The ocean’s biological carbon pump (BCP) plays a key role in global carbon cycling by transporting biologically-fixed carbon from the surface to the deep ocean. Prior analyses of the BCP in Earth System Model (ESM) simulations have typically evaluated particulate organic carbon (POC) flux at a fixed export depth horizon of 100 m. However, this overlooks spatial and temporal variations in the depth that sinking POC must penetrate to reach the mesopelagic, or to sequester carbon from the atmosphere on climate-relevant timescales. We use depth-resolved POC flux output from eight Coupled Model Intercomparison Project Phase 6 (CMIP6) ESMs to compare global and regional changes in POC flux at five export depth horizons ‒100 m, the base of the euphotic zone (EZ depth), the particle compensation depth (PCD), the maximum annual mixed layer depth (MLDmax), and 1000 m ‒ under the high-emissions scenario SSP5-8.5. We also examine the relationship among net primary production, export efficiency from the surface ocean, and transfer efficiency to depth in key regions of the ocean, identifying model- and region-specific variations in the mechanistic drivers of POC flux changes in the deep ocean. Globally and spatially, trends in POC flux magnitude and decline are similar at the four surface export depth horizons, and multimodel variability in POC flux change by 2100 is greatest at the 1000 m export depth horizon (+4% to -55%). This indicates the importance of improving model parameterizations of transfer efficiency and of POC flux to the deep ocean.

The North Atlantic subpolar gyre experienced strong freshening in recent years starting around 2012. Here, we investigate the imprint of this freshwater anomaly on the water column hydrography and transport variability of the Irminger Current (IC). The IC transports warm and saline waters northward along the western flank of the Reykjanes Ridge as part of the upper limb of the Atlantic Meridional Overturning Circulation (AMOC). To investigate if the salinity anomaly spread and propagated downward, we used high-resolution mooring data from the IC covering the period 2014 – 2022 combined with hydrographic sections from the Irminger Sea and Iceland Basin. We found that the IC experienced a strong freshening starting in summer 2016. By 2018, this salinity anomaly covers the whole water column down to 1500 m depth and freshened the IC until 2022. In 2022, the IC was at its freshest state observed since the early 1990’s. Hydrographic sections across the adjacent basins showed that the recent freshening spread across the Irminger Sea and was also comparable to its fresh state in the early 1990’s. The salinity anomaly increased the freshwater transport of the IC by a factor of three from 2014-2015 to 2021-2022 and caused a decrease in density over much of the water column. This resulted in an increase in the transport of waters lighter than 27.55 kg m-3, potentially strengthening the upper limb of the AMOC.

Transformation of light to dense waters by atmospheric cooling is key to the Atlantic Meridional Overturning in the Subpolar Gyre. Convection in the center of the Irminger Gyre determines the transformation of the densest waters east of Greenland. We present a 19-year (2002-2020) weekly time series of hydrography and convection in the central Irminger Sea based on (bi-)daily mooring profiles supplemented with Argo profiles. A 70-year annual time series of shipboard hydrography shows that this mooring period is representative of longer term variability. The depth of convection varies strongly from winter to winter (288-1500 dbar), with a mean March climatogical mixed layer depth of 470 dbar and a mean maximum density reached of 27.70 ± 0.05 kg m-3. The densification of the water column by local convection directly impacts the sea surface height in the center of the Irminger Gyre and thus large-scale circulation patterns. Both the observations and a Price-Weller-Pinkel (PWP) mixed layer model analysis show that the main cause of interannual variability in mixed layer depth is the strength of the winter atmospheric surface forcing. Its role is three times as important as that of the strength of the maximum stratification in the preceeding summer. Strong stratification as a result of a fresh surface anomaly similar to the one observed in 2010 can weaken convection by approximately 170 m on average, but changes in surface forcing will need to be taken into account as well when considering the evolution of Irminger Sea convection under climate change.

