Precursor definitions:
Warm Water Volume (WWV) is defined as an average of D20 anomaly across 
the equatorial Pacific (120E – 80W, 5S-5N) (Meinen and McPhaden 2000). 

Central tropical Pacific (CTP) index is calculated as the averaged D20
anomaly in the central tropical Pacific (160W-110W, 10S-10N) (Wen et al. 2014) 

Univariate Regression prediction models are construced using leave-one-year-out 
cross valiation over the full period by iteratively recomputing the coefficents 
with the target prediction year removed (Wen et al. 2021).

References:
Meinen, C. S., and M. J. McPhaden, 2000: Observations of warm water volume 
changes in the equatorial Pacific and their relationship to El Niño and La Niña.
J. Climate, 13, 3551–3559, https://doi.org/10.1175/1520-0442(2000)013<3551:OOWWVC>2.0.CO;2.

Wen, C., A. Kumar, Y. Xue, and M. J. McPhaden, 2014: Changes in tropical Pacific
thermocline depth and their relationship to ENSO after 1999. J. Climate, 27, 7230–7249,
https://doi.org/10.1175/JCLI-D-13-00518.1.

Wen, C., Kumar, A., L’Heureux, M., Xue, Y. and Becker, E., 2021: The Importance of Central
Pacific Meridional Heat Advection to the Development of ENSO. Journal of Climate, 34(14), 
pp.5753-5774. https://doi.org/10.1175/JCLI-D-20-0648.1.