You pass variables into the closure scope with the use
keyword:
$new_title = "test seo title";
add_filter( 'wpseo_title', function( $arg ) use ( $new_title ) {
return $new_title;
});
The argument in function($arg) will be sent by the apply_filters() call, eg the other plugin, not by your code.
See also: Passing a parameter to filter and action functions