This function calculates the SST anomaly for every month of SST data and then writes that out to a csv file that will later be used to produce a SST anomaly figure. The approach used here is based on Reed et al. 2016, Nature Communications. Using the first 15 full years of data from the SST data set (2003-2017), an average SST value is generated for every month of the year. Then the average SST value for the appropriate month is subtracted from every value in the SST dataset.