Structure and transport properties of jets in the atmosphere and oceans

Lead Research Organisation: University of St Andrews
Department Name: Mathematics and Statistics

Abstract

Over the last several years, the strongly inhomogeneous nature of
atmospheric and oceanic mixing in diverse situations has become
increasingly apparent, as high resolution numerical simulations and
observations begin to represent accurately the detailed spatial
structure of air and water masses and their constituents. Mixing is
now known to be confined to distinct latitudinal regions, often
separated by sharp gradients that indicate dynamical transport
barriers. Inhomogeneous mixing by waves and eddies in atmospheres and
oceans is intrinsically linked to the presence of zonally aligned
jets, which not only arise as a result of the eddy mixing, but also
organize the mixing in distinct latitudinal regions. The combined
effect is a dynamical feedback that is now known to operate under very
general conditions. Inhomogeneous mixing is important for the
transport of constituents such as water vapour, carbon dioxide, ozone,
heat, and salinity; their inhomogeneous re-distribution impacts both
global radiative balances and regional climate change.

Despite recent advances, a complete understanding of the way zonal
jets organize inhomogeneous mixing, in particular the vertical
structure of such mixing, remains elusive. Progress in understanding
the horizontal structure of jets and mixing has been made recently, in
particular by focusing on the potential vorticity, a key dynamical
quantity that contains information about both horizontal rotational
motion and density stratification. The aim of this project is to
build on that recent work to develop a complete theory for the
vertical structure of jets and mixing. In doing so, it will
contribute to our understanding both of the structure of the dominant
jet structures in the atmosphere and oceans, as well as providing
predictions of how they will reach dynamical equilibrium under
different forcing conditions, conditions that may change in a changing
climate. It is anticipated that the new theory will allow us to
assess the robustness of predictions made by climate models, which are
now beginning to accurately represent the complexities inherent in jet
structures.

As well as advancing our fundamental understanding of basic dynamical
processes, we will study four specific issues of current importance in
climate science: (i) systematic transport of trace chemicals within
the stratosphere; (ii) the coupling of the stratospheric and
tropospheric circulations; (iii) the consequences of a climatic shift
in the tropospheric jet stream; and (iv) inhomogeneous transport and
mixing associated with jets in the Southern Ocean. The project
highlights how advances in fundamental science can be effectively
combined with directed goals driven by specific applications.

Planned Impact

In addition to the academic beneficiaries above, the following
beneficiaries can be identified:

1. Climate scientists who use general circulation models as their
main tool for prediction of the future climate and analysing the
effect of mitigation scenarios. They will benefit from the improved
understanding of atmospheric and oceanic transport processes developed
by project components A1-A3 and as well as the specific applications
B1-B4.

2. Policy makers who act on the advice of the climate scientists in 1.
They will benefit from more reliable predictions derived from improved
climate models.

3. The general public whose lives will be affected by the future
climate and mitigation strategies. In the longer term, the public
will benefit from improved mitigation driven by better-informed
decisions of the policy makers in 2.

4. The insurance industry. It will benefit from an improved
understanding of extreme weather phenomenon and predictions of storm
frequency and intensity in future climates, as developed in project
component B3.

5. The fishing industry. It will benefit from advances in our
understanding of ocean transport of heat and nutrients (on which the
distribution of marine life in the Earth's oceans depends), as
developed in project component B4.

Publications

10 25 50
 
Description The project has identified the existence of a self-similar population of coherent vortices in the two-dimensional turbulent inverse energy cascade and has developed a theory predicting the population and describing its evolution. An extensive set of numerical experiments were carried out to verify the robustness of the vortex population across a range of relevant forcing and dissipation mechanisms. A connection has been made with vortex populations in decaying turbulence and a new form of extended scale invariance has been uncovered.

The project has identified parameters for which jets and eddies may coexist in forced circulations with non-trivial vertical structure. In flows in which compressible effects are important development of jets from turbulent forcing has been found to occur more readily, and over a wider range of physical parameters, than in incompressible flows. Coherent jets in the compressible case are found despite strongly meandering motions. A new theory has been developed to explain the partition of kinetic and potential energy in strong jet dominated flows, taking large jet meanders into account. The theory agrees well with high resolution numerical experiments. The theory has implications for the equilibration of forced systems with dissipation mechanisms relevant to those found across a range of planetary atmospheres and oceans.

A new theory has been developed to describe the lateral wave transfer in the winter stratosphere, between the wave guides of the polar vortex edge and the subtropical barrier. The new theory has been applied to an analysis of the vortex response to topographic wave forcing in the presence of idealized anomalies to the tropical flow and the effect of these on wave transfer from the vortex edge to the tropics has been quantified.
Exploitation Route Some of the results have already been published or submitted for publication. Other aspects will be developed further following the end date of the project and submitted for publication.
Sectors Environment