Some considerations on the spectral features of meridional heat transport by transient eddies



The present study analyses extreme events in meridional atmospheric heat transport due to transient eddies in the time–frequency domain. The data used are the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis data, at the 850 mb pressure level and with daily, 0.7° latitude and longitude resolution.

Fast-growing atmospheric modes are associated with large heat transport, which would suggest a link between transport extremes and growing baroclinic systems (defined here as motions in the 2.5–6 day band). However, by analyzing wavelet power spectra of transport extremes and of the corresponding meridional velocity and moist static energy temporal anomalies, this is found not to be the case. In fact, baroclinic systems provide only a modest contribution to the integrated power of the extreme heat transport event spectrum. The transport extremes are driven by very precise phase and coherence relationships between the velocity and moist static energy anomalies, acting over a broad range of frequencies (2.5–30 days). Planetary-scale motions (k = 0–4) with periods beyond 6 days play a key role in this framework.