The Climate Trio: stochastic climate variability, seasonal cycle, and El Niño

Seminar by Prof. Malte Stuecker from University of Hawaii

11 November 2022
KST 16:30 – 17:30

The Seminar is being held in Room 1010 (Jasmin) – Integrated mechanical engineering building. Click here for the campus map.

Climate variability has distinct spatial patterns with the strongest signal of sea surface temperature (SST) variance residing in the tropical Pacific. This interannual climate phenomenon, the El Niño-Southern Oscillation (ENSO), impacts weather patterns across the globe via atmospheric teleconnections. Pronounced SST variability, albeit of smaller amplitude, also exists in the other tropical basins as well as in the extratropical regions. To improve our physical understanding of internal climate variability across the global oceans, we here propose a conceptual model hierarchy that captures the essence of observed SST variability from near-annual to multi-decadal timescales. The building blocks consist of the classic stochastic climate model formulated by Hasselmann, a deterministic low-order model for ENSO variability, and the effect of the seasonal cycle on both of these models. This model hierarchy allows us to trace the impacts of seasonal processes on the statistics of observed and simulated climate variability. One of the important outcomes of ENSO’s interaction with the seasonal cycle is the generation of a frequency cascade leading to deterministic climate variability on a wide range of timescales, including the near-annual ENSO combination mode.

Using the aforementioned building blocks, we arrive at a succinct conceptual model that delineates ENSO’s ubiquitous climate impacts and allows us to revisit ENSO’s observed statistical relationships with other spatio-temporal coherent patterns of climate variability – so called modes of variability. We demonstrate the importance of correctly accounting for different seasonal phasing in the linear growth/damping rates of different climate phenomena, as well as the seasonal phasing of ENSO teleconnections and of atmospheric noise forcings. We discuss how previously some of ENSO’s relationships with other modes of variability have been misinterpreted due to non-intuitive seasonal cycle effects on both power spectra and lead/lag correlations. It is unveiled that ENSO’s impacts on climate variability outside the tropical Pacific are oftentimes larger than previously thought and that accurately accounting for them has important implications. For instance, it has been shown that improved seasonal prediction skill can be achieved in the Indian Ocean by fully considering ENSO’s seasonally modulated remote impacts. These results move us to refocus our attention to the tropical Pacific for understanding the global patterns of climate variability.

Finally, we briefly discuss our ongoing research using this framework to delineate the physical drivers of the global spatial pattern of future SST variance changes.