Hi-- we've blogged before on alternative measures (which we monitor regularly).  Regarding this event, the SOI is very negative right now (both versions, even the station based one).  I honestly do not see any issues with the tropical response at this point and we are seeing what we need to see.  Keep in mind the atmosphere will always be noisy (e.g. jump around from month to month) b/c it is susceptible to influence from MJO and other tropical 'weather.'   This is why we prioritize the ocean based indices, which are more resilient to these influences.