The following was a short exegetical essay for Dr. Richard E. Averbeck’s Hebrew Exegesis course at Trinity Evangelical Divinity School.
In 2:13d-f Joel provides the basis (note the causal כִּי) for his previous exhortation of repentance (vv.12-13) by citing the well-known confessional formula of Ex 34:6-7. Rather than finding hope in their present circumstances (disastrous and fickle), he sources confidence that YHWH may relent from this impending disaster in YHWH’s unchanging character (Crenshaw, 136). However, in citing Ex 34:6-7, Joel adds an additional participial phrase—וְנִחָ֖ם עַל־הָרָעָֽה, relenting from disaster (cf. רָעָה with this sense of calamity in Jer 1:14; Am 3:6) not found in the original formula.
Many redaction-critical scholars are agnostic towards the originality of this formula used 8 times in the OT (Ex 34:6-7; Num 14:18; Neh 9:17; Psalm 86:15; 103:8; 145:8; Joel 2:13; Jon 4:2) and alluded to once in Nah 1:3. For example, Dentan believes none of its occurrences are preexilic (36). Barton claims “that [it] is not original to any of the passages where it now occurs,” and that therefore, “there is no way of telling which of the other uses of the passage Joel is quoting. . . .” (25). However, this redaction-critical view can be discarded given the central role of the formula in the Ex 34 narrative (unlikely a later addition) and its reference as early as Num 14:18. Surely a Jewish scribe would have not been so presumptuous as to put words in the very mouth of YHWH (Ex 34:6-7)! Therefore, since Ex 34:6-7 is the original use of this formula, Joel’s וְנִחָ֖ם עַל־הָרָעָֽה is in fact an expansion, about which the significance needs investigation.
Out of the 7 citations and 1 allusion to Ex 34:6-7 in the OT, only Jon 4:2 contains the additional וְנִחָ֖ם עַל־הָרָעָֽה. This has led to theories of literary dependency. As Wolff states,
“That this agreement is not a matter of chance is shown by the further concurrence of v 14a with Jonah 3:9a (cf. Jon 3:10b), but especially by the relationship of thematic contexts: the announcement of the catastrophe awakens repentance in the hearer, and this repentance leads to God’s revoking what he had threatened. . . . The inescapable relationship in vocabulary and themes goes back to a common root” (49).
Crenshaw claims, “it is not clear who borrowed from whom” (137); and Garrett suggests, “there is no unambiguous evidence about the direction of the borrowing, and we should hesitate about drawing conclusions from these parallels” (347). However, as Joel characteristically references outside sources, the (most likely) postexilic Joel is probably dependent on the preexilic Jonah at this point.
Therefore, the significance of the additional וְנִחָ֖ם עַל־הָרָעָֽה, just as in Jonah 4:2 (cf. the literary dependency) is as follows: Joel contextualizes the theological richness of the ancient formula for his contemporary context—if they repent, YHWH himself may repent (note the human-divine שׁוב connection in vv.12, 13 and 14). The addition ofוְנִחָ֖ם עַל־הָרָעָֽה builds upon the confessional formula. Certainly a God of mercy, grace, hesed, and patience may forgive Joel’s contemporary audience by relenting of this disaster. Further, Joel’s expanded edition of this formula is grounded in YHWH’s previous actions which demonstrate this character (contrary to Barton , Allen  and Dentan  who understand this formula as a expression of YHWH’s attributes in a mere abstract sense). For example, in Ex 34, YHWH proclaims Himself as a God of second chances directly after giving Israel a second chance. And as Moses intercedes on behalf of the people in Num 14, he cites Ex 34:6-7 claiming, “as you have promised” and “just as you have forgiven this people, from Egypt until now”; he establishes his petition in the previous action of God. In a similar way, Joel adds Jonah’s interpretive וְנִחָ֖ם עַל־הָרָעָֽה to historic confession in order to apply the formula afresh in his contemporary exhortation of repentance.