How can I change the function being used depending on whether value in an array is positive or not in R

I have an array (Tw) of this size: 2160 x 1080 x 12

Tw=array(-10:10, c(2160,1080,12))

I also have a temperature array (Ta)

Ta=array(-40:40, c(2160,1080,12))

I want to apply a slightly different function to Ta, depending on whether Tw>=0 or Tw <0.

t.es <- function(T) 6.107 * exp(17.38 * T/(239. + T))    # Tw >= 0

Or

t.es <- function(T) 6.107 * exp(22.44 * T/(272.4 + T))    # Tw <= 0

I tried using an if statement, but it doesn't appear to be working...it seems to just be applying one of the functions:

if(Tw>=0){
  t.es <- function(T) 6.107 * exp(17.38 * T/(239. + T))    # Tw >= 0 
  es <- t.es(Ta) 
}else{
  t.es <- function(T) 6.107 * exp(22.44 * T/(272.4 + T))    # Tw <= 0
  es<- t.es(Ta) 
}

How can I do this?