Abstract:A dynamically and data-consistent ocean state estimate during 1993-2010 is analyzed for bidecadal changes in the mechanisms of heat exchange between the upper and lower oceans. Many patterns of change are consistent with prior studies. However, at various levels above 1800 m the global integral of the change in ocean vertical heat flux involves the summation of positive and negative regional contributions and is not statistically significant. The nonsignificance of change in the global ocean vertical heat tran… Show more
“…Ocean turbulence on scales of roughly 1–200 km is characterized by vigorous eddies, fronts, filaments, and other structures, which collectively make an important contribution to material transport. In the context of climate, it is well established that the mesoscale flows, roughly on the scales of 20–200 km, are of first‐order importance to the global ocean heat budget, especially for the vertical transport of heat (Wolfe et al, ; Griffies et al, ; Liang et al, ), and can drive global‐scale variations in ocean heat content (Liang et al, ; Busecke and Abernathey, ). In the Southern Ocean, the westerlies tend to steepen the isopycnals via Ekman pumping and make the water column more baroclinically unstable.…”
Biological productivity in the Southern Ocean is limited by iron availability. Previous studies of iron supply have focused on mixed-layer entrainment and diapycnal fluxes. However, the Southern Ocean is a region highly energetic mesoscale and submesoscale turbulence. Here we investigate the role of eddies in supplying iron to the euphotic zone, using a flat-bottom zonally re-entrant model, configured to represent the Antarctic Circumpolar Current region, that is coupled to a biogeochemical model with a realistic seasonal cycle. Eddies are admitted or suppressed by changing the model's horizontal resolution. We utilize cross spectral analysis and the generalized Omega equation to temporally and spatially decompose the vertical transport attributable to mesoscale and submesoscale motions. Our results suggest that the mesoscale vertical fluxes provide a first-order pathway for transporting iron across the mixing-layer base, where diapycnal mixing is weak, and must be included in modeling the open-Southern Ocean iron budget.
Plain Language SummaryOcean currents at the surface on the spatial scales of 1-200 km are energetic due to heating by the sun and stirring by the winds. These currents contribute to the climate system by transporting heat and carbon horizontally towards the poles and vertically into the deep ocean. By running a numerical simulation at very high spatial resolution that resolve these currents, we show that these currents are also responsible for transporting iron from the ocean interior to the surface in the Southern Ocean, where phytoplankton growth is limited by the lack of iron-a key nutrient for most living organisms on Earth. Our results highlight the importance of accurately representing these ocean currents and associated iron transport, in order to understand the Southern Ocean ecosystem and its impact on the climate via photosynthesis, the process in which carbon dioxide is converted to organic carbon and oxygen is produced as a bi-product.
“…Ocean turbulence on scales of roughly 1–200 km is characterized by vigorous eddies, fronts, filaments, and other structures, which collectively make an important contribution to material transport. In the context of climate, it is well established that the mesoscale flows, roughly on the scales of 20–200 km, are of first‐order importance to the global ocean heat budget, especially for the vertical transport of heat (Wolfe et al, ; Griffies et al, ; Liang et al, ), and can drive global‐scale variations in ocean heat content (Liang et al, ; Busecke and Abernathey, ). In the Southern Ocean, the westerlies tend to steepen the isopycnals via Ekman pumping and make the water column more baroclinically unstable.…”
Biological productivity in the Southern Ocean is limited by iron availability. Previous studies of iron supply have focused on mixed-layer entrainment and diapycnal fluxes. However, the Southern Ocean is a region highly energetic mesoscale and submesoscale turbulence. Here we investigate the role of eddies in supplying iron to the euphotic zone, using a flat-bottom zonally re-entrant model, configured to represent the Antarctic Circumpolar Current region, that is coupled to a biogeochemical model with a realistic seasonal cycle. Eddies are admitted or suppressed by changing the model's horizontal resolution. We utilize cross spectral analysis and the generalized Omega equation to temporally and spatially decompose the vertical transport attributable to mesoscale and submesoscale motions. Our results suggest that the mesoscale vertical fluxes provide a first-order pathway for transporting iron across the mixing-layer base, where diapycnal mixing is weak, and must be included in modeling the open-Southern Ocean iron budget.
