Dept. of Geosciences Colloquium: Significance of the Baroclinic Nature of the NAO and Maritime Blocking Highs
Prof. Hisashi Nakamura, University of Tokyo
It has been widely accepted that atmospheric low-frequency anomalies over the extratropical oceans, including blocking highs and the North Atlantic Oscillation (NAO), tend to have deep equivalent barotropic structure without tilting vertically, maintained primarily by migratory synoptic-scale disturbances. A careful inspection nonetheless reveals that those anomalies do exhibit a subtle vertical tilt, but its dynamical implications are still unknown. Using reanalysis data, the present study demonstrates that this vertical tilt is of vital dynamical significance for the wintertime NAO and blocking anomalies in the North Pacific and western North Atlantic. In the lower troposphere, the tilted anomalies are found to yield anomalous thermal advection across the pronounced climatological-mean thermal gradient associated with a background westerly jetstream, thereby acting to reinforce the associated persistent thermal anomalies at lower levels. The resultant conversion of available potential energy from the background state is a larger energy source for maintaining the quasi-stationary anomalies than the barotropic energy conversion from the background westerlies and the feedback forcing from migratory disturbances. In summer, the baroclinic energy conversion and feedback from high-frequency eddies are more comparable in magnitude.
Martineau, et al., 2020: Importance of a vertically tilting structure for energizing the North Atlantic Oscillation. Sci. Rep., 10, 12671.
Martineau, et al., 2020: Modulations of North American and European weather variability and extremes by interdecadal variability of the atmospheric circulation over the North Atlantic sector. J. Climate, 33, 8125-8146.
Martineau et al., 2022: Baroclinic blocking, Geophys. Res. Lett., 50, e2022GL097791.
Event Organizer: Dr. Roy Barkan