Complex sentence: The wave moved westward across the Atlantic Ocean, and on entering the northwest Caribbean Sea interacted with other weather systems; the resultant system was declared a tropical depression on September 17 in the central Gulf of Mexico.
Split into simpler sentences: The wave moved westward across the Atlantic Ocean, and on entering the northwest Caribbean Sea interacted with other weather systems. The resultant system was declared a tropical depression on September 17 in the central Gulf of Mexico.