Tropical―Extratropical Interactions of Intraseasonal Oscillations

Tropical–extratropical interactions of intraseasonal oscillations (ISOs), based on 30 years (1979–2009) of northern winter observations and theory, are compared. The phase relationships between the tropical signal of the leading theoretical ISO for a January 1979 basic state and the development of Pacific–North America (PNA)-like and North Atlantic Oscillation (NAO) teleconnection patterns are found to compare closely with those for the observed Madden–Julian oscillation (MJO). For both observations and theory positive NAO occurs 5–15 days after MJO convection [negative outgoing longwave radiation (OLR) and positive precipitation] and negative upper-troposphere velocity potential ISO anomalies are focused over the central Indian Ocean. The fluxes of wave activity, based on the upper-troposphere streamfunction of the leading theoretical mode, indicate strong tropical–extratropical interactions and have very similar structures to those obtained by H. Lin et al. based on observations of extratropical anomalies associated with MJO convection. The second leading theoretical ISO mode for January 1979 has quite similar properties to the leading ISO mode but has a longer period of 44.5 days compared with 34.4 days and a more distinct quadrupole streamfunction structure straddling the equator. Theoretical ISO modes for other observed basic states, including January 1988 and the 30-yr average of January 1980–2009, again link the tropical ISO signal with Northern Hemisphere teleconnection patterns, particularly the NAO. The growth rates of ISO modes increase with stronger baroclinicity of the basic-state zonal winds in the main jet streams and, importantly, with increased tropical–extratropical interactions because of stronger meridional winds.