Cooperative Institute for Research in Environmental Sciences

Abstracts: 11

NOAA-CIRES Joint Institute Research on Multidecadal Climate Variability and Change

M. Hoerling, M. Alexander, A. Capotondi, H. Diaz, H.P. Huang, X. Quan, D. Sun, R. Webb, and K. Weickmann

On decadal to centennial time scales, joint institute research at CDC includes process and model simulation studies to elucidate the relation between changes in the atmosphere and those in the ocean. Joint institute scientists seek to provide physical and dynamical understanding of observed long-term climate variations and change through analysis of hierarchies of general circulation model (GCM) experiments. These include atmospheric models forced by sea-surface temperatures (SSTs), ocean models forced by wind stress, and coupled ocean-atmosphere GCMs, including runs forced by greenhouse gases. Key objectives include determining fundamental processes responsible for decadal climate variability and change, and assessing whether the latter are due to human influences or natural variability.

According to the Third Assessment Report of the IPCC, it is now very likely that global temperatures during the 1990s were the highest since 1861. The same appears to be true for tropical sea surface temperatures, and the extent of areal coverage of the so-called oceanic warm pool (SSTs > 28.5C). Joint institute scientists are diagnosing relationships between this tropical ocean warming, the global atmospheric circulation and recent climate change, and also are seeking to identify how slow changes in climate affect interannual variability. One key question is how warm pool changes over the equatorial west Pacific impact the statistics of El Niño/Southern Oscillation (ENSO) in the eastern Pacific. Another research focus is to assess whether the occurrence of strong El Niño events in recent decades is a signature of climate change or merely random fluctuations. The global impacts of ENSO have also changed in recent decades. A fundamental question being pursued is whether the ENSO teleconnections diagnosed from historical data of the 19th and 20th centuries will continue to be useful in depicting ENSO impacts in this unprecedented new era of human-induced climate change.

CDC joint institute scientists are also studying the origin and climatic impact of midlatitude ocean changes. Most dramatic among these is the multi-decadal variability in SSTs over the Pacific poleward of 30N, an index commonly know as the Pacific-Decadal Oscillation (PDO). The apparent long time scale of this oceanic behavior differs significantly from the ENSO time series. Nonetheless, CDC research indicates a strong relation between the two on interannual time scales, and an intriguing question is the extent to which they are coupled on multi-decadal scales. Likewise, the low frequency variations of North Pacific SSTs since 1950 have atmospheric counterparts, including changes in the oceanic storm tracks and the strength of the upper tropospheric jet. Joint institute scientists are also studying air-sea interactions over the North Pacific, and assessing to what extent diagnostically-identified relations imply predictability.