As the roots of the Climate Explorer are in ENSO teleconnections, the following monthly time series are always available to correlate when the time scale is monthly. For other time scales only time is available at the moment.

• NINO1+2, NINO3, NINO3.4, NINO4 (relative to 20S-20N, i.e., without global warming trend): relative SST anomaly indices from ERSST v5 1854-now. See van Oldenborgh et al 2021 for more details on the relative index. Niño indices based on HadSST can be selected via monthly indices. NINO1+2 is the SST anomaly over 10°S-EQ, 80°-90°W; NINO3 over 5°S-5°N, 90°-150°W; NINO4 covers 5°S-5°N, 160°E-150°W. The NINO3.4 index is between NINO3 and NINO4 at 5°S-5°W, 120°-170°W.
  NINO1+2 is an indicator for coastal El Niño, which influences the coasts of Ecuador and northern Peru. NINO3 is an indicator of variability in the cold tongue, whereas NINO4 sits on the edge of the warm pool. NINO3.4 combines both ENSO mechanisms and often is the best overall indicator of the strength of the basin El Niño or La Niña. Note that these series are not very homogeneous. As SST observations have become much better over time the variability has increased, particularly since the 1970s.
  Note that you can use the original Niño indices rather than the system-defined relative Niño indices by selecting them from the "Monthly climate indices" page and generating a user-defined time series.
• SOI: the Southern Oscillation Index from CRU 1866-2006 is an atmospheric indicator for the strength of El Niño or La Niña. It is defined as the normalised pressure difference between Darwin and Tahiti. As these pressures are also influenced by more local weather it contains more noise than the SST-based indices. However, it is also more homogeneous: the observation method has not changed much since the middle of the 19th century. Unfortunately CRU does not seem to update the series any more. The CPC pattern-based SOI runs over 1948-now and can be selected under monthly indices.
• NAO: the North Atlantic Oscillation index from CRU, based on the pressure difference between Iceland and Gibraltar. The latter point is not in the center of action, but has a much longer time series. This time series is updated irregularly by CRU. The CPC pattern-based NAO index 1948-now can be selected under monthly indices.
• CO2: annual mean CO2 concentration. The observed part is the global marine series from ESRL extended with Mauna Loa for the last few months before taking the annual average. The historical part is the one specified in the CMIP5 forcing.
• smoothed GMST: the GISTEMP land/sea global mean temperature smoothed with a four-year running mean. The "trends in extremes" page always uses the annual mean version, even if it is selected under "monthly".
• Time: centered on 1-jan-2000. Very useful to do trend analyses; the regression against time is the trend.

You can also correlate with a user-defined time serie, which is generated by selecting it. If it is of type "index" it will atomatically be added to the list, otherwise add it to the list explicitly.