Relative phase between tidal and solar cycles influences the heating of a coral atoll lagoon

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.