Complex sentence: The Subantarctic is a region in the Southern Hemisphere immediately north of the Antarctic and covering the many islands of the southern parts of the Indian Ocean, Atlantic Ocean and Pacific Ocean, which are north of the Antarctic Convergence.
Split into simpler sentences: The Subantarctic is a region in the southern hemisphere, located immediately north of the Antarctic region. The subantarctic region includes many islands in the southern parts of the Indian Ocean, Atlantic Ocean and Pacific Ocean, especially those situated north of the Antarctic Convergence.