The Relationship Between Convectively Coupled Waves and the East Pacific ITCZ

Longstanding climate model biases in tropical precipitation exist over the east Pacific (EP) Ocean, especially during boreal winter and spring when models have excessive Southern Hemisphere (SH) precipitation near the intertropical convergence zone (ITCZ). In this study, we document the impact of convectively coupled waves (CCWs) on EP precipitation and the ITCZ using observations and reanalyses. We focus on the months when SH precipitation peaks in observations: February, March, and April (FMA). CCWs explain up to 93% of total precipitation variance in the southern EP (EQ-10S) during FMA. We further investigate three high frequency wave bands: Mixed Rossby–gravity waves and tropical depression type disturbances (MRG-TD type, 41%), Kelvin waves (17.2%), and n=0 eastward inertia-gravity waves (IG0, 16.9%). Caution must be taken comparing percentages between wave bands as the background variance inflates larger spectral widths (e.g., MRG–TD type). Compared to their warm pool counterparts, these three CCWs have a more zonally elongated and meridionally narrower precipitation structure with circulations that resemble past observational studies and/or shallow water theory. We quantify the contribution of all CCWs to four different daily ITCZ "states'': Northern Hemisphere (NH) (nITCZ), SH (sITCZ), double (dITCZ), and equatorial (eITCZ) using a new precipitation-based ITCZ states algorithm. We find that the percent of total precipitation variance explained by each of the CCWs is heightened for sITCZs and eITCZs and diminished for nITCZs. Lastly, we find that nITCZs are most prevalent weeks after strong CCW activity happens in the NH, whereas CCWs and sITCZs peak simultaneously in the SH.