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Hi Joe,

Thanks for the comment! To clarify, Rossby waves and the jet stream are not the same entity, though the two phenomena are typically (though not always) collocated with each other. Rossby waves can and do exist independently of the jet stream. Furthermore, Rossby waves can both interact with and modulate the jet stream and vice versa. 

Regarding upper air mass divergence and the formation of high pressure ridges, this is a common source of atmospheric Rossby waves. As these ridges build, so do stationary wave activity within them, which is eventually dispersed downstream as a Rossby wave. During ENSO, positive SST anomalies serve to promote convection and thus the development of anomalous convective heating in the upper atmosphere. This anomalous convective heating forces the formation of an upper level high pressure ridge as well as the build-up of stationary wave activity fluxes, which are then dispersed downstream as a Rossby wave. To summarize, during the different phases of ENSO positive SST anomalies can be located in either the eastern Pacific (El Niño) or western Pacific (La Niña), consequently dictating where high pressure ridges and subsequent Rossby waves are favored to form.

In reply to by Joe Witte