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<p>The northward displacement of the Intertropical Convergence Zone is a defining feature of the tropical climate that global models struggle to reproduce. While often attributed to interhemispheric energy imbalance, other processes may also shape its annual-mean structure. Here we use a hierarchy of models—from a fully coupled model to simplified frameworks—to examine how seasonal insolation variations influence the symmetry of the tropical rain band. Using idealized obliquity experiments alongside observations from 1979 to 2024, we show that amplifying seasonal insolation, while preserving hemispheric symmetry in the annual mean, shifts the rain belt towards a more symmetric state. Enhanced austral-summer heating raises southern tropical sea surface temperatures above a convective threshold, triggering a transient southern rain band amplified by wind–evaporation–surface temperature feedback, while convection remains suppressed outside this season south of the Equator. This seasonally selective response thus affects the annual mean. The rain-belt position thus reflects the nonlinear integration of seasonal dynamics rather than a simple response to annual-mean forcing. Consistently, many models overestimate southeastern tropical Pacific sea surface temperature seasonality, contributing to the double rain-belt bias. Our findings highlight the need for improved representation of seasonal processes in models to better capture annual-mean rainfall patterns.</p>

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Reduced tropical rain-band asymmetry through the seasonal cycle

  • 期刊论文

  • 发表日期：

    2026年7月14日

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  • Pengcheng Zhang,
  • Shang-Ping Xie,
  • Nicholas J. Lutsko,
  • Yuko M. Okumura,
  • Da Yang,
  • Tiffany A. Shaw

摘要

The northward displacement of the Intertropical Convergence Zone is a defining feature of the tropical climate that global models struggle to reproduce. While often attributed to interhemispheric energy imbalance, other processes may also shape its annual-mean structure. Here we use a hierarchy of models—from a fully coupled model to simplified frameworks—to examine how seasonal insolation variations influence the symmetry of the tropical rain band. Using idealized obliquity experiments alongside observations from 1979 to 2024, we show that amplifying seasonal insolation, while preserving hemispheric symmetry in the annual mean, shifts the rain belt towards a more symmetric state. Enhanced austral-summer heating raises southern tropical sea surface temperatures above a convective threshold, triggering a transient southern rain band amplified by wind–evaporation–surface temperature feedback, while convection remains suppressed outside this season south of the Equator. This seasonally selective response thus affects the annual mean. The rain-belt position thus reflects the nonlinear integration of seasonal dynamics rather than a simple response to annual-mean forcing. Consistently, many models overestimate southeastern tropical Pacific sea surface temperature seasonality, contributing to the double rain-belt bias. Our findings highlight the need for improved representation of seasonal processes in models to better capture annual-mean rainfall patterns.