An Extratropical Pathway for the Madden–Julian Oscillation’s Influence on North Atlantic Tropical Cyclones
A plausible mechanism is proposed by which tropical forcing over the eastern Pacific can affect North Atlantic tropical cyclones by altering the behavior of extratropical Rossby waves.
The National Oceanic and Atmospheric Adminstration (NOAA) satellite image of Hurricanes (tropical cyclones) Franklin and Idalia in August 2023.
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 oceanic 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.
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.
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.