Edit in JSFiddle

var meses = ["Ene", "Feb", "Mar", "Abr", "May", "Jun", "Jul", "Ago", "Sep", "Oct", "Nov", "Dic"],
    fecha = $("input[name=fecha]");

$("#datepicker").datepicker({
    onSelect: function(date){
        var picker = date.split("/"),
            nuevo = picker[1] + "-" + meses[picker[0] - 1] + "-" + picker[2];
        
        fecha.val(nuevo);        
    }
});

<div id="datepicker"></div>
Fecha: <input type="text" name="fecha" readOnly>