Presenter: Lukas Lobert (MIT-WHOI Joint Program in Oceanography/Applied Ocean Science and Engineering)

Description:
Climate projections strongly indicate a more meandering polar jet stream altering large-scale atmospheric patterns in the mid-latitudes. This change likely affects local oceanic conditions, e.g., along the U.S. shelf south of New England. High-wind events during the fall predominantly cause the rapid breakdown of stratification on the New England shelf which initiates the fall phytoplankton bloom in one of the most productive regions worldwide. While the leading-order dynamics of destratification have been identified, variability remains high if the different types of wind forcing patterns are treated alike. Since climate change will not affect all wind forcing patterns similarly, the categorization and impact assessment of high-wind patterns is a necessary first step towards predicting how the annual stratification breakdown's timing might change. In this study, we propose a categorization scheme for high-wind events based on local wind conditions and sea level pressure patterns across Northeast America. The categorized patterns are then compared with associated changes in shelf stratification to determine their impact. According to observations from the OOI Coastal Pioneer Array (2015-2019), impactful wind forcing patterns primarily fall into two categories: a) Propagating cyclonic storm systems with strong rotating winds and b) persistent large-scale high-pressure systems over East Canada with a stable downwelling-favorable wind component. The observed stratification changes are composed of opposite temperature and salinity contributions which provide hints about the underlying dynamics. Cyclonic storms are associated with the strongest local wind energy inputs resulting into enhanced mechanical mixing and surface cooling. In contrast, Canadian Highs have steady downwelling-favorable winds that seem to drive a noticeable 2-layer Ekman response advecting salty slope water onto the shelf. Patterns that cluster during the early fall season are particularly effective since they occur when stratification is still enhanced. With a robust categorization scheme in mind, researchers will be able to investigate how a changing large-scale atmospheric forcing will affect local stratification patterns on the New England shelf.