Goal 4: Diagnose dynamics of vertical transport processes at submesoscales to mesoscales.
How does this goal help address the hypothesis?
Vertical transport occurs when vertical velocity w aligns with a transported scalar C so that the product <w'C'> is nonzero. (Here the primes represent the perturbations from the mean, which is denoted by angle brackets). Hence, when warm water aligns with downwelling, heat is transported downward. We will estimate these fluxes, primarily for heat and chlorophyll, over a range of scales, thereby assessing the importance of sebmesoscale fluxes. Measuring vertical fluxes is a challenging goal, but even partial progress will be an important advance.
What needs to be done to achieve this goal and why?
We propose indirect measurements of vertical velocity and flux made possible by the unique observations. Surface velocity divergence D, measured by DopplerScatt, equals -∂W/∂z, the vertical derivative of vertical velocity. We will form proxies for vertical velocity WD=D h, where h is a vertical length scale, and for flux <W'DC'> where C is temperature, chlorophyll, or other biogeochemical properties, such as particulate organic carbon (POC), measured by aircraft remote sensing. We will form estimates of h by, first, combining the in situ measurements of W with DopplerScatt estimates of D, and second, by directly evaluating h in numerical models. Given a recipe for h, we will evaluate the flux proxy using our aircraft measurements, paying particular attention to its statistical stabliity, convergence and sensitivity to instrumental noise. We will make parallel evaluations using numerical models, both as a reality check on the proxy and a test of the model accuracy. We will assess the importance of the submesoscale, our primary hypothesis, by comparing this proxy with Omega equation estimates of vertical velocity and flux (Pallàs-Sanz et al. 2010) from the mesoscale surveys, with the small-scale turbulent flux estimates from the Seagliders, and by directly examiming the contributions of different horizontal scales to the flux proxies.
