Sea temperature
About 90% of the heat accumulated on the planet has been stored mainly in the ocean in recent decades, meaning that the heat content of the ocean is a good indicator of warming. Sea temperature is an indicator of the heat content of the ocean, measured in degrees Celsius (°C).
- Both the sea surface temperature of the Bay of Biscay and the sea temperature at different depths are increasing.
- This warming of the sea temperature is leading to the 14°C isotherm moving deeper.
Relationship of the indicator to climate change
On a global scale, the top 75 metres of the water column experienced a temperature warming of 0.09-0.13°C per decade over the period 1971-2010 associated with global warming (IPCC-AR5 2013). More specifically, the entire water column in the Bay of Biscay has warmed by approximately 0.2°C per decade between 1990 and 2009, linked to warming of the atmosphere (Bode et al. 2012). On the Basque coast, the surface sea has been warming since 1980 (Goikoetxea et al. 2009, Chust et al. 2022).
Trend analyses have shown a gradual change associated with climate change from the 1980s onwards, with sea surface warming up to a depth of 100 m in the bay (0.10-0.25°C per decade) (Chust et al. 2022).
Figure 1. Time series for the sea surface temperature (SST) in the Bay of Biscay between 1981 and 2021 from satellites with OSTIA sensors.
Analyses of the time series for the sea surface temperature in the Bay of Biscay show a significant increasing trend with a rate of 0.191°C ± 0.036°C per decade (p<0.0001) (Figure 1) between 1981 and 2021.
Figure 2. Time series for the depth of the 14°C isotherm at station D2 between 1986 and 2021.
The time series analysis of sea temperature in the column indicates that the temperature at different depths (0-100 m) has a significant positive trend on average (p=0.0001) at a rate of 0.137°C ± 0.036°C per decade between 1986 and 2021 (Figure 2).
Figure 3. Time series for the depth of the 14°C isotherm at station D2 between 1993 and 2020.
The trend for the depth of the 14 °C isotherm indicates an increase in the depth of the isotherm (i.e. the isotherm is now deeper) by 4.71 m with a standard deviation of 1.93 m per decade (p=0.0154) between 1993 and 2020 (Figure 3).
In conclusion, the time series analysis shows that the sea surface temperature of the Bay of Biscay and the sea temperature at different depths off the Basque coast are increasing. This warming of the sea in the area is a result of climate change and is leading to the depth of the 14°C isotherm also increasing.
The trend in sea temperature can be measured using a number of different indicators:
- Sea surface temperature by means of satellite sensors for the Bay of Biscay
- Temperature in the water column from the surface to a depth 100 metres on the continental shelf off the coast of Pasaia.
- Depth of the 14 °C isotherm
In order to analyse the trend of the sea surface temperature, data are downloaded from the Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) which provides daily data on the sea surface temperature and has a time series dating back to 1981. The source of the data is the Copernicus Climate Change Service (C3S)
Temperature data have been collected in the water column from the surface to a depth of 100 metres by measuring conductivity, temperature and depth (CTD) on a monthly basis since 1986 in order to analyse the trends for the temperature in the water column and the depth of the 14 °C isotherm. The station is located on the continental shelf off Pasaia at a depth of 110 m and is called D2.
The trend analysis for sea surface temperature is carried out using generalised additive models (GAMs) (Simpson, 2018). These models are methods adapted for irregular data that make it possible to analyse time series. In order to analyse the temperature in the water column and the depth of the 14 °C isotherm, which are multivariate cases as the variable is measured at different depths, the general trend of the variable was estimated by taking into account each depth as a random effect and the year and seasonality as fixed effects, using mixed-effects models (Zuur et al., 2009).
Bode, A., A. Lavín, and L. Valdés. 2012. Cambio climático y oceanográfico en el Atlántico del norte de España. Instituto Español de Oceanografía, Madrid (Spain).
Norte de España. Instituto Español de Oceanografía, Madrid (Spain). Chust, G., M. González, A. Fontán, M. Revilla, P. Alvarez, M. Santos, U. Cotano, M. Chifflet, A. Borja, I. Muxika, Y. Sagarminaga, A. Caballero, I. de Santiago, I. Epelde, P. Liria, L. Ibaibarriaga, R. Garnier, J. Franco, E. Villarino, X. Irigoien, J. A. Fernandes-Salvador, A. Uriarte, X. Esteban, D. Orue-Echevarria, T. Figueira, and A. Uriarte. 2022. Climate regime shifts and biodiversity redistribution in the Bay of Biscay. Science of the Total Environment 803:149622.
Goikoetxea, N., Á. Borja, A. Fontán, M. González, and V. Valencia. 2009. Trends and anomalies in sea-surface temperature, observed over the last 60 years, within the southeastern Bay of Biscay. Continental Shelf Research, 100 Years of Research within the Bay of Biscay - XI International Symposium on Oceanography of the Bay of Biscay 29:1060-1069.
IPCC-AR5. 2013. The Physical Science Basis. Working Group I. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
Simpson, G.L. 2018. Modelling Palaeoecological Time Series Using Generalised Additive Models. Frontiers in Ecology and Evolution, 6.
Zuur, A.F., Ieno, E.N. and Walker, N.J. (2009) Mixed effects models and extensions in ecology with R. New York: Springer Science.
Chust, G., M. González, A. Fontán, M. Revilla, P. Alvarez, M. Santos, U. Cotano, M. Chifflet, A. Borja, I. Muxika, Y. Sagarminaga, A. Caballero, I. de Santiago, I. Epelde, P. Liria, L. Ibaibarriaga, R. Garnier, J. Franco, E. Villarino, X. Irigoien, J. A. Fernandes-Salvador, A. Uriarte, X. Esteban, D. Orue-Echevarria, T. Figueira, and A. Uriarte. 2022. Climate regime shifts and biodiversity redistribution in the Bay of Biscay. Science of the Total Environment 803:149622.
Globally (1)
+0.2ºC
Per decade
Europe (1)
+0.2ºC -+0.3ºC
Per decade
Bay of Biscay (2)
+0.1ºC -+0.25ºC
Per decade
14°C isotherm deepening
(1) Copernicus