Students of haiku traditionally write these poems by using a season word (kigo in Japanese) and a cutting word (kireji in Japanese), and also often employ objective imagery based on the five senses (thus avoiding excesses of commentary, judgment, and interpretation). The word “kana” is actually a cutting word, and is one of many in Japanese haiku (in An Introduction to Haiku [New York: Doubleday, 1958], Harold G. Henderson lists at least four, and elsewhere there are lists of as many as eighteen). It’s a meaningless syllable, sort of like spoken punctuation, that, in this case, conveys a sense of surprise or delight.
Robert Hass’s description of “kana” (or perhaps he meant kireji in general) as an intensification is helpful, but it is only part of what cutting words are. Consider Bashō’s most famous poem:
mizu no oto
a frog leaps into
the water’s sound
In the first line, “ya” is a cutting word, a grammatical intensifier, and it too expresses wonder and contemplation (“kana” isn’t the only cutting word that does that). But cutting words are actually functioning with a larger purpose in haiku than to intensify (whether intensifying surprise or something else). Rather, a cutting word divides the haiku into two parts, like a caesura (hence the literal meaning of kireji as “cutting word”). There isn’t really an equivalent to cutting words in English, but leading haiku poets (such as many poets in Cor van den Heuvel’s The Haiku Anthology [third edition, Norton, 1999]) create an equivalent by presenting two grammatically disjointed fragments in their haiku, with the cut between them sometimes indicated by a dash or other punctuation (or an indent). The point is to create a juxtaposition between two parts of the poem, with the relationship between the two parts creating something larger than the sum of the parts—letting readers “leap” between the two parts to figure out the relationship between them, to figure out something that’s deliberately been left out. That’s really the point of cutting words, and “kana” is simply one example of many that help to convey a tone of delight.
Thus there is much more to “kana” than is hinted at in this article [Chris Dombrowski’s “Kana”], and “kana” is merely one example of many cutting words used in Japanese. It may impart some degree of tone, but it is not really equivalent to an aesthetic ideal (as this article seems to suggest) along the lines of wabi, sabi, yugen, makoto, karumi, or other Japanese aesthetic terms. Still, haiku itself is a delightful poetry of wonder and appreciation, which can happen whether a haiku uses “kana” or not.
Note: In An Introduction to Haiku (New York: Doubleday, 1958), Harold G. Henderson says of “kana” that it is “A kireji usually used at the end of a haiku. It has an effect somewhat like that of an exclamation mark or a preceding ‘Ah!’ or ‘Oh!’ As normal Japanese sentences end with a verb, kana may be considered as in a sense substituting for it, and so also have some of the effect of a series of dots [ellipsis]” (187–188). Also, “kana” still functions as a cutting word even when it appears at the end of a haiku, serving to “cut” between the poem (thus in one part rather than two) and implied content that comes after or outside the poem.