[1] Strengthened stratification of the upper ocean, associated with anthropogenic or climatedriven warming, is generally expected to inhibit marine primary productivity in lightreplete, nutrient-limited environments, essentially, in the low and middle latitude ocean, based on the supposition that increased water column stability will inhibit vertical mixing and consequently the upward entrainment of deep nutrients into the euphotic zone. Herein, we examine the local stratification control of productivity on interannual timescales over the global subtropical and tropical ocean by directly comparing in situ measures of stratification (from hydrographic profile data) with contemporaneous values of ocean chlorophyll (from satellite data). In the subtropical ocean, we find no evidence of a strong local correlative relationship between these properties over the observational record, a result that challenges the widely held view that stratification variability is a primary driver of interannual variability in nutrient supply and productivity in these waters. A strong negative relationship is observed, however, in the tropical Pacific, suggesting that previously reported correlations between globally averaged stratification and productivity variability are driven by strong associations in this region. An examination of the long-term changes in our profile data also reveals trends of decreasing stratification scattered across the lowlatitude and mid-latitude ocean, driven by faster rates of warming in the subsurface relative to the surface. This observation seemingly undercuts a fundamental assumption of the paradigm of local stratification control, namely that increases in upper ocean heat content necessarily produce strengthened stratification.Citation: Dave A. C., and M. S. Lozier (2013), Examining the global record of interannual variability in stratification and marine productivity in the low-latitude and mid-latitude ocean,

Recent observational studies linking variability in global ocean productivity with upper ocean warming are based on the paradigm that warming produces a more stable water column which, in turn, inhibits primary productivity for a large fraction of the global ocean, namely the tropics and subtropics. Though seemingly straightforward, this paradigm relies on the assumption that an increase in the stratification of the upper ocean water column decreases the vertical mixing or overturning of the surface waters such that the supply of nutrients to the euphotic zone is reduced, and so too the primary productivity. Here we show, using observational data in the North Atlantic subtropical gyre, that while upper ocean stratification and primary productivity are strongly linked on seasonal time scales, they have at most a weak correlative relationship on interannual time scales over the modern observational record. We suggest that interannual variability in ocean biomass and primary productivity depends on a host of variables that are not easily predicted from the expected temperature response to climate variability and change. These variables include the strength of local and remote wind and buoyancy forcing and the surface or subsurface advective supply of nutrients.

[1] Strengthened stratification of the upper ocean due to global warming is generally expected to inhibit marine primary productivity in the subtropics, based on the supposition that increased water column stability will decrease vertical mixing and consequently the entrainment of deep nutrients into the euphotic zone. A recent analysis of observational data from the subtropical North Atlantic, however, demonstrates that productivity in this region is not correlated with stratification on interannual time scales over the modern observational record, but is instead impacted by other dynamics that affect vertical mixing and nutrient supply. Herein, we examine data from the Hawaiian Ocean Time series program's Station ALOHA (A Long-Term Oligotrophic Habitat Assessment) in the subtropical North Pacific. We find that stratification and productivity are not strongly correlated at this location over the observational record. In contrast to the North Atlantic, the weakness of correlation observed at ALOHA may reflect the strongly stratified ecosystem of the eastern subtropical North Pacific and a lack of sufficiently strong interannual forcing in this region. Although basin-wide climate processes (namely El Niño-Southern Oscillation and Pacific Decadel Oscillation) have previously been suggested to impact local stratification and vertical nutrient supply at ALOHA, we find no evidence of a strong or consistent linkage. Comparing local ecosystem variability to the recently identified North Pacific Gyre Oscillation, however, we observe a correlation with local subsurface productivity and salinity. The correlations have similar structure in both space (i.e., depth) and time and are possibly linked to dynamics associated with the formation and advection of water masses in the central gyre.

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The oligotrophic regions of the subtropical gyres cover a significant portion of the global ocean, and exhibit considerable but poorly understood intraseasonal, interannual, and longer‐term variations in spatial extent. Here using historical observations of surface ocean nitrate, wind, and currents, we have investigated how horizontal and vertical supplies of nitrate control seasonal changes in the size and shape of oligotrophic regions of the subtropical North Atlantic. In general, the oligotrophic region of the subtropical North Atlantic is associated with the region of weak vertical supply of nitrate. Though the total vertical supply of nitrate here is generally greater than the total horizontal supply, we find that seasonal expansion and contraction of the oligotrophic region is consistent with changes in horizontal supply of nitrate. In this dynamic periphery of the subtropical gyre, the seasonal variations in chlorophyll are linked to variations in horizontal nitrate supply that facilitate changes in intracellular pigment concentrations, and to a lesser extent, phytoplankton biomass. Our results suggest that horizontal transports of nutrient are crucial in setting seasonal cycles of chlorophyll in large expanses of the subtropical North Atlantic, and may play a key and underappreciated role in regulating interannual variations in these globally important marine ecosystems.

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Previously reported global‐scale correlations between interannual variability in upper ocean stratification and chlorophyll a (a proxy for phytoplankton biomass) have been shown to be driven by strong associations between the two properties in the central and western equatorial Pacific. Herein, we present evidence that these correlations are not causal but instead result from the advection of heat, salt, and nutrients in the region. Specifically, we demonstrate that stratification and chlorophyll are simultaneously influenced by shifts in the horizontal advective inputs of cold/saline/nutrient‐rich waters from upwelling regions to the east and warm/fresh/nutrient‐poor waters to the west. We find that horizontal advection contributes substantially to the annual surface layer nutrient budget and, together with vertical advection, significantly impacts interannual variability in chlorophyll. These results highlight the importance of a three‐dimensional framework for examining nutrient supply in the upper ocean—a crucial requirement for assessing future marine ecosystem responses to a changing climate.

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