Nili Harnik's homepage:

“European weather Extremes: DrIvers, Predictability and Impacts” (EDIPI) project.

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 University

Tel Aviv, 69978, Israel

harnik at tauex . tau . ac . il

Hiring: I have no active call at the moment but am always interested in adding new excellent students or postdoc to my group so please email me if you ae interested.

Publications Courses Group members Models and data

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.

Shear flow instabilities:

Instability of shear flows is one of the main sources of disturbances in the Atmosphere and oceans, giving rise to synoptic weather systems patterns and ocean eddies.

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.

Ongoing project: Processes Controlling Stratospheric Dynamic Variability, and Implications for Ozone Levels and Downward Coupling to the Troposphere – joint with Katja Matthes at GEOMAR, Kiel, funded by the German Israeli Foundation. We are currently investigating the influences of various natural and anthropogenic forcings on the downward coupling to the stratosphere. In a separate project we are studying the interactions between ozone, radiation and dynamics and the implication for stratospheric planetary waves in a hierarchy of models and in observations.