Rapid Dynamical Evolution of ITCZ Events over the East Pacific

Abstract

The latitudinal location of the east Pacific Ocean intertropical convergence zone (ITCZ) changes on time scales of days to weeks during boreal spring. This study focuses on tropical near-surface dynamics in the days leading up to the two most frequent types of ITCZ events, nITCZ (Northern Hemisphere) and dITCZ (double). There is a rapid, daily evolution of dynamical features on top of a slower, weekly evolution that occurs leading up to and after nITCZ and dITCZ events. Zonally-elongated bands of anomalous cross-equatorial flow and off-equatorial convergence rapidly intensify and peak one day before or the day of these ITCZ events, followed one or two days later by a peak in near-equatorial zonal wind anomalies. In addition, there is a wide region north of the southeast Pacific subtropical high where anomalous northwesterlies strengthen prior to nITCZ events and southeasterlies strengthen before dITCZ events. Anomalous zonal and meridional near-surface momentum budgets reveal that the terms associated with Ekman balance are of first-order importance preceding nITCZ events, but that the meridional momentum advective terms are just as important before dITCZ events. Variations in cross-equatorial flow are promoted by the meridional pressure gradient force (PGF) prior to nITCZ events and the meridional advection of meridional momentum in addition to the meridional PGF before dITCZ events. Meanwhile, variations in near-equatorial easterlies are driven by the zonal PGF and the Coriolis force preceding nITCZ events and the zonal PGF, the Coriolis force, and the meridional advection of zonal momentum before dITCZ events.

Dynamical Importance of the Trade Wind Inversion in Suppressing the Southeast Pacific ITCZ

“…Figures 4b and 4c illustrates that SST‐only SBLM simulations produce a year‐round double ITCZ and excessive equatorial divergence compared to Full T v SBLM simulations. In observations and reanalyzes, a double ITCZ peaks during February through April (Gu et al., 2005; Gonzalez et al., 2022; Liu & Xie, 2002; C. Zhang, 2001). Surface wind convergence in SST‐only SBLM simulations is much more interhemispherically symmetric especially during May through December despite there being a robust NH ITCZ in both wind convergence and precipitation observations (Waliser & Gautier, 1993).…”

“…J. Zhang et al, 2019). We plan to extend our methods to understand not only modeled climatological ITCZ variability but also subseasonal (Gonzalez et al, 2022;Haffke et al, 2016) and interannual variability (R. Xie & Yang, 2014;Yang & Magnusdottir, 2016).…”

“…While coupled models may suffer more from double ITCZ biases than atmosphere‐only models, models with prescribed SSTs can still have significant issues with low‐level stratocumulus clouds (Lin, 2007; Xiang et al., 2017, 2018; Woelfle et al., 2019; G. J. Zhang et al., 2019). We plan to extend our methods to understand not only modeled climatological ITCZ variability but also subseasonal (Gonzalez et al., 2022; Haffke et al., 2016) and interannual variability (R. Xie & Yang, 2014; S.‐P. Xie et al., 2018; Yang & Magnusdottir, 2016).…”

“…However, how significant the off-equatorial convergence and equatorial divergence pattern biases are in SST-only SBLM simulations relative to Full T v SBLM simulations is not as clear. This is relevant since SH convergence is present all year in the Full T v SBLM simulations and in observations (Gonzalez et al, 2022;Liu & Xie, 2002) but it is relatively weak for most of the year in observations, especially compared to NH convergence. Figure 5a suggests there is indeed a substantial pattern problem in SST-only SBLM simulations in that SH convergence is from two times to an order of magnitude too strong compared to NH convergence for all months except from February through April.…”

“…The east Pacific Ocean intertropical convergence zone (ITCZ) is highly modulated by variations in the tropical boundary layer winds, which often produce horizontal convergence that is co‐located with ITCZ precipitation (Gonzalez et al., 2022; Lindzen & Nigam, 1987; Liu & Xie, 2002). The cause of these boundary layer wind variations is commonly diagnosed through the zonal and meridional momentum budgets (Back & Bretherton, 2009a; Gonzalez & Schubert, 2019; Gonzalez et al., 2022; Holton, 1975; Holton et al., 1971; Lindzen & Nigam, 1987; Mahrt, 1972a, 1972b; McGauley et al., 2004; Raymond et al., 2006; Sobel & Neelin, 2006; Tomas et al., 1999). A leading term in boundary layer momentum budgets is the pressure gradient force (PGF), especially in regions where there are strong sea surface temperature (SST) gradients (Back & Bretherton, 2009b; Duffy et al., 2020; Lindzen & Nigam, 1987).…”