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<p>Earlier studies have shown a declining precipitation trend over the Indian state of Kerala during the southwest monsoon season since the early twentieth century. However, a notable change in trend is witnessed in this region during the 1990–2020 period relative to the previous decade. This study provides some insights using observations and ERA5 reanalysis. Seasonal rainfall amounts for the state of Kerala during the study period (1990–2020) show significant zonal variations, with rising trends over the midland (elevations 30–200&#xa0;m asl) and highland (exceeding 400&#xa0;m asl) regions and a falling trend over the lowland. In monthly data, near-compensating precipitation trends are noted from the first and second halves of the monsoon season (i.e., a negative precipitation trend during June and July, followed by a positive trend in August and September.). This pattern signals a slowing down of the overall declining trend in Kerala’s monsoon rainfall since the 1990s. Circulation trends appear to support this pattern, with June and July showing deficient moisture convergence over the Kerala region, while August and September exhibit enhanced moisture convergence. Additionally, increased moisture transport in August and September favors stronger ascent and deeper convective cloud development in the region. Observations also indicate a shift in Kerala’s rainfall patterns in recent decades relative to the 1960–1989 period. We investigated the climate change signals by decomposing precipitation changes into dynamic and thermodynamic contributions using a water vapor vertical advection budget framework. Our analysis demonstrates that the precipitation changes during 1990–2020 are predominantly driven by alterations in large-scale atmospheric circulation rather than by variations in moisture availability when compared with the earlier period. Contrary to earlier assessments in the scientific literature regarding highland rainfall in the Western Ghats, recent decades have shown greater rainfall variability and more frequent intense rain events in the highlands of Kerala. This also motivates the critical need for a larger observation network, particularly in mid- and highland regions of Kerala, to further understand the impact of complex mesoscale terrain on zonal variations in rainfall as the climate changes.</p>

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On the slowdown of southwest monsoon rainfall decline in recent decades over Kerala

  • 期刊论文

  • 发表日期：

    2025年5月12日

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  • M. J. K. Reji,
  • H. Varikoden,
  • R. K. Vellore,
  • A. G. Turner,
  • T. Gokul,
  • R. Krishnan

摘要

Earlier studies have shown a declining precipitation trend over the Indian state of Kerala during the southwest monsoon season since the early twentieth century. However, a notable change in trend is witnessed in this region during the 1990–2020 period relative to the previous decade. This study provides some insights using observations and ERA5 reanalysis. Seasonal rainfall amounts for the state of Kerala during the study period (1990–2020) show significant zonal variations, with rising trends over the midland (elevations 30–200 m asl) and highland (exceeding 400 m asl) regions and a falling trend over the lowland. In monthly data, near-compensating precipitation trends are noted from the first and second halves of the monsoon season (i.e., a negative precipitation trend during June and July, followed by a positive trend in August and September.). This pattern signals a slowing down of the overall declining trend in Kerala’s monsoon rainfall since the 1990s. Circulation trends appear to support this pattern, with June and July showing deficient moisture convergence over the Kerala region, while August and September exhibit enhanced moisture convergence. Additionally, increased moisture transport in August and September favors stronger ascent and deeper convective cloud development in the region. Observations also indicate a shift in Kerala’s rainfall patterns in recent decades relative to the 1960–1989 period. We investigated the climate change signals by decomposing precipitation changes into dynamic and thermodynamic contributions using a water vapor vertical advection budget framework. Our analysis demonstrates that the precipitation changes during 1990–2020 are predominantly driven by alterations in large-scale atmospheric circulation rather than by variations in moisture availability when compared with the earlier period. Contrary to earlier assessments in the scientific literature regarding highland rainfall in the Western Ghats, recent decades have shown greater rainfall variability and more frequent intense rain events in the highlands of Kerala. This also motivates the critical need for a larger observation network, particularly in mid- and highland regions of Kerala, to further understand the impact of complex mesoscale terrain on zonal variations in rainfall as the climate changes.