On the structure and dynamics of the Asian monsoon anticyclone
Creators: name: Rupp, Philip
Publisher: name: Apollo - University of Cambridge Repository
Year: 2019
Subjects: monsoon
Asian monsoon
stratosphere
strat-trop-coupling
dynamics
atmosphere
anticyclone
shedding
vortices
eddies
localisation
weastward
eastward
instability
absolute
convective
vertical extent
tropopause
transition
trop-strat
tropics
sub-tropics
circulation
friction
idealised
modelling
re-analysis
GCM
PV
shallow water
heating
baroclinic instability
mid-latitude dynamics
jet

description: The Asian summer monsoon circulation has been the subject of continuous research for several decades. Much emphasis has been given to the lower level cyclone, but while various models and mechanisms have been proposed to explain certain features of the upper level anticyclone, like the zonal scale or time dependence, a comprehensive theory combining these characteristics is still lacking. We re-visited the steady and linear 2D monsoon model proposed by Gill and Matsuno, which potentially captures the time-mean response of the flow, but fails to account for any kind of temporal evolution and requires a strong mechanical friction throughout the atmosphere, which is a questionable assumption for the upper troposphere/lower stratosphere region. We then re-modelled the monsoon flow as a response to a localised heat source using a numerical model that is able to capture the time dependence, non-linearity and three-dimensional nature of the system, which we see as necessary to successfully model the monsoon anticyclone. In addition to the explicit heating driving the monsoon we included a simple representation of mid-latitude dynamics into the model by imposing a relaxation towards a baroclinically unstable state. The simulated flow therefore includes mid-latitude westerly jets and baroclinic eddies. We then explored the parameter space by varying for example the forcing magnitude or the meridional temperature gradient of the basic state and investigated changes of the response. This way it was possible to identify various parameter combinations for which the response shows similar structures and behaviours to what has been observed in re-analysis data in relation to the monsoon anticyclone. This includes a finite zonal length scale of the response, as well as temporal evolution in the form of east- and westward shedding of eddies. Many characteristics of the behaviour of the upper level monsoon anticyclone can potentially be investigated by considering the dynamical evolution of a single fluid layer forced by a steady mass source. Such a single-layer model can be used to perform more in-depth studies than are possible in a 3D model. Using this approach we explained a variety of phenomena related to westward eddy shedding from the monsoon anticyclone as a consequence of an absolute instability of the flow field. By performing a comprehensive spatial stability analysis of an idealised representation of the system we were able to develop a theory for the transition between different shedding states. Adapting a simple analytic theory based on a centre-of-mass approach further allowed us to explain the near-steady propagation of the eddies shed from the monsoon. We also showed that the eddy propagation in our system is not modified by the fact that eddies are not well-separated and shed as a series of vortices, in contrast to findings of previous authors. By extending the theory and including a zonal background wind we were further able to explain some aspects of the changes due to thermal damping and the interaction of the flow with a zonal mean background wind like the mid-latitude jet. The presented study covers various observed features and behaviours of the Asian monsoon anticyclone and shows how they arise in simple dynamical models for certain ranges of values of external parameters.
description: The European project StratoClim (7th Framework Programme, project no. 603557), the ACCI project of the European Research Council (project no. 267760), as well as the Cambridge Philosophical Society and the Queens' College Cambridge