Planetary-scale baroclinic instability and the MJO

Eastward propagating planetary waves of zonal wavenumber one in the zonal w ind (u) with phase speeds in the range of 1-10 m s(-1), and also with frequ encies in the 30-60-day range, are studied using 39 boreal winter (austral summer) seasons teach of length 180 days) from the reanalyses of the Nation al Centers for Environmental Prediction. The purpose of the paper is to stu dy the relationship between these low phase speed waves in the extratropics (which are candidates for instabilities) and the low-frequency tropical wa ves associated with the Madden-Julian oscillation (MJO). Planetary waves dominate the zonal wavenumber spectrum of all (eastward plu s westward) transient fluctuations with phase speeds of 1-10 m s(-1) at upp er levels. Using the theory of Y. Hayashi to separate out standing oscillations, it is found that eastward propagating waves for zonal wavenumber one have varian ce maxima at 63 degreesN, 32 degreesN, 13 degreesN, 32 degreesS, and 52 deg reesS at upper levels. As a percentage of the total variance for phase spee ds 1-10 m s(-1) and zonal wavenumber one, the eastward propagating waves ha ve strong maxima at 200 hPa in the Tropics (13 degreesN, 13 degreesS) and a t lower levels at 13 degreesS (indicative of the MJO). The standing wave va riance is maximum in northern midlatitudes. The eastward propagating wave v ariance for zonal wavenumber two has similar properties. For zonal wavenumber one, eastward propagating waves at 52 degreesN and 300 hPa are highly coherent with mid- and upper-level waves at 32 degreesN, wi th a nearly perfectly out-of-phase relationship. For a base point at 32 deg reesN, 200 hPa, we find a strong coherence maximum for phase speeds of 1-10 m s(-1) (coherence squared greater than 0.7) with upper levels in the Trop ics (13 degreesN), accompanied by a 180 degrees relative phase shift. Coher ence and phase plots with a base point at 13 degreesN and 200 hPa show not only strong coherence with waves at 32 degreesN, but also with waves at 13 degreesS at 700 hPa. While results for the MJO averaging (corresponding to periods of 30-60 days) are generally similar, there is in addition a strong cross-equatorial coherence between fluctuations at 13 degreesN and 13 degr eesS at 200 hPa, and stronger coherence with the lower tropical troposphere . The strong coherence between waves in the subtropics (32 degreesN) and th e Tropics (13 degreesN) indicates a potential role for dynamical instabilit y in the organization of the MJO. Coherence and phase diagnostics for base points in the Southern Hemisphere have generally the same character, although the subtropical-tropical cohere nce is less dramatic (but still significant). Coherence and phase results f or eastward propagating zonal wavenumber two are generally similar. These results represent, for midlatitudes, an extension of earlier work of C. R. Mechoso and D. L,. Hartmann to phase speeds less than 10 m s(-1), for which an interpretation in terms of baroclinic instability becomes viable; a much larger dataset is also used here. The strong coherence found betwee n the subtropics and Tropics lend support to the notion that planetary wave baroclinic instability and the MJO are connected with each other, as sugge sted by J. S. Frederiksen and C. S. Frederiksen. An origin of the MJO in wh ich subtropical jet instability helps to organize tropical convection is su ggested.