Eddy life cycles and the midlatitude respons to ENSO

Eddy life cycles and the midlatitude response to El Nino.

Various observational studies suggest that changes in synoptic scale eddy momentum fluxes are central to the dynamics of the leading modes of variability in midlatitudes. In addition, there is evidence that momentum fluxes play an important role in the response of midlatitudes to external forcing (e.g. the stratosphere, ENSO, midlatitude SST). In particular, Seager et al (2004) showed that the midlatitude cooling during El Nino is due to changes in eddy momentum fluxes in response to the tropical heating and corresponding strengthening of the subtropical jet stream. Seager et al further suggested that this observed Tropical Modulation of Midlatitude Eddies (TMME) is a linear response of the eddies to changes in the index of refraction.

Observational and model studies have shown, however, that momentum fluxes can be larger during the nonlinear stage of an eddy life cycle. Moreover, there are various types of nonlinear wave life cycles, which differ in the way in which the eddies exchange momentum with the mean flow. We investigate the nonlinear response of eddies to changes in the basic state, and the possibility that the midlatitude response to ENSO is due to a change in the type of eddy life cycle.

We will present some results from a series of nonlinear eddy life cycle integrations using idealized basic states, in which we use the wave geometry diagnostic of Harnik and Lindzen (2001) to determine wave propagation haracteristics in the meridional direction. Our results suggest the wave life cycle is determined by the wave geometry of the mean flow during the linear growth stages, with the most important parameter being the phase speed of the waves. This raises the need to understand what controls the phase speed of baroclinically unstable waves. We will also present results from a series of integrations using ENSO-like anomalies on climatology, which support the hypothesis that a change in the type of eddy life cycle leads to the observed midlatitude cooling during El Nino.