The Effect of Atmosphere‐Ocean Coupling on the Sensitivity of the ITCZ to Convective Mixing. (30th November 2020)

• Record Type: Journal Article
• Authors: Talib, J.; Woolnough, S. J.; Klingaman, N. P.; Holloway, C. E.
• Is Part Of: Journal of advances in modeling earth systems. Volume 12:Number 12(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-11-30
• Subjects: Intertropical Convergence Zone -- convection -- atmosphere‐ocean coupling -- aquaplanet -- ocean dynamics -- tropical rainfall; Geological modeling -- Periodicals; Climatology -- Periodicals; Geochemical modeling -- Periodicals; 551.5011
• DOI: 10.1029/2020MS002322
• Languages: English
• ISSNs: 1942-2466

Abstract

Abstract: The Intertropical Convergence Zone (ITCZ) is a discontinuous, zonal precipitation band that plays a crucial role in the global hydrological cycle. Previous studies using prescribed sea surface temperature (SST) aquaplanets show the ITCZ is sensitive to convective mixing, but such a framework is energetically inconsistent. Studies also show that atmosphere‐ocean coupling reduces the sensitivity of the ITCZ to hemispherically asymmetric forcing. We investigate the effect of atmosphere‐ocean coupling on the sensitivity of the ITCZ to convective mixing using an idealized modeling framework with an Ekman‐driven ocean energy transport (OET). Coupling reduces the sensitivity of the ITCZ location to convective mixing due to SST changes. In prescribed‐SST simulations reducing convective mixing promotes a double ITCZ, while in coupled simulations, it increases the meridional SST gradient which promotes an equatorward ITCZ shift. Prescribing OET in additional experiments has a minimal effect on the sensitivity of the ITCZ location to mixing but does increase the sensitivity of the ITCZ intensity by constraining the net‐downward surface energy flux. Decreasing convective mixing increases net‐downward shortwave cloudy‐sky radiation associated with increased latent heat fluxes and an intensified ITCZ. For simulations analyzed the atmospheric energy input framework is inadequate to study ITCZ dynamics due to the contribution of transient eddies to the atmospheric energy transport. Prescribing SST or OET may strengthen the sensitivity of the ITCZ to a change in parameterization or atmospheric forcing. Future modeling studies investigating the precipitation response to such changes should be aware of the potential sensitivity of their results to atmosphere‐ocean interactions. Plain Language Summary: The Intertropical Convergence Zone (ITCZ) is a discontinuous tropical rainfall band with over three billion livelihoods dependent on its seasonal cycle. However, even the latest state‐of‐the‐art climate models poorly simulate the real‐world ITCZ. Previous modeling studies which fix sea surface temperatures and have no land show that the ITCZ is sensitive to the model's representation of deep clouds. However, it is yet to be determined whether ocean dynamics that are influenced by the atmosphere removes this sensitivity. In this study we couple the atmosphere to an ocean model with a circulation that depends on near‐surface winds. Through doing so we highlight that interactions between the atmosphere and ocean reduce changes in ITCZ location when varying the representation of deep clouds. This is predominately associated with sea surface temperature changes rather than changes in ocean circulation. However, fixing the ocean circulation and only allowing sea surface temperatures to vary, can cause different changes in ITCZ intensity when changing the representation of deep clouds. We therefore highlight that fixing either sea surface temperatures or the ocean circulation can affect how the ITCZ responds to changes in model design. Key Points: Atmosphere‐ocean coupling reduces the sensitivity of the simulated ITCZ to convective mixing Sea surface temperature variations are predominately responsible for the reduced sensitivity Atmospheric energy input framework to understand ITCZ behavior has significant limitations due to strong transient eddies