A water mass found at intermediate depths in the arctic domain in the North Atlantic Ocean. It is identified by a temperature minimum at a depth of about 75 to 150 m as well as temperature and salinity maximums at depths ranging from about 250 to 400 m, with the extremes being the product of winter cooling and sinking in the arctic domain. It is useful to separate this water mass into lower and upper AIW.

The lower AIW contains the temperature and salinity maximums but generally not the temperature minimum, with temperatures ranging from 0 to 3° C and salinities greater than 34. 9, with the maximums clear signs that this water mass is produced by the cooling and sinking of Atlantic Water (AW). The upper AIW is defined as including the denser portion of the water associated with the temperature minimum, including much of the water column from the minimum up to the temperature maximum. It is characterized by temperatures less than 2° C in the salinity range 34. 7 to 34. 9 (with a lower limit of 34. 6 suggested by some).

The definitions for upper and lower AIW deliberately overlap in density, with upper AIW in the Iceland Sea having a temperature of 0° C, S = 34. 88 and σt= 28. 03 as opposed to a portion of the lower AIW in the northern Greenland Sea having T = 3° C, S = 35. 05, and σt= 27. 95. This is only true of the northeastern Greenland Sea, however. Elsewhere, upper AIW always overlies lower AIW.