Analysis of the impact of vertical variation and temporal frequency of the chlorophyll forcing field on modelled temperature in the Mediterranean Sea and potential implications for regional climate projections

Abstract

The urgency of climate change calls for the exploration of a variety of multi-forcing scenarios based on Shared Socio-economic Pathways. Ensuring the reliability of the climate projections is therefore an imperative prerequisite. In this paper, we examined the impact of the vertical variability and temporal frequency of the chlorophyll field used to force the NEMOMED12 ocean circulation model in the absence of a biogeochemical model on some key physical characteristics, mainly seawater temperature. Our analysis reveals that forcing by a chlorophyll field that is homogeneous in the vertical direction favours heat accumulation below the Deep Chlorophyll Maximum, leading to a positive temperature bias increasing with time. The extrapolation of the trend determined over the 11-year simulations leads to a bias in temperature as high as +1 °C after 100 years in the intermediate layer. Comparison with in situ data clearly shows that forcing the model with a realistically varying Chl field over the vertical allows the model to better represent temperature and avoid the presence of this bias. Additionally, we find that using the same chlorophyll field saved at different time frequencies, namely daily, monthly and monthly climatology, to force the NEMOMED12 model also creates temperature differences between simulations that increase with time, especially in the intermediate layer. The simulation forced by the daily chlorophyll is warmer in the surface layers than the two others, and we suggest that this is due to the asymmetric role of chlorophyll extremes on heat distribution. Finally, using a monthly chlorophyll climatology to force the NEMOMED12 ocean circulation model seems to be sufficient for physical modelling of the Mediterranean basin if the vertical variability of the Chl field is realistic.