Manfredi Manizza

The Arctic Ocean has experienced intensifying phytoplankton spring blooms near the sea ice edge. Here, we investigate the drivers of such near‐ice blooms in the Greenland Sea by comparing satellite and in situ observations with the ECCO‐Darwin physical–biogeochemical state estimate. Satellite data reveal pronounced spring blooms within ~ 200 km of the ice, a signal that is not reproduced in ECCO‐Darwin. Observations further reveal that primary production is tightly linked to submesoscale meltwater eddies shed from the ice edge, which are not currently resolved by ECCO‐Darwin. We argue that the relatively coarse spatial resolution, together with biases in sea ice concentration and sea surface salinity lead to the underestimation of near‐ice blooms in ECCO‐Darwin. These results highlight (1) a critical role for fine‐scale processes in driving Arctic Ocean primary production and (2) the need for improved freshwater forcing, stratification, and small‐scale circulation in Arctic Ocean state estimates.

While the ocean is a critical regulator of atmospheric nitrous oxide (N ₂ O) levels, its specific contribution remains poorly constrained. Here, we implemented an N ₂ O module into the Estimating Circulation and Climate of the Ocean‐Darwin (ECCO‐Darwin) to investigate global sea‐air N ₂ O fluxes, trends and interannual variabilities. From 1992 to 2017, the model yielded a global the oceanic N ₂ O efflux of 2.43 ± 0.43 Tg N yr ⁻¹ , with a declining trend of −0.021 ± 0.0031 Tg N yr ⁻² , attributed mainly to a reduction in N ₂ O production induced by strengthened upper‐ocean stratification. The Eastern Tropical Pacific (ETP), Indian Ocean (IO) and Southern Ocean (SO) emerge as the primary three hotspots for N ₂ O outgassing, where interannual variability is modulated by regional climate modes. In the ETP and IO, N ₂ O fluxes strongly correlate with the El Niño Southern Oscillation index, with correlation coefficients of r = −0.93 and 0.53, respectively. During El Niño, reduced wind speeds and the deepening of the thermocline lead to decreased N ₂ O outgassing in the ETP. El Niño events also tend to result in a positive Indian Ocean Dipole, which causes the thermocline in the Eastern Tropical IO to shoal, resulting in increased N ₂ O outgassing. In the SO, the interannual variability of N ₂ O fluxes is positively correlates with the Southern Annular Mode (SAM) index ( r = 0.76). When the SAM is positive, westerly winds shift poleward, strengthening Ekman pumping and resulting in more subsurface N ₂ O‐laden water being upwelled and outgassed. Our study provides a mechanistic understanding of the critical role of ocean‐atmosphere dynamics and climate variability in modulating oceanic N ₂ O emissions.

Although the global greening associated with climate change is well documented on land, similar trends in the ocean have not been thoroughly identified. Using satellite observations of ocean chlorophyll a (Chl) concentration, we show that the surface ocean experienced a poleward greening from 2003 to 2022. Contemporaneously, the subtropical regions of the Northern Hemisphere experienced a decrease in Chl. As such, the latitudinal disparity in Chl, as documented by an inequality index, has been increasing over the past two decades, particularly in the Northern Hemisphere. Rising water temperatures may primarily influence the Chl trends. The increasing Chl inequality—marked by “greener green and bluer blue” waters—has the potential to cascade to higher trophic levels, with implications for the fisheries and economies of coastal nations.

Phytoplankton primary production is a crucial component of Arctic Ocean (AO) biogeochemistry, playing a pivotal role in carbon cycling by supporting higher trophic levels and removing atmospheric carbon dioxide. The advent of satellite observations measuring chlorophyll a concentration (Chl_a) has provided unprecedented insights into the distribution of AO phytoplankton, enhancing our ability to assess oceanic net primary production (NPP). However, the optical properties of AO waters differ significantly from those of the lower‐latitude waters, complicating remotely sensed Chl_a retrievals. To mitigate these deficiencies, solar‐induced chlorophyll fluorescence (SIF) has emerged as a valuable tool for gaining physiological insights into the direct photosynthetic processes of the AO. However, the temporal coverage of satellite SIF data makes long‐term analysis of Chl_a photosynthetic activity challenging. In this study, we leverage satellite‐based SIF measurements from 2018 to 2021 to assess their correlation with a set of predictive factors influencing phytoplankton photosynthesis. Generally, observed SIF over the AO showed a higher correlation with normalized fluorescence line height (NFLH) compared to Chl_a. We extended the temporal coverage of the original SIF data to encompass the period from 2004 to 2020. The extended record revealed noticeable differences between SIF, and satellite‐based Chl_a, and NFLH observations. Our novel data set offers a pathway forward to monitor the physiological interactions of phytoplankton with climate changes, promising to significantly improve our understanding of Arctic waters productivity. The application of this data is expected to provide new insights into how phytoplankton respond to environmental shifts, contributing to a more nuanced understanding of their role in high‐latitude marine ecosystems.

