Tropical Multidecadal and Interannual Climate Variability in the NCEP–NCAR Reanalysis

Abstract

The leading tropical multidecadal mode (TMM) and tropical interannual (ENSO) mode in the 52-yr (1949– 2000) NCEP–NCAR reanalysis are examined for the December–February (DJF) and June–August (JJA) seasons based on seasonal tropical convective rainfall variability and tropical surface (land + ocean) temperature variability. These combined modes are shown to capture 70%–80% of the unfiltered variance in seasonal 200-hPa velocity potential anomalies in the analysis region of 30°N–30°S. The TMM is the dominant mode overall, accounting for 50%–60% of the total unfiltered variance in both seasons, compared to the 22%–24% for ENSO. The robustness of the tropical multidecadal mode is addressed, and the results are shown to compare favorably with observed station data and published results of decadal climate variability in the key loading regions. The temporal and spatial characteristics of this mode are found to be distinct from ENSO. The TMM captures the global climate regimes observed during the 1950s–60s and 1980s–90s, and the 1970s transition between these regimes. It provides a global-scale perspective for many known aspects of this decadal climate variability (i.e., surface temperature, precipitation, and atmospheric circulation) and links them to coherent multidecadal variations in tropical convection and surface temperatures in four core regions: the West African monsoon region, the central tropical Pacific, the Amazon basin, and the tropical Indian Ocean. During JJA, two distinguishing features of the tropical multidecadal mode are its link to West African monsoon variability and the pronounced zonal wavenumber-1 structure of the 200-hPa streamfunction anomalies in the subtropics of both hemispheres. During DJF a distinguishing feature is its link between anomalous tropical convection and multidecadal variations in the North Atlantic Oscillation (NAO). For the linear combination of the TMM and ENSO the strongest regressed values of the wintertime NAO index are found when their principal component (PC) time series are out of phase. In the Tropics and subtropics the linearly combined signal for the TMM and ENSO is strongest when their PC time series are in phase and is weakest when they are out of phase. This result suggests a substantial modulation of the ENSO teleconnections by the background flow. It indicates stronger La Niña teleconnections during the 1950s–60s, compared to stronger El Niño teleconnections during the 1980s–90s. Although this study addresses the linear ENSO–TMM interference, the results also suggest that interactions between the two modes may help to explain the stronger El Niño episodes observed during the 1980s–90s compared to the 1950s–60s.