Last month was the warmest November since records began in 1884.
The mean temperature was 23.1 degrees Celsius, up almost two degrees from the average 21.4 for the month.
The cold northeastern monsoon, which usually dominates southern China in November, was weak last month and gave way to a warm southerly airstream for several days.
Although El Nino did not have a direct effect, Observatory meteorologists said the phenomenon was affecting global atmospheric conditions and Hong Kong was no exception.
'I won't say El Nino had a direct effect on the November mean temperature, but overall, the global phenomenon is affecting Hong Kong in several important respects,' scientific officer Chan Sai-tick said.
He said unusual features included the low number of typhoons in the South China Sea this year, down from an average of 12 to seven, and the increase in rainfall from an average of 2,214 millimetres to more than 3,300 mm this year.