Planetary-Scale Baroclinic Instability and the MJO

Abstract

Eastward propagating planetary waves of zonal wavenumber one in the zonal wind (u) with phase speeds in the range of 1–10 m s−1, and also with frequencies in the 30–60-day range, are studied using 39 boreal winter (austral summer) seasons (each of length 180 days) from the reanalyses of the National Centers for Environmental Prediction. The purpose of the paper is to study the relationship between these low phase speed waves in the extratropics (which are candidates for instabilities) and the low-frequency tropical waves associated with the Madden–Julian oscillation (MJO). Planetary waves dominate the zonal wavenumber spectrum of all (eastward plus westward) transient fluctuations with phase speeds of 1–10 m s−1 at upper levels. Using the theory of Y. Hayashi to separate out standing oscillations, it is found that eastward propagating waves for zonal wavenumber one have variance maxima at 63°N, 32°N, 13°N, 32°S, and 52°S at upper levels. As a percentage of the total variance for phase speeds 1–10 m s−1 and zonal wavenumber one, the eastward propagating waves have strong maxima at 200 hPa in the Tropics (13°N, 13°S) and at lower levels at 13°S (indicative of the MJO). The standing wave variance is maximum in northern midlatitudes. The eastward propagating wave variance for zonal wavenumber two has similar properties. For zonal wavenumber one, eastward propagating waves at 52°N and 300 hPa are highly coherent with mid- and upper-level waves at 32°N, with a nearly perfectly out-of-phase relationship. For a base point at 32°N, 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 Tropics (13°N), accompanied by a 180° relative phase shift. Coherence and phase plots with a base point at 13°N and 200 hPa show not only strong coherence with waves at 32°N, but also with waves at 13°S 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°N and 13°S at 200 hPa, and stronger coherence with the lower tropical troposphere. The strong coherence between waves in the subtropics (32°N) and the Tropics (13°N) indicates a potential role for dynamical instability 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 coherence is less dramatic (but still significant). Coherence and phase results for 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 between the subtropics and Tropics lend support to the notion that planetary wave baroclinic instability and the MJO are connected with each other, as suggested by J. S. Frederiksen and C. S. Frederiksen. An origin of the MJO in which subtropical jet instability helps to organize tropical convection is suggested.