[A-CG34] Projection and detection of global environmental change

[ACG34-03] Low-latitude mesopelagic nutrient recycling controls on productivity and export

*Keith Bradley Rodgers¹, Olivier Aumont², Katsuya Toyama³, Laure Resplandy⁴, Masao Ishii³, Toshiya Nakano⁶, Daisuke Sasano⁵, Daniele Bianchi⁷, Ryohei Yamaguchi⁸ (1.IBS Center for Climate Physics, Busan, South Korea, 2.Sorbonne Universités, Paris, France, 3.Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, Japa, 4.Princeton University Department of Geosciences, Princeton, USA, 5.Atmosphere and Ocean Department, Japan Meterological Agency, Tokyo, Japan, 6.Nagasaki Ocean Academy, Nagasaki, Japan, 7.Department of Atmospheric and Ocean Sciences, University of California, Los Angeles, Los Angeles, USA, 8.Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology)

Keywords:Earth system modeling, Marine ecosystems, Climate Change, Ocean biogeochemical modeling

Analyses of CMIP6 Earth system model extensions of projections to 2300 under sustained high emissions (SSP5-8.5) reveal remarkable divergence in the forced changes to low latitude (30°S-30°N) primary production and export. Important differences are seen in not only the amplitude but also the sign of the aggregated low-latitude response. To date, attribution of such changes in CMIP extension runs that consider multi-century projections has consisted of invoking the long-standing paradigm of Sarmiento et al. (2004), namely by the study of Moore et al. (2018) using CESM1. Specifically, it has been argued that the Southern Ocean-sourced Subantarctic Mode Water (SAMW) supply of preformed nutrients will be disrupted as a consequence of shifting Southern Ocean winds, thereby starving the low-latitudes of nutrients needed to sustain primary production and export.

Here we argue instead that it is the combined effect of local low-latitude remineralization and vigorous renewal of thermocline waters through the shallow overturning within the low-latitude mesopelagic domain that sustains 72% of low-latitude primary production and 55% of low-latitude export. This is identified through a sensitivity study with a state-of-the-art forward ocean biogeochemical model. The results of the sensitivity study are used to interpret observation-based products (including GLODAPv2) that indicate that the regenerated fraction of macronutrients is greater than 50% of much of the low-latitude mesopelagic domain spanning 150m-870m. Further model sensitivity experiments indicate reveal the SAMW supply of nutrients from the Southern Ocean only supplies 7% of low latitude export, with low-latitude mesopelagic remineralization being 7 to 8 times more important.

The results with the sensitivity analyses strongly point to a local low-latitude mechanism as the source of the disparity between the longer CMIP6 projections of primary production to 2300. In particular, the results implicate temperature-dependence of remineralization as playing a first-order role in determining the response sustained by low-latitude water mass renewal, and thereby as the source of uncertainty in projections of primary production. This underscores the importance of further developing our understanding of mesopelagic remineralization and its sensitivity to ocean warming for predicting future ecosystem changes.