Nili Harnik's homepage:
GOAT – Geophysical Observational Analysis Tool: http://www.goat-geo.org/
A new flexible MATLAB-based tool to analyze observations and model output, written by Ori Adam, which I helped initiate and test. Recommended for teaching, a quick peek at the data and complex data analysis.
Prof. Nili
Harnik
Department
of Geosciences
Tel Aviv, 69978, Israel
harnik at post .
tau . ac . il
Seeking PhD student or
a postdoc to study the unusual
merging of the African and Atlantic jets, in the framework of
dynamical extremes.
Background in
atmospheric dynamics and experience running numerical models, in particular
coupled atmosphere-ocean models are highly beneficial. If interested, please
email me a CV, list
of publications and names of references
Publications Courses Group members
Research interests: The
large scale circulation of the troposphere and stratosphere. Waves and
instabilities in geophysical flows.
General motivation:
The
atmospheric patterns of wind, pressure, and temperature, and their temporal
variations, influence our daily lives through the changes in weather, and on
seasonal and longer time scales, through changes in climate conditions. The
global circulation of the atmosphere, which consists of a few different
regions, is one of the main features determining the climate zones on Earth.
Understanding the circulation of the atmosphere, and the ability to simulate it
numerically and predict the weather, have advanced greatly in the past half
century or so. The vast growth of data from these efforts has strengthened our
need for a fundamental physical understanding of theis circulation, in
particular, it is important to understand its natural variability, in relation
to the response to external forcing and man made changes.
Research in my group centers on developing an understanding of the main atmospheric wind and temperature patterns, their natural variations and response to different forms of forcing, on daily, seasonal, annual and longer time scales
An
idealized view of the general circulation:
The interaction between the meridional circulation
cells, the zonal jet streams, and baroclinic eddies, gives rise to different
dynamical regimes. These regimes are characterized by the type of jet stream:
A) A diabatically driven subtropical jet at the edge of the Hadley cell. B) A
strong meandering eddy driven jet. C) A
merged jet which is both thermally and eddy driven.
Modified Quasi Geostrophic (MQG) model (Lachmy and
Harnik, 2014): The simplest model which can study the maintenance and
transitions between the global circulation regimes – a spherical 2-level QG
model, modified to include zonal mean ageostrophic advection of momentum and
temperature. This allows a diabatically driven Hadley cell, and reproduces the
known jet stream regimes and their main characteristics. We find that wave
amplitudes play a central role in determining the jet regime, and use the model
to study the maintenance and transition between the different dynamical
regimes, and their influence on other circulation features. Learn
more
The subtropical jet stream: Lachmy and Harnik, (2014) discuss what maintains the zonal mean winter jet at the
edge of the Hadley cell, when the strong winter eddies force the jet in the
middle of the Ferrel cell? Learn more
The unusual merging of the Atlantic and African jet
during winter 2009-10: Many papers have
discussed the unusually cold and snowy Northern Hemisphere winter of 2009-10
with a persistently negative NAO. One aspect which did not receive attention is
the merging of the Atlantic and African jets into one unusually zonal jet. Harnik et al (2014) suggest that during this winter, the jet transitioned to a
merged state. Read more
The influence of the type of jet stream on the
distribution of extreme weather events:
The different kinds of jet streams are associated with a different interaction
with the synoptic scale eddies. As such, we expect the distribution of extreme
events to also change with jet stream regime. Read
more
The midlatitude influence of ENSO: In a different set of studies (summarized nicely in Richard Seager’s web page here) we examined how the modulations of the tropical heating
during ENSO subtly but systematically modify the jet stream and correspondingly
the structure of the storm track eddies, within a given dynamical regime (the
Pacific merged jet). We find that during El Nino, the changes in the
subtropical jet stream modify midlatitude eddy fluxes, and correspondingly, the
eddy driven Ferrel cell, which can explain the colder/wetter midlatitudes
during El Nino and warmer/dryer midlatitudes during La Nina. In
Harnik et al (2010), we further used a hierarchy of models, we obtain the
sequence of events that lead from a tropical SST anomaly to the quasi
equilibrium change in the mean and transient atmospheric circulation.
Fundamental mechanism: There are two basic theories of the mechanism behind
Rossby-wave based shear flow instability – a mutual amplification of counter
propagating Rossby waves, and Overreflection of Rossby waves. In Harnik and
Heifetz (2007) we
related these two fundamental theories, in terms of Kernel Rossby Waves – the
fundamental building blocks of shear instability. This framework also explains
stratified shear flow instabilities which involve gravity rather than Rossby
waves (Harnik et al, 2008; Rabinovich et al, 2011).
Nonlinear equilibration: Recently we have been studying the nonlinear evolution of
these basic shear flow instabilities. In Harnik
et al (2014) we
examine the
fundamental and complex interrelation between the mean flow, Rossby waves and
vortices in a particularly simple setup of asymmetric barotropic instability.
In this flow, PV gradients and corresponding Rossby waves exist throughout the
nonlinear evolution. Using an extensive parameter sweep, we combine constraints of linear stability with conservation
of wave activity and circulation, to obtain a theory for the equilibrated mean
flow and wave amplitudes. Learn more including a beautiful movie of this nonlinear
evolution in which dragon-head structures form in the equilibrated stage.
The large
scale circulation of the stratosphere:
The stratospheric winter variability and correspondingly its downward coupling to the troposphere, can be roughly divided to two kinds: 1) Absorptive winters: with a sudden stratospheric warming, a weak vortex and persistent downward coupling of the zonal mean flow. 2) Reflective winters: with a strong polar vortex, downward wave reflection, and short time scale downward wave coupling.
A wave geometry diagnostic: Separates the classical index of refraction into vertical and meridional components. Learn More
Downward
wave reflection and the influence on downward coupling to the troposphere:
The importance of downward wave coupling varies
seasonally, and is different between the two hemispheres. Learn More
Downward
reflection or a sudden warming? Many
of the episodes of downward reflection occur when an upward propagating wave
decelerates the flow in the upper stratosphere (a stratopause
warming?), forming negative meridional PV gradients and a downward reflecting
surface. What determines if an upward pulse of wave activity will break the
vortex apart as in a sudden warming, or just form a reflecting surface and get
reflected down?
Harnik (2009) shows a major factor is the length of the upward wave pulse- long pulses cause sudden warmings while short pulse (less than a week) lead to reflection.