Altimetry, SST and ocean colour unveil the effects of planetary waves on phytoplankton
National Oceanography Centre, Southampton,
SO14 3ZH Southampton,
Fifteen years of altimetry have dramatically increased our knowledge of planetary waves in the ocean, highlighting the ubiquity of the waves and reshaping the relevant theory. Prompted by this success, research has more recently widened into detecting the signature of the waves in other datasets. While their demonstrated presence in the SST record was somehow expected on the basis of dynamical considerations and in view of the long known correlation between temperature and height, the recent discovery of the almost ubiquitous signature of planetary waves in maps of phytoplankton chlorophyll from ocean colour satellites raises a number of intriguing questions. What is causing the westward propagating signals in the ocean colour records? Are there purely mechanical reasons for it, or is there a contribution due to biological processes? And if the latter is true, what is the importance of planetary waves for primary production and for the global carbon cycle?
The above questions can be addressed by contrasting the altimetric observations with those in other datasets. In this paper we will first review a number of studies that have highlighted the wave signature in SST and ocean colour; then we will discuss the different mechanisms that have been suggested to explain the formation of such a signature. These mechanisms range from purely physical, such as the horizontal advection of phytoplankton due to geostrophic velocities associated with the waves, to purely biological such as the increased growth due to upwelled nutrients. We will then show a comparative analysis of the wave signatures in altimetry and in the other datasets, based on cross-spectra, and we will demonstrate how this analysis, in combination with process modelling, is capable to shed some light on which mechanisms predominate in different regions of the world’s ocean. We will conclude by discussing those questions that still remain open, and in particular the significance of planetary wave-induced production in the oceanic carbon pool.