POSEIDON & Climate monitoring: oceanic extremes

POSEIDON & Climate monitoring: oceanic extremes

Date
23/11/2022

Written by D. Denaxa and the POSEIDON scientific team
 

Deeper understanding of the ocean climate, including anthropogenic climate change and natural variability, is becoming more and more imperative. Monitoring the sea surface temperature –being a major climate change indicator– is of fundamental importance especially in highly vulnerable areas to climate change, such as the Mediterranean Sea [6, 11].  


The Mediterranean Sea has been experiencing unprecedented surface warming during the past decades. In fact, it has largely exceeded the observed global sea surface warming trends [1, 15, 16]. Extreme oceanic events such as marine heatwaves (discrete warm ocean events lasting at least five consecutive days) have attracted great interest within the climate change community [2, 7, 8, 9, 12]. The POSEIDON team has also been studying the temporal evolution of these episodes over the past decades in the Mediterranean Sea. Results reveal a significant increase in their frequency of occurrence, their mean intensity (mean temperature difference with respect to climatology) and duration (Figure 1), in all Mediterranean sub-basins. The eastern Mediterranean Sea in particular presents the highest positive trends of occurrence frequency and event duration. To better illustrate this, less than one event per year would occur in the basin during the 70s, while during the last decade approximately five marine heatwaves per year are detected, each one lasting of about four times longer. Several studies based on observational and modelled temperature datasets point out the eastern Mediterranean basin as the area exhibiting the strongest surface warming [13, 15].


Availability of high quality oceanographic and meteorological observational data allows for properly reporting and analysing such events. In turn, such studies may provide significant baseline-knowledge for impact assessment. For instance, temperature recordings derived from two POSEIDON buoy stations moored in the southern Aegean (Ε1-Μ3Α and Ηeraklion Coastal Buoy (HCB)) have captured record-breaking sea surface temperature anomalies in mid May 2020 [3]. This specific event evolved in the eastern Mediterranean as a marine heatwave of category Extreme (Figure 2a,b). Atmospheric parameters recorded by these buoy stations as well as buoy measurements of temperature at several depths were additionally used to investigate potential driving mechanisms and the sub-surface conditions during the event, respectively. Temperature profiles from Argo floats recording in the eastern Mediterranean at that time provided further insight into the upper ocean stratification conditions before the onset, throughout the event-duration and after its decline (Figure 2c).
 

Marine heatwaves posing a high impact on marine life is already a fact. Massive geographical shifts, impacts on primary productivity, mass mortality events and several other direct and indirect threats for the marine ecosystems have been attributed to thermal stress under a warming Mediterranean climate [4, 5, 10, 14, 17, 18]. On top of this, future projections suggest that anthropogenic forcing will further increase marine heatwaves by 2100 in the basin, especially under high-emission scenarios [19].


On these grounds, it is crucial to promote infrastructures and research studies aiming to enrich the existing knowledge and increase awareness on the emergency character of this topic. Most importantly, maintaining a high quality network of continuously monitoring instruments is crucial for the implementation of reliable scientific climate-related studies. 

Figure 1 Marine Heatwave (MHW) properties in the Mediterranean Sea for the period 1975-2020 based on ECMWF ERA5 sea surface temperature data: (a) occurrence frequency (b) mean intensity (c) mean duration. Different colours (blue, magenda, red, black) correspond to the western, central, eastern sub-basins and the entire Mediterranean basin, respectively. All linear trend values of MHW properties displayed in legends are statistically significant (95% confidence level). MHW identification methodology and definition of MHW properties follow Hobday et al., 2016 & Hobday et al., 2018.  

Figure 2 (a) Temperature measurements from POSEIDON buoy Ε1-Μ3Α and Copernicus satellite data (product SST_MED_SST_L4_NRT_OBSERVATIONS_010_004) at the platform location (35.7263°N - 25.1307°E) during May 2020. Blue marks stand for daily mean buoy SSTs at 1m and the associated error bars define the corresponding diurnal range. Orange marks represent mean daily water temperature at 20m from the same buoy. Green dots represent the satellite nighttime SST. Dashed purple line shows the mean daily wind speed evolution throughout the event. (b) Application of the Hobday et al., 2016 methodology for the May 2020 MHW detection at the same location as in (a), using historical and NRT Copernicus satellite SST data (c) Temperature profiles from Argo float 6902847. Different colors (blue, red, purple) represent different time periods relative to the MHW evolution (before, during and after, respectively). The path followed by the float within the days considered in this graph extends from 34.8271°N - 22.7087°E (May 6) to 34.4326°N - 22.954°E (May 26) (Figure published in Denaxa et al., 2022)


