Behavior of Free-Tropospheric Transients in Relation to Sub-seasonal Rainfall in the East Pacific

Previous research done by the authors (in revision) affirms the hypothesis that free-tropospheric variance play a role in adjusting sub-seasonal rainfall states in the ITZC by altering the SLP gradient in the East Pacific. This incites questioning about how exactly different tropical transient interact with the large-scale dynamics and local thermodynamics to suppress and enhance rainfall. The aim of this research is to unearth the relationship between the SLP gradient and convection on the high-frequency temporal scale, which necessitates the consideration of sub-seasonal transients.

This research is influenced by the work of Huaman et al. (2021), which studied the effect of three passing easterly waves (EWs) during the OTREC period on ITCZ structure and found that each wavefront corresponded with a strengthening of the deep convective mode in the vertical profile of the ITCZ, followed by an increase in column moisture that strengthens the shallow mode. Using a spectral identification method established by Wheeler and Kiladis (1999) and outgoing longwave radiation (OLR) data from ERA5, we identify sub-seasonal frequency waves over an approximately 20-year period corresponding to the TRMM precipitation dataset. In the East Pacific, disturbances contributing to the majority of precipitation variance on the synoptic scale are convectively coupled equatorial waves, such as EWs, Kelvin and Rossby waves and, to a lesser degree, the Madden-Julian Oscillation (MJO) disturbance. Identification of wave events allows for compositing of 850 hPa and 1000 hPa geopotential fields from ERA5, which can inform us the effect that free-tropospheric disturbance have on the boundary-layer dynamics.

Initial results shows that several CCEWs troughs, especially EWs, correspond to a deepening of the SLP gradient, strengthening the low-level moisture flux into the northern ITCZ region. This research suggest that sub-seasonal variance of precipitation in this region can be explained at least partially through the effects on low-level dynamics by CCEWs.

References:

Huaman, L., Maloney, E. D., Schumacher, C., & Kiladis, G. N. (2021, November). Easterly Waves in the East Pacific during the OTREC 2019 Field Campaign. Journal of the Atmospheric Sciences, 78 (12), 4071–4088. Retrieved 2022-04-24, from https://journals.ametsoc.org/view/journals/atsc/78/12/
JAS-D-21-0128.1.xml (Publisher: American Meteorological Society Section:Journal of the Atmospheric Sciences) doi: 10.1175/JAS-D-21-0128.1

Wheeler, M., & Kiladis, G. N. (1999, February). Convectively Coupled Equatorial Waves: Analysis of Clouds and Temperature in the Wavenumber–Frequency Domain. Journal of the Atmospheric Sciences, 56 (3), 374–399. Retrieved 2022-09-15, from https://journals.ametsoc.org/view/journals/atsc/56/3/1520-0469 1999 056 0374 ccewao 2.0.co 2.xml (Publisher: American Meteorological Society Section: Journal of the Atmospheric Sciences) doi:10.1175/1520-0469(1999)056⟨0374:CCEWAO⟩2.0.CO;2