Plain Language SummaryOcean currents at the surface on the spatial scales of 1-200 km are energetic due to heating by the sun and stirring by the winds. These currents contribute to the climate system by transporting heat and carbon horizontally towards the poles and vertically into the deep ocean. By running a numerical simulation at very high spatial resolution that resolve these currents, we show that these currents are also responsible for transporting iron from the ocean interior to the surface in the Southern Ocean, where phytoplankton growth is limited by the lack of iron-a key nutrient for most living organisms on Earth. Our results highlight the importance of accurately representing these ocean currents and associated iron transport, in order to understand the Southern Ocean ecosystem and its impact on the climate via photosynthesis, the process in which carbon dioxide is converted to organic carbon and oxygen is produced as a bi-product.
“…Lateral mixing in the upper ocean plays an important role in Earth's climate system. For instance, near‐surface lateral mixing processes help determine the ocean's rate of uptake of tracers such as heat and anthropogenic CO 2 (Abernathey & Ferreira, ; Balwada et al, ; Gnanadesikan et al, ; Khatiwala et al, ; Liang et al, ; Marshall et al, ). Lateral mixing also strongly influences the global water mass distribution and overturning circulation (Groeskamp et al, , ).…”
We investigate the role of small‐scale, high‐frequency motions on lateral transport in the ocean, by using velocity fields and particle trajectories from an ocean general circulation model (MITgcm‐llc4320) that permits submesoscale flows, inertia‐gravity waves, and tides. Temporal averaging/filtering removes most of the submesoscale turbulence, inertia‐gravity waves, and tides, resulting in a largely geostrophic flow, with a rapid drop‐off in energy at scales smaller than the mesoscales. We advect two types of Lagrangian particles: (a) 2‐D particles (surface restricted) and (b) 3‐D particles (advected in full three dimensions) with the filtered and unfiltered velocities and calculate Lagrangian diagnostics. At large length/time scales, Lagrangian diffusivity is comparable for filtered and unfiltered velocities, while at short scales, unfiltered velocities disperse particles much faster. We also calculate diagnostics of Lagrangian coherent structures:rotationally coherent Lagrangian vortices detected from closed contours of the Lagrangian‐averaged vorticity deviation and material transport barriers formed by ridges of maximum finite‐time Lyapunov exponent. For temporally filtered velocities, we observe strong material coherence, which breaks down when the level of temporal filtering is reduced/removed, due to vigorous small‐scale mixing. In addition, for the lowest temporal resolution, the 3‐D particles experience very little vertical motion, suggesting that degrading temporal resolution greatly reduces vertical advection by high‐frequency motions. Our study suggests that Lagrangian diagnostics based on satellite‐derived surface geostrophic velocity fields, even with higher spatial resolutions as in the upcoming Surface Water and Ocean Topography mission, may overestimate the presence of mesoscale coherent structures and underestimate dispersion.
“…The upward diffusive salt flux mainly occurs in the high-latitude North Atlantic and the Southern Ocean (Fig. 5e, f), where the isopycnal mixing dominates the vertical diffusive fluxes 33 .Fig. 5Spatial distribution of the ocean vertical salt fluxes.…”
Salinity is an essential proxy for estimating the global net freshwater input into the ocean. Due to the limited spatial and temporal coverage of the existing salinity measurements, previous studies of global salinity changes focused mostly on the surface and upper oceans. Here, we examine global ocean salinity changes and ocean vertical salt fluxes over the full depth in a dynamically consistent and data-constrained ocean state estimate. The changes of the horizontally averaged salinity display a vertically layered structure, consistent with the profiles of the ocean vertical salt fluxes. For salinity changes in the relatively well-observed upper ocean, the contribution of vertical exchange of salt can be on the same order of the net surface freshwater input. The vertical redistribution of salt thus should be considered in inferring changes in global ocean salinity and the hydrological cycle from the surface and upper ocean measurements.
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