Abstract
Planetary wave breaking (PWB) over the subtropical North Atlantic is observed over 45 winters (December 1958–March 2003) using NCEP–NCAR reanalysis data. PWB is manifested in the rapid, large-scale and irreversible overturning of potential vorticity (PV) contours on isentropic surfaces in the subtropical upper troposphere. As breaking occurs over the subtropical North Atlantic, an upper-tropospheric PV tripole anomaly forms with nodes over the subtropical, midlatitude, and subpolar North Atlantic. The northern two nodes of this tripole are quite similar to the spatial structure of the North Atlantic Oscillation (NAO), with positive polarity. Nonlinear reflection is identified in approximately a quarter of all PWB events. Following breaking, two distinct circulation regimes arise, one in response to reflective events and the other in response to nonreflective events. For reflective events, anomalies over the North Atlantic rapidly propagate away from the breaking region along a poleward arching wave train over the Eurasian continent. The quasi-stationary wave activity flux indicates that wave activity is exported out of the Atlantic basin. At the same time, the regional poleward eddy momentum flux goes through a sign reversal, as does the polarity of the NAO. For nonreflective events, the dipole anomaly over the North Atlantic amplifies. Diagnostics for nonreflective events suggest that wave activity over the Azores gets absorbed, allowing continued enhancement of both the regional poleward eddy momentum flux and the positive NAO.
Highlights
Quasi-stationary planetary waves, as other Rossby waves, depend on a background potential vorticity (PV) gradient for their restoring mechanism
By using composites in our analysis, we acknowledge that strong perturbations are likely canceled by weaker perturbations. We address this point by examining the impact Planetary wave breaking (PWB) has on the genesis of a persistent positive North Atlantic Oscillation (NAO) (PPN)
Upon analyzing daily fields of PV on several isentropic surfaces in the upper troposphere/lower stratosphere over the course of 45 winters, we identified 576 cases of anticyclonic planetary wave breaking over the subtropical North Atlantic
Summary
Quasi-stationary planetary waves, as other Rossby waves, depend on a background potential vorticity (PV) gradient for their restoring mechanism. As a wave train approaches the weak background flow over the subtropical Atlantic, it amplifies and imposes a strong poleward eddy momentum flux. Feldstein (2003) found that the life cycle of the (high frequency) NAO has a time scale of around 2 weeks and is driven primarily by the nonlinear eddy vorticity flux. These results are supported by simple dynamical modeling studies that find that the spatial and temporal characteristics of the in-. We note strikingly different responses in the NAO life cycle following the breaking event, dependent on whether wave activity is absorbed locally or reflected poleward into midlatitudes.
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