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.
On the surface they seem to get along, but actually they don't talk much.
His explanation seems logical on the surface.
表面上 describes how things appear on the surface, while 実際に describes how things truly are in practice.
Compound of 表面 (ひょうめん, 'surface') and the suffix 上 (じょう, 'on, in terms of').