H. Reed Ogrosky

The Walker circulation, diabatic heating, and outgoing longwave radiation

For the tropical atmosphere on planetary scales, it is common to model the circulation using strong damping. Here with new data analysis techniques, evidence suggests that damping can actually be neglected. Specifically, near the equator, the east-west overturning circulation is in agreement with the undamped wave response to atmospheric heating. To estimate the heating, satellite observations of outgoing longwave radiation (OLR) are used. Frequently, OLR is used as a heuristic indicator of cloudiness. Here the results further suggest that OLR variations are actually proportional to diabatic heating variations, with a proportionality constant of 18 W m−2 (K d−1)−1. While the agreement holds best over long time averages of years or decades, it also holds over shorter periods of one season or 1 month. Consequently, it is suggested that the strength of the Walker circulation—and its evolution in time—could be estimated using satellite data.

Assessing the Equatorial Long-Wave Approximation: Asymptotics and Observational Data Analysis*

AbstractEquatorial long-wave theory applies where a small horizontal aspect ratio between meridional and zonal length scales is assumed. In an idealized setting, the theory suggests that (i) meridional wind is small, (ii) geostrophic balance holds in the meridional direction, and (iii) inertio-gravity waves are small in amplitude or “filtered out.” In this paper a spectral data analysis method is used to quantitatively assess the spatial and temporal scales on which each of these aspects of long-wave dynamics is observed in reanalysis data. Three different perspectives are used in this assessment: primitive variables, characteristic variables, and wave variables. To define each wave variable, the eigenvectors and theoretical wave structures of the equatorial shallow-water equations are used. Evidence is presented that the range of spatial and temporal scales on which long-wave dynamics holds depends on which aspect of the dynamics is considered. For example, while meridional winds are an order of magnitud...

Identifying Convectively Coupled Equatorial Waves Using Theoretical Wave Eigenvectors

AbstractConvectively coupled equatorial waves (CCEWs) are often identified by space–time filtering techniques that make use of the eigenvalues of linear shallow water theory. Here, instead, a method is presented for identifying CCEWs by projection onto the eigenvectors of the theory. This method does not use space–time filtering; instead, wave signals corresponding to the first baroclinic Kelvin, Rossby, and mixed Rossby–gravity (MRG) waves are constructed from reanalysis data by a series of projections onto (i) vertical and meridional modes and (ii) the wave eigenvectors. In accordance with the theory, only dry variables, that is, winds and geopotential height, are used; no proxy for convection is used. Using lag–lead regression, composites of the structures associated with each eigenvector signal during boreal summer are shown to contain all the features of the theory as well as some additional features seen in previous observational studies, such as vertical tilts. In addition, these composites exhibit...