Transitions Between Circulation Regimes: The Role of Tropical Heating

Four Euro-Atlantic (EA) circulation regimes are identified using cluster analysis applied to 500 hPa geopotential heights from the ERA-Interim (ERAI) reanalysis. These are the positive and negative phases of the North Atlantic Oscillation (NAO+, NAO−), Scandinavian Blocking (SB), and the Atlantic Ridge (AR). This paper studies transitions between these four regimes, the signature of tropical heating preceding these transitions, and the identification of transitions for which this forcing plays a role. The findings can further our understanding of when transitions occur. To address these questions, we examine the relationship of heating to the Madden–Julian Oscillation (MJO), the El Niño Southern Oscillation (ENSO), shifts in the Intertropical Convergence Zone (ITCZ), and possible stratospheric influences. Mid-latitude diabatic heating is also examined to determine shifts in the storm tracks. We use the ERAI reanalysis to estimate diabatic heating, streamfunction, Rossby wave activity, and stratospheric zonal winds. We find that Indian Ocean tropical heating enhances the transition from the SB regime to the NAO+ regime. In contrast, western Pacific heating seems to force transitions from all other regimes into the NAO− regime. The flux of Rossby wave activity indicates that in some transitions, mid-latitudes play a role in forcing tropical heating. The majority of the transitions examined show indications of tropically forced behavior. Less than half showed evidence that mid-latitude dynamics were the primary cause of the transition. Nearly half of the transitions appeared to be related to phases of the MJO. We also found that intensification of heating in the eastern equatorial Pacific and equatorial Atlantic (ITCZ) plays a role. Transitions during the early and late parts of the season, along with the role of ENSO, are found to be modest factors.