Large Wave Events in a Changing Climate

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At the altitude of the jet streams the atmospheric circulation over the middle latitudes tends to be quite wavy, with high and low pressure centers moving westward roughly along the jet axes. But at times the waves slow down and grow quite large, forming a "block" in the jet-level flow. The blocked flow results in a stagnation of weather systems, which can lead to prolonged cold spells, excessive precipitation, and other forms of extreme weather. Naturally blocking has been a topic of intensive research for some time, but a satisfactory explanation for it has been elusive. One reason is that that much of the relevant wave dynamics theory assumes that the waves have small amplitudes, which is not the case for blocking. To remedy this mismatch the PI and his colleagues have developed a bulk measure of the waviness around a latitude circle called Finite Amplitude Wave Activity (FAWA), along with its longitudinally varying counterpart, Local Wave Activity (LWA, see AGS-1563307). FAWA and LWA are well suited to the study of blocking because they can be applied throughout the lifecycle of a block, even when the blocking pattern is quite pronounced.Recently the PI and his group developed a simple theory of blocking based on the relationship between mean jet speed and LWA. The idea of the theory is that an increase in wave activity slows down the jet stream, which slows down the propagation of waves along the jet. The slowdown leads to a pile-up of LWA which increases waviness and further slows down the jet. The formation of blocks by accumulation of LWA is analogous to the formation of a traffic jam, in which a slow-down on a highway leads to accumulation of cars which further slows highway speed until a jam is produced. The analogy is exact in the sense that the formation of a block by the piling up of LWA can be modeled using the classical traffic flow equation derived in papers from the the 1930s and 1950s.Here the PI uses his traffic jam theory of blocking to perform a diagnostic study of wave-mean flow interactions leading to blocks, using concepts from traffic flow theory to understand block formation. For instance the "capacity" of a jet stream, meaning the amount of LWA that can travel along a jet without initiating a slowdown in wave propagation, can be defined in analogy with the capacity of a highway for traffic. Quantities like capacity can be easily calculated from weather maps and from model simulations.Further work applies the FAWA/LWA framework to Sudden Stratospheric Warming (SSW) events, in which the stratospheric vortex circling the North Pole breaks down and stratospheric temperatures rise dramatically, sometimes over 100F within a few days. SSWs are thought to occur because the vortex is disrupted by upward-propagating waves, thus SSWs are a logical target for analysis using the FAWA/LWA framework.The work has broader impacts due to the extreme weather associated with blocks and the impact it has on human activities. Blocking events are difficult to predict, and simulations from state-of-the-art forecast models are generally deficient in blocking. The understanding developed in this work may thus be important for improved simulation and prediction of blocking. SSWs are equally challenging from a forecast perspective, and they can also affect surface weather in various ways. Codes for the calculation of FAWA and LWA from meteorological data are made available to the research community via github to promote their adoption and use. Education and outreach are conducted through a biennial summer school and a local community science center. In addition, the project supports two graduate students and a postdoctoral fellow, thereby fostering the next generation of scientists in this research area.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.