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We report the significant impact of near-inertial waves (NIWs) on vertical mixing and air-sea carbon dioxide (CO2) fluxes in the Southern Ocean using a biogeochemical model coupled to an eddy-rich ocean circulation model. The effects of high-frequency processes are quantified by comparing the fully coupled solution (ONLINE) to two offline simulations based on 5-day-averaged output of the ONLINE simulation: one that uses vertical mixing archived from the ONLINE model (CTRL) and another in which vertical mixing is recomputed from the 5-day average hydrodynamic fields (5dAVG). In this latter simulation, processes with temporal variabilities of a few days including NIWs are excluded in the biogeochemical simulation. Suppression of these processes reduces vertical shear and vertical mixing in the upper ocean, leading to decreased supply of carbon-rich water from below, less CO2 outgassing in austral winter, and more uptake in summer. The net change amounts up to one third of the seasonal variability in Southern Ocean CO2 flux. Our results clearly demonstrate the importance of resolving high-frequency processes such as NIWs to better estimate the carbon cycle in numerical model simulations. ©2019. American Geophysical Union. All Rights Reserved., NSF MOBY project (OCE-1048926), NSF MOBY project (OCE-1048897), National Research Foundation of Korea (NRF) (grant no. NRF-2019R1C1C1003663), Yonsei University Research Fund (2018-22-0053)

Anthropogenic influences have led to a strengthening and poleward shift of westerly winds over the Southern Ocean, especially during austral summer. We use observations, an idealized eddy-resolving ocean sea ice channel model, and a global coupled model to explore the Southern Ocean response to a step change in westerly winds. Previous work hypothesized a two time scale response for sea surface temperature. Initially, Ekman transport cools the surface before sustained upwelling causes warming on decadal time scales. The fast response is robust across our models and the observations: We find Ekman-driven cooling in the mixed layer, mixing-driven warming below the mixed layer, and a small upwelling-driven warming at the temperature inversion. The long-term response is inaccessible from observations. Neither of our models shows a persistent upwelling anomaly, or long-term, upwelling-driven subsurface warming. Mesoscale eddies act to oppose the anomalous wind-driven upwelling, through a process known as eddy compensation, thereby preventing long-term warming. ©2019

The seasonal ice zone around both the Arctic and the Antarctic coasts is typically characterized by warm and salty waters underlying a cold and fresh layer that insulates sea-ice floating at the surface from vertical heat fluxes. Here, we explore how a mesoscale eddy field rubbing against ice at the surface can, through Ekman-induced vertical motion, bring warm waters up to the surface and partially melt the ice. We dub this the “Eddy-Ice-Pumping” (EIP) mechanism. When sea-ice is relatively motionless, underlying mesoscale eddies experience a surface drag that generates Ekman upwelling in anticyclones and downwelling in cyclones. An eddy composite analysis of a Southern Ocean eddying channel model, capturing the interaction of the mesoscale with sea-ice, shows that within the compact ice zone, the mixed layer depth (MLD) is shallow in anticyclones (∼20 m) due to sea-ice melt and deep in cyclones (∼50–200m) due to brine rejection. “EIP” warms the core of anticyclones without significantly affecting the temperature of cyclones, producing a net upward vertical heat flux that reduces the mean sea-ice thickness by 10% and shoals the MLD by 60% over the course of winter and spring. In the following months, the sea-ice thickness recovers with an overshoot, due to strong negative feedbacks associated with atmospheric cooling and salt stratification. Consequently, the effect of “EIP” does not accumulate over the years, but modulates the seasonal cycle of ice within the compact ice zone.

