"El Niño brings surface heat from the west, and stratifies the ocean layers"

• arrange or classify

  "stratifying patients into well-defined risk groups"

• place (seeds) close together in layers in moist sand or peat to preserve them or to help them germinate

  "rose seeds stratified in November may be sown outdoors"

• (of seeds) be germinated by stratifying

  "if you want to sow your own mistletoe, collect ripe berries and store them in moist sand to stratify"