The warmer the ocean surface, the shalowwer the mixed layer. How much of this thrue?

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Slide 1

© Raquel Somavilla Cabrillo (www.fromtheblueside.com) The warmer the ocean surface, the shallower the mixed layer. How much of this is true?

Slide 2

Very often, we see pictures like the one below reflecting the fact that surface temperatures are increasing. Ocean surface warming © Raquel Somavilla Cabrillo (www.fromtheblueside.com)

Slide 3

Satellite based SST (sea surface temperature) time-series show warming trends at different locations, some of them coinciding with the position of long term oceanographic time-series like the ones at , and the Santander station at mid-latitudes in the eastern North Atlantic (+0.15°/decade). the Bermuda station (BATS) in the subtropical North Atlantic (+0.18°/decade) , the Hawaii station (HOTS) in the subtropical North Pacific (+0.11°/decade) Ocean surface warming © Raquel Somavilla Cabrillo (www.fromtheblueside.com)

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Ocean surface warming Ocean surface warming is commonly associated with a more stratified ocean, less productive and oxygenated. Such assertion is mainly made based on its consistency with increased near-surface stratification and shallower mixed layers under global warming scenarios. However, what do observations show nowadays? As we have seen above, the ocean surface is getting warmer. Do we find shallower mixed layers and enhanced stratification where surface temperatures are increasing? © Raquel Somavilla Cabrillo (www.fromtheblueside.com)

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Upper ocean vertical structure There is not a standard criterion to define the extension of the mixed layer (MLD), the stratification of the upper layer and, in general, the upper ocean vertical structure. Most usually, the MLD is determined setting a threshold value either for temperature (ΔT) or for density. The depth at which the difference between the variable and that of the surface exceeds the threshold value is taken as the MLD (e.g: MLDΔT=0.2ºC) . In our study, we use a different approach, and each profile (grey line) is fitted to an ‘ideal’ functional form (red line)1. Through this approximation, apart from the MLD, other relevant parameters such as the stratification (main pycnocline gradient) and the distance between the MLD and the main pycnocline are obtained. MLDΔT=0.2ºC MLDfitted profile 1. Gonzalez-Pola et al., 2007. doi: 10.1016/j.dsr.2007.08.007 © Raquel Somavilla Cabrillo (www.fromtheblueside.com)

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Upper ocean vertical structure Here you see more examples, with the actual profiles of the Santander standard section in grey, its best fit to the ‘ideal’ functional form in red, and the MLDs obtained represented by the horizontal black lines. So, when you applied the algorithm to a long term hydrographic time-series …. © Raquel Somavilla Cabrillo (www.fromtheblueside.com)

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Upper ocean vertical structure … you obtain long time-series of the MLD, stratification, etc., and from them you can derive e.g. its nice seasonal cycle but also study the long term trends. © Raquel Somavilla Cabrillo (www.fromtheblueside.com)

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Shallower MLD? Here, you see the MLD time-series at BATS oceanographic station in the North Atlantic subtropical Gyre where the ocean surface is warming at a rate of 0.18°C/decade MLD(m) It seems to occur while the MLD is deepening at a rate of 10 m/decade. Linear trend in observed MLD Linear trend in estimated MLD from atmospheric forcing © Raquel Somavilla Cabrillo (www.fromtheblueside.com)

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Shallower MLD? That was just an example, but I can tell you that ocean surface warming is found concurrent with deepening MLDs also at others locations. Thus, the analysis of long term hydrographic time-series at mid-latitudes does not support the simplified relationship of ´the warmer the ocean surface, the shallower the mixed layer’, and further investigation is needed. I will be working on that. © Raquel Somavilla Cabrillo (www.fromtheblueside.com)

Slide 10

Many thanks for your attention © Raquel Somavilla Cabrillo (www.fromtheblueside.com)

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