To predict ecosystem change in the Arctic Ocean, understanding Arctic phytoplankton phenology is essential. We develop a marine ecosystem model focusing on phytoplankton dynamics and the competition for multiple resources based on knowledge from in situ data obtained in the Chukchi Sea. The model is designed to include the unique ecological characteristics of the Arctic Ocean. To the best of our knowledge, this is the first model successfully simulating the current paradigm for Arctic phytoplankton phenology, including both under‐ice blooms and succession of phytoplankton groups. Sensitivity experiments show that the dominance of diatoms can be sustained by lateral transport of a high‐silicate water mass (Pacific Winter Water). Experiments for future scenarios show that sea ice retreat drives a decrease in large diatoms and an increase in the other relatively small non‐diatom groups. These results suggest that dominant phytoplankton groups can be shifted from diatoms to non‐diatoms under ongoing Arctic Ocean sea ice retreat.

Nitrous oxide (N ₂ O) is a greenhouse gas and stratospheric ozone‐depleting substance with large and growing anthropogenic emissions. Previous studies identified the influx of N ₂ O‐depleted air from the stratosphere to partly cause the seasonality in tropospheric N ₂ O (aN ₂ O), but other contributions remain unclear. Here, we combine surface fluxes from eight land and four ocean models from phase 2 of the Nitrogen/N ₂ O Model Intercomparison Project with tropospheric transport modeling to simulate aN ₂ O at eight remote air sampling sites for modern and pre‐industrial periods. Models show general agreement on the seasonal phasing of zonal‐average N ₂ O fluxes for most sites, but seasonal peak‐to‐peak amplitudes differ several‐fold across models. The modeled seasonal amplitude of surface aN ₂ O ranges from 0.25 to 0.80 ppb (interquartile ranges 21%–52% of median) for land, 0.14–0.25 ppb (17%–68%) for ocean, and 0.28–0.77 ppb (23%–52%) for combined flux contributions. The observed seasonal amplitude ranges from 0.34 to 1.08 ppb for these sites. The stratospheric contributions to aN ₂ O, inferred by the difference between the surface‐troposphere model and observations, show 16%–126% larger amplitudes and minima delayed by ∼1 month compared to Northern Hemisphere site observations. Land fluxes and their seasonal amplitude have increased since the pre‐industrial era and are projected to grow further under anthropogenic activities. Our results demonstrate the increasing importance of land fluxes for aN ₂ O seasonality. Considering the large model spread, in situ aN ₂ O observations and atmospheric transport‐chemistry models will provide opportunities for constraining terrestrial and oceanic biosphere models, critical for projecting carbon‐nitrogen cycles under ongoing global warming.

While the preindustrial ocean was assumed to be in equilibrium with the atmosphere, the modern ocean is a carbon sink, resulting from natural variability and anthropogenic perturbations, such as fossil fuel emissions and changes in riverine exports over the past two centuries. Here we use a suite of sensitivity experiments based on the ECCO-Darwin global-ocean biogeochemistry model to evaluate the response of air-sea CO2 flux and carbon cycling to present-day lateral fluxes of carbon, nitrogen, and silica. We generate a daily export product by combining point-source freshwater discharge from JRA55-do with the Global NEWS 2 watershed model, accounting for lateral fluxes from 5171 watersheds worldwide. From 2000 to 2019, carbon exports increase CO2 outgassing by 0.22 Pg C yr-1 via the solubility pump, while nitrogen exports increase the ocean sink by 0.17 Pg C yr-1 due to phytoplankton fertilization. On regional scales, exports to the Tropical Atlantic and Arctic Ocean are dominated by organic carbon, which originates from terrestrial vegetation and peats and increases CO2 outgassing (+10 and +20%, respectively). In contrast, Southeast Asia is dominated by nitrogen from anthropogenic sources, such as agriculture and pollution, leading to increased CO2 uptake (+7%). Our results demonstrate that the magnitude and composition of riverine exports, which are determined in part from upstream watersheds and anthropogenic perturbations, substantially impact present-day regional-to-global-ocean carbon cycling. Ultimately, this work stresses that lateral fluxes must be included in ocean biogeochemistry and Earth System Models to better constrain the transport of carbon, nutrients, and metals across the land-ocean-aquatic-continuum.