Notebook 7 - Climate Indeces

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• Reconstruct well-known climate indices (ENSO, NAO etc.)
• Identify teleconnections by using MCA on Pacific SST and continental temperature

ENSO

NAO

Reference: https://climate.copernicus.eu/esotc/2022/about-data#ERA5

A daily time series of the North Atlantic Oscillation (NAO) Index is used in the report. In step 1, Empirical Orthogonal Function (EOF) analysis is applied to daily area-weighted 500 hPa geopotential height (Z500) anomalies from the ERA-Interim reanalysis for the Euro-Atlantic region (30°N to 88.5°N, 80°W to 40°E) for the period 1980–2008. Only data for October–April are used for this part of the analysis. The NAO pattern is defined as the first mode obtained from this EOF analysis. In step 2, a daily time series of the NAO index is then generated by projecting the daily Z500 anomalies from ERA5 onto the NAO pattern defined in step 1. The time series is then standardised by its standard deviation during 1991–2020. Note that the daily anomalies used in step 2 are calculated with respect to the daily climatology for the 1991–2020 period. Further details can be found in this ECMWF Newsletter article.

Skandinavian Blocking Index

The calculation of the daily Scandinavian Blocking (SB) Index used in the report is based on an EOF analysis similar to one described above for the NAO Index. However, the leading 20 EOF patterns are retained (instead of only the first EOF for the NAO Index). A clustering analysis of these EOFs is then performed, which attributes each analysis time to a specific cluster in the EOF phase space and allows the computation of the pattern of the cluster mean anomaly in physical space. Further detail and results from this clustering analysis can be found in this ECMWF Newsletter article.

AO index

The method used to derive the Arctic Oscillation (AO) Index used in the ESOTC 2022 report is different from the method used for the ESOCT 2020 and ESOTC 2021 reports. For the ESOTC 2022, a daily AO index is derived from daily area-weighted anomalies of ERA5 1000 hPa geopotential height north of 65°N. The anomalies are relative to the 1991–2020 reference period. The resulting time series is then standardized by its standard deviation during 1991–2020.