adverb, noun which may take the genitive case particle 'no'
on the surface; ostensibly; superficially
Describes how something appears outwardly, often implying that the reality may be different. Used both as an adverb and as a no-adjective.
表面上は仲が良さそうだが、実はあまり話さない。
On the surface they seem to get along, but actually they don't talk much.
彼の説明は表面上は筋が通っている。
His explanation seems logical on the surface.
Compound of 表面 (ひょうめん, 'surface') and the suffix 上 (じょう, 'on, in terms of').