Kelvin/Rossby Wave Partition of Madden-Julian Oscillation Circulations

Patrick Haertel

doi:10.3390/cli9010002

Abstract

The Madden Julian Oscillation (MJO) is a large-scale convective and circulation system that propagates slowly eastward over the equatorial Indian and Western Pacific Oceans. Multiple, conflicting theories describe its growth and propagation, most involving equatorial Kelvin and/or Rossby waves. This study partitions MJO circulations into Kelvin and Rossby wave components for three sets of data: (1) a modeled linear response to an MJO-like heating; (2) a composite MJO based on atmospheric sounding data; and (3) a composite MJO based on data from a Lagrangian atmospheric model. The first dataset has a simple dynamical interpretation, the second provides a realistic view of MJO circulations, and the third occurs in a laboratory supporting controlled experiments. In all three of the datasets, the propagation of Kelvin waves is similar, suggesting that the dynamics of Kelvin wave circulations in the MJO can be captured by a system of equations linearized about a basic state of rest. In contrast, the Rossby wave component of the observed MJO’s circulation differs substantially from that in our linear model, with Rossby gyres moving eastward along with the heating and migrating poleward relative to their linear counterparts. These results support the use of a system of equations linearized about a basic state of rest for the Kelvin wave component of MJO circulation, but they question its use for the Rossby wave component.

Highlights

The Madden Julian Oscillation (MJO) is a large-scale tropical convective and circulation system that moves slowly eastward over the warm equatorial waters of the Indian and Western Pacific Oceans [1,2,3]
We examine three sets of data relevant to MJO circulation: (1) a modeled linear response to an MJOlike heating; (2) a composite MJO based on atmospheric sounding data [25]; and (3) a composite MJO simulated with a Lagrangian Atmospheric Model (LAM) [26]
A birds-eye view of the atmospheric response to the heating reveals positive tropospheric temperature anomalies co-located with low-level easterlies to the east of the heating, which is the signature of an equatorial Kelvin wave, and cyclonic gyres forming to the west of the heating indicative of an equatorial Rossby wave (Figure 1b [30])

Summary

Introduction

The Madden Julian Oscillation (MJO) is a large-scale tropical convective and circulation system that moves slowly eastward over the warm equatorial waters of the Indian and Western Pacific Oceans [1,2,3]. Does the MJO cause strong variations in winds and precipitation in the equatorial region [8], but it modulates the frequency and intensity of tropical cyclones in the Atlantic, Indian, and Pacific Oceans [9,10], as well as the timing and intensity of Asian and North American monsoons [11,12,13]. Dozens of different theories have been proposed to explain the growth and propagation of the MJO, mentioning many different physical processes, such as enhanced surface evaporation due to the MJO’s perturbation winds [14,15], frictional convergence to the east of the MJO’s convective center [16,17], radiation [18], baroclinic instability [19], and upscale transports by smaller disturbances [20]. While there has been some improvement in MJO representations in climate models, many still produce MJO’s that are too weak, lack eastward propagation, and/or have the wrong period [22,23,24]

Methods

Results

Conclusion