The subpolar North Atlantic plays an outsized role in the atmosphere-to-ocean carbon sink. The central Irminger Sea is home to well-documented deep winter convection and high phytoplankton production, which drive strong seasonal and interannual variability in regional carbon cycling. We use observational data from moored carbonate system chemistry sensors and annual turn-around cruise samples at the Ocean Observatories Initiative’s Global Irminger Sea Array to construct a near-continuous time series of mixed layer dissolved inorganic carbon (DIC), pCO2, and total alkalinity from summer 2015 to summer 2022. We use these carbonate system chemistry time series to deconvolve the physical and biological drivers of surface ocean carbon cycling in this region on seasonal, annual, and interannual time scales. We find high annual net community production within the seasonally-varying mixed layer, averaging 9.7±1.7 mol m-2 yr-1 with high interannual variability (range of 6.0 to 13.7 mol m-2 yr-1). The highest daily net community production rates occur during the late winter and early spring, prior to the observed high chlorophyll concentrations associated with the spring phytoplankton bloom. As a result, the winter and early spring play a much larger role in biological carbon export from the mixed layer than traditionally thought.

The export flux of organic carbon from the upper ocean is the starting point of the transfer and long term storage of photosynthetically-fixed carbon in the deep ocean. This “biological carbon pump” is a significant component of the global carbon cycle, reducing atmospheric CO2 levels by ~ 200 ppm. Carbon exported out of the upper ocean also fuels the productivity of the mesopelagic zone, including significant fisheries. Here we show that, despite its importance, export flux is poorly constrained in Earth System Models, with the modelled range in projected future global-mean changes due to climate change spanning +1.8 to -41%. Fundamental constraints to understanding export flux arise because a myriad of interconnected processes make the biological carbon pump challenging to both observe and model. Our synthesis prioritises the processes likely to be most important to include in modern-day estimates (particle fragmentation and zooplankton vertical migration) and future projections (phytoplankton and particle size spectra, and temperature-dependent remineralisation) of export. We also identify the observations required to achieve more robust characterisation, and hence improved model parameterization, of export flux, and thus decrease uncertainties in current and future estimates of this important planetary carbon flux.

Global Earth system model simulations of ocean carbon export flux are commonly interpreted only at a fixed depth horizon of 100-m, despite the fact that the maximum annual mixed layer depth (MLDmax) is a more appropriate depth horizon to evaluate export-driven carbon sequestration. We compare particulate organic carbon (POC) flux and export efficiency (e-ratio) evaluated at both the MLDmax and 100-m depth horizons, simulated for the 21st century (2005-2100) under the RCP8.5 climate change scenario with the Biogeochemical Elemental Cycle model embedded in the Community Earth System Model (CESM1-BEC). These two depth horizon choices produce differing baseline global rates and spatial patterns of POC flux and e-ratio, with the greatest discrepancies found in regions with deep winter mixing. Over the 21st century, enhanced stratification reduces the depth of MLDmax, with the most pronounced reductions in regions that currently experience the deepest winter mixing. Simulated global mean decreases in POC flux and in e-ratio over the 21st century are similar for both depth horizons (8-9% for POC flux and 4-6% for e-ratio), yet the spatial patterns of change are quite different. The model simulates less pronounced decreases and even increases in POC flux and e-ratio in deep winter mixing regions when evaluated at MLDmax, since enhanced stratification over the 21st century shoals the depth of this horizon. The differing spatial patterns of change across these two depth horizons demonstrate the importance of including multiple export depth horizons in observational and modeling efforts to monitor and predict potential future changes to export.

The Kuroshio current separates from the Japanese coast to become the Kuroshio Extension (KE) characterized by a strong latitudinal density front, high levels of mesoscale (eddy) energy, and high chlorophyll (CHL). Recent work has also shown that the KE carries subsurface nutrients into the region horizontally. While satellite measurements of CHL show evidence of the impact of eddies on the standing stock of phytoplankton, there have been very limited in situ estimates of productivity over synoptic scales in this region. Here, we present highly spatially resolved estimates of net community production (NCP) for the KE region derived from underway O2/Ar measurements made in spring, summer, and early autumn. We find large seasonal differences in the relationships between NCP, CHL, and sea level anomaly (SLA, a proxy for local thermocline depth deviations driven by mesoscale eddies). The KE front is a pronounced hotspot of NCP in spring when NCP is almost completely decorrelated with CHL. Conversely, we find that NCP and CHL are strongly correlated in summer away from the front. We explore the mechanistic underpinnings of the relationship between NCP and CHL and suggest that the KE nutrient stream as well as vertical motions associated with mesoscale eddies might be a key factor in supporting an NCP hotspot that is seasonally decoupled from CHL at the KE front. Our observations also highlight seasonal and regional (de)coupling between NCP and CHL which may impact the accuracy of CHL-based estimates of productivity.