Sensitivity of Coupled Tropical Pacific Model Biases to Convective Parameterization in CESM1

• Published in: Journal of advances in modeling earth systems 2018-01, Vol.10 (1), p.126-144
• Main Authors: Woelfle, M. D., Yu, S., Bretherton, C. S., Pritchard, M. S.
• Format: Article
• Language: English
• Subjects: Advection; Cold; Cold regions; cold tongue; Convection; Convective momentum transport; Convergence zones; Earth; Equator; hindcast; Intertropical convergence zone; ITCZ; Low-level jets; Momentum; Momentum transport; Ocean currents; Oceans; Parameterization; Pressure gradients; Rain; Sea surface; Sea surface temperature; Simulation; superparameterization; Surface temperature; Surface wind; Temperature; Transport; Tropical climate; Vertical mixing; Wind; Wind shear; Wind stress; Zonal winds
• ISSN: 1942-2466; 1942-2466
• DOI: 10.1002/2017MS001176

Six month coupled hindcasts show the central equatorial Pacific cold tongue bias development in a GCM to be sensitive to the atmospheric convective parameterization employed. Simulations using the standard configuration of the Community Earth System Model version 1 (CESM1) develop a cold bias in equatorial Pacific sea surface temperatures (SSTs) within the first two months of integration due to anomalous ocean advection driven by overly strong easterly surface wind stress along the equator. Disabling the deep convection parameterization enhances the zonal pressure gradient leading to stronger zonal wind stress and a stronger equatorial SST bias, highlighting the role of pressure gradients in determining the strength of the cold bias. Superparameterized hindcasts show reduced SST bias in the cold tongue region due to a reduction in surface easterlies despite simulating an excessively strong low‐level jet at 1‐1.5 km elevation. This reflects inadequate vertical mixing of zonal momentum from the absence of convective momentum transport in the superparameterized model. Standard CESM1simulations modified to omit shallow convective momentum transport reproduce the superparameterized low‐level wind bias and associated equatorial SST pattern. Further superparameterized simulations using a three‐dimensional cloud resolving model capable of producing realistic momentum transport simulate a cold tongue similar to the default CESM1. These findings imply convective momentum fluxes may be an underappreciated mechanism for controlling the strength of the equatorial cold tongue. Despite the sensitivity of equatorial SST to these changes in convective parameterization, the east Pacific double‐Intertropical Convergence Zone rainfall bias persists in all simulations presented in this study. Key Points In the first six months of initialized CESM1 runs, wind stress errors drive cold biases in central equatorial Pacific Ocean temperature Pacific cold tongue temperature bias is sensitive to convective parameterization and the representation of convective momentum transport Atmospheric superparameterization with no convective momentum transport induces low‐level wind shear bias that reduces cold tongue bias