header.js

import Ember from 'ember';

export default Ember.Controller.extend({
  sessionService: Ember.inject.service(),
  
  currentSiteChanged: Ember.observer('session.current_site', function(){
    var current_site = this.get('session.current_site');
    console.log('currentSiteObserver', current_site);
    this.get('sessionService').set('currentSite', current_site);
  }),

});

index.js

import Ember from 'ember';

export default Ember.Controller.extend({
  sessionService: Ember.inject.service(),
  currentSite: Ember.computed.alias('sessionService.currentSite'),
  dashboardData: function(){
    var currentSite = this.get('currentSite');
    console.log("in dashboardData", currentSite);
    //other code that uses the currentSite
  }.property('currentSite'),
});

session-service.js

import Ember from 'ember';

export default Ember.Service.extend({
  currentSite: null,

  setCurrentSite: function(){
    var currentSite = this.get('session.current_site');
    this.set('currentSite', currentSite );
  }.on('init'),
});