A mechanistic model of mid-latitude decadal climate variability

Abstract

A simple heuristic model of coupled decadal ocean-atmosphere modes in middle latitudes is developed. Previous studies have treated atmospheric intrinsic variability as a linear stochastic process modified by a deterministic coupling to the ocean.The present paper takes an alternative view: based on observational, as well as process modeling results, it represents this variability in terms of irregular transitions between two anomalously persistent, high-latitude and low-latitude jet-stream states. Atmospheric behavior is thus governed by an equation analogous to that describing the trajectory of a particle in a double-well potential, subject to stochastic forcing. Oceanic adjustment to a positional shift in the atmospheric jet involves persistent circulation anomalies maintained by the action of baroclinic eddies; this process is parameterized in the model as a delayed oceanic response. The associated sea-surface temperature anomalies provide heat fluxes that affect atmospheric circulation by modifying the shape of the double-well potential. If the latter coupling is strong enough, the model's spectrum exhibits a peak at a periodicity related to the ocean's eddy-driven adjustment time. A nearly analytical approximation of the coupled model is used to study the sensitivity of this behavior to key model parameters.

Low-frequency variability and heat transport in a low-order nonlinear coupled ocean–atmosphere model

“…9. The former periodicity agrees roughly with the variability peak in the coupled modes of the slightly more highly resolved atmosphere-ocean model of Kravtsov et al [32,33]. In the latter, the near-decadal peak was associated with alternations between two atmospheric regimes characterized by the subtropical jet's latitude; this bimodality of the jet position and intensity does not seem to play a key role in the present model.…”

“…We saw that our coupled model successfully simulates semi-permanent highs and lows that have some similarity with the Azores High and the Icelandic Low, as well as multi-annual LFV. One might thus wonder whether it might generate spatial features similar to the North Atlantic Oscillation (NAO) and display its decadal variability, as proposed by Feliks et al [18] or by Kravtsov et al [32,33]. Figure 8 shows the geopotential height difference between two points of the spatial domain -located at (π/n, π/4) and π/n, 3π/4), in the model's nondimensional coordinates -within the atmosphere, for different values of the radiative meridional gradient C o and of the coupling parameter d. These points correspond roughly to high-and low-pressure centers in the model atmosphere.…”

“…From a dynamical point of view, one possible answer to these questions translates into a search for the presence of coupled modes between the ocean and the atmosphere, such as found for instance in observational data [9]. In the context of coupled global climate models, the answer, however, differs from one investigation to another or from a modeling setup to another; this answer depends, to a large extent, on whether the forcing is generated by deterministic or stochastic, linear or nonlinear processes [10,33,39].…”

Final Technical Report for Collaborative Research: Regional climate-change projections through next-generation empirical and dynamical models, DE-FG02-07ER64429

“…This PM variability accounts for as much as 50% of the total PM variance in the midwest region throughout the year. Kravtsov et al (2008a): Previous studies have treated atmospheric intrinsic variability as a linear stochastic process modified by a deterministic coupling to the ocean. We here represent this variability in terms of irregular transitions between two anomalously persistent, high-latitude and low-latitude jetstream states.…”

Final Technical Report for "Collaborative Research: Regional climate-change projections through next-generation empirical and dynamical models"

“…This PM variability accounts for as much as 50% of the total PM variance in the midwest region throughout the year. Kravtsov et al (2008a): Previous studies have treated atmospheric intrinsic variability as a linear stochastic process modified by a deterministic coupling to the ocean. We here represent this variability in terms of irregular transitions between two anomalously persistent, high-latitude and low-latitude jet-stream states.…”