Relative phase between tidal and solar cycles influences the heating of a coral atoll lagoon
Authors/Creators
- 1. University of California, Irvine
- 2. University of California, San Diego
- 3. Taiwan National Academy of Marine Research*
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.
Notes
Funding provided by: National Science Foundation
Crossref Funder Registry ID: https://ror.org/021nxhr62
Award Number: NSF-OCE1753317
Methods
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)
Files
Dongsha_Heat_Budget_Data_Archive_.zip
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