Dear Crystal

You answer is correct. But the simple sentence is different than what you wrote. It should be:-

Main independent clause : El Nino is a phenomenon.

The dependent clause: in which changes in the ocean and atmosphere combine to allow the warm water that is accumulated in the western Pacific to flow back to the east.

Note that "in which" is introducing another clause which by itself can stand alone but the whole clause including "in which" cannot stand alone. Read the line below :-

Changes in ocean and atmosphere combine to allow the water that is accumulated in the western Pacific to flow back to the east.


Above, "Changes" are the subject doing the action "combine" and the effect of action is taken by "water that is accumulated in the western Pacific". Note that "that is accumulated in the western Pacific" cannot be deleted as the modifier is an essential modifier and without which you will not know WHICH water the sentence is referring to.

Hope this makes it clear.

Thank you for your reply. I understand what you say but I think two comprehension both ok. Can you tell me why I am wrong? Thanks in advance!

EI Nino, the periodic abnormal warming of the sea surface off Peru, is a phenomenon in which changes in the ocean and atmosphere combine to allow the warm water that is accumulated in the western Pacific to flow back to the east.

Here, the entire clause in blue is an essential modifier serving to describe not El Nino but a phenomenon.
Since this clause is an ESSENTIAL modifier, it cannot be removed without altering the meaning of the sentence.
We can, however, remove the non-essential modifier in red.
The result is the core meaning of the sentence:
El Nino is a phenomenon in which changes in the ocean and atmosphere combine to allow the warm water that is accumulated in the western Pacific to flow back to the east.

The simple sentence structure is "EI Nino is a phenomenon to allow the warm water to flow back to the east". Am I correct?

In the OA, to allow does not refer to a phenomenon.
Rather, to allow is an ADVERB serving to modify combine, expressing WHY changes in the ocean and atmosphere COMBINE.
Question: WHY do changes in the ocean and atmosphere combine?
Answer: They combine TO ALLOW the warm water that is accumulated in the western Pacific to flow back to the east.