Advisor: Raymond G. Najjar

Abstract: Closures of upper-ocean heat budgets at National Oceanic and Atmospheric Administration (NOAA) North Pacific moorings KEO (32.3°N, 144.6°E) and Papa (50.1°N ,144.9°W) suggest seasonal and regional variability in turbulent diffusion below the mixed layer. Motivated by this variability, this investigation combines two years of hourly Acoustic Doppler Current Profiler velocities with subsurface density profiles at KEO and Papa to compute hourly Richardson number time series below the mixed layer. Results suggest Richardson numbers at KEO and Papa most likely indicate shear-based turbulence below the mixed layer (Ri << 1) from early to late spring, preceding the development of the seasonal pycnocline. Moreover, Richardson numbers at KEO are found to be significantly smaller, sometimes by an order of magnitude, than at Papa throughout the two-year investigation and are likely attributed to stronger velocity shears below the mixed layer at KEO. Peaks in prevalence of turbulence, defined as the number of hours per day when Ri < 1, exhibit periodicity of 6.5 days, matching that of peaks in diffusivities. Peaks in prevalence of turbulence and diffusivities also appear co-located at KEO during late summer and early fall, suggesting shear-based turbulence might be at times a first-order turbulent diffusion process near western boundary currents. Diffusivities at both moorings are also found to correlate negatively with surface heat flux and positively with wind stress throughout this investigation. These correlations are statistically significant at the 5% level and increase confidence local meteorology is yet another process responsible for variability observed in turbulent diffusion below the mixed layer.