Ocean Heat Storage

Ocean Heat Storage And Implications On Sea Level Rise Using CCSM4 Model Output For 1993 – 2016

Student : Maurice Huguenin

Supervisor: Prof. Dr. Reto Knutti, Dr. Iselin Medhaug

Ocean heat uptake and storage is a key component in regulating the global climate system and is leading to sea level rise by thermal expansion. This thesis presents an analysis based on the output from the general circulation model CCSM4 for the time period 1993 – 2016 and shows that ocean heat storage is highest in the Southern and North Atlantic Ocean. A total of 2.24×10^23 Joule have been accumulated during the last 24 years, 70% is stored in the upper 700 m. A comparison with observed data sets shows that the model slightly overestimates (underestimates) global 700 m (2000 m) heat storage for 1993 – 2015 (2005 – 2015). Modelled upper 700 m heat accumulation is dominated by the Southern Ocean’s latitude band +/-30 degrees South. A considerable warming trend in the ocean waters, high surface absorption of heat around Antarctica in connection with a consistent northward transport of water masses transfers the warming signal into depths of 800 – 1000 m below warmer subtropical water. Due to low initial temperature conditions of 2 – 3 degrees Celsius during heat uptake, thermosteric sea level in this region is minimal. On the contrary, modelled Atlantic Ocean heat storage infuences sea level rise trends in its basin by up to 5 mm/year or twice the amount of the observed total mean annual sea level rise for this time-period. High heat uptake and subsequent deepwater formation in high latitudes transports 0.66×10^23 Joule, or 40% of the global heat anomaly, into depths of 1500 – 2500 m although this basin only covers 17% of the ocean. The main components governing thermosteric sea level rise are identified as the initial temperature and salinity conditions during heat uptake, the magnitude of the ocean water’s warming trends and the strength of the model’s ocean circulation which transports the heat signal into the ocean’s interior.