Spatial structure of diurnal variability from profiling float arrays

Relevance. This proposal is submitted to the Climate Variability and Predictability Program (CVP)-Observing and Understanding Processes Affecting the Propagation of Intraseasonal Oscillations in the Maritime Continent (MC) Region. This project addresses NOAA’s and CVP’s goals by observing the upper ocean at high temporal and vertical resolution during the active and suppressed phases of a Madden-Julian Oscillation (MJO). This coupled-air sea phenomenon affects El Nino, monsoons, cyclones, and atmospheric rivers either directly or through atmospheric teleconnections. Global climate models have difficulty accurately predicting the propagation of MJO through the varied topography and enclosed seas of the MC. Better MJO prediction requires high spatial and temporal resolution of the diurnal cycle in observations and models because of diurnal effects on sea surface temperature (SST). Amidst a varying background (mesoscale, MJO phase, and lateral gradients in diurnal and mean structure of winds, precipitation, and stratification), arrays of state-of-the-art autonomous profiling floats will observe subsurface processes affecting (a) diurnal SST variability and (b) thin salinity-controlled upper ocean mixed layers, which in turn produce enhanced SST variability.
Diurnal cycle. The Years of the Maritime Continent (YMC) Science Plan notes, that models poorly predict the observed diurnal variability in the MC and the barrier effect of the MC on MJO. Diurnal SST both modulates and is modulated by MJO: diurnal SST affects the initiation, propagation, and strength of MJO, while MJO affects diurnal insolation and heat fluxes. The diurnal cycle of mixing raises and lowers the mixed layer depth. The diurnal wind’s propagation across the MC’s enclosed seas is affected by SST and geography of islands and seas. Therefore, spatially and temporally extensive measurements of the diurnal cycle in the upper ocean are needed. Subsurface processes affecting SST. Fresh water input from June–October into the South China Sea (SCS), for example, produces a mixed layer controlled by salinity (S) and a roughly isothermal layer beneath, which isolates the mixed layer from cooler thermocline waters. Heat fluxes are concentrated into a thin mixed layer and enhance SST variability. Therefore, accurate SST prediction requires high vertical and temporal resolution observations and modeling of processes affecting not only SST, but also subsurface S and stratification. The spatial S structure depends on lateral stirring and vertical mixing of fresher surface waters. Spatially extensive measurements of S variability at different sites and times are needed to forecast SST.
Plan. These measurements will augment intensive observations in the SCS by PISTON and around the southern MC by Australia’s YMC program in 2018. Floats will be seeded in the SCS and around the MC, in a region of varying S, diurnal amplitude, and winds. Eight floats will be deployed but not recovered from each of R/V Thompson in PISTON and R/V Investigator in YMC in a nominal 50 km box with the flow dispersing the floats over a larger area. The SOLO-II float is a proven tool in the ARGO program and will be optimized here for rapid, shallow profiling. Arrays of SOLO-II floats will yield extensive 3-D coverage of the upper ocean over 30 days via clean, near-surface, high-vertical resolution profiles of T and S from 0–50 m every 25 minutes and produce ∼3500 profiles/float. Alternatively 250-m profiles provide 90-day endurance and ∼1600 profiles/float. Float arrays provide spatial coverage as does the drift of each float. In total, up to 57,000 T and S profiles to 50 m will be obtained (or 26,000 profiles to 250 m).