Influence of the Madden-Julian Oscillation on the Tropical Tropopause Layer during the 2011-12 DYNAMO Field Campaign

Observations in the tropical tropopause layer (TTL) show time variations across multiple scales that are not fully understood, including the intraseasonal variability of the Madden-Julian oscillation (MJO). In this study, we investigate the evolution of TTL properties and their vertical structure during the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign from October-December 2011. This time period is particularly interesting in that two prominent MJO passages were seen over the tropical Indian Ocean. We focus on two sites. Gan Island, Maldives (0.7S, 73.2E) provides a better understanding of the response of the TTL to MJO dynamics in the region of initiation. In comparison, Manus Island, Papua New Guinea (2.1S, 147.4E) provides insight to a more developed stage of the MJO. We use multiple datasets, including high vertical resolution, three-hourly atmospheric soundings over the three-month period. The presence of deep convective and cirrus clouds are determined using CALIPSO satellite data in addition to ground-based radar.

Results indicate an increase in deep convection and higher cloud top heights during the active phase of the MJO. Thin tropopause-level cirrus are present during periods of upper-level cold anomalies prior to the active phase, and may have a significant impact on TTL radiative heating rates. Characteristics of the observed background wave structures are analyzed, and suggest convectively coupled equatorial waves with downward phase propagation. Bandpass filtering is used to isolate low-frequency variability, Kelvin wave activity, and higher-frequency gravity waves excited by MJO convection. Further analyses of the observed wave structures reveal striking differences between the two MJO active periods, in terms of scale and lower stratospheric wave energy.