How the ocean subsurface changes (0-1500m) during the recent warming slowdown and the impact of observation system change

Abstract:

The earth system experiences continuous heat input, but a “climate hiatus” of upper ocean waters has been observed in this century. This leads to a question: where is the extra heat going? Using four in situobservation datasets, we explore the ocean subsurface temperature changes from 2004 to 2013. The observations all show that the ocean has continued to gain heat in this century, which is indicative of anthropogenic global warming. However, a distinctive pattern of change in the interior ocean is observed. The sea surface (1-100 m) temperature has decreased in this century, accompanied by warming in the 101-300 m layer. This pattern is due to the changes in the frequency of El Niño and La Niña events (ENSO characteristics), according to both observations and CMIP5 model simulations. In addition, we show for the first time that the ocean subsurface within 301-700 m experienced a net cooling (or insignificant warming), indicative of another instance of variability in the natural ocean. The strong ocean cooling (freshening) in Northwest Pacific gyre region is mostly responsible for the 301-700m cooling (freshening). Furthermore, the ocean layer of 701-1500 m has experienced significant warming.

Besides, spurious OHC estimate can be obtained due to the change of the observation system from historical ship-based system to Argo-based system. The authors will show how the observation system change could impact the OHC calculation in Indian Ocean and Atlantic Ocean. For instance: a very quick but spurious warming rate could be obtained from late 1990s to early 2000s in Indian Ocean. This is a crucial issue during the interpretation of the observation-based results.