Good question.  From Zeng-Zhen Hu:

Hi Sam,
Here are the steps I used:
(a) Calculate the regressions of zonal wind stress anomalies onto the Niño3 index (1981-2010);
(b) Project observed wind stress in Nino3.4/Nino4 regions onto the regression pattern;
(c) Compared the observed SSTA and projected SSTA  in Nino3.4/Nino4 regions to check the coupling strength.

Here is the reference:
Lloyd, J., E. Guilyardi, H. Weller, and J. Slingo, 2009: The role of atmosphere feedbacks during ENSO in the CMIP3 models. Atmos. Sci. Lett., 10: 170–176.