The Relationship Between Convectively Coupled Tropical 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 tropical waves (CCTWs) 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). CCTWs explain 53% of total precipitation variance in the SH during FMA, led by Kelvin waves (8.9%), tropical depression-type disturbances (TD-type, 8.3%), and n=0 eastward inertia-gravity waves (IG0, 8.1%). Compared to their warm pool counterparts, Kelvin waves, IG0, and TD-type all 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 CCTWs 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 CCTWs is heightened for sITCZs and eITCZs and diminished for nITCZs. Furthermore, there is a broad correlation between ITCZ states and the CCTW variance activity on interannual timescales such that the CCTW activity peaks in the same hemisphere as the ITCZ. Lastly, we find that nITCZs are most prevalent weeks after strong CCTW activity happens in the NH whereas CCTWs and sITCZs peak simultaneously in the SH.