North Pacific Gyre Oscillation (NPGO) Index in the CESM Large Ensemble

Co-Author: Riley Brady (riley.brady@colorado.edu)

Co-Author: Nikki Lovenduski

Co-Author: Emanuele Di Lorenzo

We have calculated the North Pacific Gyre Oscillation (NPGO) for Large Ensemble members 001—105 over the historical period (1920-2005) and under RCP8.5 forcing (2006-2100). A NetCDF file is available for each ensemble member for each period, containing a principal component time series (scaled by the square root of its eigenvalues), an EOF pattern expressing the correlation between North Pacific SSTa and the principal component time series, and a float representing the amount of SSTa variance explained by the NPGO mode.

NPGO website and observational data: http://www.oces.us/npgo
NCAR/CISL LENS NPGO data location: /glade/p/work/rbrady/NPGO/

Python Code: https://github.com/bradyrx/public_code/blob/master/NPGO_Index.py

Procedure:

Regrid SST to a 1x1 degree (360x180) global map for each ensemble member.
Deseasonalize and detrend SST to produce SSTa. This is done by removing the ensemble mean at each grid cell at each time step.
Subset the Northeast Pacific over 180-110W and 25-62N.
Compute the EOF of the JFM (winter) seasonal means, taking the second mode as the NPGO.
Project monthly SSTa back onto EOF2 to reconstruct a monthly time series of NPGO.

References:

Di Lorenzo, Emanuele, et al. "North Pacific Gyre Oscillation links ocean climate and ecosystem change." Geophysical Research Letters 35.8 (2008).
Di Lorenzo, Emanuele, and Nathan Mantua. "Multi-year persistence of the 2014/15 North Pacific marine heatwave." Nature Climate Change 6.11 (2016): 1042-1047.
Joh, Y., and E. Di Lorenzo. "Increasing coupling between NPGO and PDO leads to prolonged marine heatwaves in the Northeast Pacific." Geophysical Research Letters 44 (2017).