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Antarctic sea ice gradually increased from the late 1970s until 2016, when it experienced an abrupt decline. A number of mechanisms have been proposed for both the gradual increase and abrupt decline of Antarctic sea ice, but how each mechanism manifests spatially and temporally remains poorly understood. Here, we use a statistical method called low-frequency component analysis to analyze the spatial-temporal structure of observed Antarctic sea-ice concentration variability. The identified patterns reveal distinct modes of low-frequency sea ice variability. The leading mode, which accounts for the large-scale, gradual expansion of sea ice, is associated with the Interdecadal Pacific Oscillation and resembles the observed sea-surface temperature trend pattern that climate models have trouble reproducing. The second mode is associated with the central Pacific El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode, and accounts for most of the sea ice variability in the Ross Sea. The third mode is associated with the eastern Pacific ENSO and Amundsen Sea Low, and accounts for most of the pan-Antarctic sea-ice variability and almost all of the sea ice variability in the Weddell Sea. This mode is associated with periods of abrupt Antarctic sea-ice decline and is related to a weakening of the circumpolar westerlies, which favors surface warming through a shoaling of the ocean mixed layer and decreased northward Ekman heat convergence. Broadly, these results suggest that climate model biases in long-term Antarctic sea-ice and global sea-surface temperature trends are related to each other and that eastern Pacific ENSO variability causes abrupt sea ice changes. [ABSTRACT FROM AUTHOR]

Wind work at the air-sea interface is the transfer of kinetic energy between the ocean and the atmosphere and, as such, is an important part of the atmosphere-ocean coupled system. Since wind work involves winds and ocean currents that span a broad range of spatial and temporal scales, a comprehensive study would require access to observations of a wide range of space and time scales. In the absence of appropriate global observations, our study makes use of a new, global, coupled ocean-atmosphere simulation with horizontal grid spacing of 2-5 km for the ocean and 7 km for the atmosphere. Here we develop a methodology, both in physical and spectral space, to diagnose different components of wind work in terms of forcing distinct classes of oceanic motions, including mean currents, time-dependent large-scale currents and mesoscale eddies, and internal gravity waves such as near-inertial waves. The total simulated wind work has a magnitude of 5.21 TW, a value much larger than reported by previous modeling studies. The total wind work is first decomposed into time-mean and time-dependent components, with the former accounting for 2.23 TW (43%) and the latter 2.98 TW (57%). The time-dependent wind work is then decomposed into two components, a high-frequency component that forces internal gravity waves and a low-frequency component that forces mesoscale eddies and large-scale currents. The high-frequency component is positive at scales between 10 km and 1000 km and represents 75% of the total time-dependent component. The low-frequency component is found to be positive for spatial scales larger than 275 km and ten times larger than the negative part associated with smaller spatial scales. The negative wind work acts as a surface drag that slows down surface currents and damps mesoscale eddies whereas the positive low-frequency part accelerates large-scale currents. The complex and consequential interplay of surface winds and currents in the numerical simulation motivates the need for a winds-and-currents satellite mission to directly observe these wind work components. [ABSTRACT FROM AUTHOR]

The eddy heat flux generated by statistically equilibrated baroclinic turbulence supported on a uniform, horizontal temperature gradient is examined using a two-layer [beta]-plane quasigeostrophic model. The dependence of the eddy diffusivity of temperature, [D.sub.[tau]], on external parameters such as [beta], bottom friction [kappa], the deformation radius [lambda], and the velocity jump 2U, is provided by numerical simulations at 110 different points in the parameter space [[beta].sub.*] = [beta][[lambda].sup.1]/U and [[kappa].sub.*] = [kappa][lambda]/U. There is a special 'pivot' value of [[beta].sub.*], [[beta].sup.piv.sub.*] [approximately equal to] 11/16, at which [D.sub.[tau]] depends weakly on [[kappa].sub.*]. But otherwise [D.sub.[tau]] has a complicated dependence on both [[beta].sub.*], and [[kappa].sub.*], highlighted by the fact that reducing K, leads to increases (decreases) in [D.sub.[tau]] if [beta] is less than (greater than) [[beta].sup.piv.sub.*]. Existing heat-flux parameterizafions, based on Kolmogorov cascade theories, predict that [D.sub.[tau]] is nonzero and independent of [[kappa].sub.*] in the limit [[kappa].sub.*] [right arrow] 0. Simulations show indications of this regime provided that [[kappa].sub.*] [less than or equal to] 0.04 and 0.25 [less than or equal to] [[beta].sub.*], [less than or equal to]0.5. All important length scales in this problem, namely the mixing length, the scale of the energy containing eddies, the Rhines scale, and the spacing of the zonal jets, converge to a common value as bottom friction is reduced. The mixing length and jet spacing do not decouple in the parameter regime considered here, as predicted by cascade theories. The convergence of these length scales is due to the formation of jet-scale eddies that align along the eastward jets. The baroclinic component of these eddies helps force the zonal mean flow, which occurs through nonzero Reynolds stress correlations in the upper layer, as opposed to the barotropic mode. This behavior suggests that the dynamics of the inverse barotropic cascade are insufficient to fully describe baroclinic turbulence.