An interdecadal American rainfall mode

Abstract

Low‐frequency climate variability across the American continents and surrounding oceans is analyzed by application of singular value decomposition (SVD) to gauge‐based rainfall and environmental anomaly fields in the period 1901–2002. A 5‐year filter is used to maintain a focus on interdecadal cycles. The rainfall regime of particular interest (mode 1) is when West Africa and the Caribbean share positive loading and North and South America share negative loading. Wavelet cospectral energy is found at ∼8, 24, and 50 years for Caribbean/West African zones and 16 and 32 years for North/South America. West Africa and South America exhibit antiphase multidecadal variability, while North America and the Caribbean rainfall exhibit quasi‐decadal cycles. The rainfall associations are nonstationary. In the early 1900s, Caribbean and South American rainfall were antiphase. Since 1930 low‐frequency oscillations of North American (West African) rainfall have been positively (negatively) associated with South America. Low‐frequency oscillations of North American rainfall have been consistently antiphase with respect to Caribbean rainfall; however, West Africa rainfall fluctuations have been in phase with the Caribbean more in the period 1920–1950 than at other times. Hemispheric‐scale environmental SVD patterns and scores were compared with the leading rainfall modes. The north‐south gradient modes in temperature are influential in respect of mode 1 rainfall, while east‐west gradients relate to mode 2 (northern Brazil) rainfall. The ability of the GFDL2.1 coupled (ocean‐atmosphere) general circulation model to represent interdecadal rainfall modes in the 20th century was evaluated. While mode 2 is reproduced, mode 1 remains elusive.