ELFT HÍRADÓ

[StatFizSzeminar <statfiz AT glu.elte.hu>]

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               ELTE Institute of Physics
            SEMINARS IN STATISTICAL PHYSICS
             
             A HELYSZÍN: Északi tömb 4.70


            Henk A. Dijkstra Lecture Series
           on Nonlinear Physical Oceanography


1 March (Thu), 16:15-17:45

The use of implicit models to understand the physics of the 
climate system

Abstract: Over the last decades a hierarchy of implicit 
ocean-climate models has been developed making use of 
sophisticated linear systems solvers (e.g., MRILU) and 
generalized eigenproblem solvers (e.g. JDQZ). In this 
presentation, I will give an overview of what one has learned 
from these models regarding the physics of the climate system 
(a few examples), describe the current capabilities of these 
models and discuss their (potential) future development. 

Ref: H.A. Dijkstra, Nonlinear Physical Oceanography, 2nd 
edition, Springer, 2005. 



2 March (Fri), 11:15-12:45

A nonlinear theory of Kuroshio path variability

Abstract: The bimodal behavior of the path of the Kuroshio in 
the North Pacific has fascinated physical oceanographers since 
indications of this phenomenon were found. Why would a western 
boundary current switch between a large-meander and a 
small-meander state and back? Why does this phenomenon not 
appear in other western boundary currents, such as the Gulf 
Stream? For the Kuroshio, both large- and small meander states 
can persist over a period ranging from a few years to a decade. 
With the analysis of satellite data and those of in situ 
measurements, quite a detailed description of the different 
states and their transition behavior is now available. There is 
still, however, no consensus on which processes cause the 
low-frequency variability in the Kuroshio. It appears that 
direct interpretation of the observations often has been based 
on mechanisms involving external causes (such as atmospheric 
forcing) and linear ocean dynamics while ocean modelers have 
tended to suggest mechanisms which involve elements of 
nonlinear ocean dynamics. A theory of the Kuroshio path 
variability should explain at least (i) why the Kuroshio can be 
in different states and provide a mechanism for the different 
spatial patterns of these states, and explain (ii) the physics 
of the transition time scale between the two states. Based on a 
(stochastic) dynamical systems analysis of a hierarchy of 
models (quasi-geostrophic, shallow-water, primitive equations), 
a nonlinear theory will be presented which is able to provide 
answers to the issues (i-ii) above. 

Ref: S. Pierini, Dijkstra, H.A. and A. Riccio, A nonlinear 
theory of the Kuroshio Extension Bimodality, J. Phys. 
Oceanography 39, 2212-2229 , (2009). 



2 March (Fri), 16:15-17:45

North Atlantic Multidecadal Variability: a synthesis view

Abstract: A dynamical systems framework, in many ways similar 
to that for El Nino/Southern Oscillation, is presented to 
understand the multidecadal variability in the North Atlantic. 
A so-called minimal primitive equation model is first used to 
represent the multidecadal variability in the North Atlantic 
climate system. Within this minimal model, we identify a normal 
mode of multidecadal variability that can destabilize the 
background climate state through a so-called Hopf bifurcation. 
Next, it is argued that noise is setting the amplitude of the 
sea surface temperature variability associated with this 
normal mode. Finally, it will argued that the mechanism found 
in the minimal model is responsible for the multidecadal 
variability in GCMs and in available observations. 

Ref: L. M. Frankcombe, A. von der Heydt, H. A. Dijkstra, North 
Atlantic Multidecadal Climate Variability: An investigation of 
dominant time scales and processes, J. Climate, 23, 3626-3638, 
(2010). 



3 March (Sat), 11:15-12:45

Will the Atlantic Ocean Circulation collapse before the end of 
this century?

Abstract: The Atlantic Ocean, in particular its Meridional 
Overturning Circulation (MOC), is sensitive to the patterns of 
atmospheric forcing, in particular to that of the freshwater 
flux. Relatively small changes in atmospheric conditions can 
therefore lead to a spectacular reduction (collapse) of the 
Atlantic MOC. As the time scale of a collapse is only a few 
decades, the associated changes in heat transport may have a 
large impact on European climate and society. In the latest 
IPCC assessment (AR4), climate models project that changes in 
freshwater input due to global warming will only lead to at 
most 10% change in the strength of the Atlantic MOC at the end 
of this century. However, there have been recent developments 
which indicate that the Atlantic MOC may be more sensitive 
than these models suggest. These new results will be the main 
focus of this presentation. 

Ref: Huisman, S., M. den Toom, Dijkstra, H. A. and S. 
Drijfhout. An indicator of the multiple equilibria regime of 
the Atlantic Meridional Overturning Circulation. J. Phys. 
Oceanography, 40, 551-567, (2010).



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