Observed variability of intertropical convergence zone over 1998—2018

Chunlei Liu,Richard P Allan,Xiaoli Feng,Xiaoqing Liao,Yazhu Yang,Jianjun Xu,Juliao Qiu,Jingchao Long,Ning Cao

doi:10.1088/1748-9326/aba033

Abstract

The intertropical convergence zone (ITCZ) accounts for more than 30% of the global precipitation and its variability has a great effect on the people living in the tropical area. It is the manifestation of the Hadley circulation, tropical dynamic and thermodynamic coupling and the air-sea interaction. Therefore, it is essential to understand the changes and variability of the ITCZ, including its position and intensity. Using observed precipitation from GPCP and TRMM 3B43, as well as the ERA5 reanalysis data, we examine the ITCZ variations over the global and seven regions from 1998–2018. These data sets show consistent ITCZ climatology and inter-annual variability. Except over Atlantic and eastern Pacific where ITCZ stays in the northern hemisphere, the ITCZ crosses the equator after equinoxes over other sections. There is no overall significant shift of annual mean ITCZ position over the globe and seven regions, except over the America section where the GPCP data show a significant increase trend of 0.8° decade−1 and over the western Pacific section where ERA5 data show a significant decrease trend of −1.2° decade−1. The ITCZ positions over the globe and Africa are related to both ENSO and NAO, while ITCZ position over eastern Pacific is significantly affected by ENSO and the Atlantic ITCZ is mainly related to NAO. Except for the western Pacific, all other sections are significantly related to local meridional surface temperature gradient, particularly over America, Atlantic and eastern Pacific. The meridional gradient variation of the vertically integrated moist static energy has generally good agreement with the shift of the intertropical convergence zone, particularly for the seasonal climatology over the Africa region. The relationship between ITCZ position and intensity shows complicated patterns, with positive correlation over the globe, but different correlations over different sections. The two observed data sets are more or less consistent, but ERA5 shows discrepancies over some sections. It is also found that the local meridional temperature gradient has more influences on the ITCZ positions than the global one.

Highlights

The intertropical convergence zone (ITCZ) is the location with strong convection and maximum rainfall (Philander et al 1996) and controls the tropical precipitation over both ocean and land (Nicholson 2018)
Using observed precipitation from GPCP and TRMM 3B43, as well as the ERA5 reanalysis data, we examine the ITCZ variations over the global and seven regions from 1998–2018
It has been difficult for models to represent the ITCZ position correctly (Flato et al 2013, Shonk et al 2018), the double ITCZ persists in modern models where the precipitation is overestimated over southern tropical oceans and underestimated over equatorial Pacific (Li and Xie 2014, Zhang et al 2015, Adam et al 2016, Loeb et al 2016), which may be associated with the southeastern Pacific sea surface temperature (SST) bias (Ma et al 1996, Song and Zhang 2016), tropical North Atlantic SST bias (Song and Zhang 2017) and model resolution (Song and Zhang 2020)