Annual and semiannual cycles of midlatitude near-surface temperature and tropospheric baroclinicity: reanalysis data and AOGCM simulations

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.

Northern Hemisphere Extratropical Turbulent Heat Fluxes in ASRv2 and Global Reanalyses

“…10d), where lower amplitudes in ASRv2 are related to fluctuations of the LE in response to daily variability of weather patterns. Better agreement among reanalyses is found in midlatitudes over the North Atlantic open water, where the atmospheric flow exhibits a much weaker intraseasonal variability compared to the North Pacific (Yashayaev and Zveryaev 2001;Lembo et al 2017).…”

The role of semiannual cycle in modulating seasonality changes of surface air temperature over China and its mechanism

“…The direction of anomalous meridional wind eld and anomalous temperature eld both control the strength of the semiannual cycle. It is well known that temperature gradient can generate transient eddies through baroclinic instability and baroclinic activity also has a semiannual cycle (Lembo et al, 2017). Therefore, the baroclinic semiannual activity can preferentially enhance the eddy activities in winter and in turn intensify the temperature semiannual cycle as feedback.…”

“…However, the larger contribution is from the wave-zonal ow interaction with accelerations of the westerly ow by Kelvin waves and the easterly ow by planetary Rossby waves (Hirota, 1980). In the midlatitude and polar regions, the semiannual cycle is mainly associated with the activity of the transient eddy (Lembo et al, 2017). Besides, there are observed differences in the semiannual cycle between the Northern and Southern Hemisphere due to their different portion of land surface and the semiannual cycle is a coupled ocean-atmosphere phenomenon in mid to high latitudes (van Loon, 1967;Meehl, 1991).…”