Prof. Nili Harnik's Home page
Large-scale atmospheric dynamics group
Dynamical regimes of the global circulation
An idealized global circulation model consists of an interaction between the meridional circulation cells, the zonal jet streams, and baroclinic eddies, which 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. We have examined the dynamics of this three-way balance in a series of papers (Lachmy and Harnik, 2014, 2016, 2020), using a Modified Quasi Geostrophic (MQG) model.
The unusual merging of the Atlantic and African jet during winter 2009-10:
The unusually cold and snowy Northern Hemisphere winter of 2009-10, which had a persistently negative NAO, was unusual in that the Atlantic and African jets merged into a single jet flowing straight from the Atlantic onto North African and the Mediterranean. Harnik et al (2014) show that this merging is consistent with the characteristically Eddy-driven Atlantic jet, transitioning to a merged state. This happens when mid latitude eddies are anomalously weak and tropical Pacific heating is anomalously strong. See more details here.
Current project on the rare winter-long merging of the African and Atlantic jets:
Sohan Suresan's PhD project is examining various aspects of this rare winter-long jet merging. In Sohan et al (2024) the effect of such merging on the distribution of extreme events is examined in ERA5 data. He is also examining this merging in large ensambles of Earth System Model runs. Preliminary results indicate that this rar merging will become more prevalent in a warmer future. We are also using the PLanet SIMulator (PLASIM) coupled to the rare event algorithm used in Ragone and Bouchet (2021), to simulate many winter-long jet mergings. The algorithm is able to simulate winter-long jet merging, with the estimated return time curve shown blow. This is the first instance of using the rare event algorithm to study jet stream dynamical regimes.