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MEAS Roscoe Braham, Jr., Seminar

March 25, 2019 | 3:30 pm - 4:30 pm

Speaker: Prof. Noboru Nakamura, University of Chicago |

Seminar Title: Disruption of the Jet Stream and Threshold Behavior of Rossby Waves |

Abstract: Weather anomalies in the midlatitudes are often linked to disruptions of the jet stream.  Atmospheric blocking in the troposphere and stratospheric sudden warming are cases in point.  Despite the advances in numerical weather prediction, these major disruption events remain difficult to predict, and climate simulations do not converge on their response to climate perturbations.  One of the main obstacles is the lack of theory for the onset criteria of the disruption events.  Here we argue that these events arise from threshold behaviors of finite-amplitude Rossby waves interacting with westerly winds. Our recent work on blocking demonstrates that, as the eastward propagating Rossby waves attain large amplitude, they decelerate the eastward flow of the jet, which in turn causes the waves to stagnate and accumulate even more wave amplitude.  When (and where) a critical amplitude (‘capacity’) is reached through this positive feedback, the jet stream transitions abruptly into a very wavy state, which characterizes the onset of blocking. We hypothesize that climate change affects the frequency of the disruption events by moderating the threshold condition.  In this talk I will discuss the response of blocking statistics to variations to i) stationary wave amplitude, ii) transient eddy forcing, and iii) background jet speed, using a semi-empirical 1D model. Our parameter sweep suggests that in the present-day climate blocking frequency is sensitive to stationary wave amplitude and jet speed, corroborating the low confidence in climate simulations on the response of blocking to climate change, but it also provides a theoretical basis for understanding model biases and trends in reanalysis data.  If time permits I will also touch on the latest theoretical advances in stratospheric sudden warming using the same dynamical framework.

 

Details

Date:
March 25, 2019
Time:
3:30 pm - 4:30 pm
Event Categories:
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Venue

1216 Jordan Addition
2720 Faucette Drive
Raleigh, NC 27695 United States
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