Quasi-periodic, global oscillations in sea level pressure on intraseasonal timescales
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The sea level pressure (SLP) variability in 30–60 day intraseasonal timescales is investigated using 25 years of reanalysis data addressing two issues. The first concerns the non-zero zonal mean component of SLP near the equator and its meridional connections, and the second concerns the fast eastward propagation (EP) speed of SLP compared to that of zonal wind. It is shown that the entire globe resonates with high amplitude wave activity during some periods which may last for few to several months, followed by lull periods of varying duration. SLP variations in the tropical belt are highly coherent from 25°S to 25°N, uncorrelated with variations in mid latitudes and again significantly correlated but with opposite phase around 60°S and 65°N. Near the equator (8°S–8°N), the zonal mean contributes significantly to the total variance in SLP, and after its removal, SLP shows a dominant zonal wavenumber one structure having a periodicity of 40 days and EP speeds comparable to that of zonal winds in the Indian Ocean. SLP from many of the atmospheric and coupled general circulation models show similar behaviour in the meridional direction although their propagation characteristics in the tropical belt differ widely.
KeywordsSea level pressure (SLP) Intraseasonal oscillation Madden Julian oscillation (MJO) Tropical–extra-tropical connection Global oscillation
The NCEP reanalysis data and the interpolated OLR data are provided by NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, and accessed from the website http://www.cdc.noaa.gov/. We thank Matthews Wheeler for providing MJO Index and Brian Hoskin for some useful suggestions. We acknowledge the modeling groups listed in Table 1 for making their simulations available for analysis, the Program for Climate Model Diagnosis and Intercomparison (PCMDI) for collecting and archiving the AMIP and CMIP model outputs, and the WCRP’s Working Group on Coupled Modelling (WGCM) for organizing the model data analysis activity. We thank Ravi. S. Nanjundiah for providing the SFM data. We thank the anonymous referees whose comments have helped in improving the scientific content of the paper.
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