Using observations and simple models to quantify convective momentum transport in 2-day waves and the Madden Julian Oscillation

Accurately simulating large-scale tropical convective disturbances in global models continues to be a challenge. An important question with implications to this endeavor is: what role does convective momentum transport (CMT) play in generating or damping circulations in these disturbances? Recent studies give conflicting answers to this question. For example, some have suggested that CMT provides positive feedback to low-level westerlies during their onset within the Madden-Julian Oscillation (MJO), while others suggest that CMT weakens low-level westerlies during the onset period. Moreover, different proponents of the positive-feedback hypothesis emphasize momentum transport by different types of organized convection: synoptic scale waves or mesoscale convective systems.

In this study the authors use two approaches to quantify CMT within 2-day waves and the Madden-Julian oscillation. First, they simulate linear responses to the large-scale heatings of these disturbances as observed during TOGA COARE, and then determine the momentum forcing needed to accurately reproduce the observed temperature/heating/wind phase and amplitude relationships in the simulations. Second, they calculate the momentum transport associated with the large-scale circulations of the disturbances themselves using statistical composites of their observed vertical structures. Not only do these analyses quantify the total CMT in these disturbances. but they also compare the relative contributions of synoptic versus meso- and smaller-scale components. Moreover, the modeling study partitions the disturbances' large-scale circulations into components generated by heating and momentum forcing.