Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling
03 June 2020

Regional MJO Modulation of Northwest Pacific Tropical Cyclones Driven by Multiple Transient Controls

Science

The boreal summer MJO has been widely acknowledged for its ability to modulate tropical cyclone (TC) formations in the Northwest Pacific, yet the underlying drivers of this modulation remain uncertain. As a result, there remains an unsatisfying lack of consensus on which phases of the MJO are favorable/unfavorable for cyclogenesis and why. Here, we attempt to reconcile apparent differences in past studies by assessing the MJO-TC modulation at finer spatial scales than has been previously possible. Researchers combined observations of environmental conditions in each MJO phase with a novel TC-generating model developed by Kerry Emanuel at MIT, resulting in 4,000 TC tracks in each individual phase – substantially more than is available in current TC observations.

Impact

With the expanded sample size, we are able to robustly assess sub-basin responses of TCs to the MJO’s passage for the first time, revealing two action centers that are especially susceptible to the oscillation. The South China Sea region and the West-Central Pacific are found to respond to the MJO out of phase with one another, favoring cyclogenesis in Phases 3-5 and 1-2/7-8 respectively. This pattern of out-of-phase modulation is consistent across two definitions of the MJO as well as in a regional analysis of the Genesis Potential Index (GPI), emphasizing the importance of sub-basin analysis when defining the MJO-TC modulation in this particular basin.

GPI is particularly useful as it can be decomposed to assess the drivers of the modulation, which reveals transient support from both dynamics and thermodynamics associated with the MJO. The South China Sea is found to be particularly susceptible to the MJO due to initial reductions in wind shear that allow the surface ocean layer to warm ahead of the arrival of the convective packet and increased relative humidity. The West-Central Pacific is instead governed first by increased humidity in Phases 7-8, followed by shear support in Phases 1-2. These findings are especially crucial to consider in light of numerous climate models suggesting future MJO amplification as a result of climate change, including more frequent/faster-moving events that stretch further into the Central Pacific.

Summary

The Madden‐Julian Oscillation (MJO) is widely acknowledged for its ability to modulate Northwest Pacific tropical cyclones (TCs) but a complete understanding of the underlying mechanisms remains uncertain. Beyond established effects of the MJO's relative humidity envelope, other dynamical factors have recently been invoked via new genesis potential indices and high‐resolution modeling studies. Here we revisit the ability of the MJO to modulate West Pacific TCs through a quasi‐explicit cyclone downscaling strategy driven by composited observations, paired later with a genesis index to investigate regional drivers of modulation. We reveal two distinct spatial modes of TC modulation in which the MJO's dynamic and thermodynamic effects act in tandem to increase TCs. In the South China Sea, for instance, shear reductions associated with the MJO's circulation lead to increasing potential intensity ahead of the arrival of a positive humidity anomaly, all of which combine for an extended period of cyclogenesis favorability.

Contact
Mike Pritchard
University of California – Irvine
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