。。。is a phenonmenon in which changes in the ocean and atmosphere combine to allow the warm water that has accumulated in the western Pacific to flow back to the east.
...is a phenomenon
in which changes in the ocean and atmosphere combine to allow the warm water that has accumulated in the western Pacific to flow back to the east...
深色部分in which...为定语从句，其中changes为主语，combine是谓语动词， to allow...表目的的状语从句修饰combine； that has accumulated in the western Pacific 定语从句修饰the warm water, to flow...作为宾补出现， allow sth to do.
这里 combine 和 to 不是一个词组了，应该分开，combine 是前面从句中的内容，相当于 changes in the ocean and atmosphere combine in the phenonmenon ；而 to 是从句带的目的状语。
这样看这个句子：
。。。is a phenonmenon in which changes in the ocean and atmosphere combine to allow the warm water that has accumulated in the western Pacific to flow back to the east.
