Complex sentence: Toward the end of September 2001, a poorly defined tropical wave moved westward across the tropical Atlantic Ocean through an area of hostile wind shear, which was caused by a large upper-level low within a trough to the northeast of the Lesser Antilles.
Split into simpler sentences: Toward the end of September 2001, a poorly defined tropical wave moved westward across the tropical Atlantic Ocean through an area of hostile wind shear. This was caused by a large upper-level low within a trough to the northeast of the Lesser Antilles.