WHOI PO

Title: The Fresh Blob

Abstract: It has been claimed that the North Atlantic Warming Hole (NAWH) or “Cold Blob” reflects a decrease in ocean heat transport via the Atlantic Meridional Overturning Circulation (AMOC). The associated SST anomaly has been used in dynamical systems models to predict the (imminent) demise of AMOC. Others have suggested that the NAWH is a purely atmospheric effect, stemming from changes in atmospheric cooling due to a shift in the Atlantic storm track. We examine the NAWH in a coupled simulation of the Norwegian Earth System Model. The cold anomaly coincides with a freshwater anomaly, which spreads into the subpolar gyre from the Labrador and Nordic Seas. This reduces the mixed layer depth, enhancing heat loss to the atmosphere. Simultaneously, turbulent eddy fluxes in the mixed layer act to reduce lateral density gradients by injecting cold water to the region. The latter is aided by atmospheric cooling. Thus heat fluxes to the atmosphere are essential to the formation, but the blob is inherently an advective feature and is not possible in a slab ocean. Furthermore, both the cold blob and AMOC are affected by the same freshwater anomaly. So though the blob does not stem directly from reduced ocean heat transport, it does weaken in concert with AMOC.