Estimates of the Global Ocean Diapycnal/Vertical Diffusivity
Assessment of Fundamental Ocean Dynamics using ECCO version 4
How does the ocean affect and response to the changing climate?
Changes of the Oceanic Vertical Heat Transport
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 non-significance of change in the global ocean vertical heat transport from an ocean state estimate, “data” from which are of global coverage and are regularly “sampled” spatially and temporally, raises the question whether an adequate observational data base exists to assess changes in the upper ocean heat content over the past few decades. Also, whereas the advective term largely determines the spatial pattern of the change in ocean vertical heat flux, its global integral is not significantly different from zero. In contrast, the diffusive term, although regionally weak except in high-latitude oceans, produces a statistically significant extra downward heat flux during the 00s. This suggests that besides ocean advection, ocean mixing processes, including isopycnal, diapycnal as well as convective mixing, are important for the decadal variation of the heat exchange between upper and deep oceans as well. Furthermore, our analyses indicate that focusing on any particular region in explaining changes of the global ocean heat content could be misleading and not necessarily correspond to changes in the global mean.
Estimated values of recent oceanic heat uptake are of order of a few tenths of a watt/m2, and are a very small residual of air-sea exchanges with annual average regional magnitudes of hundreds of watt/m2. By using a dynamically consistent state estimate, the redistribution of heat within the ocean is calculated over a 20-year period. The results support an inference that the near-surface thermal properties of the ocean are a consequence, at least in part, of internal redistributions of heat, some of which must reflect water that has undergone long trajectories since last exposure to the atmosphere. The small residual heat exchange with the atmosphere today is unlikely to represent the interaction with an ocean that was in thermal equilibrium at the start of global warming. An analogy is drawn with carbon-14 “reservoir ages” which range over hundreds to a thousand years.
An assessment is made of the mean and variability of the net air–sea heat flux, Qnet, from four products (ECCO, OAFlux–CERES, ERA-Interim, and NCEP1) over the global ice-free ocean from January 2001 to December 2010. For the 10-yr “hiatus” period, all products agree on an overall net heat gain over the global ice-free ocean, but the magnitude varies from 1.7 to 9.5 W m−2. The differences among products are particularly large in the Southern Ocean, where they cannot even agree on whether the region gains or loses heat on the annual mean basis. Decadal trends of Qnet differ significantly between products. ECCO and OAFlux–CERES show almost no trend, whereas ERA-Interim suggests a downward trend and NCEP1 shows an upward trend. Therefore, numerical simulations utilizing different surface flux forcing products will likely produce diverged trends of the ocean heat content during this period. The downward trend in ERA-Interim started from 2006, driven by a peculiar pattern change in the tropical regions. ECCO, which used ERA-Interim as initial surface forcings and is constrained by ocean dynamics and ocean observations, corrected the pattern. Among the four products, ECCO and OAFlux–CERES show great similarities in the examined spatial and temporal patterns. Given that the two estimates were obtained using different approaches and based on largely independent observations, these similarities are encouraging and instructive. It is more likely that the global net air–sea heat flux does not change much during the so-called hiatus period.
Estimates are made of the global ocean vertical velocity, w, from a dynamically and kinematically consistent ocean state estimate (ECCO, version 4, release 1). Conventional patterns of vertical velocity, Ekman pumping, appear in the upper ocean, with topographic dominance at depth. Intense and vertically coherent upwelling and downwelling occur in the North Atlantic and the Southern Ocean, acting as “pipes” to connect the atmosphere and the upper ocean to the deep and abyssal oceans and being a mechanism for fast response of the deep ocean to the changing surface climate. Comparison of Eulerian and eddy-induced components shows compensation almost everywhere, but the eddy effect is only important in limited regions, particularly in the Southern Ocean. Vertical velocity is primarily determined by the Eulerian component, and related to winds and large-scale topographic features. Except for a universal annual cycle, the temporal variation of the vertical velocity presents varying characteristic timescales in different regions. The complex spatial structure of w ultimately permits regional tests of basic oceanographic concepts such as Sverdrup balance.
Twenty-year means of the residual vertical velocity from ECCO v4. Publications: Liang, X., M. Spall and C. Wunsch, 2017: Global ocean vertical velocity from a dynamically consistent ocean state estimate, J. Geophys. Res, In Press.
