Tropopause Polar Vortices
Tropopause Polar Vortices (TPVs) are coherent, closed circulations that form and primarily reside in the polar regions, distant from midlatitude jets. TPVs are centered on the tropopause, but their associated wind field extends throughout the upper troposphere and lower stratosphere (UTLS). Spatially, TPVs are a meso- to sub-synoptic scale phenomenon, with radii on the order of hundreds to occasionally thousands of kilometers. Nevertheless, TPVs often remain coherent for weeks to months at a time, pushing them well into the synoptic temporal scale and creating potential long-term forecasting opportunities. TPV strength is largely governed by diabatic processes, including longwave radiative cooling, which strengthens vortices and latent heating, which weakens them. While TPVs play a crucial role in high-latitude weather, helping to strengthen Arctic cyclone, they also commonly enter the midlatitudes. Previous studies have linked TPVs to Rossby wave initiations, cold air outbreaks, and severe weather events.
Our research investigates several key questions: How do TPVs form, strengthen, move, and dissipate? How do TPVs interact with Arctic cyclones (and by extension sea ice) and influence the Arctic climate system? What role do they play in midlatitude weather predictability? Understanding these connections is essential for improving forecast skill in polar and midlatitude regions.
Arctic Cyclones
Arctic cyclones (ACs) are synoptic-scale surface lows that are present in the polar regions, whether forming locally or moving in from the midlatitudes. Summer is a unique season for Arctic cyclones as they are more frequently generated within the Arctic, often forming when a TPV moves over the Arctic Frontal Zone or over marginal sea ice regions. Several summer AC cases have been observed with lifetimes over almost two weeks. We suspect that this longevity can be at least partially explained by connections with TPVs, with some studies suggesting that intensifying Arctic cyclones can exhibit an equivalent barotropic structure (vertical alignment between the surface low and TPV) for extended periods of time.
During winter, Arctic cyclones are less frequent and most often move into the Arctic from midlatitudes. Fundamental questions exist regarding their large horizontal scale (reaching up to 5000 km), which contrasts with Rossby radius theory given the large planetary vorticity of high latitudes. During all seasons, ACs can also affect the underlying sea ice through a combination of dynamic and thermodynamic processes, and these are cyclones commonly linked with Very Rapid Ice Loss Events (VRILEs).
Our research focuses on interactions between ACs and TPVs in all seasons and addresses the following questions. What role do TPVs play in AC genesis? What characteristics of TPVs (size, mesoscale structure, relative position) are most important for AC development? How can we forecast significant AC events with greater lead time? We are also interested in questions of AC sea ice interactions. What are the dominant mechanisms of action by which ACs modify sea ice? Under what conditions do ACs have the most impact on sea ice? To what degree to ACs form and destroy sea ice versus advecting it from one region to another? You can read more about sea ice below.
Sea Ice Dynamics
The Arctic provides favorable conditions for the growth and evolution of Tropopause Polar Vortices (TPVs). The dynamics of TPVs lead to surface cyclone development, which in turn plays a significant role in the dynamics of Arctic sea ice.
Understanding these atmosphere-ice interactions is crucial for improving our ability to predict both short-term weather events and longer-term changes in the Arctic climate system. Our research examines the feedback mechanisms between atmospheric circulation patterns and sea ice distribution.
Antarctic Ensemble Data Assimilation
Our group utilizes ensemble data assimilation techniques to help build our understanding of the processes that are represented in numerical models and improve the capability of numerical weather prediction.
The Antarctic is of particular interest to our group since it is the most data-sparse region of the globe. By applying advanced data assimilation methods, we can better constrain model forecasts and gain insights into the fundamental dynamics of Antarctic weather systems.
Publications
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Cavallo, S. M., and G. J. Hakim, 2013: Physical mechanisms of tropopause polar vortex intensity change. J. Atmos. Sci., 70 (11), 3359–3373, https://doi.org/10.1175/JAS-D-13-088.1.
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Cavallo, S. M., and G. J. Hakim, 2012: Radiative impact on tropopause polar vortices over the arctic. Mon. Wea. Rev., 140 (5), 1683–1702, https://doi.org/10.1175/MWR-D-11-00182.1.
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Cavallo, S. M., and G. J. Hakim, 2010: Composite structure of tropopause polar cyclones. Mon. Wea. Rev., 138 (10), 3840–3857, https://doi.org/10.1175/2010MWR3371.1.
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Cavallo, S. M., and G. J. Hakim, 2009: Potential vorticity diagnosis of a tropopause polar cyclone. Mon. Wea. Rev., 137 (4), 1358–1371, https://doi.org/10.1175/2008MWR2670.1.