Redirection problem after using ajax to POST and model save

Hi, I have a basic sign up form and my javascript is as follow:

$("#signUpButton").click(function() {
    var confirmation = $('#rform-confirmation').is(':checked');
    //console.log(confirmation);

    if(confirmation) {
        var firstname = $("#rform-firstname").val();
        var lastname = $('#rform-lastname').val();
        var email = $('#rform-email').val();
        var password = $('#rform-password').val();
        var rpassword = $('#rform-rpassword').val();
        var tokenKey = $('#rform-csrf').attr('name');
        var token = $('#rform-csrf').val();

        console.log(tokenKey);
        console.log(token);

        //debugger;

        $.ajax({
            url: '/register/doRegister',
            type: 'POST',
            data:{
                firstname: firstname,
                lastname: lastname,
                email: email,
                password: password,
                rpassword: rpassword,
                tokenKey: tokenKey,
                token: token
            },
            success: function(res) {
                console.log(res);
            }
        });
    } else {
        alert('Please agree to the Terms of Service.');
        return false;
    }
});

Now I have this in my controller:

$send = $user->addUser($email, $pwd, $fname, $lname);
if($send){
     $this->flash->success('You have registered successfully');
     $this->response->redirect('/signin');
}

The functionality works fine (ie. it saves into the model and everything else) but it just reloads the page and somehow I can see that in the url bar in the browser shows the information of the POST. The flash message will come after I physically clicked on another link. I tried doing different redirect path in $this->response->redirect('signin'); or $this->response->redirect($this->get->url('signin'); still does not work. Though if I use the same redirection in other function they work perfectly.