The eastern subpolar North Atlantic upper ocean salinity undergoes decadal fluctuations. A large fresh anomaly event occurred during 2012--2016. Using the ECCO state estimate, we diagnose and compare mechanisms of this low salinity event with that of the 1990s fresh anomaly event. To avoid erroneous interpretations of physical mechanisms due to reference salinity values in the freshwater budget, we perform a salt mass content budget analysis of the eastern subpolar North Atlantic. It shows that the recent fresh anomaly occurs due to the circulation of anomalous salinity by mean currents entering the eastern subpolar basin from its western boundary via the North Atlantic Current. This is in contrast to the early 1990s, when the dominant mechanism governing the fresh anomaly was the transport of the mean salinity field by anomalous currents across the southern boundary of the subpolar North Atlantic.

Examination of historical simulations from CMIP6 models shows substantial pre-industrial to present-day changes in ocean heat (ΔH), salinity (ΔS), oxygen (ΔO2), dissolved inorganic carbon (ΔDIC), chlorofluorocarbon-12 (ΔCFC12), and sulfur hexafluoride (ΔSF6). The spatial structure of the changes and the consistency among models differ among tracers: ΔDIC, ΔCFC12, and ΔSF6 all are largest near the surface, are positive throughout the thermocline with weak changes below, and there is good agreement amongst the models. In contrast, the largest ΔH, ΔS, and ΔO2 are not necessarily at the surface, their sign varies within the thermocline, and there are large differences among models. These differences between the two groups of tracers are linked to climate-driven changes in the ocean transport, with this tracer “redistribution” playing a significant role in changes in ΔH, ΔS, and ΔO2 but not the other tracers. Tracer redistribution is prominent in the southern subtropics, in a region where apparent oxygen utilization and ideal age indicate increased ventilation time scales. The tracer changes are linked to a poleward shift of the peak Southern Hemisphere westerly winds, which causes a similar shift of the subtropical gyres, and anomalous upwelling in the subtropics. This wind - tracer connection is also suggested to be a factor in the large model spread in some tracers, as there is also a large model spread in wind trends. A similar multi-tracer analysis of observations could provide insights into the relative role of the addition and redistribution of tracers in the ocean.

Computational Oceanography is the study of ocean phenomena by numerical simulation, especially dynamical and physical phenomena. Progress in information technology has driven exponential growth in the number of global ocean observations and the fidelity of numerical simulations of the ocean in the past few decades. The growth has been exponentially faster for ocean simulations, however. We argue that this faster growth is shifting the importance of field measurements and numerical simulations for oceanographic research. It is leading to the maturation of Computational Oceanography as a branch of marine science on par with observational oceanography. One implication is that ultra-resolved ocean simulations are only loosely constrained by observations. Another implication is that barriers to analyzing the output of such simulations should be removed. Although some specific limits and challenges exist, many opportunities are identified for the future of Computational Oceanography. Most important is the prospect of hybrid computational and observational approaches to advance understanding of the ocean.

Mixing parameters can be inaccurate in ocean data assimilation systems, even if there is close agreement between observations and mixing parameters in the same modeling system when data are not assimilated. To address this, we investigate whether there are additional observations that can be assimilated by ocean modeling systems to improve their representation of mixing parameters and thereby gain knowledge of the global ocean’s mixing parameters. Observationally-derived diapycnal diffusivities–using a strain-based parameterization of finescale hydrographic structure–are included in the Estimating the Circulation & Climate of the Ocean (ECCO) framework and the GEOS-5 coupled Earth system model to test if adding observational diffusivities can reduce model biases. We find that adjusting ECCO-estimated and GEOS-5-calculated diapycnal diffusivity profiles toward profiles derived from Argo floats using the finescale parameterization improves agreement with independent diapycnal diffusivity profiles inferred from microstructure data. Additionally, for the GEOS-5 hindcast, agreement with observed mixed layer depths and temperature/salinity/stratification (i.e., hydrographic) fields improves. Dynamic adjustments arise when we make this substitution in GEOS-5, causing the model’s hydrographic changes. Adjoint model-based sensitivity analyses suggest that the assimilation of dissolved oxygen concentrations in future ECCO assimilation efforts would improve estimates of the diapycnal diffusivity field. Observationally-derived products for horizontal mixing need to be validated before conclusions can be drawn about them through similar analyses.

Arctic Ocean freshwater storage increased since the mid 1990s, but the cause was unknown. Now a recent paper by Jahn & Laiho [2020, 2020GL088854] uses ensemble runs of a coupled climate model to suggest that the observed increase is anthropogenic. The paper quantifies when the anthropogenic signals should emerge from the noise of natural variability. This result contextualizes research on the Arctic Ocean freshwater system and sketches an unprecedented opportunity. Future work should elucidate mechanisms, seek to observe the anthropogenic freshwater changes, and investigate the impacts on biogeochemistry and the North Atlantic Ocean circulation.

