If model parameterizations of unresolved physics, such as the variety of upper ocean mixing processes, are to hold over the large range of time and space scales of importance to climate, they must be strongly physically based. Observations, theories, and models of oceanic vertical mixing are surveyed. Two distinct regimes are identified: ocean mixing in the boundary layer near the surface under a variety of surface forcing conditions (stabilizing, destabilizing, and wind driven), and mixing in the ocean interior due to internal waves, shear instability, and double diffusion (arising from the different molecular diffusion rates of heat and salt). Mixing schemes commonly applied to the upper ocean are shown not to contain some potentially important boundary layer physics. Therefore a new parameterization of oceanic boundary layer mixing is developed to accommodate some of this physics. It includes a scheme for determining the boundary layer depth h, where the turbulent contribution to the vertical shear of a bulk Richardson number is parameterized. Expressions for diffusivity and nonlocal transport throughout the boundary layer are given. The diffusivity is formulated to agree with similarity theory of turbulence in the surface layer and is subject to the conditions that both it and its vertical gradient match the interior values at h. This nonlocal “K profile parameterization” (KPP) is then verified and compared to alternatives, including its atmospheric counterparts. Its most important feature is shown to be the capability of the boundary layer to penetrate well into a stable thermocline in both convective and wind-driven situations. The diffusivities of the aforementioned three interior mixing processes are modeled as constants, functions of a gradient Richardson number (a measure of the relative importance of stratification to destabilizing shear), and functions of the double-diffusion density ratio, Rρ. Oceanic simulations of convective penetration, wind deepening, and diurnal cycling are used to determine appropriate values for various model parameters as weak functions of vertical resolution. Annual cycle simulations at ocean weather station Papa for 1961 and 1969–1974 are used to test the complete suite of parameterizations. Model and observed temperatures at all depths are shown to agree very well into September, after which systematic advective cooling in the ocean produces expected differences. It is argued that this cooling and a steady salt advection into the model are needed to balance the net annual surface heating and freshwater input. With these advections, good multiyear simulations of temperature and salinity can be achieved. These results and KPP simulations of the diurnal cycle at the Long-Term Upper Ocean Study (LOTUS) site are compared with the results of other models. It is demonstrated that the KPP model exchanges properties between the mixed layer and thermocline in a manner consistent with observations, and at least as well or better than alternatives.

Oceanic vertical mixing: a review and a model with a nonlocal boundary layer parameterization

The potential for sea ice-albedo feedback to give rise to nonlinear climate change in the Arctic Ocean – defined as a nonlinear relationship between polar and global temperature change or, equivalently, a time-varying polar amplification – is explored in IPCC AR4 climate models. Five models supplying SRES A1B ensembles for the 21 st century are examined and very linear relationships are found between polar and global temperatures (indicating linear Arctic Ocean climate change), and between polar temperature and albedo (the potential source of nonlinearity). Two of the climate models have Arctic Ocean simulations that become annually sea ice-free under the stronger CO 2 increase to quadrupling forcing. Both of these runs show increases in polar amplification at polar temperatures above-5 o C and one exhibits heat budget changes that are consistent with the small ice cap instability of simple energy balance models. Both models show linear warming up to a polar temperature of-5 o C, well above the disappearance of their September ice covers at about-9 o C. Below-5 o C, surface albedo decreases smoothly as reductions move, progressively, to earlier parts of the sunlit period. Atmospheric heat transport exerts a strong cooling effect during the transition to annually ice-free conditions. Specialized experiments with atmosphere and coupled models show that the main damping mechanism for sea ice region surface temperature is reduced upward heat flux through the adjacent ice-free oceans resulting in reduced atmospheric heat transport into the region.

Geophysical fluid dynamics.

Cryptic genetic variation can facilitate adaptation in evolving populations. To elucidate the underlying genetic mechanisms, we used directed evolution in Escherichia coli to accumulate variation in populations of yellow fluorescent proteins and then evolved these proteins toward the new phenotype of green fluorescence. Populations with cryptic variation evolved adaptive genotypes with greater diversity and higher fitness than populations without cryptic variation, which converged on similar genotypes. Populations with cryptic variation accumulated neutral or deleterious mutations that break the constraints on the order in which adaptive mutations arise. In doing so, cryptic variation opens paths to adaptive genotypes, creates historical contingency, and reduces the predictability of evolution by allowing different replicate populations to climb different adaptive peaks and explore otherwise-inaccessible regions of an adaptive landscape.

/pdf/cryptic-genetic-variation-accelerates-evolution-by-opening-necdfsr9vz.pdf

Cryptic genetic variation accelerates evolution by opening access to diverse adaptive peaks.

