A14B-01 INVITED 16:00h
Mid-Latitude Ocean-Atmosphere Coupling on Decadal Time Scales
The mid-latitude natural variability of the climate on decadal scales --- manifested in North-Atlantic, Pacific-Decadal, and Antarctic-Annular "modes" --- has the default mechanistic paradigm of being atmospherically controlled with passive thermal adjustment by the ocean. A related conventional wisdom is that the mid-latitude atmosphere is insensitive to oceanic temperature variations. In this talk several reasons to question this paradigm will be discussed. In particular, evidence will be presented from idealized, highly turbulent, quasigeostrophic computational simulations that there is an important oceanic circulation control on the coupled decadal variability. Its origin is in the intrinsic, low-frequency, large-scale variability of the inertial recirculation gyres of the wind-driven circulation. This in turn creates oceanic temperature anomalies to which the atmosphere responds in a nonlinear way to make irregularly spaced transitions between two different zonal flow states, viz., high-latitude and low-latitude jet locations. The atmospheric transitions alter the wind forcing patterns, with a decadal adjustment response time for the inertial recirculation gyres. The transitions lead to a statistically significant, broad-band signal in the period range of 5-20 years evident in both oceanic and atmospheric variables. Accompanying atmosphere-only and ocean-only simulations both have weaker decadal variability compared to the coupled one. Implications for observational analysis and standard climate models are assessed.
A14B-02 INVITED 16:20h
The use of systems and control techniques in equation free modeling of complex multiscale processes
In current modeling , the best available descriptions of a system often come at a fine level (atomistic, stochastic, microscopic, individual-based) while the questions asked and the tasks required by the modeler (prediction, parametric analysis, optimization and control) are at a much coarser, averaged, macroscopic level. Traditional modeling approaches start by first deriving macroscopic evolution equations from the microscopic models, and then bringing our arsenal of mathematical and algorithmic tools to bear on these macroscopic descriptions. Over the last few years, and with several collaborators, we have developed and validated a mathematically inspired, computational enabling technology that allows the modeler to perform macroscopic tasks acting on the microscopic models directly. We call this the ``equation-free" approach, since it circumvents the step of obtaining accurate macroscopic descriptions. Te backbone of this approach is the design of (computational) experiments. Traditional continuum numerical algorithms can be viewed as a set protocols for experimental design (where "experiment" means a computational experiment set up and performed with a model at a different level of description). Ultimately, what makes it all possible is the ability to initialize computational experiments at will. Short bursts of appropriately initialized computational experimentation -through matrix-free numerical analysis and systems theory tools like feedback, variance reduction and estimation- bridges microscopic simulation with macroscopic modeling.
A14B-03 16:40h
Bio-physical feedbacks in the tropical Pacific
We study the influence of phytoplankton on the tropical Pacific heat budget. A hybrid coupled model for the tropical Pacific is used which is based on a primitive equation reduced gravity multi-layer ocean model, a dynamic ocean mixed layer, an atmospheric mixed layer and a statistical atmosphere. The statistical atmosphere relates deviations of the sea surface temperature from its mean to wind-stress anomalies and allows for the rectification of the annual cycle and the El Ni\~no-Southern Oscillation (ENSO) phenomenon through the positive Bjerknes-feedback. Furthermore, a 9-component ecosystem model is coupled to the physical variables of the ocean. The simulated chlorophyll concentrations can feedback onto the ocean heat budget by their optical properties which modify solar light absorption in the surface layers. It is shown that both, the surface layer concentration as well as the vertical profile of chlorophyll have a significant effect on the simulated mean state, the tropical annual cycle and ENSO. This study supports a previously suggested hypothesis which predicts an influence of phytoplankton concentration of the tropical Pacific climate mean state and its variability. The bio-climate feedback diagnosed here works as follows: Maxima in the subsurface chlorophyll concentrations lead to an enhanced subsurface warming due to the absorption of photosynthetically available shortwave radiation. This warming triggers a deepening of the mixed layer in the eastern equatorial Pacific and eventually a reduction of the surface ocean currents. The weakened south-equatorial current generates an eastern Pacific surface warming. Due to the deepening of the mixed layer, also the strength of the simulated annual cycle is diminished. This in trurn leads to an increase in ENSO variability.
