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  STL[result] = STL[fRef] + ( 1STL[fPert] - w.box) + STL[fPertfRef] ).w.box

where STL[result] is the resultant SST values, w is the weight applied to the perturbation and "box" represents the mask created in the sst_modif.mv script (see below). The values of 'box' will be 1 in the defined rectangular mask, otherwise 0.

...

This will replace the SST values in the tropical band 20N-20S, for all longitudes, with the climatological SST from the ERA-Interim file. The change in SST will be merged to the reference value over a 'halo' of 5 degrees latitude. Note that if the weight was set to zero, this would effectively not make any change to the SST in this tropical band.

By changing the value of 'weight' the observed SST values can be changed to be a fraction

To replace the SST with climatology over a limited range of longitudes, say, just the central and east Pacific:

box=[10,-180,-10,-80]  #  N, W, S, E

Example 2: Increasing the SST anomaly in the Pacific

In this example, we don't want to replace the SST with the climatology, but increase the Pacific SST anomaly compared to the climatology by a factor of 2

...

.

This requires changes to the script as follow:

fInit="ICMCLob01INIT"
fRef="ICMCLob01INIT"
fPert="/perm/rd/openifs/oifs_workshop_2017/expts-inidata/erai-climate-sst/sst.ci.stl1.erai.climatology.grb"
fLsm="ICMGGob01INIT"
weight=-1				# !!! not 2 !!!
box=[10,-180,-10,-80]  #  N, W, S, E
halo=5

The value of -1 for weight means the modified values of SST, inside the box mask become (referring to the equation above):

STL[result] = STL[fRef] + ( STL[fPert] - STL[fRef] ).(-1)
            = STL[fRef] + ( STL[fRef] - STL[fPert] )

so that the SST anomaly with respect to the climatology is added to the existing SST field over the Pacific, effectively doubling the strength of the El Nino signal there. 

Running the script

To run the script, type the command:

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