Abstract

Abstract. Seasonal variability in near-surface air temperature and baroclinicity from the ECMWF ERA-Interim (ERAI) reanalysis and six coupled atmosphere–ocean general circulation models (AOGCMs) participating in the Coupled Model Intercomparison Project phase 3 and 5 (CMIP3 and CMIP5) are examined. In particular, the annual and semiannual cycles of hemispherically averaged fields are studied using spectral analysis. The aim is to assess the ability of coupled general circulation models to properly reproduce the observed amplitude and phase of these cycles, and investigate the relationship between near-surface temperature and baroclinicity (coherency and relative phase) in such frequency bands. The overall results of power spectra agree in displaying a statistically significant peak at the annual frequency in the zonally averaged fields of both hemispheres. The semiannual peak, instead, shows less power and in the NH seems to have a more regional character, as is observed in the North Pacific Ocean region. Results of bivariate analysis for such a region and Southern Hemisphere midlatitudes show some discrepancies between ERAI and model data, as well as among models, especially for the semiannual frequency. Specifically, (i) the coherency at the annual and semiannual frequency observed in the reanalysis data is well represented by models in both hemispheres, and (ii) at the annual frequency, estimates of the relative phase between near-surface temperature and baroclinicity are bounded between about ±15° around an average value of 220° (i.e., approximately 1-month phase shift), while at the semiannual frequency model phases show a wider dispersion in both hemispheres with larger errors in the estimates, denoting increased uncertainty and some disagreement among models. The most recent CMIP climate models (CMIP5) show several improvements when compared with CMIP3, but a degree of discrepancy still persists though masked by the large errors characterizing the semiannual frequency. These findings contribute to better characterizing the cyclic response of current global atmosphere–ocean models to the external (solar) forcing that is of interest for seasonal forecasts.

Highlights

  • The seasonal cycle of the heating of the atmosphere is one of the most prominent features of the Earth’s climate (e.g., Kiehl and Trenberth, 1997; Trenberth and Stepaniak, 2004)

  • The baroclinicity, geopotential height (GPH), and mean sea level pressure (MSLP) variance cycles have been reversed so that their maxima occur in summer as for the other fields

  • For Atmospheric Model Intercomparison Project (AMIP) runs (CanCM4, FGOALS-g2, GFDL-CM3, INMCM4, MIROC5, MPI-ESM-MR) we have considered the common time section 1979–2009, while for ERA-20CM the period is 1979–2011

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Summary

Introduction

The seasonal cycle of the heating of the atmosphere is one of the most prominent features of the Earth’s climate (e.g., Kiehl and Trenberth, 1997; Trenberth and Stepaniak, 2004). Observations show that at midlatitudes the annual harmonic is by far the largest component of the seasonal cycle, while other sub-harmonics capture only the finer structure of the cyclic variation. Closer to the equatorial regions, the seasonal cycle has a more complicated behavior and the annual and semiannual harmonics are both large components. There are observed differences in the seasonality of atmospheric temperature and eddy activity between the Northern and the Southern Hemisphere (hereafter NH and SH), with the NH exhibiting stronger seasonal variation due to the larger portion of land surface (Peixoto and Oort, 1992)

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