Abstract
Interannual oscillatory modes, atmospheric and oceanic, are present in several large regions of the globe. We examine here low-frequency variability (LFV) over the entire globe in the Community Earth System Model (CESM) and in the NCEP-NCAR and ECMWF ERA5 reanalyses. Multichannel singular spectrum analysis (MSSA) is applied to these three datasets. In the fully coupled CESM1.1 model, with its resolution of 0.1 times 0.1 degrees in the ocean and 0.25 times 0.25 degrees in the atmosphere, the fields analyzed are surface temperatures, sea level pressures and the 200-hPa geopotential. The simulation is 100-year long and the last 66 yr are used in the analysis. The two statistically significant periodicities in this IPCC-class model are 11 and 3.4 year. In the NCEP-NCAR reanalysis, the fields of sea level pressure and of 200-hPa geopotential are analyzed at the available resolution of 2.5 times 2.5 degrees over the 68-years interval 1949–2016. Oscillations with periods of 12 and 3.6 years are found to be statistically significant in this dataset. In the ECMWF ERA5 reanalysis, the 200-hPa geopotential field was analyzed at its resolution of 0.25 times 0.25 degrees over the 71-years interval 1950–2020. Oscillations with periods of 10 and 3.6 years are found to be statistically significant in this third dataset. The spatio-temporal patterns of the oscillations in the three datasets are quite similar. The spatial pattern of these global oscillations over the North Pacific and North Atlantic resemble the Pacific Decadal Oscillation and the LFV found in the Gulf Stream region and Labrador Sea, respectively. We speculate that such regional oscillations are synchronized over the globe, thus yielding the global oscillatory modes found herein, and discuss the potential role of the 11-year solar-irradiance cycle in this synchronization. The robustness of the two global modes, with their 10–12 and 3.4–3.6 years periodicities, also suggests potential contributions to predictability at 1–3 years horizons.
Highlights
Introduction and motivationInterannual and interdecadal oscillations with a regional extent have been found in several areas of the Earth’s oceans and atmosphere
The present results suggest that these near-decadal oscillatory modes in the North Atlantic basin (NAB) are, like the Pacific Decadal Oscillation (PDO) in the North Pacific, a manifestation of the 11-years global mode in the Atlantic sector
We studied in this paper the interannual variability in a high-end coupled ocean–atmosphere model, the CESM1.1 model (Hurrell et al 2013; Small et al 2014), and in the NCEP-National Center for Atmospheric Research (NCAR) (Kalnay et al 1996; Kistler et al 2001) and ERA5 (Hersbach et al 2020) reanalyses
Summary
Introduction and motivationInterannual and interdecadal oscillations with a regional extent have been found in several areas of the Earth’s oceans and atmosphere. The extent to which these oscillations are due to purely oceanic (Dijkstra and Ghil 2005; Berloff et al 2007, and references therein) purely atmospheric (e.g., Frankignoul and Hasselmann 1977; Frankignoul et al 1997) or coupled (Vannitsem et al 2015, and references therein) processes is still a matter of controversy Over and near the Pacific Ocean basin, the Pacific Decadal Oscillation (PDO) was extensively studied; see, for instance, the Newman et al (2016) review. These authors affirm that, in all likelihood, the PDO is not due to a single mechanism but rather to the combination of several different basin-scale processes. The spatial patterns corresponding to these processes are not identical and their characteristic time scales are quite different, but they all project strongly onto the PDO pattern; see Chao et al (2000)
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