Vertical Structure of the High-Level Cross-Equatorial Flow and Response to ENSO in Boreal Summer

Abstract

Cross-equatorial flow (CEF) represents a fundamental atmospheric phenomenon in the tropical troposphere. While low-level CEF (LCEF) has been extensively studied, the understanding of high-level CEF (HCEF) remains insufficient. Based on three reanalysis datasets (ERA-5, JRA-55, and MERRA-2) and CMIP6 models, this study conducts a comprehensive analysis of HCEF, including its vertical structure, multiple time-scales variability, and relationship with the El Niño-Southern Oscillation (ENSO). The results indicate that HCEF has four primary channels located over the eastern Pacific, the Atlantic, West Africa, and the Indian Ocean-Maritime Continent, respectively. While interannual variability of HCEF is relatively consistent across the datasets, substantial discrepancies exist in its decadal variability and trend. The linkage between HCEF and the underlying LCEF exhibits dataset dependence and channel difference at interannual scales. The eastern Pacific HCEF (EPHCEF) is directly modulated by two types of ENSO (eastern Pacific ENSO and central Pacific ENSO) due to sea surface temperature anomalies that excite anomalous convection, which in turn modifies patterns of upper tropospheric convergence and divergence, thereby driving HCEF anomalies. During eastern Pacific ENSO events, Atlantic HCEF (ALHCEF) and West Africa HCEF (WAHCEF) also exhibit significant interannual variability through a physical process similar to that affecting EPHCEF. Statistical analysis suggests that the Atlantic Niño is more likely to be the primary driver of their variability compared with ENSO. The simulation results of CMIP6 for EP ENSO are similar to those of the reanalysis datasets, but show systematic biases for CP ENSO. These findings significantly advance our understanding of the tropical upper tropospheric circulation.