Ultrathin Membranes: A New Opportunity for Ultrafast and Efficient Separation

[1] Variability in the equatorial Indian Ocean on intraseasonal time scales (defined as periods of 30–110 days) is investigated using satellite and in situ observations and a simple analytical linear long-wave equatorial β-plane model. Despite the extreme simplicity of the model, which includes just the two gravest baroclinic mode Kelvin waves and first meridional mode Rossby waves, simulated surface zonal velocity and sea surface height compare very well with observations. Both observations and model are characterized by a red shift in the velocity spectrum relative to the wind forcing spectrum, which is attributable to a combination of factors, including (1) the near resonant excitation of Kelvin waves by eastward propagating winds, (2) constructive interference between wind-forced waves and Rossby waves reflected from the eastern boundary, and (3) the favored excitation of low-frequency waves whose zonal wavelengths are long compared to the zonal fetch of the wind. We decomposed variability in two broad period bands, namely, 30–70 days and 70–110 days, for detailed analysis. At periods of 30–70 days, zonal velocity tends to be stationary in the directly forced region along the equator owing to the competing contributions of Kelvin and Rossby waves. In contrast, at 70–110 day periods, zonal velocity propagates westward despite eastward propagation of zonal wind stress because of the combined influence of eastern boundary generated and wind-forced Rossby waves. Kelvin waves reflected from the western boundary are negligibly small, indicating that basin mode resonances are not prominent as has been previously suggested.

The dynamics of wind-driven intraseasonal variability in the equatorial Indian Ocean

Strong large-scale winds can relay their energy to the ocean bottom and elicit an almost immediate intraseasonal barotropic (depth independent) response in the ocean. The intense winds associated with the Madden-Julian Oscillation over the Maritime Continent generate significant intraseasonal basin-wide barotropic sea level variability in the tropical Indian Ocean. Here we show, using a numerical model and a network of in-situ bottom pressure recorders, that the concerted barotropic response of the Indian and the Pacific Ocean to these winds leads to an intraseasonal see-saw of oceanic mass in the Indo-Pacific basin. This global-scale mass shift is unexpectedly fast, as we show that the mass field of the entire Indo-Pacific basin is dynamically adjusted to Madden-Julian Oscillation in a few days. We find this large-scale ocean see-saw, induced by the Madden-Julian Oscillation, has a detectable influence on the Earth’s polar axis motion, in particular during the strong see-saw of early 2013. Intense winds over the Maritime Continent associated with the Madden-Julian Oscillation lead to a large-scale redistribution of oceanic mass in the Indo-Pacific basins, and have a noticeable impact on the Earth’s angular momentum.

/pdf/madden-julian-oscillation-winds-excite-an-intraseasonal-see-4qy63z7uio.pdf

Madden-Julian oscillation winds excite an intraseasonal see-saw of ocean mass that affects Earth’s polar motion

Investigating the robustness of the intraseasonal see-saw in the Indo-Pacific barotropic sea level across models

Recent discovery of a see-saw in intraseasonal barotropic sea level driven by boreal winter Madden-Julian Oscillation winds in the tropical Indo-Pacific basin has renewed interest in the barotropic dynamics in the tropics, which was otherwise known to be prominently a region of baroclinic dynamics at intraseasonal timescales. In this study, using a reference model simulation and several sensitivity experiments, we provide comprehensive details of the dynamics associated with this see-saw. It is found that the narrow Indonesian straits are instrumental in relaying barotropic excitations from one basin to the other. The intraseasonal barotropic circulation associated with this see-saw dynamics appears well organized at basin scale, both in the Indian Ocean and in the Pacific Ocean. During the positive phase of the see-saw, the Madden-Julian Oscillation winds over the Maritime Continent drive a basin-wide anti-clockwise circulation of ∼2 Sv around the Australian Continent with the eastern arm of this circulation located along the west African coast. Concurrently, it also drives a clockwise circulation in the southern Pacific Ocean. This circulation pattern reverses during the negative phase of the see-saw. In the Indonesian Throughflow region, the relatively narrow Ombai Strait and Lombok Strait facilitate this circulation in comparison to the wider Timor Passage. We discuss and point out the significant differences in this flow pattern with the mean flow across the Indonesian seas. We also address how some regions in the northern Pacific Ocean are decoupled from this large-scale dynamics. We show that it takes about a week for both the basins to adjust and to establish the see-saw, once the Madden-Julian Oscillation winds reach the Maritime Continent. 

The MJO-driven Indo-Pacific barotropic see-saw

The Anomalous 2012–13 Boreal Winter Oceanic Excitation of Earth’s Polar Motion

Showcasing model performance across space and time using single diagrams

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