Blooms in Hawaii: An investigation using a high-resolution decadal reanalysis of ROMS+COBALT

The North Pacific Subtropical Gyre (NPSG) is the largest marine ecosystem on Earth comprising about 12% of the Pacific Ocean. The region around the main Hawaiian Islands is oligotrophic and subject to short-lived, stochastic bloom events and numerous mesoscale processes have been proposed to facilitate the vertical injection of nutrients into the euphotic zone. Using the Regional Ocean Modeling System (ROMS) and the ecosystem model Carbon, Ocean Biogeochemsitry and Lower Trophics (COBALT) with a 4 km horizontal resolution we reconstruct the physical and biogeochemical conditions around Hawaii based on a 10-year state-estimate assimilating 35 million physical observations. Our model results for the current period (2010-2017) exhibit promising agreement between simulated values and observations at Station ALOHA for physical and biogeochemical parameters. The overall levels and the amplitude of the seasonal cycles are well captured for many parameters. We explore the role of mesoscale processes such as eddy-induced uplift of isopycnals, frontogenesis and straining as well as the impact of large-scale dynamics in triggering bloom events and in shaping the seasonal cycle of primary productivity in the area.