add_filter('woocommerce_get_catalog_ordering_args', 'dvk_woocommerce_catalog_orderby', 99);
function dvk_woocommerce_catalog_orderby( $args ) {
    $args['meta_key'] = '_sku';
    $args['orderby'] = 'meta_value_num';
    $args['order'] = 'dsc';
    return $args;
}