Mesoscale physical–biological–biogeochemical linkages in the open ocean: An introduction to the results of the E-Flux and EDDIES programs

Mesoscale currents, fronts, and eddies are ubiquitous and energetic features of ocean circulation. These phenomena, sometimes referred to as the ‘‘internal weather of the sea,’’ accommodate a diverse set of physical, chemical, and biological interactions that influence marine biogeochemistry on a wide range of timescales. These biogeochemical processes include the ‘‘biological pump,’’ i.e., the transfer or flux of biologically produced organic matter and associated elements from the surface ocean to depth (Ducklow et al., 2001; Volk and Hoffert, 1985). Within 80% of the world’s oceans, productivity of the autotrophic organisms that contribute to the biological pump are typically limited by major nutrients (e.g., nitrogen, phosphorus, and silica) or trace metals (e.g., iron). Primary production in such oligotrophic regions therefore depends mostly on intense recycling of nutrients within the surface sunlit waters, with only a small fraction supported by that entering from the atmosphere, or from the physical transport of nutrients from nutrient-rich deep waters below. Evidence that mesoscale and submesoscale phenomena play a role in the latter process dates back more than two decades (Angel and Fasham, 1983; Franks et al., 1986; Ring Group, 1981; Tranter et al., 1980; Venrick, 1990; Woods, 1988). In the open ocean there are several common types of eddies. In the northern hemisphere, cyclones tend to create a doming of the seasonal and main pycnoclines, while anticyclones depress both. Mode-water eddies (MWEs) are anticyclonic in rotation; however, the seasonal pycnocline domes while the main thermocline is depressed (McGillicuddy et al., 1999). Wind-driven eddies in the lee of Hawaii can be either cyclonic or anticyclonic; cyclonic lee eddies have surface intensified features that dome the seasonal thermocline (Lumpkin, 1998; Patzert, 1969). In the Sargasso Sea, cyclones and MWEs have been hypothesized to reconcile differences between tracer-based estimates of new production and traditional shipboard methods that may miss eddy-induced biological activity (Jenkins, 1988). Estimates of the integrated impact of eddydriven nutrient fluxes vary considerably, from less than 10% to more than 50% of annual new production