141 research outputs found
Coherent Resonat millenial-scale climate transitions triggered by massive meltwater pulses
The role of mean and stochastic freshwater forcing on the generation of millennial-scale climate variability in the North Atlantic is studied using a low-order coupled atmosphereâoceanâsea ice model. It is shown that millennial-scale oscillations can be excited stochastically, when the North Atlantic Ocean is fresh enough. This finding is used in order to interpret the aftermath of massive iceberg surges (Heinrich events) in the glacial North Atlantic, which are characterized by an excitation of DansgaardâOeschger events. Based on model results, it is hypothesized that Heinrich events trigger DansgaardâOeschger cycles and that furthermore the occurrence of Heinrich events is dependent on the accumulated climatic effect of a series of DansgaardâOeschger events. This scenario leads to a coupled oceanâice sheet oscillation that shares many similarities with the Bond cycle. Further sensitivity experiments reveal that the timescale of the oscillations can be decomposed into stochastic, linear, and nonlinear deterministic components. A schematic bifurcation diagram is used to compare theoretical results with paleoclimatic data
An approach for projecting the timing of abrupt winter Arctic sea ice loss
Abrupt and irreversible winter Arctic sea ice loss may occur under anthropogenic warming due to the disappearance of a sea ice equilibrium at a
threshold value of CO2, commonly referred to as a tipping point. Previous work has been unable to conclusively identify whether a tipping
point in winter Arctic sea ice exists because fully coupled climate models are too computationally expensive to run to equilibrium for many
CO2Â values. Here, we explore the deviation of sea ice from its equilibrium state under realistic rates of CO2Â increase to
demonstrate for the first time how a few time-dependent CO2Â experiments can be used to predict the existence and timing of sea ice tipping
points without running the model to steady state. This study highlights the inefficacy of using a single experiment with slow-changing CO2
to discover changes in the sea ice steady state and provides a novel alternate method that can be developed for the identification of tipping
points in realistic climate models.</p
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Controlling Spatiotemporal Chaos in a Realistic El Niño Prediction Model
A method for controlling low-order chaotic behavior of continuous spatiotemporal systems is developed and demonstrated in a complex, realistic 3D partial differential equation model that is used successfully for predicting El Niño events in the equatorial Pacific. An unstable periodic orbit that involves a full-domain oscillation is stabilized using a feedback control applied to a single degree of freedom at a carefully chosen single âchoke pointâ in space. A general criterion is presented for determining the optimal points in reconstructed delay-coordinate phase space at which to apply the feedback control.Earth and Planetary Science
The role of regional feedbacks in glacial inception on Baffin Island: the interaction of ice flow and meteorology
Over the past 0.8 million years, 100 kyr ice ages have dominated
Earth's climate with geological evidence suggesting the last glacial
inception began in the mountains of Baffin Island. Currently,
state-of-the-art global climate models (GCMs) have difficulty simulating
glacial inception, possibly due in part to their coarse horizontal resolution
and the neglect of ice flow dynamics in some models. We attempt to address
the role of regional feedbacks in the initial inception problem on Baffin
Island by asynchronously coupling the Weather Research and Forecast (WRF) model,
configured as a high-resolution inner domain over Baffin and an outer
domain incorporating much of North America, to an ice flow model using the
shallow ice approximation. The mass balance is calculated from WRF
simulations and used to drive the ice model, which updates the ice extent
and elevation, that then serve as inputs to the next WRF run. We drive the
regional WRF configuration using atmospheric boundary conditions from 1986
that correspond to a relatively cold summer, and with 115 kya insolation.
Initially, ice accumulates on mountain glaciers, driving downslope ice flow
which expands the size of the ice caps. However, continued iterations of the
atmosphere and ice models reveal a stagnation of the ice sheet on Baffin
Island, driven by melting due to warmer temperatures at the margins of the
ice caps. This warming is caused by changes in the regional circulation that
are forced by elevation changes due to the ice growth. A stabilizing feedback
between ice elevation and atmospheric circulation thus prevents full
inception from occurring.</p
Destabilization of the thermohaline circulation by transient perturbations to the hydrological cycle
We reconsider the problem of the stability of the thermohaline circulation as
described by a two-dimensional Boussinesq model with mixed boundary conditions.
We determine how the stability properties of the system depend on the intensity
of the hydrological cycle. We define a two-dimensional parameters' space
descriptive of the hydrology of the system and determine, by considering
suitable quasi-static perturbations, a bounded region where multiple equilibria
of the system are realized. We then focus on how the response of the system to
finite-amplitude surface freshwater forcings depends on their rate of increase.
