Identify what is the central idea that the author develops throughout this
essay? what key points support that central idea?

Cause of he different elevations  hope this ?

Water flows along the east coast and across the western tropical pacific towards south america, while they both are directed towards that way,  el niño does not directly affect the southern flow of water along the coast. 

Tropical climate is experienced in the nothern countries of south america

Well, a and b together would be negative. b alone would also be negative.