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Regional Intensification of the Tropical Hydrological Cycle During ENSO

Stephens, G. L., M. Hakuba, M. J. Webb, M. Lebsock, Q. Yue, B. Kahn, S. M. Hristova-Veleva, A. Rapp, C. Stubenrauch, G. S. Elsaesser, and J. Slingo (2018), Regional Intensification of the Tropical Hydrological Cycle During ENSO, Geophys. Res. Lett., 45, 4361-4370, doi:10.1029/2018GL077598.

This study provides observational evidence for feedbacks that amplify the short-term hydrological response associated with the warm phase of the El Niño-Southern Oscillation. Our analyses make use of a comprehensive set of independent satellite observations collected over decades to show that much larger local changes to cloud (~50%/K) and precipitation (~60%/K) occur than would be expected from the guidance of Clausius-Clapeyron theory (~7%/K). This amplification comes from atmospheric feedbacks involving shifts in the patterns of latent and radiative heating that mutually act on the dynamics enhancing changes to the hydrological cycle. We also confirm the existence of an opposing negative flux feedback at the ocean surface, driven largely by solar radiation changes, that opposes the surface warming. Estimates of the strength of this and other feedback factors associated with warming in the Niño3 region are provided from observations. These observations are also used to examine comparative processes and feedbacks in model experiments from the Coupled Model Intercomparison Project Phase 5 Atmospheric Model Intercomparison Project. Plain Language Summary It is a widely held paradigm of a warming world the “wet gets wetter and dry drier.” It is also argued that in this context precipitation in wet areas is expected to increase at a rate of approximately 7%/K, which is in accordance with expected increases in water vapor availability from elementary theory (the so-called Clausius-Clapeyron response). One of the main wet regions of the planet is the region of deep tropical convection organized into broad convective zones referred to as the Inter-Tropical Convergence Zone, and this region undergoes variability that is often used to test ideas thought to be relevant to climate change. Our study pieces together several independent measurements collected over multiple decades to reveal a strong, positive feedback on tropical convection associated with the short-term climate variations of the El Niño/Southern Oscillation. The feedback is a result of coupled dynamical-radiative processes that combine to produce intensification of the tropical hydrological cycle that is more than twice that expected from the CC response alone.

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