We show that it is possible to define a robust separation between slow and fast
regimes of forcing. Such separation is obtained by singling out an estimate of
the critical growth rate for the anomalous forcing, which can be related to the
characteristic advective time scale of the system.Comment: 37 pages, 8 figures, submitted to Clim. Dy
Consequences of pacing the Pleistocene 100 kyr ice ages by nonlinear phase locking to Milankovitch forcing
Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 21 (2006): PA4206, doi:10.1029/2005PA001241.The consequences of the hypothesis that Milankovitch forcing affects the phase (e.g., termination times) of the 100 kyr glacial cycles via a mechanism known as ânonlinear phase lockingâ are examined. Phase locking provides a mechanism by which Milankovitch forcing can act as the âpacemakerâ of the glacial cycles. Nonlinear phase locking can determine the timing of the major deglaciations, nearly independently of the specific mechanism or model that is responsible for these cycles as long as this mechanism is suitably nonlinear. A consequence of this is that the fit of a certain model output to the observed ice volume record cannot be used as an indication that the glacial mechanism in this model is necessarily correct. Phase locking to obliquity and possibly precession variations is distinct from mechanisms relying on a linear or nonlinear amplification of the eccentricity forcing. Nonlinear phase locking may determine the phase of the glacial cycles even in the presence of noise in the climate system and can be effective at setting glacial termination times even when the precession and obliquity bands account only for a small portion of the total power of an ice volume record. Nonlinear phase locking can also result in the observed âquantizationâ of the glacial period into multiples of the obliquity or precession periods.E.T. is funded by NSF Paleoclimate program, grant
ATM-0455470 and by the McDonnell Foundation. P.H. is supported by the
NOAA Postdoctoral Program in Climate and Global Change. C.W. is
supported by the National Ocean Partnership Program (NOPP). M.E.R. is
supported by NSF grant ATM-0455328
The stochastic resonance mechanism in the Aerosol Index dynamics
We consider Aerosol Index (AI) time-series extracted from TOMS archive for an area covering Italy . The missing of convergence in estimating the embedding dimension of the system and the inability of the Independent Component Analysis (ICA) in separating the fluctuations from deterministic component of the signals are evidences of an intrinsic link between the periodic behavior of AI and its fluctuations. We prove that these time series are well described by a stochastic dynamical model. Moreover, the principal peak in the power spectrum of these signals can be explained whereby a stochastic resonance, linking variable external factors, such as Sun-Earth radiation budget and local insolation, and fluctuations on smaller spatial and temporal scale due to internal weather and antrophic components
Characterizing, modelling and understanding the climate variability of the deep water formation in the North-Western Mediterranean Sea
Observing, modelling and understanding the climate-scale variability of the deep water formation (DWF) in the North-Western Mediterranean Sea remains today very challenging. In this study, we first characterize the interannual variability of this phenomenon by a thorough reanalysis of observations in order to establish reference time series. These quantitative indicators include 31 observed years for the yearly maximum mixed layer depth over the period 1980â2013 and a detailed multi-indicator description of the period 2007â2013. Then a 1980â2013 hindcast simulation is performed with a fully-coupled regional climate system model including the high-resolution representation of the regional atmosphere, ocean, land-surface and rivers. The simulation reproduces quantitatively well the mean behaviour and the large interannual variability of the DWF phenomenon. The model shows convection deeper than 1000 m in 2/3 of the modelled winters, a mean DWF rate equal to 0.35 Sv with maximum values of 1.7 (resp. 1.6) Sv in 2013 (resp. 2005). Using the model results, the winter-integrated buoyancy loss over the Gulf of Lions is identified as the primary driving factor of the DWF interannual variability and explains, alone, around 50 % of its variance. It is itself explained by the occurrence of few stormy days during winter. At daily scale, the Atlantic ridge weather regime is identified as favourable to strong buoyancy losses and therefore DWF, whereas the positive phase of the North Atlantic oscillation is unfavourable. The driving role of the vertical stratification in autumn, a measure of the water column inhibition to mixing, has also been analyzed. Combining both driving factors allows to explain more than 70 % of the interannual variance of the phenomenon and in particular the occurrence of the five strongest convective years of the model (1981, 1999, 2005, 2009, 2013). The model simulates qualitatively well the trends in the deep waters (warming, saltening, increase in the dense water volume, increase in the bottom water density) despite an underestimation of the salinity and density trends. These deep trends come from a heat and salt accumulation during the 1980s and the 1990s in the surface and intermediate layers of the Gulf of Lions before being transferred stepwise towards the deep layers when very convective years occur in 1999 and later. The salinity increase in the near Atlantic Ocean surface layers seems to be the external forcing that finally leads to these deep trends. In the future, our results may allow to better understand the behaviour of the DWF phenomenon in Mediterranean Sea simulations in hindcast, forecast, reanalysis or future climate change scenario modes. The robustness of the obtained results must be however confirmed in multi-model studies
A practical indicator for surface ocean heat and freshwater buoyancy fluxes and its application to the NCEP reanalysis data
The buoyancy flux at the air/sea interface plays a key role in water mass transformation and mixing as it modifies surface water density and in turn drives overturning and enhances stratification. It is the interplay of these two independent heat and freshwater buoyancy flux components that is of central importance when analysing mechanisms of the ocean/atmosphere interaction. Here, a diagnostic quantity (ÎB) is presented that allows to capture the relative contribution of both components on the buoyancy flux in one single quantity. Using NCEP reanalysis of heat and freshwater fluxes (1948â2009) demonstrates that ÎB is a convenient tool to analyse both the temporal and spatial variability of their corresponding buoyancy fluxes. For the global ocean the areal extent of buoyancy gain and loss regions changed by 10%, with the largest extent of buoyancy gain during the 1970â1990 period. In the subpolar North Atlantic, and likewise in the South Pacific, decadal variability in freshwater flux is pronounced and, for the latter region, takes control over the total buoyancy flux since the 1980s. Some of the areal extent time series show a significant correlation with large-scale climate indices
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