Earth's axis of rotation about its center of mass is tilted relative to the axis of rotation of the earth about the sun. This means the angle of the sun's rays with respect to the horizon will change with a yearly cycle.
Summer occurs in the hemisphere for which the sun's rays are most directly overhead.
So summer in one hemisphere must correspond to winter in the other. Geopotential gradient is the sum of gravity and centrifugal force. This centrifugal force is zero at the poles, so gravity and geopential gradient are the same at the pole. This is the primary reason you weigh slightly more on the poles than on the equator. Sketch a curve that shows the undulations in the geoid that would occur over the ocean as one progresses over abbysal plains, then over a mid ocean ridge, then over more abbysal plains, and finally over a deep ocean trench.