Quantifying mesoscale-driven nitrate supply: A case study

摘要

The supply of nitrate to surface waters plays a crucial role in maintaining marine life. Physical processes at the mesoscale (similar to 10-100 km) and smaller scale have been advocated to provide a major fraction of the global supply. While observational studies have focused on well-defined features, such as isolated eddies, the vertical circulation and nutrient supply in a typical 100-200 km square of ocean will involve a turbulent spectrum of interacting, evolving, and decaying features. A crucial step in closing the ocean nitrogen budget is to be able to rank the importance of mesoscale fluxes against other sources of nitrate for surface waters for a representative area of open ocean. While this has been done using models, the vital observational equivalent is still lacking. To illustrate the difficulties that prevent us from putting a global estimate on the significance of the mesoscale observationally, we use data from a cruise in the Iceland Basin where vertical velocity and nitrate observations were made simultaneously at the same high spatial resolution. Local mesoscale nitrate flux is found to be an order of magnitude greater than that due to small-scale vertical mixing and exceeds coincident nitrate uptake rates and estimates of nitrate supply due to winter convection. However, a nonzero net vertical velocity for the region introduces a significant bias in regional estimates of the mesoscale vertical nitrate transport. The need for synopticity means that a more accurate estimate cannot be simply found by using a larger survey area. It is argued that time series, rather than spatial surveys, may be the best means to quantify the contribution of mesoscale processes to the nitrate budget of the surface ocean.