PNNL

The Science

Skillful predictions of tropical cyclone (TC) activity on subseasonal time scales (7–60 days) may help mitigate their destructive impacts. As the dominant component of intraseasonal (30–90 days) variability in the tropical atmosphere, the Madden–Julian oscillation (MJO) has been known to modulate TC activity. The MJO, a constantly moving atmospheric disturbance, occurs in eight phases that are defined based on which geographic region convection is detected over. In this study, a plausible mechanism was proposed through which tropical convection associated with the MJO over the eastern Pacific (phases 6–7) affects the behavior of Rossby waves (planetary-scale perturbations in the atmosphere created by Earth’s rotation which have significant influence on global weather) to thereby suppress North Atlantic TC activity. The link between these disparate phenomena, known as teleconnection, may provide useful subseasonal predictability for North Atlantic TC activity in the northern hemisphere summertime.

The Impact

At a one-to-five-week (subseasonal) timescale, Atlantic TC activity is influenced by the MJO, but extratropical weather perturbations are also known to contribute to TC variability. This study identified significant changes in North Atlantic TC activity during MJO phases 2–3 and 6–7 when convective signals were present over Africa, the Indian Ocean, or tropical Pacific regions, respectively. Moreover, anticyclonic Rossby wave breaking (AWB)—extratropical weather perturbations that destabilize the atmosphere—plays a key role during phases 6–7 to suppress basin-wide TC activity; hurricane number, accumulated cyclone energy (a metric for overall cyclone activity at a given time), and rapid intensification probability decrease by ~50-80%. Simultaneously, large-scale environmental variables become extremely unfavorable for TC formation and intensification due to the presence of AWB. By proposing a plausible mechanism by which MJO and extratropical weather disturbances can jointly affect North Atlantic TCs, this study improves efforts to predict Atlantic TCs and subseasonal forecasting of extreme weather events.

Summary

This study investigates the combined impacts of the Madden-Julian Oscillation (MJO) and extratropical anticyclonic Rossby wave breaking (AWB) on subseasonal Atlantic TC activity. AWB events significantly restrain the development and genesis of TCs during phases 6–7, while their impact is less pronounced in phases 2–3. During phases 6–7, AWB events coincide with the development of a stronger anticyclone in the lower troposphere. This system transports drier, more stable extratropical air toward the equator and leads to enhanced cooling of tropical sea surface temperatures, which decreases the amount of “fuel” for TC activity. Consequently, AWB events during phases 6–7 disrupt the development of nearby TCs more than during phases 2–3. Active MJO convection predominates in the eastern tropical Pacific prior to AWB events in phases 6–7, generating more eastward-propagating Rossby waves. As these Rossby wave packets reach the northwestern subtropical Atlantic, transient eddy forcing slows down the westerly mean flow between 25° and 35° N. Variations in the background zonal flow make the propagation of Rossby waves difficult, ultimately resulting in wave breaking.

PNNL Contact

L. Ruby Leung, Pacific Northwest National Laboratory, ruby.leung@pnnl.gov 
 

Funding

This research was supported by the Department of Energy, Office of Science, as part of research in the Regional and Global Model Analysis program area, Earth and Environmental System Modeling Program.