Ocean model utility dependence on horizontal resolution

• Main Author: Sonnewald, Maike
• Format: Thesis
• Language: English
• Published: 2016
• Subjects: Southern Ocean; Weller
• Online Access: https://eprints.soton.ac.uk/397412/; https://eprints.soton.ac.uk/397412/1/Sonnewald%252C%2520M_PhD_Thesis_June_2016.pdf

This thesis examines the change in ocean model utility with changing horizontal resolution. Oceans are a crucial part of the climate system, with numerical models offering important insights into our mechanistic understanding. We use a 30 year integration (1978 to 2007) of the NEMO model at 1º, 1/4º and 1/12º to investigate the impact of modelling choices associated with horizontal resolution changes. Changes in degrees of freedom associated with the increasing resolution allow alternative energy dissipation pathways, with potential impact on model accuracy. We develop a measure of utility based on an estimate of the accuracy, as well as a penalisation which scales with resolution. Overall, accuracy is thought to increase with resolution, and we examine the associated change in utility on a range of model fields. The exploration of the NEMO model assesses the surface mixed layer, deep (>2000m) to surface (<2000m) communication through the ocean interior and the changes in the meridional overturning with topographic interactions. Assessing these areas, we illustrate potential changes in the energy pathways in the system. We investigate the surface in terms of the mixed layer depth globally, but also investigating a case study in the Southern Ocean. We find that the mixed layer does not change significantly with resolution, and that NEMO compares well with observations. Minor changes with resolution are attributed to increased numbers of fronts with increasing resolution. When the mixed layer is assessed, we see no significant change with resolution, and so find that 1º has the highest utility. For our case study, we investigate the zonally asymmetric deepening of the mixed layer in the Southern Ocean. We find that the stratification set by the advection is key, and confirm this using the 1D Price-Weller-Pinkel model. The communication between the surface and the deep ocean is assessed by looking at the steric height variability, and specifically its covariance between the surface and the deep. We find that ...