Hydrodynamics of a Coral Atoll Lagoon and Its Connection to Large-Scale Climate Variability
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Poemnamthip-ppoem-PhD-EAPS-2026-Thesis.pdf
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Author(s)
Poemnamthip, Phadtaya
Advisor(s)
Zhang, Weifeng (Gordon)
Date Issued
May 2026
Publisher
Massachusetts Institute of Technology
Abstract
Coral atoll lagoons are dynamic environments in which hydrodynamic processes play a critical role in regulating environmental conditions that influence reef ecosystems. This thesis investigates local hydrodynamic variability over different time scales, the influence of large-scale climate variability, and water dispersal timescales in the semi-enclosed lagoon of Kanton Island, Republic of Kiribati, using a high-resolution three-dimensional numerical model. The baseline hydrodynamic variability is characterized across tidal to seasonal timescales, showing that tidal flows through the ~500 m-wide single, bifurcated channel dominate circulation in lagoon regions near the channel and maintain water properties there close to those of the open ocean. The tidal influence weakens toward the more isolated lagoon regions farther to the east, particularly Line Reef and Back Lagoon regions. In these regions, wind-driven flows, lateral stirring, and air–sea interactions, including solar radiation, evaporation, and precipitation, govern variability in circulation, temperature, and salinity. These different forcing mechanisms across the lagoon drive pronounced spatial heterogeneity in the lagoon hydrodynamic conditions.
The impact of the El Niño–Southern Oscillation (ENSO) on lagoon hydrography and circulation is then examined. Lagoon temperature near the channel is primarily controlled by open-ocean sea surface temperature, whereas local air–sea heat exchange modulates temperature in the shallow central and back lagoon regions farther from the channel. During El Niño events, enhanced precipitation and cloud cover reduce incoming shortwave radiation, producing episodic cooling and altering salinity and density gradients, which modify stratification and reverse baroclinic flow tendencies. Surface flows in the lagoon are determined by both wind-driven and baroclinic exchange flows, with their relative importance varying across ENSO phases. Differences between El Niño events further demonstrate that lagoon hydrodynamics depend not only on the large -scale atmospheric and oceanic forcings but also on the interaction of the forcings, particularly precipitation and winds, with the local geometry and bathymetry. Water dispersal timescales in the lagoon are investigated using water age as a diagnostic parameter. Simulations reveal a consistent spatial pattern of the lagoon water age from <5 days near the channel to >50 days in the back lagoon, with relatively small vertical variation. Water age differs substantially between different ENSO phases, particularly in the central and back lagoon, where El Niño conditions are associated with lower water age and enhanced temporal variability. Analysis indicates that the lagoon water age is controlled by a balance between wind-driven flow and baroclinic flow induced by cross-lagoon pressure gradients, with wind and rainfall regulating both the magnitude and direction of these flows. Together, these results provide a dynamic understanding of hydrodynamic variability and water dispersal in a coral atoll lagoon and highlight the importance of local hydrodynamic processes in shaping reef environmental conditions and assessing coral reef resilience under future climate variability.
The impact of the El Niño–Southern Oscillation (ENSO) on lagoon hydrography and circulation is then examined. Lagoon temperature near the channel is primarily controlled by open-ocean sea surface temperature, whereas local air–sea heat exchange modulates temperature in the shallow central and back lagoon regions farther from the channel. During El Niño events, enhanced precipitation and cloud cover reduce incoming shortwave radiation, producing episodic cooling and altering salinity and density gradients, which modify stratification and reverse baroclinic flow tendencies. Surface flows in the lagoon are determined by both wind-driven and baroclinic exchange flows, with their relative importance varying across ENSO phases. Differences between El Niño events further demonstrate that lagoon hydrodynamics depend not only on the large -scale atmospheric and oceanic forcings but also on the interaction of the forcings, particularly precipitation and winds, with the local geometry and bathymetry. Water dispersal timescales in the lagoon are investigated using water age as a diagnostic parameter. Simulations reveal a consistent spatial pattern of the lagoon water age from <5 days near the channel to >50 days in the back lagoon, with relatively small vertical variation. Water age differs substantially between different ENSO phases, particularly in the central and back lagoon, where El Niño conditions are associated with lower water age and enhanced temporal variability. Analysis indicates that the lagoon water age is controlled by a balance between wind-driven flow and baroclinic flow induced by cross-lagoon pressure gradients, with wind and rainfall regulating both the magnitude and direction of these flows. Together, these results provide a dynamic understanding of hydrodynamic variability and water dispersal in a coral atoll lagoon and highlight the importance of local hydrodynamic processes in shaping reef environmental conditions and assessing coral reef resilience under future climate variability.
MIT Department
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
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