Spelling variations in English
A variety of spelling variations for the word "nowruz" exist in English-language usage. Random House (unabridged) provides the spelling "nowruz". Merriam-Webster (2006) recognizes only the spelling "nauruz" (and a contestant in the final session of the 2006 Scripps National Spelling Bee, Allion Salvador, in the United States was disqualified on that basis. In the USA, many respected figures in the field of language such as Dr. Yarshater at Columbia University have suggested to use Nowruz.
Source: encyclopedia.thefreedictionary.com