Predictability of weather and climate

Abstract:

The representation of midlatitude winter atmospheric synoptic variability in centennial reanalysis products, which assimilate surface observations only, and atmospheric model simulations constrained by observation-based data sets is assessed. Midlatitude waves activity in twentieth century reanalyses (20CR, ERA-20C) and atmospheric model simulations are compared with those estimated from observationally complete reanalysis products. All reanalyses are in good agreement regarding the representation of the synoptic variability during the last decades of the twentieth century. This suggests that the assimilation of surface observations can generate high-quality extratropical upper air fields. In the first decades of the twentieth century a suppression of high-frequency variability is apparent in the centennial reanalysis products. This behavior does not have a counterpart in the atmospheric model integrations. Since the latter differ from one of the reanalysis products considered here (ERA-20C) only in the assimilation of surface observations, it seems reasonable to attribute the high-frequency variability suppression to the poor coverage of the observations assimilated.

Abstract:

The response of the period of the quasi-biennial oscillation (QBO) to increases in tropical upwelling are considered using a one-dimensional model. We find that the imposition of the annual cycle in tropical upwelling creates substantial variability in the period of the QBO. The annual cycle creates synchronisation regions in the wave forcing space, within which the QBO period locks onto an integer multiple of the annual forcing period. Outside of these regions, the QBO period undergoes discrete jumps as it attempts to find a stable relationship with the oscillator forcing. The resulting set of QBO periods can be either discrete or broad-banded, depending on the intrinsic period of the QBO.

We use the same model to study the evolution of the QBO period as the strength of tropical upwelling increases as would be expected in a warmer climate. The QBO period lengthens and migrates closer towards 36 and 48 month locking regions as upwelling increases. The QBO period does not vary continuously with increased upwelling, however, but instead transitions through a series of 2- and 3-cycles before becoming locked to the annual cycle. Finally, some observational evidence for the cyclical behaviour of the QBO periods in the real atmosphere is presented.

Abstract:

In the 2013/14 winter, the eastern USA was exceptionally cold, the Bering Strait region was exceptionally warm, California was in the midst of drought and the UK suffered severe flooding. It has been suggested that elevated SSTs in the tropical West Pacific (TWPAC) were partly to blame due to their producing a Rossby wavetrain that propagated into the extratropics. We find that seasonal forecasts with the tropical atmosphere relaxed towards a reanalysis give 2013/14 winter-mean anomalies with strong similarities to those observed in the Northern Hemisphere, indicating that low-latitude anomalies had a role in the development of the extremes. Relaxing just the TWPAC produces a strong wavetrain over the North Pacific and North America in January, but not in the winter-mean. This suggests that anomalies in this region alone had a large influence, but cannot explain the extremes through the whole winter. We also examine the response to applying the observed TWPAC SST anomalies in two atmospheric general circulation models. We find that this does produce winter-mean anomalies in the North Pacific and North America resembling those observed, but that the tropical forcing of Rossby waves due to the applied SST anomalies appears stronger than that in reanalysis, except in January. Therefore both experiments indicate that the TWPAC influence was important, but the true strength of the TWPAC influence is uncertain. None of the experiments indicate a strong systematic impact of the TWPAC anomalies on Europe.

Abstract:

Characteristic timescales for the Northern Annular Mode (NAM) and Southern Annular Mode (SAM) variability are diagnosed in historical simulations submitted to the Coupled Model Intercomparison Project Phase 5 (CMIP5) and are compared to the European Centre for Medium-Range Weather Forecasts ERA-Interim data. These timescales are calculated from geopotential height anomaly spectra using a recently developed method, where spectra are divided into low-frequency (Lorentzian) and high-frequency (exponential) parts to account for stochastic and chaotic behaviors, respectively. As found for reanalysis data, model spectra at high frequencies are consistent with low-order chaotic behavior, in contrast to an AR1 process at low frequencies. This places the characterization of the annular mode timescales in a more dynamical rather than purely stochastic context. The characteristic high-frequency timescales for the NAM and SAM derived from the model spectra at high frequencies are ∼5 days, independent of season, which is consistent with the timescales of ERA-Interim. In the low-frequency domain, however, models are slightly biased toward too long timescales, but within the error bars, a finding which is consistent with previous studies of CMIP3 models. For the SAM, low-frequency timescales in November, December, January, and February are overestimated in the models compared to ERA-Interim. In some models, the overestimation in the SAM austral summer timescale is partly due to interannual variability, which can inflate these timescales by up to ∼40% in the models but only accounts for about 5% in the ERA-Interim reanalysis.