The Antarctic Circumpolar Current has highly energetic mesoscale phenomena, but their impacts on phytoplankton biomass, productivity, and biogeochemical cycling are not understood well. We analyze satellite observations and an eddy-rich ocean model to show that they drive chlorophyll anomalies of opposite sign in winter versus summer. In winter, deeper mixed layers in positive sea surface height (SSH) anomalies reduce light availability, leading to anomalously low chlorophyll concentrations. In summer with abundant light, however, positive SSH anomalies show elevated chlorophyll concentration due to higher iron level, and an iron budget analysis reveals that anomalously strong vertical mixing enhances iron supply to the mixed layer. Features with negative SSH anomalies exhibit the opposite tendencies: higher chlorophyll concentration in winter and lower in summer. Our results suggest that mesoscale modulation of iron supply, light availability, and vertical mixing plays an important role in causing systematic variations in primary productivity over the seasonal cycle., National Science Foundation (U.S.) (OCE-1048926)

We investigate the role of mesoscale eddies in modulating air-sea CO2 flux and associated biogeochemical fields in Drake Passage using in situ observations and an eddy-resolving numerical model. Both observations and model show a negative correlation between temperature and partial pressure of CO2 (pCO2) anomalies at the sea surface in austral summer, indicating that warm/cold anticyclonic/cyclonic eddies take up more/less CO2. In austral winter, in contrast, relationships are reversed: warm/cold anticyclonic/cyclonic eddies are characterized by a positive/negative pCO2 anomaly and more/less CO2 outgassing. It is argued that DIC-driven effects on pCO2 are greater than temperature effects in austral summer, leading to a negative correlation. In austral winter, however, the reverse is true. An eddy-centric analysis of the model solution reveals that nitrate and iron respond differently to the same vertical mixing: vertical mixing has a greater impact on iron because its normalized vertical gradient at the base of the surface mixed layer is an order of magnitude greater than that of nitrate. This article is protected by copyright. All rights reserved.

The role of mesoscale eddies in the uptake of anthropogenic chlorofluorocarbon-11 (CFC-11) gas is investigated with a 1/20° eddy-resolving numerical ocean model of a region of the Southern Ocean. With a relatively fast air-sea equilibrium time scale (about a month), the air-sea CFC-11 flux quickly responds to the changes in the mixed layer CFC-11 partial pressure (pCFC-11). At the mesoscale, significant correlations are observed between pCFC-11 anomaly, anomalies in sea surface temperature (SST), net heat flux, and mixed layer depth. An eddy-centric analysis of the simulated CFC-11 field suggests that anticyclonic warm-core eddies generate negative pCFC-11 anomalies and cyclonic cold-core eddies generate positive anomalies of pCFC-11. Surface pCFC-11 is modulated by mixed layer dynamics in addition to CFC-11 air-sea fluxes. A negative cross correlation between mixed layer depth and surface pCFC-11 anomalies is linked to higher CFC-11 uptake in anticyclones and lower CFC-11 uptake in cyclones, especially in winter. An almost exact asymmetry in the air-sea CFC-11 flux between cyclones and anticyclones is found.

[1] The Pacific sardine (Sardinops sagax) showed significant differences in spawning habitat area, spawning habitat quality and availability of survivor habitat as the Pacific Ocean went through the La Nina state in April 2002 to a weak El Nino in April 2003. During another El Nino/Southern Oscillation transition period in 2006–2007 when the El Nino state retreated and the La Nina returned, a similar pattern in spawning habitat quality was seen. The coupling between the atmospheric forcing, the physical ocean states and the properties of the sardine egg spawning are investigated using dynamically consistent data-assimilation fits of the available physical oceanographic observations during these months. Fits were executed using the Regional Ocean Modeling System four-dimensional variational assimilation platform along with adjoint model runs using a passive tracer to deduce source waters for the areas of interest. Analysis using the data-assimilation model runs reveals that unusually strong equatorward wind-forcing drives offshore transport during the La Nina conditions, which extends the spawning habitat for sardine further offshore. A statistical model of sardine spawning habitat shows better habitat quality during the El Nino conditions, which is associated with higher egg densities and corresponded to higher daily egg production. Concentration of eggs is also increased by convergence of water. The results of the source waters analysis using the adjoint data assimilation model support the idea that offshore transport extends the spawning habitat, and show that higher levels of nutrient are brought into the spawning habitat with high concentration of sardine eggs.