High variability in cross-shore thermally driven exchange

High variability in cross-shore thermally driven exchange Hugo N. Ulloa (1) , Kristen A. Davis (2) , Geno Pawlak (1) and Stephen G. Monismith (3) (1) Mechanical and Aerospace Engineering, University of California, San Diego, USA and Environmental Engineering, University of California, Irvine, USA (3) Department of Civil and Environmental Engineering, Stanford University, USA hulloasanchez@eng.ucsd. edu (2) Civil Abstract The variability of cross-shore thermally driven exchange was examined using ensemble-averages of observations from the Kilo Nalu Observatory on the south shore of Oahu Hawaii. The cross- shore vertical shear, δV z , top-bottom temperature difference, ∆T z , and cross-shore advective heat flux, Q ex , were analyzed to evaluate the influence of the surface heat flux, the cross-shore wind and the M 2 tide in the cross-shore exchange variability. The M 2 affects the exchange through the effects of tidally driven alongshore flow on turbulent diffusivity and on Coriolis driven cross-shore accelerations. Lunar phase ensemble-averages are compared with a theoretical model to show that the interaction of the diurnal wind pattern and the tidally driven alongshore flow can lead to significant cross-shore exchange variability at sub-diurnal time scales. Introduction Nearshore physical processes can modulate the mass, momentum and heat transport in complex ecosystems such as coral reefs (Monismith, 2007). A key physical process in these environments is the cross-shore exchange, between nearshore and the adjoining ocean, because it regulates the transport of nutrients, phytoplankton and waste (e.g., Genin et al., 2009; Paul et al., 1997). Cross-shore exchange can be driven by a variety of mechanisms, including Ekman transport, buoyancy driven flows, cross-shelf wind driven surface currents, internal waves, and surface and internal tides, all of which have different spatial and temporal scales. The role of these various processes will depend on local characteristics, such as bathymetry, shelf width, stratification, etc (Monismith et al., 2006). In particular, field studies have identified that diurnal surface heat flux is an important mechanism in driving for cross-shore flows at diurnal time scale in coastal regions, where the bed depth increases toward offshore (Monismith et al., 1990, 2006; Molina et al., 2014). Here, the heat flux acting on the surface with a diurnal periodicity causes shallower waters to heat and cool more rapidly than deeper waters, thus setting up a cross-shore temperature gradient due to the change in depth. This thermal structure drives cross-shore baroclinic flow that exchanges momentum and heat between the reef and the adjoining ocean. The cross-shore flow pattern associated with the cooling/heating diurnal phase response is shown in schematic form in figure 1; the fluid response associated with the cooling phase (0-12 hr) leads to colder offshore flow at the bottom, balanced by warmer onshore flow at the surface, with the opposite flow pattern for the heating phase (12-24 hr) (Farrow, 2004; Monismith et al., 2006). Field observations of the velocity and thermal structure at Kilo Nalu Observatory (KNO) on the south shore of Oahu, Hawaii show that thermally driven baroclinic exchange is a dom- inant mechanism for cross-shore transport for this tropical forereef environment, modulated principally by the diurnal surface heat flux signal, the wind speed/direction and tides (Molina et al., 2014). Velocity and temperature data were collected from a bottom mounted acoustic Doppler current profiler (ADCP, 1200 kHz RDI Workhorse) and a thermistor chain (T-chain, Precision Measurement Engineering Inc.) deployed at 12 m depth and 370 m from shore at KNO site during January to September of 2010. Local atmospheric data, including pressure, relative humidity, short wave radiation, air temperature, wind speed/direction were available for this period. Currents, tidal height and temperature data were averaged into 20 min ensembles and atmospheric data were averaged into 1 hr ensembles. The bathymetry slope at KNO is VIII th Int. Symp. on Stratified Flows, San Diego, USA, Aug. 29 - Sept. 1, 2016