How would you describe the mood of the poem "Dover Beach"?

Mood is defined as the emotional atmosphere an author creates in order to make the reader feel a certain way in response to the text.
In Matthew Arnold’s “Dover Beach,” the speaker creates a somber mood via diction, imagery, and simile.
Examples of diction that stand out include “grating roar,” “eternal,” “misery,” “melancholy,” “struggle,” and “ignorant.” Each of these in context has a strongly negative connotation, which indicates that the speaker is not happily describing the beauty of the beach at night. Instead, the landscape scene inspires the speaker to ponder the great mysteries of life.
The strongest image that contributes to the somber mood of the poem comes in the second stanza, where the speaker describes the pebbles of the shore:

At their return, up the high strand,Begin, and cease, and then again begin,With tremulous cadence slow, and bringThe eternal note of sadness in.

This image suggests that the pebbles are thrown to and fro, helpless in the roughness of the surf. The sound the waves make as they fling these pebbles about reminds the speaker of the infinite hopelessness of life, which throws people into situations in which they are powerless.
Finally, the ending simile helps to cement the somber mood. The speaker compares human existence “as on a darkling plain / Swept with confused alarms with struggle and flight, / Where ignorant armies clash by night.” This comparison suggests that people go through life blindly, trying to fight their way through insurmountable obstacles in order to find meaning.
Ultimately, Arnold’s lyric poem is a meditation on the futile struggle of the human experience, and he communicates this in creating a somber, stoic mood.