Just use
global sink
(global is just a special upvar
case)
variables that end with (...)
are (associative) arrays, using numbers as key is not recommended, it is better to use a list instead, e.g.
set mylist {}
# append some elements
lappend mylist "foo" "bar" "baz"
# get the 2nd element
puts [lindex $mylist 1]
# set the 3rd element to "Hello World"
lset mylist 2 "Hello World"