References

  1. Bulgin, C. E., Merchant, C. J., & Ferreira, D. (2020). Tendencies, variability and persistence of sea surface temperature anomalies. Scientific reports, 10(1), 1-13.
  2. Darmaraki, S., Somot, S., Sevault, F., & Nabat, P. (2019). Past variability of Mediterranean Sea marine heatwaves. Geophysical Research Letters, 46, 9813– 9823. https://doi.org/10.1029/2019GL082933
  3. Denaxa D., Korres G., Sotiropoulou M., Perivoliotis L. (2022) Extreme Marine Heatwave in the eastern Mediterranean in May 2020. In: Copernicus Ocean State Report, Issue 6, Journal of Operational Oceanography, 15:sup1, s119–s126; DOI: 10.1080/1755876X.2022.2095169
  4. Frölicher, T.L., Fischer, E.M. & Gruber, N. Marine heatwaves under global warming. Nature 560, 360–364 (2018). https://doi.org/10.1038/s41586-018-0383-9
  5. Garrabou J, Coma R, Bensoussan N, Bally M, Chevaldonné P, Cigli- ano M, Diaz D, Harmelin JG, Gambi M, Kersting D (2009) Mass mortality in Northwestern Mediterranean rocky benthic com- munities: effects of the 2003 heat wave. Global Change Biol 15(5):1090–1103
  6. Giorgi F (2006) Climate change hot-spots. Geophysical research let- ters 33(8)
  7. Ibrahim, O., Mohamed, B., & Nagy, H. (2021). Spatial variability and trends of marine heat waves in the eastern mediterranean sea over 39 years. Journal of Marine Science and Engineering, 9(6), 643.
  8. Holbrook, N. J., Scannell, H. A., Sen Gupta, A., Benthuysen, J. A., Feng, M., Oliver, E. C., ... & Wernberg, T. (2019). A global assessment of marine heatwaves and their drivers. Nature Communications, 10(1), 1-13.
  9. Juza M, Fernández-Mora À and Tintoré J (2022) Sub-Regional Marine Heat Waves in the Mediterranean Sea From Observations: Long-Term Surface Changes, Sub-Surface and Coastal Responses. Front. Mar. Sci. 9:785771. doi: 10.3389/fmars.2022.785771
  10. Leach, T. S., BuyanUrt, B., & Hofmann, G. E. (2021). Exploring impacts of marine heatwaves: paternal heat exposure diminishes fertilization success in the purple sea urchin (Strongylocentrotus purpuratus). Marine Biology, 168(7), 1-15.
  11. Lionello, P., Malanotte-Rizzoli, P., & Boscolo, R. (Eds.). (2006). Mediterranean climate variability. Elsevier.
  12. Oliver, E. C., Donat, M. G., Burrows, M. T., Moore, P. J., Smale, D. A., Alexander, L. V., ... & Wernberg, T. (2018). Longer and more frequent marine heatwaves over the past century. Nature communications, 9(1), 1-12.
  13. Pastor F, Valiente JA, Khodayar S. A Warming Mediterranean: 38 Years of Increasing Sea Surface Temperature. Remote Sensing. 2020; 12(17):2687. https://doi.org/10.3390/rs12172687
  14. Pearce, A. F., & Feng, M. (2013). The rise and fall of the “marine heat wave” off Western Australia during the summer of 2010/2011. Journal of Marine Systems, 111, 139-156.
  15. Pisano A, Marullo S, Artale V, Falcini F, Yang C, Leonelli FE, Santoleri R, Buongiorno Nardelli B. New Evidence of Mediterranean Climate Change and Variability from Sea Surface Temperature Observations. Remote Sensing. 2020; 12(1):132. https://doi.org/10.3390/rs12010132
  16. Shaltout, M.; Omstedt, A. Recent sea surface temperature trends and future scenarios for the Mediterranean Sea. Oceanologia 2014, 56, 411–443
  17. Smale, D. A., Wernberg, T., Oliver, E. C. J., Thomsen, M., Harvey, B. P., Straub, S. C., Burrows, M. T., Alexander, L. V., Benthuysen, J. A., Donat, M. G., Feng, M., Hobday, A. J., Holbrook, N. J., Perkins-Kirkpatrick, S. E., Scannell, H. A., Gupta, A. S., Payne, B. L., & Moore, P. J. (2019). Marine heatwaves threaten global biodiversity and the provision of ecosystem services. Nature Climate Change. https://doi.org/10.1038/s41558-019-0412-1
  18. Wernberg T, Bennett S, Babcock RC, de Bettignies T, Cure K, Depc- zynski M, Dufois F, Fromont J, Fulton CJ, Hovey RK et al. (2016) Climate-driven regime shift of a temperate marine ecosystem. Science 353(6295):169–172
  19. Oliver, E. C., Burrows, M. T., Donat, M. G., Sen Gupta, A., Alexander, L. V., Perkins-Kirkpatrick, S. E., ... & Smale, D. A. (2019). Projected marine heatwaves in the 21st century and the potential for ecological impact. Frontiers in Marine Science, 6, 734.