The West Antarctic Ice Sheet contains enough ice to raise global sea level by several metres and, because it is grounded mainly below sea level, it is sensitive to ocean warming. Accelerated thinning of glaciers that discharge into the Amundsen Sea over the past decades has been proposed to be related to the presence of warmer waters beneath the ice shelves. Three deep troughs crosscut the continental shelf of the Amundsen Sea, forming passages through which warm ocean waters can access the ice shelves, but oceanographic data has been limited. Here we present direct measurements from an ocean mooring and ship transect of the temperatures, salinities and velocities from one of these troughs in the central Amundsen Sea during the year 2010. The data show persistent inflow towards the ice shelf of relatively warm and salty water at the bottom of the trough throughout the year, and outflow of colder water above. Superposed on this background flow are barotropic current fluctuations that do not contribute significantly to the overall transport. In contrast to numerical models, which show seasonal inflow changes in response to regional winds, we find that warm water is supplied to the Central Amundsen Shelf without strong seasonal variability. © 2012 Macmillan Publishers Limited. All rights reserved.