Redistribution and Dissipation of the Internal Wave Energy
Once produced, the energy associated with internal tides can be transferred to other frequencies within the internal wave band through a variety of nonlinear mechanisms, including the triad interactions of parametric subharmonic instability (PSI), elastic scattering, and induced diffusion. In particular, recent modelling studies show that near-inertial oscillations (NIO) can be generated by pure tidal forcing. Because observed NIO tend to have relatively large-shears, the extent to which they are tidally produced becomes of interest in the mixing process, and more generally their role in supporting the background internal wave continuum. In a recent work, Prof. Wunsch and I studied the redistribution and instability of internal wave energy arising from the conversion at topography of the barotropic tide in a set of numerical experiments. A two-dimensional nonhydrostatic model with 100m horizontal resolution and 10-25m vertical resolution is used to represent the detailed processes near an idealized ridge. Conventional internal wave beams at the tidal frequency, M2, appear. At the ridge crest, apparent Kelvin-Helmholtz instability occurs that both mixes the fluid and generates strong near-inertial oscillations radiating in near-horizontal beams. Resonant triad interaction between the tidal and inertial motions in turn produces beams at a frequency tidal minus f, (M2-f), with high shear, thus effectively mixing fluid at considerable heights above the ridge crest.
Influence of Mesoscale Eddies on the Generation of Internal Tides
Both tides and mesoscale eddies are ubiquitous in the World Ocean and both are considered as important energy sources for deep ocean mixing. Even though a variety of studies focusing on the energy pathways from tides or mesoscale eddies to ocean turbulence have been carried out, the interaction of mesoscale eddies and internal tides as well as its effects on the deep ocean mixing have not received much attention until recently. In my work, I examine the temporal variation of the semidiurnal internal tides in the deep ocean over a segment of the East Pacific Rise between 9 and 10N using observations from a set of moored instruments. Both the kinetic energy and vertical shear of the semidiurnal internal tides show significant subinertial variability. By comparing with background forcing, it is found that the subinertial variation of the semidiurnal internal tides is related to both the spring-neap tidal cycles and the low-frequency flows, especially the component crossing the ridge crest. The observations also suggest that the semidiurnal internal tides at the axial stations are likely generated locally, and it is highly probable that the subinertial variation of the semidiurnal internal tides is due to the eddy-modulated internal tide generation. These findings underscore the fact that the impacts of low-frequency flows should be taken into account when simulating internal tides as well as understanding tide-induced ocean mixing. Publications: Liang, X., 2014, Semidiurnal tidal currents in the deep ocean near the East Pacific Rise between 9 and 10N, J. Geophys. Res., 119:4264-4277
Influence of Mesoscale Eddies on the Deep Ocean Circulation
Since subinertial variability (20-150 days) plays an important role in dispersing mass, heat and other ocean tracers, gaining a better understanding of it is essential for investigating ocean circulation and marine biogeochemistry. However, due to a lack of in situ data from the deep ocean, most previous work has focused on signals near the sea-surface, that is, the subinertial variability in the deep eastern tropical Pacific has not yet been intensively studied. In my work, I examine the subinertial variability of deep-ocean currents near the crest of the East Pacific Rise using observations from a collection of moored instruments augmented with sea-surface height data. The results reveal low-frequency currents with characteristic time scales of 1--3 months and maximum speeds up to 10 cm/s. Furthermore, the subinertial velocities at depth are significantly correlated with geostrophic near-surface currents estimated from sea-surface height data. It suggests that the subinertial velocity field near the EPR crest is a superposition of velocities associated with eddies propagating westward across the ridge and "topographic flows", such as trapped waves and boundary currents. Considering the ubiquitousness of eddies in the ocean we expect that the circulation near other portions of the global mid-ocean ridge system is similarly dominated by mesoscale variability and topographic effects. This is particularly important for dispersal of larvae and geochemical tracers associated with hydrothermal sources that are found primarily along the crest of mid-ocean ridges.
Influence of Mesoscale Eddies on the Deep Ocean Near-inertial Oscillations and Mixing
Recent theoretical and numerical studies suggest a connection between mesoscale eddies and diapycnal mixing in the deep ocean, especially near rough topography in regions of strong geostrophic flow. However, unambiguous observational evidence for such a connection has not yet been found and it is still unclear what physical processes are responsible for transferring energy from the mesoscale to small-scale processes. In my work, I present observations that clearly demonstrate energy transfer near the crest of the East Pacific Rise (EPR) from low-frequency geostrophic flows, including mesoscale eddies, to near-inertial oscillations, finescale variability and mixing. In particular, our measurements imply a significant increase in diapycnal diffusivity near the seafloor during episodes of increased subinertial flow. Our findings are expected to be useful for validating and improving numerical-model parameterizations of turbulence and mixing in the ocean. Furthermore, since the frequency and intensity of mesoscale eddies depend on the state of the climate, the observed eddy modulation of turbulence and mixing connect climate change and climate variability to physical and biogeochemical dynamics in the deep ocean and implies an unexplored feedback mechanism potentially affecting the global climate system. Publications: Liang X. and A. Thurnherr. Eddy-modulated internal waves and mixing on a mid-ocean ridge. J. Phys. Oceanogr., 2012, 42(7):1242-1248.