Journal
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS
Volume 127, Issue 4, Pages -Publisher
AMER GEOPHYSICAL UNION
DOI: 10.1029/2021JC017912
Keywords
coastal ocean; thermal exchange; numerical modeling; buoyancy; ROMS; diurnal processes
Categories
Funding
- National Science Foundation (NSF) [1436254]
- NSF [1436522]
- Directorate For Geosciences [1436254] Funding Source: National Science Foundation
- Division Of Ocean Sciences [1436254] Funding Source: National Science Foundation
- Division Of Ocean Sciences
- Directorate For Geosciences [1436522] Funding Source: National Science Foundation
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Numerical modeling is used to study how cross-shore flow is affected by steady alongshore currents and shear-generated turbulence. The results show that shear-driven vertical mixing reduces temperature gradients and decreases cross-shore thermal exchange with increasing alongshore current speed. Alongshore flow can enhance or diminish the baroclinic cross-shore exchange flow, depending on its strength, ultimately leading to a dominance of Ekman dynamics for exchange.
Idealized numerical modeling of thermally driven baroclinic exchange is performed to understand how cross-shore flow is modulated by steady alongshore currents and associated shear-generated turbulence. In general, we find that shear-driven vertical mixing reduces the temperature gradients responsible for establishing the baroclinic flow, such that cross-shore thermal exchange diminishes with alongshore current speed. Circulation in a base-case simulation of thermal exchange with no alongshore forcing contains a cooling response consisting of a midday flow in the form of a downslope current with a compensating onshore near-surface flow driving cross-shore exchange, followed by an afternoon warming response flow via an offshore-directed surface warm front, with a compensating return flow at the bottom. Nighttime convective cooling enhances vertical mixing and decelerates the warming response, and the diurnal cycle is renewed. In this base-case scenario, representative of tropical reef environments with optically clear water and weak alongshore flow, surface heating and cooling can drive cross-shore circulation with O(1) cm s(-1) velocities. Alongshore flow forcing is implemented to induce upwelling- and downwelling-favorable cross-shore circulation. For mild alongshore forcing, the baroclinic cross-shore exchange flow is enhanced due to an increase in the horizontal temperature gradient. Stronger alongshore flow leads to diminished thermally driven exchange, ultimately reaching a regime where the cross-shore exchange is due predominantly to Ekman dynamics. Though exchange velocities are relatively small (O(1) cm s(-1)), these persistent exchange flows are capable of flushing the nearshore region multiple times per day, with important implications for water properties of nearshore ecosystems.
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