Defining spring transition: regional indices for the California Current System

While the physical and ecological importance of the spring transition in the California Current System (CCS) is well recognized, there is no widely agreed upon metric for tracking the phenomenon. Most metrics reflect oceanographic events of the northern CCS; few pertain to southern regions. Our goal was to compare 2 commonly used methods for identifying spring transition in the CCS that use data on wind-driven upwelling and coastal sea levels to 2 newly developed methods that use data on sea levels as well as satellite-derived sea-surface temperatures (SSTs). More specifically, we assessed whether methods typically used in northern regions of the CCS could be applied to other regions. To demonstrate the biological implications of those methods, we evaluated relationships between timing of spring transition and recruitment of 2 groundfish species, Pacific ocean perch Sebastes alutus and sablefish Anoplopoina fimbria. Our results suggest that while dramatic changes in wind-derived upwelling and coastal sea levels consistently indicate spring transition in the northern CCS, this is not the case for central and southern regions. In those regions, spring transition may be better represented by the rate of change in sea levels and/or changes in spatial patterns of SSTs. Only metrics based on wind-driven upwelling and sea levels were related to groundfish recruitment; when transitions in upwelling winds and sea levels were delayed, recruitment tended to be poor. We advise caution when identifying dates of spring transition and applying them to analyses of ecological phenomena; a combination of several methods may be required to reveal the multidimensional physical and biological changes that occur during that transition.