The global ocean overturning circulation carries warm, salty water to high latitudes, both in the Arctic and Antarctic. Interaction with the atmosphere transforms this inflow into three distinct products: sea ice, surface Polar Water, and deep Overflow Water. The Polar Water and Overflow Water form estuarine and thermal overturning cells, stratified by salinity and temperature, respectively. A conceptual model specifies the characteristics of these water masses and cells given the inflow and air/sea/land fluxes of heat and freshwater. The model includes budgets of mass, salt, and heat, and parametrizations of Polar Water and Overflow Water formation, which include exchange with continental shelves. Model solutions are mainly controlled by a linear combination of air/sea/ice heat and freshwater fluxes and inflow heat flux that approximates the meteoric freshwater flux plus the sea ice export flux. The model shows that for the Arctic, the thermal overturning is likely robust, but the estuarine cell appears vulnerable to collapse via a so-called heat crisis that violates the budget equations. The system is pushed towards this crisis by increasing Atlantic Water inflow heat flux, increasing meteoric freshwater flux, and/or decreasing heat loss to the atmosphere. The Antarctic appears close to a so-called Overflow Water emergency with weak constraints on the strengths of the estuarine and thermal cells, uncertain sensitivity to parameters, and possibility of collapse of the thermal cell.

Mixing parameters can be inaccurate in ocean data assimilation systems, even if there is close agreement between observations and mixing parameters in the same modeling system when data are not assimilated. To address this, we investigate whether there are additional observations that can be assimilated by ocean modeling systems to improve their representation of mixing parameters and thereby gain knowledge of the global ocean’s mixing parameters. Observationally-derived diapycnal diffusivities–using a strain-based parameterization of finescale hydrographic structure–are included in the Estimating the Circulation & Climate of the Ocean (ECCO) framework and the GEOS-5 coupled Earth system model to test if adding observational diffusivities can reduce model biases. We find that adjusting ECCO-estimated and GEOS-5-calculated diapycnal diffusivity profiles toward profiles derived from Argo floats using the finescale parameterization improves agreement with independent diapycnal diffusivity profiles inferred from microstructure data. Additionally, for the GEOS-5 hindcast, agreement with observed mixed layer depths and temperature/salinity/stratification (i.e., hydrographic) fields improves. Dynamic adjustments arise when we make this substitution in GEOS-5, causing the model’s hydrographic changes. Adjoint model-based sensitivity analyses suggest that the assimilation of dissolved oxygen concentrations in future ECCO assimilation efforts would improve estimates of the diapycnal diffusivity field. Observationally-derived products for horizontal mixing need to be validated before conclusions can be drawn about them through similar analyses.

The Arctic and subarctic oceans exhibit distinct decadal variations in freshwater and heat content. We describe freshwater and heat budgets with the ECCOv4 reanalysis product and compare budget variability and mechanisms within the subpolar North Atlantic Ocean, Nordic Seas and Labrador Sea from 1992 to 2015. For all regions, changes in freshwater content are largely anti-correlated with changes in heat content. Since 1995, the subpolar North Atlantic Ocean has undergone a decade of warming and salinification followed by ongoing cooling and freshening. The recent increase in freshwater content and the reduction in heat in the subpolar North Atlantic can largely be attributed to anomalous circulation of mean salinity and temperature, respectively. Interannual variability in heat and freshwater mostly corresponds to boundary fluxes from the subtropics. Meanwhile the Nordic Seas have undergone an overall warming and salinification from the mid-1990s to 2015. Salinification is primarily driven by reduced sea ice flux through Fram Strait, while warming is due to changes in both sea surface heating and advective flux. In the last five years, Labrador Sea freshwater convergence remained unchanged, as increased inflow via the Baffin Island Current is balanced by increased outflow via the Labrador Current. Hence the observed freshening of the Arctic Ocean is expected to be an increasingly important source of future freshwater increases in the subpolar North Atlantic. This stands in contrast to variability in freshwater flux from the subtropical North Atlantic, which is associated with variability in the Atlantic Meridional Overturning Circulation.

The response time of the southern subtropical oceans to an increase in the wind stress is examined in a climate model perturbation simulation where there is an abrupt increase in the wind stress. The ocean response time is shown to vary among fields: The intensification of the gyres and vertical movement of isopycnals happens over 5-10 years, while the change in ideal age, temperature, and salinity in mode and intermediate waters occurs much slower, with the response time exceeding 100 years at depths of 500-1000 m. While the response time for ideal age is longer than that for the surface circulation it is notable that it is much younger than the ideal age itself. The different response times indicate that changes in the winds over the southern oceans and in the horizontal circulation / density structure will occur near simultaneously, but there may be a substantial lag in subsurface changes from wind changes and these may persist for decades even if no further changes in the winds occurs