$.ajaxでjsonpの場合は同期設定にできない
http://api.jquery.com/jQuery.ajax/
By default, all requests are sent asynchronously (i.e. this is set to true by default). If you need synchronous requests, set this option to false. Cross-domain requests and dataType: "jsonp" requests do not support synchronous operation. Note that synchronous requests may temporarily lock the browser, disabling any actions while the request is active.
ちなみに$.getJSONには同期/非同期の設定がない。