ELFT HÍRADÓ

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

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                    ELTE TTK Fizikai Intézet
                STATISZTIKUS FIZIKAI SZEMINÁRIUM


                        2014. november 3-5.

                         Yosef Ashkenazy

            Ben-Gurion University of the Negev, Israel

                "Deep ocean dynamics under present
                    and past climate conditions"


Oceans in general and the deep ocean in particular play an important role in 
the
climate system through various mechanisms on many temporal and spatial scales.
During the three lectures we will review and explore the mechanisms associated
with deep ocean dynamics at different time periods of Earth history. We will
start by describing classical theories for the thermohaline circulation and 
will
then show the effect of winds on this circulation and its relation to 
millennium
time scale oceanic variability. We then will explore ocean dynamics under
complete ice cover, mimicking the most drastic climate events in Earth 
history,
the Snowball Earth glaciations. We will then present the effect of wind stress
variability on deep ocean dynamics. Detailed description and relevant
publications related to each of the topics described above is followed. 


1) A wind-induced thermohaline circulation hysteresis
   and millennial variability regimes
Monday 3 November, 12:00, room 7.59 of the Northern Block

http://www.bgu.ac.il/~ashkena/Papers/JPO_37_2446_2007.pdf

The multiple equilibria of the thermohaline circulation (THC: used here in the
sense of the meridional overturning circulation) as function of the surface
freshwater flux has been studied intensively following a Stommel paper from
1961. It is shown here that multistability and hysteresis of the THC also 
exist
when the wind stress amplitude is varied as a control parameter. Both the
Massachusetts Institute of Technology ocean general circulation model (MITgcm)
and a simple three-box model are used to study and explain different dynamical
regimes of the THC and THC variability as a function of the wind stress
amplitude. Starting with active winds and a thermally dominant thermohaline
circulation state, the wind stress amplitude is slowly reduced to zero over a
time period of 40 000 yr (40 kyr) and then increased again to its initial 
value
over another 40 kyr. It is found that during the decreasing wind stress phase,
the THC remains thermally dominant until very low wind stress amplitude at 
which
pronounced Dansgaard-Oeschger-like THC relaxation oscillations are initiated.
However, while the wind stress amplitude is increased, these relaxation
oscillations are present up to significantly larger wind stress amplitude. The
results of this study thus suggest that under the same wind stress amplitude,
the THC can be either in a stable thermally dominant state or in a pronounced
relaxation oscillations state. The simple box model analysis suggests that the
observed hysteresis is due to the combination of the Stommel hysteresis and 
the
Winton and Sarachik "deep decoupling" oscillations. 


2) Dynamics of a Snowball Earth ocean
Tuesday 4 November, 14:00, room 4.52 ("Sas Elemér") of the Northern Block

http://www.bgu.ac.il/~ashkena/Papers/Nature-495-2013.pdf
http://www.bgu.ac.il/~ashkena/Papers/JPO-2014.pdf
http://www.bgu.ac.il/~ashkena/Papers/JOC-submitted-2014.pdf

Geological evidence suggests that marine ice extended to the Equator at least
twice during the Neoproterozoic era (about 750 to 635 million years ago),
inspiring the Snowball Earth hypothesis that the Earth was globally 
ice-covered.
In a possible Snowball Earth climate, ocean circulation and mixing processes
would have set the melting and freezing rates that determine ice thickness,
would have influenced the survival of photosynthetic life, and may provide
important constraints for the interpretation of geo-chemical and
sedimentological observations. Here we show that in a Snowball Earth, the 
ocean
would have been well mixed and characterized by a dynamic circulation, with
vigorous equatorial meridional overturning circulation, zonal equatorial 
jets, a
well developed eddy field, strong coastal upwelling and convective mixing. 
This
is in contrast to the sluggish ocean often expected in a Snowball Earth 
scenario
owing to the insulation of the ocean from atmospheric forcing by the thick ice
cover. As a result of vigorous convective mixing, the ocean temperature,
salinity and density were either uniform in the vertical direction or weakly
stratified in a few locations. Our results are based on a model that couples 
ice
flow and ocean circulation, and is driven by a weak geothermal heat flux 
under a
global ice cover about a kilometre thick. Compared with the modern ocean, the
Snowball Earth ocean had far larger vertical mixing rates, and comparable
horizontal mixing by ocean eddies. The strong circulation and coastal 
upwelling
resulted in melting rates near continents as much as ten times larger than
previously estimated. 


3) Energy transfer of surface wind induced currents to the deep ocean
   via resonance with the Coriolis force
Wednesday 5 November, 11:00, room 2.54 ("Novobátzky Károly")
of the Northern Block

http://www.bgu.ac.il/~ashkena/Papers/JGR-2014.pdf
http://www.bgu.ac.il/~ashkena/Papers/JPO-41-2011.pdf
http://www.bgu.ac.il/~ashkena/Papers/JGR_114_C09009_2009.pdf

There are two main comparable sources of the energy of the deep ocean-winds 
and
tides. There are various mechanisms for the indirect transformation of surface
winds into the deep ocean. Here we show, using oceanic GCM, that the wind
directly supply energy down to the bottom of the ocean when it is stochastic 
or
when it is periodic with a frequency that is equal to the Coriolis frequency.
Basically, under such conditions one of the wind components resonates with the
Coriolis frequency. Using the finite depth Ekman layer model we show that when
the wind stress is purely periodic with a frequency that is equal to the
Coriolis frequency, the surface current speed is proportional do the depth of
the ocean, the current speed decreases linearly with depth, and the total
kinetic energy is proportional to the cube of the depth of the ocean. When the
wind stress is stochastic, the second moment of the surface current speed
increases logarithmically as a function of depth, the current speed decreases
linearly with depth in the deep ocean, and the total kinetic energy is
proportional to the depth of the ocean. Our results suggest that (i) the wind
contribution to the energy of the deep ocean may be significantly larger than
that of tides and that (ii) the seminal, infinitely deep, depth depended Ekman
layer model is ill-defined when forced by stochastic (or periodic with a
frequency that is equal to the Coriolis frequency) wind unless a friction term
is added. 


                honlap: http://glu.elte.hu/~statfiz

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