And he uses tone.

It seems to me that the tone that Hemingway sets in the novel reflects the mood that he was in when he wrote it. I don't know enough about Hemingway to decide his mood, but based on the novel, I'm going to say that he felt detached. Hemingway, throughout the novel, sets the tone of detached through characters and setting. The characters such as Jake, Mike, Bill, and Wilson-Harris all are war veterans who feel detached after the war. They speak and act as if, since the war, they are not normal people in society. I think Hemingway must have felt detached from some part of his life too.
In the final chapter, on page 241, Hemingway puts Jake in a detached area to further the tone. "Off on the right, almost closing the harbor, was a green hill with a castle. The raft rocked with the motion of the water. On the other side of the narrow gap that led into the open sea was another high headland. I thought I would like to swim across the bay but I was afraid of cramp." This final setting to the book really sets a tone of detached, because there is something that Jake wants to do, but because he is so far away, a fear keeps him from accomplishing that task.
Hemingway, by setting this tone, is able to elicit a certain emotional reaction of pity from the reader. The reader feels the same detachment that the characters do, causing them to empathize with them.