Role of the climatological intertropical convergence zone in the seasonal footprinting mechanism of the El Niño-Southern Oscillation

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  • 1 Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, South Korea.
  • 2 Department of Atmospheric Sciences/Irreversible Climate Change Research Center, Yonsei University, Seoul, South Korea
  • 3 Department of Atmosphere Nanjing University of Information Science & Technology Nanjing, Jiangsu, China
  • 4 Institute for Convergence Research and Education in Advanced Technology, Yonsei University, Seoul, South Korea
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Abstract

The North Pacific Oscillation (NPO), a primary atmospheric mode over the North Pacific in boreal winter, is known to trigger the El Niño-Southern Oscillation (ENSO) in the following winter, the process of which is recognized as the seasonal footprinting mechanism (SFM). Based on the analysis of model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5), we found that the SFM acts differently among models, and the correlation between the NPO and subsequent ENSO events, called the SFM efficiency, depends on the background mean state of the model. That is, SFM efficiency becomes stronger as the climatological position of the Pacific Intertropical Convergence Zone (ITCZ) moves poleward, representing an intensification of the northern branch of the ITCZ. When the Pacific ITCZ is located poleward, the wind-evaporation-sea surface temperature (SST) feedback becomes stronger as the precipitation response to the SST anomaly is stronger in higher latitudes compared to that of lower latitudes. In addition, such active ocean-atmosphere interactions enhance NPO variability, favoring the SFM to operate efficiently and trigger an ENSO event. Consistent with the model results, the observed SFM efficiency increased during the decades in which the northern branch of the climatological ITCZ was intensified, supporting the importance of the tropical mean state of precipitation around the Pacific ITCZ.

Corresponding author: Prof. Jong-Seong Kug (jskug1@gmail.com). Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, South Korea

Abstract

The North Pacific Oscillation (NPO), a primary atmospheric mode over the North Pacific in boreal winter, is known to trigger the El Niño-Southern Oscillation (ENSO) in the following winter, the process of which is recognized as the seasonal footprinting mechanism (SFM). Based on the analysis of model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5), we found that the SFM acts differently among models, and the correlation between the NPO and subsequent ENSO events, called the SFM efficiency, depends on the background mean state of the model. That is, SFM efficiency becomes stronger as the climatological position of the Pacific Intertropical Convergence Zone (ITCZ) moves poleward, representing an intensification of the northern branch of the ITCZ. When the Pacific ITCZ is located poleward, the wind-evaporation-sea surface temperature (SST) feedback becomes stronger as the precipitation response to the SST anomaly is stronger in higher latitudes compared to that of lower latitudes. In addition, such active ocean-atmosphere interactions enhance NPO variability, favoring the SFM to operate efficiently and trigger an ENSO event. Consistent with the model results, the observed SFM efficiency increased during the decades in which the northern branch of the climatological ITCZ was intensified, supporting the importance of the tropical mean state of precipitation around the Pacific ITCZ.

Corresponding author: Prof. Jong-Seong Kug (jskug1@gmail.com). Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, South Korea
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