The intricacies of identifying equatorial waves

Abstract

Equatorial waves (EWs) are synoptic-to planetary-scale propagating disturbances at low latitudes with periods from a few days to several weeks. Here, this term includes Kelvin waves, equatorial Rossby waves, mixed Rossby-gravity waves, and inertio-gravity waves, which are well described by linear wave theory, but it also other tropical disturbances such as easterly waves and the intraseasonal Madden-Julian Oscillation with more complex dynamics. EWs can couple with deep convection, leading to a substantial modulation of clouds and rainfall. EWs are amongst the dynamic features of the troposphere with the longest intrinsic predictability, and models are beginning to forecast them with an exploitable level of skill. Most of the methods developed to identify and objectively isolate EWs in observations and model fields rely on (or at least refer to) the adiabatic, frictionless linearized primitive equations on the sphere or the shallow-water system on the equatorial 𝛽-plane. Common ingredients to these methods are zonal wave-number-frequency filtering (Fourier or wavelet) and/or projections onto predefined empirical or theoretical dynamical patterns. This paper gives an overview of six different methods to isolate EWs and their structures, discusses the underlying assumptions, evaluates the applicability to different problems, and provides a systematic comparison based on a case study (February 20-May 20, 2009) and a climatological analysis (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018). In addition, the influence of different input fields (e.g., winds, geopotential, outgoing long-wave radiation, rainfall) is investigated. Based on the results, we generally recommend employing a combination of wave-number-frequency filtering and spatial-projection methods (and of different input fields) to check for robustness of the identified signal. In cases of disagreement, one needs to carefully investigate which assumptions made for the individual methods are most probably not fulfilled. This will help in choosing an approach optimally suited to a given problem at hand and avoid misinterpretation of the results.

Equatorial waves as useful precursors to tropical cyclone occurrence and intensification

“…A compound study conditional on equatorial waves and other factors, including long-term climate variability (e.g., ENSO) and large-scale circulations (e.g., monsoon trough), is desirable for a future investigation. Furthermore, the equatorial wave parameters used in this study (e.g., amplitude, wavelength, and phase speed) may vary with the wave identification methods and the wave band filter 10 , and this could be another source of uncertainty in the TC-wave relationship. Additionally, we cannot entirely rule out the effect of TCs on equatorial wave identification, although the impact is expected to be small in the pre-TC stage.…”

“…In these studies, the hypothesized mechanism is that equatorial waves are associated with enhanced convection in the background, favouring TC generation. In these studies, the TCG-wave relationship is basically determined from rainfall, which includes the TC rainfall, because the waves are extracted by applying a spectral filter to convection variables (i.e., precipitation and outgoing longwave radiation, OLR) in a space-time domain 10 . However, in addition to the convectively coupled equatorial waves, which have strong precipitation and OLR signals, there are many other equatorial waves, such as the equatorial waves identified by the horizontal velocity structures.…”

Link Between the Time‐Space Behavior of Rainfall and 3D Dynamical Structures of Equatorial Waves in Global Convection‐Permitting Simulations

“…Extracting the EW signal from observational or model data is a challenge (Knippertz et al., 2022; Žagar et al., 2022). In a recent methodological intercomparison study, Knippertz et al.…”

“…Due to the fundamental differences between the approaches, Knippertz et al. (2022) recommended using a combination of two different approaches to check for the robustness of the identified signals.…”

A method for estimating global subgrid-scale gravity-wave temperature perturbations in chemistry-climate models

“…Examples of waves leading to such temperature perturbations include Kelvin, Rossby and gravity waves (Madden, 1979;Fritts & Alexander, 2003;Das & Pan, 2013). While the planetary scale Kelvin and Rossby waves can be resolved by current chemistry-climate models (Eyring et al, 2016;Knippertz et al, 2022), a significant fraction of the gravity wave spectrum occurs on the subgrid scales of current models.…”