Abstract
During boreal summer, convective activity over the eastern Pacific (EPAC) inter-tropical convergence zone (ITCZ) exhibits vigorous intraseasonal variability (ISV). Previous observational studies identified two dominant ISV modes over the EPAC, i.e., a 40-day mode and a quasi-biweekly mode (QBM). The 40-day ISV mode is generally considered a local expression of the Madden-Julian Oscillation. However, in addition to the eastward propagation, northward propagation of the 40-day mode is also evident. The QBM mode bears a smaller spatial scale than the 40-day mode, and is largely characterized by northward propagation. While the ISV over the EPAC exerts significant influences on regional climate/weather systems, investigation of contemporary model capabilities in representing these ISV modes over the EPAC is limited. In this study, the model fidelity in representing these two dominant ISV modes over the EPAC is assessed by analyzing six atmospheric and three coupled general circulation models (GCMs), including one super-parameterized GCM (SPCAM) and one recently developed high-resolution GCM (GFDL HIRAM) with horizontal resolution of about 50 km. While it remains challenging for GCMs to faithfully represent these two ISV modes including their amplitude, evolution patterns, and periodicities, encouraging simulations are also noted. In general, SPCAM and HIRAM exhibit relatively superior skill in representing the two ISV modes over the EPAC. While the advantage of SPCAM is achieved through explicit representation of the cumulus process by the embedded 2-D cloud resolving models, the improved representation in HIRAM could be ascribed to the employment of a strongly entraining plume cumulus scheme, which inhibits the deep convection, and thus effectively enhances the stratiform rainfall. The sensitivity tests based on HIRAM also suggest that fine horizontal resolution could also be conducive to realistically capture the ISV over the EPAC, particularly for the QBM mode. Further analysis illustrates that the observed 40-day ISV mode over the EPAC is closely linked to the eastward propagating ISV signals from the Indian Ocean/Western Pacific, which is in agreement with the general impression that the 40-day ISV mode over the EPAC could be a local expression of the global Madden-Julian Oscillation (MJO). In contrast, the convective signals associated with the 40-day mode over the EPAC in most of the GCM simulations tend to originate between 150°E and 150°W, suggesting the 40-day ISV mode over the EPAC might be sustained without the forcing by the eastward propagating MJO. Further investigation is warranted towards improved understanding of the origin of the ISV over the EPAC.
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Notes
Note that the GEOS5 model used to produce the simulation analyzed here is essentially the same model used in the production of the Modern Era Retrospective-Analysis for Research and Applications (MERRA; Bosilovich et al. 2006), though is run here at coarser resolution.
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Acknowledgments
We thank anonymous reviewers for their critical comments on an earlier version of this manuscript. Thanks also to Terry Kubar for his comments and editorial assistance. The first author (XJ) acknowledges support by NOAA CPPA program under Award NA09OAR4310191 and NSF Climate and Large-Scale Dynamics under Award ATM-0934285. K. Sperber was supported under the auspices of the US Department of Energy Office of Science, Regional and Global Climate Modeling Program by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. DK was supported by NASA grant NNX09AK34G. We thank U.S. CLIVAR MJO Working Group for coordinating this model comparison activity and modeling centers for providing the model output. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
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Jiang, X., Waliser, D.E., Kim, D. et al. Simulation of the intraseasonal variability over the Eastern Pacific ITCZ in climate models. Clim Dyn 39, 617–636 (2012). https://doi.org/10.1007/s00382-011-1098-x
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DOI: https://doi.org/10.1007/s00382-011-1098-x