Culprit of the Eastern Pacific Double-ITCZ Bias in the NCAR CESM1.2

Zonal–vertical distribution of (a) vertical velocity and wind vectors, (b) zonal wind and wind vectors, and (c) meridional wind  averaged between 138 and 158N for November from ERA-Interim. (d)–(f) The corresponding biases from the AMIP simulation.
TitleCulprit of the Eastern Pacific Double-ITCZ Bias in the NCAR CESM1.2
Publication TypeJournal Article
Year of Publication2019
AuthorsSong X.L, Zhang GJ
Volume32
Pagination6349-6364
Date Published2019/10
Type of ArticleArticle
ISBN Number0894-8755
Accession NumberWOS:000482951300003
KeywordsAtmosphere-ocean interaction; circulation; climate models; convection parameterization; coupled; heat-budget; Meteorology & Atmospheric Sciences; Model errors; models; ocean; Pacific Ocean; part i; precipitation pattern; sea-surface temperature; seasonal cycle; tropical pacific; upper-ocean biases
Abstract

The eastern Pacific double-ITCZ bias has long been attributed to the warm bias of SST in the southeastern Pacific and associated local air-sea interaction. In this study, we conducted two simulations using the NCAR CESM1.2.1 to demonstrate that significant double-ITCZ bias can still form in the eastern Pacific through air-sea coupled feedback even when there is cold SST bias in the southeastern Pacific, indicating that other nonlocal culprits and mechanisms should be responsible for the double-ITCZ bias in the eastern Pacific. Further analyses show that the oversimulated convection in the northern ITCZ region and Central America in boreal winter may result in biases in the surface wind fields in the tropical northeastern Pacific in the atmospheric model, which favor the cooling of the ocean mixed layer through enhancement of latent heat flux and Ekman upwelling. These biases are passed into the ocean model in coupled simulations and result in a severe cold bias of SST in the northern ITCZ region. The overly cold SST bias persists in the subsequent spring, leading to the suppression of convection in the northern ITCZ region. The enhanced low-level cross-equatorial northerly wind strengthens the wind convergence south of the equator and transports abundant water vapor to the convergence zone, strengthening the southern ITCZ convection. All these processes lead to the disappearance of the northern ITCZ and the enhancement of the southern ITCZ in boreal spring, forming a seasonally alternating double-ITCZ bias. This study suggests that convection biases in the northern ITCZ region and Central America in boreal winter may be a culprit for the double-ITCZ bias in the eastern Pacific.

DOI10.1175/jcli-d-18-0580.1
Impact: 

In summary, the oversimulated convection in the northern ITCZ region and Central America in boreal winter may result in biases in the surface wind fields in tropical northeastern Pacific in the atmospheric model, which favor the cooling of ocean mixed layer through the enhancement of latent heat flux and Ekman upwelling. Once the atmospheric model is coupled to the ocean model, these biases are passed into the ocean model and result in a severe cold bias of SST in the northern ITCZ region. The overly cold SST bias persists in the subsequent spring, leading to the suppression of convection in the northern ITCZ region. The enhanced low-level cross-equatorial northerly wind strengthens the wind convergence south of the equator and transports abundant water vapor to the convergence zone, strengthening the southern ITCZ convection. All these processes lead to the disappearance of northern ITCZ and the enhancement of southern ITCZ in boreal spring, forming a seasonally alternating type of double-ITCZ bias.

Student Publication: 
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