Evaluating CMIP5 simulations of mixed layer depth during summer

Abstract

The ability of CMIP5 models in simulating surface mixed layer depth (MLD) during summer is assessed using 45 climate models. Their ocean models differ greatly in terms of vertical mixing parameterizations and model configurations. In some models, effects of surface waves, Langmuir circulations, submesoscale eddies, as well as additional wind mixing are included to improve upper-ocean simulation. Similar to findings by previous studies, the summer MLDs are significantly underestimated in most of the models. Compared with the observation, only five of these models have deeper summer MLDs in the Southern Ocean, eight models have deeper summer MLDs in the central North Atlantic Ocean, and nine models have deeper summer MLDs in the central North Pacific Ocean. This underestimation of MLD is not caused by sea surface forcing, because most of the models tend to overestimate the surface wind stress, while they underestimate the net surface heat flux. Therefore, insufficient vertical mixing in the upper ocean may still be one of the potential reasons for this systematic underestimation of MLD in the climate models.

The carbon cycle in the Australian Community Climate and Earth System Simulator (ACCESS-ESM1) – Part 2: Historical simulations

“…However, in contrast to the winter, ACCESS-ESM1 appears to systematically underestimate MLDs in the high-latitude ocean in summer, 60 % (or 30-40 m) in the Southern Ocean, Pacific and Atlantic oceans. In the Southern Ocean, in particular, the underestimation of summer MLDs is consistent with Sallée et al (2013) and Huang et al (2014), who showed that most CMIP5 models underestimate summer MLDs. Huang et al (2014) attributed this to a lack of vertical mixing in CMIP5 rather than sea surface forcing related to individual models, this is consistent with Downes et al (2015), who showed that these biases are also present in the ocean-only simulations of ACCESS-ESM1.…”

North Atlantic deep water formation and AMOC in CMIP5 models

“…In particular, the parameterisation designed by Fox-Kemper et al (2011) to improve the mixed layer representation (present in the models marked with a black bullet point, Table 2) does not consistently result in better performances than those models without the parameterisation. This lack of relationship between MLD biases and vertical mixing parameterisation was already found by Huang et al (2014) for the summer MLD.…”

Modeling the interplay between sea ice formation and the oceanic mixed layer: Limitations of simple brine rejection parameterizations

“…Without BRP, the different criteria in the model agree within 10 m at the winter maximum, and overestimate the MIMOC value by 10 to 25 m. Summer minima, on the other hand, are constantly underestimated. This latter bias, common in current climate models and possibly caused by a poor or missing representation of mixing processes like surface waves and Langmuir circulations (Huang et al, 2014), is beyond the scope of the present paper. When the BRP is introduced, the gap widens between the MLDs obtained from the three criteria in the model.…”