I have some code, from the help of this post that calculates monthly averages per year for a region of interest and exports it to a csv.

var UK_SST = ee.ImageCollection('NOAA/CDR/SST_WHOI/V2').select('sea_surface_temperature')
    .filterDate(ee.Date('1990-01-01'), ee.Date('2019-01-01'))

// Pick the scale from the first image
var scale = UK_SST.first().projection().nominalScale()

// month data for dataset

var startyear = 1990; 
var endyear = 2018; 

var startmonth = 1;
var endmonth = 12;

var startdate = ee.Date.fromYMD(startyear, startmonth, 1);
var enddate = ee.Date.fromYMD(endyear , endmonth, 1);

var years = ee.List.sequence(startyear, endyear);

var months = ee.List.sequence(1, 12);
var monthlySST =  ee.FeatureCollection(
  years.map(function (y) {
    return ee.FeatureCollection(months.map(function(m) {
      var w = UK_SST.filter(ee.Filter.calendarRange(y, y, 'year'))
            .filter(ee.Filter.calendarRange(m, m, 'month'))
      var dict = w.reduceRegion({
        reducer: ee.Reducer.mean(),
        geometry: ROIEdit,
        scale: scale,
        maxPixels: 1e13
      var sst_mean = ee.Algorithms.If(
      return ee.Feature(null, {
        'sst_mean': sst_mean,
         'year': y,
         'month': m,
         'system:time_start': ee.Date.fromYMD(y, m, 1)

collection: monthlySST,
description: 'monthlySST',
fileFormat: 'CSV'

The region of interest is broken up into two regions. I have written code to add the ID name as a property to the ROI feature collection.

// adding names to ROI
var FeatCollectAsAList = ROI.toList(ROI.size());
print(FeatCollectAsAList, 'ROI');

// map a function over the list to rename the IDs
var ROIEdit = ee.FeatureCollection(FeatCollectAsAList.map(function(feat){
  var id = ee.Feature(feat).get('id');
  return ee.Feature(feat).set("system:index", id);

I can't quite figure out how to add this to the export. I tried using the set function to add a new column as 'ROI' but it just came out empty.

 return ee.Feature(null, {
    'sst_mean': sst_mean,
     'year': y,
     'month': m,
     'system:time_start': ee.Date.fromYMD(y, m, 1),
     'ROI: RIEdit.get('id')

I could just export the two regions separately and join them together but I thought it might be simple enough to add the ROI as an extra column.

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