Cook elbow macaroni in lightly salted water until al dente, drain, rinse with cold water, and cool to room temperature.
In a large mixing bowl combine the cooled pasta, diced celery, finely chopped onion, chopped roasted red peppers, sliced black olives, rinsed capers, and sliced green onions; stir to mix.
Make the sauce by whisking together mayonnaise, pickle relish, Dijon mustard, sugar, white vinegar, salt, and black pepper in a medium bowl until smooth.
Pour the dressing over the pasta mixture and stir until the pasta and vegetables are thoroughly coated.
Taste and adjust seasoning with additional salt and pepper if needed.
Transfer to a non-reactive container, cover, and chill for at least 2 hours before serving.
