CoAuthors

Peter Gaube (Applied Physics Laboratory, University of Washington, USA); Matthew Long (National Center for Atmospheric Research, USA)

Mesoscale eddies influence upper ocean ecosystems by a variety of mechanisms, including some that stimulate productivity through nutrient supply (eddy pumping, eddy-wind interaction) and/or influencing light availability (eddy impacts on mixed layer depth), as well as those that simply redistribute biogenic material (stirring and trapping). It has been known for some time that these mechanisms can be differentially expressed in space and time. However, it is only recently that regional variations in the influence of mesoscale eddies on near-surface chlorophyll have been quantified on a global basis (Gaube et al., 2014). Coherent patterns of both positive and negative correlation between anomalies of sea level (SLA) and chlorophyll (CHL) reflect variations in the relative balance between these various eddy-driven processes. Areas of positive correlation are indicative of positive CHL anomalies associated with anticyclonic eddies (positive SLA) and negative CHL anomalies with cyclonic eddies (negative SLA). Conversely, regions of negative correlation are indicative of positive CHL anomalies associated with cyclonic eddies, and negative CHL anomalies associated with anticyclones. Attribution of these correlations to specific mechanisms is not possible on the basis of observations alone, as each of these correlation patterns can be produced by multiple mechanisms. A global eddy-resolving coupled physical-biological model is able to simulate many of the observed patterns in SLA-CHL correlation, providing a framework in which to diagnose the contributions of the various processes of mesoscale physical-biological interaction. We have looked in detail at the Gulf Stream region in a basin-scale version of the model, where the SLA-correlation is negative. Both eddy pumping and eddy trapping are candidate mechanisms to produce the observed correlation, and the model suggests eddy trapping is the dominant mechanism in this region. Moreover, we find that the time-evolution of CHL in anticyclonic eddies reflects nutrient supply via eddy-wind interaction, which would tend to produce positive correlation between SLA and CHL. However, this tendency is overshadowed by the influence of trapping in the overall correlation. Analysis of the global solution continues, and we intend to focus next on an area of positive correlation in the South Indian Ocean.