Abstract
Abstract The variability of the South Pacific convergence zone (SPCZ) during the mid-Holocene is investigated using models archived in the Paleoclimate Modelling Intercomparison Project Phase II (PMIP2) database. Relative to preindustrial conditions, mid-Holocene top-of-atmosphere insolation was relatively lower during austral summer [December–February (DJF)], which is the season when the SPCZ is at its peak intensity. In response to this perturbation, the PMIP2 models simulate a displacement of the SPCZ to the southwest. This SPCZ shift is associated with a sea surface temperature (SST) dipole, with increased rainfall collocated with warm SST anomalies. Decomposing the DJF precipitation changes in terms of a diagnostic moisture budget indicates that the SPCZ shift is balanced to leading order by a change in the mean moisture convergence. Changes to the broad area of upper-level negative zonal stretching deformation, where transient eddies can become trapped and subsequently generate deep convection, support the notion that the SPCZ shift in the subtropics is tied to eddy forcing. Idealized experiments performed with an intermediate-level complexity model, the Quasi-Equilibrium Tropical Circulation Model (QTCM), suggest that the mid-Holocene change in rainfall in the SPCZ region as well as the equatorial Pacific is dominated by a change in the underlying SST. The tropical portion of the SPCZ is further remotely affected by the orbitally induced weakening of the Australian monsoon, even though this effect is weaker compared to the effect from SSTs.
Highlights
Introduction aTheories on the South Pacific convergence zone in current climateThe Pacific supports Earth’s most extensive region of tropical deep convection, which extends eastward from the west Pacific warm pool as the intertropical convergence zone (ITCZ) and diagonally to the southeast as the South Pacific convergence zone (SPCZ) (Trenberth 1976; Kodama 1992, 1993; Vincent 1994; Cai et al 2012; Matthews 2012; Widlansky et al 2013)
As inferred from both instrumental observations (Folland et al 2002) and oxygen isotope proxy records from coral reefs (Delcroix et al 2007; Linsley et al 2008), the SPCZ appears to be correlated on decadal time scales with largescale modes such as the interdecadal Pacific oscillation (IPO), with the SPCZ shifting to the northeast when the western Pacific warm pool warms and expands eastward
Simulated mid-Holocene tropical precipitation differed substantially from preindustrial conditions. Among such differences are the weakening of local summertime precipitation over the Southern Hemisphere continents and widespread increases of precipitation over tropical oceans, with the exception of the equatorial Pacific where drying occurs
Summary
To understand the mechanisms controlling changes in SPCZ precipitation between preindustrial and midHolocene conditions, we analyze here a suite of models archived in the PMIP2 database (Braconnot et al 2007). Community Climate System Model, version 3.0 Model for Interdisciplinary Research on Climate, version 3.2 (medium resolution) Meteorological Research Institute Coupled Atmosphere–Ocean General Circulation. Version 2.3.4, flux adjusted Meteorological Research Institute Coupled Atmosphere–Ocean General Circulation. We analyze simulations performed with the GFDL Coupled Model, version 2.1 (CM2.1; Delworth et al 2006). The model version used here, QTCM1v2.3, contains one mode for each of temperature, moisture, and barotropic and baroclinic velocity modes. The QTCM simulations consist of both prescribed SST and coupled atmosphere–slab ocean model simulations. For the prescribed SST simulations, the SST forcing is obtained from the ensemble mean of the PMIP2 models. The prescribed Q flux is estimated from a 10-yr run with prescribed climatological SSTs (Reynolds dataset; Reynolds and Smith 1994)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
