),Dissolved
 oxygen(mg/L),Backscatter(m⁻¹sr⁻¹),Chlorophyl(μg/L)l,CDOM (ppb),Sea
 pressure ,Depth (m),Salinity(psu),Specific conductivity(S/m),Density
 anomaly(dbar),Speed of sound(m/s),Dissolved Oxygen concentration (mg/L) *
 **RF_vel.nc:** 20o 41.976, 116o 54.145             Current Meter: Teledyne
 RDI Workhorse ADCP             measuring: E: East/West velocity per depth
 bin  (m/s)             N: North/South velocity per depth bin  (m/s)
             temp: temperature per depth bin  (C)             pressure:
 pressure at instrument (dbar)             bin size: height of depth bins
 (m)             time: timestep ( Gregorian time) *
 **sbe37_11248_nch_5p6mab.csv:** 20o 43.670, 116o 45.336 SBE 37, 5.6 mab
             measuring: time (julian days)             temperature (C)
             salinity (psu)             pressure (dbar)             oxygen
 (mg/l)             oxygen temperature (C)             nitrogen saturation 
 (mg/l)             depth (m)             density (kg/m^3)            
 conductivity (S/m) * **sbe37_14032_lnm_13p5mab.csv:** 20o 43.533, 116o
 49.274 measuring: time (Julian days)             temperature (C)
             salinity  (psu)             pressure (dbar)             depth
 (m)             density (kg/m^3)             conductivity (S/m) *
 **sbe37_15212_wwe.cnv:** 20° 41.377, 116° 52.753 SBE37 (CTD), profiling
 whole water column             measuring: time (julian days)            
 temperature (C)             salinity (psu)             pressure (dbar)
             oxygen (mg/l)             oxygen temperature (C)            
 nitrogen saturation  (mg/l)             depth (m)             density
 (kg/m^3)             conductivity (S/m) * **SBE39_1376_NCH_0p4mab.csv:**
 20o 43.670, 116o 45.336 SBE 39, 0.4 mab             measuring: temperature
 (C), pressure (dbar), time (UTC) * **SBE39_05172_SCH_5p6mab.csv:** 20o
 39.035, 116o 43.403 SBE 39, 5.6 mab             measuring: temperature
 (C), pressure (dbar), time (UTC)   * **SBE39_05173_LSM0p4mab.csv:** 20o
 38.700, 116o 49.939 SBE 39, 0.4 mab             measuring: temperature
 (C), pressure (dbar), time (UTC) * **SBE56_01123_LNM4.0mab.csv:** 20o
 43.533, 116o 49.274 \**                        **SBE 56, 4 mab
                         measuring: time (UTC), temperature (C) *
 **SBE56_03176_LSM9.5mab.csv:** 20o 38.700, 116o 49.939 SBE 56, 9.5 mab
             measuring: time (UTC), temperature (C) *
 **SBE56_03177_LNM11.5mab.csv:** 20o 43.533, 116o 49.274 SBE 56, 11.5 mab
                         measuring: time (UTC), temperature (C) *
 **SBE56_03958_LSW.csv:** 20o 39.639, 116o 46.336 SBE 56, ~10 m depth
 measuring: time (UTC), temperature (C) * **SBE56_03959_LNM14mab.csv:** 20o
 43.533, 116o 49.274 SBE 56, 14 mab             measuring: time (UTC),
 temperature (C) * **SBE56_05483_LNM7.5mab.csv:** 20o 43.533, 116o 49.274
 SBE 56, 7.5 mab                         measuring: time (UTC), temperature
 (C) * **SBE56_06957_LSM13mab.csv:** 20o 38.700, 116o 49.939 SBE 56,13 mab
             measuring: time (UTC), temperature (C) *
 **SBE56_06963_CableReefVal.csv:** 20° 42.005' ,116° 54.564' SBE
 56, ~1 m depth measuring: time (UTC), temperature (C) *
 **SBE56_06972_LSM5.5mab.csv:** 20o 38.700, 116o 49.939 SBE 56, 5.5 mab
             measuring: time (UTC), temperature (C) *
 **SBE56_08980_LNW.csv:** 20o 43.075, 116o 46.864 SBE 56, ~10 m depth
 measuring: time (UTC), temperature (C) * **SCH_vel.nc:** 20o 39.035, 116o
 43.403 \**                        **Current meter: Teledyne RDI Workhorse
 ADCP                         measuring: E: East/West velocity per depth
 bin  (m/s)                         N: North/South velocity per depth bin 
 (m/s)                         temp: temperature per depth bin  (C)
                         pressure: pressure at instrument (dbar)
                         bin size: height of depth bins (m)
                         time: timestep ( Gregorian time) *
 **SoloT_101044_SCh3mab_data.txt:** 20o 39.035, 116o 43.403 RBR SoloT, 3
 mab             measuring: time (UTC), temperature (C) *
 **SoloT_101045_LSM1p5mab_data.txt:** 20o 38.700, 116o 49.939 RBR SoloT,
 1.5 mab             measuring: time (UTC), temperature (C) *
 **SoloT_101200_SCh0p4mab_data.txt:** 20o 39.035, 116o 43.403 RBR SoloT,
 0.4 mab             measuring: time (UTC), temperature (C) *
 **SoloT_101201_NCh3mab_data.txt:** 20o 43.670, 116o 45.336 RBR SoloT 3 mab
             measuring: time (UTC), temperature (C) ## Description of the
 data and file structure Raw data files for the observational platforms
 used in this manuscript are provided and analyzed in the methods described
 above and in the paper. The locations of the various instrument platforms
 referred to in the data files are also listed in the previous section.