141 research outputs found
Relating the diffusive salt flux just below the ocean surface to boundary freshwater and salt fluxes
We detail the physical means whereby boundary transfers of freshwater and salt induce diffusive fluxes of salinity. Our considerations focus on the kinematic balance between the diffusive fluxes of salt and freshwater, with this balance imposed by mass conservation for an element of seawater. The flux balance leads to a specific form for the diffusive salt flux immediately below the ocean surface and, in the Boussinesq approximation, to a specific form for the salinity flux. This note clarifies conceptual and formulational ambiguities in the literature concerning the surface boundary condition for the salinity equation and for the contribution of freshwater to the buoyancy budget
On Pacific Subtropical Cell Variability over the Second Half of the Twentieth Century
Abstract
The evolution of the Pacific subtropical cells (STC) is presented for the period 1948–2007. Using ocean models of different resolutions forced with interannually varying atmospheric forcing datasets, the mechanisms responsible for the observed STC weakening and late recovery during the period of study are analyzed. As a result of the STC weakening (strengthening), warming (cooling) trends are found in the equatorial Pacific sea surface temperatures (SSTs). Model results agree well with observed estimates of STC transport, STC convergence, and equatorial SST anomalies. It is shown that subtropical atmospheric variability is the primary driver of the STC and equatorial SST low-frequency evolution and is responsible for both the slowdown during the second half of the twentieth century and the rebound at the end of the century. Subtropically forced STC variability is identified as a major player in the generation of equatorial Pacific decadal SST anomalies, pacing tropical Pacific natural climate variability on interdecadal time scales, as observed in historical records. The natural mode of variability has implications for the evolution of equatorial SST in the coming decades under the concomitant effects of climate change
Global Cascade of Kinetic Energy in the Ocean and the Atmospheric Imprint
We present the first estimate for the ocean's global scale-transfer of
kinetic energy (KE), across scales from 10~km to 40000~km. We show the
existence of oceanic KE transfer between gyre-scales and mesoscales induced by
the atmosphere's Hadley, Ferrel, and polar cells, and intense downscale KE
transfer associated with the Inter-Tropical Convergence Zone. We report peak
upscale transfer of 300 GigaWatts across mesoscales of 120~km in size, roughly
1/3rd the energy input by winds into the oceanic general circulation. This
"cascade" penetrates almost the entire water column, with nearly three quarters
of it occurring south of 15S. The mesoscale cascade has a self-similar
seasonal cycle with characteristic lag-time of days per
octave of length-scales such that transfer across 50~km peaks in spring while
transfer across 500~km peaks in summer. KE content of those mesoscales follows
the same self-similar cycle but peaks days after the peak
cascade, suggesting that energy transferred across a scale is primarily
deposited at a scale 4 larger
Exploring the non-stationarity of coastal sea level probability distributions
Studies agree on a significant global mean sea level rise in the 20th century
and its recent 21st century acceleration in the satellite record. At regional
scale, the evolution of sea level probability distributions is often assumed to
be dominated by changes in the mean. However, a quantification of changes in
distributional shapes in a changing climate is currently missing. To this end,
we propose a novel framework quantifying significant changes in probability
distributions from time series data. The framework first quantifies linear
trends in quantiles through quantile regression. Quantile slopes are then
projected onto a set of four polynomials quantifying how such
changes can be explained by shifts in the first four statistical
moments. The framework proposed is theoretically founded, general and can be
applied to any climate observable with close-to-linear changes in
distributions. We focus on observations and a coupled climate model (GFDL-CM4).
In the historical period, trends in coastal daily sea level have been driven
mainly by changes in the mean and can therefore be explained by a shift of the
distribution with no change in shape. In the modeled world, robust changes in
higher order moments emerge with increasing concentration. Such
changes are driven in part by ocean circulation alone and get amplified by sea
level pressure fluctuations, with possible consequences for sea level extremes
attribution studies
Preconditioning of the Weddell Sea Polynya by the Ocean Mesoscale and Dense Water Overflows
The Weddell Sea polynya is a large opening in the open-ocean sea ice cover associated with intense deep convection in the ocean. A necessary condition to form and maintain a polynya is the presence of a strong subsurface heat reservoir. This study investigates the processes that control the stratification and hence the buildup of the subsurface heat reservoir in the Weddell Sea. To do so, a climate model run for 200 years under preindustrial forcing with two eddying resolutions in the ocean (0.25° CM2.5 and 0.10° CM2.6) is investigated. Over the course of the simulation, CM2.6 develops two polynyas in the Weddell Sea, while CM2.5 exhibits quasi-continuous deep convection but no polynyas, exemplifying that deep convection is not a sufficient condition for a polynya to occur. CM2.5 features a weaker subsurface heat reservoir than CM2.6 owing to weak stratification associated with episodes of gravitational instability and enhanced vertical mixing of heat, resulting in an erosion of the reservoir. In contrast, in CM2.6, the water column is more stably stratified, allowing the subsurface heat reservoir to build up. The enhanced stratification in CM2.6 arises from its refined horizontal grid spacing and resolution of topography, which allows, in particular, a better representation of the restratifying effect by transient mesoscale eddies and of the overflows of dense waters along the continental slope
Atlantic multi-decadal oscillation covaries with Agulhas leakage
The interoceanic transfer of seawater between the Indian Ocean and the Atlantic, ‘Agulhas leakage’, forms a choke point for the overturning circulation in the global ocean. Here, by combining output from a series of high-resolution ocean and climate models with in situ and satellite observations, we construct a time series of Agulhas leakage for the period 1870–2014. The time series demonstrates the impact of Southern Hemisphere westerlies on decadal timescales. Agulhas leakage shows a correlation with the Atlantic Multi-decadal Oscillation on multi-decadal timescales; the former leading by 15 years. This is relevant for climate in the North Atlanti
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