As a newbie to Spanish I get the impression that there is no feminine form of cariño (cariña); is that so as both adjective and noun? And if so, it seems strange to me though I suppose it's just one of those things you have to accept like 'la mano.' I suppose asking why that should be would be futile.
Here's a very good explanation of nouns in apposition.
Wow, I love Lazarus' explanations. An art form to be appreciated.
Let's tone it down for a newbie.
Lazurus explained that "cariñö" was not the adjectvie form of the word, but the masculine noun form. Hence the "o" ending. Why a noun and not an adjective form? This is a noun of aposition or whatever the proper grammatical term. It similar to saying "my friend Joe" , "his wife Karen" or "my beloved/love, (not lovely) Anita" It is a noun used as an adjective since it modifies another noun or you may think of it as a noun renaming another noun.
beloved was a poor choice, but interesting, because it can be a noun or adjective solely based on how you pronouce it.
Animals with sexual reproductive organs have sex, but in Spanish all nouns have gender, which can be masculine or feminine. A chair is feminine, but a sofa is masculine; this is entirely arbitrary, since they don't have sexual organs. "Cariño" is a noun, and as such, it has a fixed gender, which has nothing to do with its sexual organs.
The adjective for "cariño" is "cariñoso", and this one changes to "cariñosa" for women, and "cariñosos/-as" in plural. "Cariño" is always masculine.