Investigating tropical cyclone impacts on MJO convection over the Indian Ocean
Thayer, Jeffrey D
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Permalink
https://hdl.handle.net/2142/117674
Description
Title
Investigating tropical cyclone impacts on MJO convection over the Indian Ocean
Author(s)
Thayer, Jeffrey D
Issue Date
2022-11-30
Director of Research (if dissertation) or Advisor (if thesis)
Hence, Deanna A
Doctoral Committee Chair(s)
Hence, Deanna A
Committee Member(s)
Nesbitt, Stephen W
Wang, Zhuo
Sriver, Ryan
Department of Study
Atmospheric Sciences
Discipline
Atmospheric Sciences
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
tropical cyclones
tropical convection
Madden-Julian Oscillation
high-resolution simulations
WRF
Indian Ocean
Abstract
Past work has not thoroughly investigated how TCs after genesis may influence MJO convection development, with TC-MJO convection interactions possibly having downstream implications for MJO forecasting. We use a high-resolution WRF simulation of one MJO event with an associated TC to examine lower-tropospheric dry-air impacts on MJO convection. With a long-term composite analysis, we broadly establish the relationship between Indian Ocean TCs and MJO convection development. We then seek to isolate and quantify TC impacts on MJO convection using semi-idealized WRF simulations with inserted idealized TCs.
With the high-resolution WRF simulation, we find significant decreases in equatorial lower-tropospheric moisture when the westerly wind burst (WWB) and TC are present. Zonal moisture advection primarily contributes to lower-tropospheric drying in the western Indian Ocean, with a non-negligible secondary contribution from meridional advection. Lower-tropospheric drying is coincident with increases in stability, leading towards significant decreases in convection depth and convection intensity over the western Indian Ocean. We assess that WWB-related dry-air advection inhibits MJO convection development, with the associated TC likely enhancing the WWB impacts.
We broadly examine Indian Ocean TC impacts on MJO convection from 1998-2018 for TC and No TC Days using composite analysis. We find significantly decreased lower-tropospheric moisture, greater stability, and decreased TRMM rainfall over the western Indian Ocean for each phase on TC Days. Significant increases in lower-tropospheric horizontal moisture advection and evaporation suggest that TCs can likely contribute to decreased MJO convection. A TC Day breakdown reveals that 60-80oE TCs can be linked with significant increases in equatorial dry-air advection and rainfall decreases over the western Indian Ocean. However, MJO strength and position changes on TC Days limit the overall robustness of our composite analysis results.
To better isolate and quantify TC impacts on MJO convection, we compare WRF simulations of the same MJO event with and without inserted idealized TCs. Simulations with idealized TCs have increased duration and eastward extent of lower-tropospheric dry air. Each perturbed run shows increased equatorial lower-tropospheric zonal dry-air advection occurring downstream of the TC insertion longitude starting 3 days after insertion. Preliminary findings suggest that increased TC size induces greater equatorial drying impacts, with TC intensity having lesser influences.
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