A14B-04 17:00h
Storage of Heat in the Glacial Deep Ocean and the Importance of Seawater Thermodynamics in Climate Change
A variety of records of both oceanic and atmospheric variability link Dansgaard/Oeschger events and Bond Cycles to changes in the overturning strength of the deep ocean. Various models have shown that the observed temperature changes and circulation switches can be forced with variations in the freshwater budget of the North Atlantic surface ocean. However, recent evidence from sediment pore fluids show that the stratification of LGM deep waters was dominated by salinity, rather than temperature. If the saltiest waters of the glacial deep ocean were produced in the Southern Ocean, than salinification of surface waters in the North Atlantic cannot produce large transients in overturning strength without some other source of buoyancy to erode the deep stratification. Here we present an energy storage mechanism, thermobaricity, and an energy source, geothermal heating, that implicate the deep ocean as the origin of the glacial rapid climate changes and the source of this buoyancy. Salinity stratification of the deep ocean during the glacial can lead to heat storage in the abyss that will decrease the deep to surface density difference and then, due to the non-linearity of the seawater equation of state, could periodically cause catastrophic convective events. This "thermobaric convection" arises from the pressure dependence of the seawater thermal expansion coefficient. Our contention is that the thermodynamics of seawater could play an important a role in glacial ocean/atmosphere reorganizations. We will outline how the pressure dependence of the seawater thermal expansion coefficient can lead to occasional rapid overturning of the deep ocean and present some new model results that examine the feasibility of this idea.
A14B-05 17:15h
Influence of Sea-ice on Inter-Annual Climate Variability in Coupled Ocean-Sea-ice-Atmosphere Simulations
There is currently considerable interest in the role of sea-ice on changes in climate and climate variability. Variations in sea-ice modify the atmosphere and oceans through its influence on albedo, ocean-atmosphere heat exchange and ocean buoyancy. Studies show clearly that interactions between sea-ice, oceans and atmosphere are likely to be important at seasonal to decadal timescales of variability. The interaction of Arctic sea-ice with Northern Hemisphere climate variability at these timescales has been investigated. The Hadley Centre ocean-atmosphere model, HadCM3, has been coupled to an elastic-viscous-plastic (EVP) sea-ice dynamics and Semnter zero-layer sea-ice thermodynamics schemes. Multi-decadal length simulations have been performed primarily using pre-industrial atmospheric forcing to determine natural variability and feedback processes inherent to the climate system. The simulations are examined in terms of patterns such as the North Atlantic Oscillation and oceanic thermohaline circulation, and how their characteristics and magnitude are influenced by key sea-ice processes, for example albedo feedbacks and brine rejection. The HadCM3 results using dynamic-thermodynamic sea-ice have also been compared to an equivalent simulation using thermodynamic sea-ice coupled to a simpler ice drift model in which sea-ice is passively advected using surface level ocean currents. By this comparison we investigate the effect of including wind-driven sea-ice motion on the mean climate and climate variability. Results suggest that extent and winter variability of Arctic sea-ice is improved. The sea-ice differences impact on high latitude temperatures and Arctic Oscillation pattern. The meridional overturning circulation becomes weaker as a result of increased ice export into the North Atlantic with a wind-driven sea-ice cover.
A14B-06 17:30h
A Stochastic Bayesian Approach to Identify the Dynamical Regimes of ENSO
Statistical inverse modeling is used to explore and quantify the relative likelihood of the different dynamic regimes exhibited within an intermediate coupled model of the tropical Pacific. This is accomplished by systematically searching model parameter space for the settings that enable the model to reproduce the observed variance, skewness, kurtosis, and decorrelation times of the Nino3 index of the past 150 years. One objective of this exercise is to relate particular features of ENSO behavior to the specification of model parameters including aspects of the climatological mean state such as the mean thermocline depth and surface wind forcing. In particular we find that the manifestation of a positive skewness in histograms of modeled SSTs is most strongly related to the specification of the mean thermocline depth. The model configuration that most resembles the statistical characteristics of the observed ENSO is a system with regular self-sustaining (non-damped) oscillations in which ENSO irregularity is entirely governed by atmospheric noise. Emphasis will be placed on the use of this statistical inverse modeling technique to shed light on the processes governing ENSO and its use in interpreting changes in ENSO behavior through time.
A14B-07 17:45h
Two-way Feedback Interaction Between the Thermohaline and Wind-driven Circulations
The thermohaline circulation (THC) affects the meridional atmospheric temperature gradient and therefore the atmospheric wind and the wind driven ocean circulation. The wind driven circulation (WDC), in turn, affects the THC by the advection of salinity anomalies into deep water formation sites. This paper considers this two way coupling between the WDC and THC, using tools from engineering feedback control. The coupled system is modeled using an enhanced version of the model of Pasquero and Tziperman (2004). For this model, the two way feedback can have a significant effect on the dynamics of the coupled system. For a reasonable choice of parameters, the feedback destabilizes the THC equilibrium for low fresh-water forcing. The dependence of stability on feedback parameters can be rapidly explored using root locus tools, which we introduce and explain. At higher fresh-water forcing, the model exhibits limit cycle behaviour, and describing function analysis is used with partial success to predict the dependence of limit cycle amplitude on feedback parameters. As the fresh water forcing is increased further, the feedback results in a new stable equilibrium instead of the large amplitude limit cycle that develops without feedback. It is expected that the analysis approaches used here may be broadly applicable to the study of feedback interconnections of other climate systems as well.