Sea surface temperature

The sea surface temperature (SST), representative of an area of 1 km², is determined from the remote sensing of the surface IR radiation and the relevant model calculations. The intensity of IR radiation emitted by the sea surface can be recorded by satellite radiometers, but only for areas that are not overcast. The accuracy of temperatures determined for cloud-free areas on the basis of remote sensing data is 0.5°C. For overcast areas the satellite information is supplemented with the results of calculations using the marine hydrodynamic model. This model is made ever more precise with the use of current data from satellite measurements during the process of measurement data assimilation. As a result of such assimilation, SSTs calculated using the model do not differ significantly from temperatures measured in situ just below the sea surface. The estimated absolute error of these temperatures is ca 1°C. The SO SatBałtyk service gives SST distributions in the Baltic in degrees Celsius [°C], four times a day, in the form of maps with a 1 km resolution.

The methodology of determining sea surface temperature In the SatBałtyk system SST is calculated by pooling the spatial distributions of this parameter remotely sensed by various satellites (NOAA, METOP, MODIS) with those calculated using the prognostic hydrodynamic models PM3D and 3D CEMBS. The algorithm for pooling data from these two sources assumes that:

1. in regions where no satellite data are available (e.g. because they are overcast), SST is determined using the model;
2. in regions distant from overcast areas, SST is recorded by a satellite;
3. in regions intermediate between regions 1 and 2, the mean weighted SST is calculated  from both satellite and model values. The weight coefficients are calculated in such a way that as an overcast area is approached, the model data are given a greater weight than the remote sensing data.

Source data on SST have a spatial resolution of 1 km (NOAA, METOP, MODIS satellites), 0.9 km (PM3D model) and 2 km (3D CEMBS model). In the SO SatBałtyk system the resolution of all maps has been unified to 1 km by spatial interpolation. Validation (assessment of accuracy)

The accuracy of SST maps is estimated from analyses of the differences between in situ SSTs and those obtained by pooling remote sensing and hydrodynamic model data. The statistical error, expressed as the standard deviation of these differences, is estimated at 0.73 °C. The systematic error (the mean difference) is -0.03 °C. For more information, see: Validation of sea surface temperature.

Citation Using the data from this collection for scientific purposes, please cite the work containing a detailed description of the method and the evaluation of the accuracy of the products: Konik M., Kowalewski M., Bradtke K., Darecki M., 2019, The operational method of filling information gaps in satellite imagery using numerical models, International Journal of Applied Earth Observations and Geoinformation, 75, 68-82, doi: 10.1016/j.jag.2018.09.002 For more information see „Rules for the use and dissemination of data” in Service Regulation

Interesting phenomena visible on temperature maps

Coastal upwelling is a current raising deep water up to the surface; such events occur in some regions of the Baltic [1]. Satellite maps of the southern shores of the Baltic often show areas with cooler surface waters, the effect of upwelling. This occurs when water from deeper layers, usually cooler, are brought to the surface under the influence of a current generated by winds blowing parallel to the coastline. In winter the temperature of upwelled waters may be higher than at the surface, so the temperature maps will show this as a warmer area.

A thermal front is a transition zone separating water masses of different temperatures; the temperature gradient across such a front is therefore steep. Thermal fronts in the Baltic are variable in size and stable over time. They usually form at the boundaries between basins of different depths (all-year or seasonal), in areas subject to upwelling (short-term but with a temporally stable position), within a hydrological front, restricting the spread of waters flowing in from rivers (short-term, with frequent changes in position frequently), around eddies etc. [2] The waters on either side of the front often differ not only in temperature but also with respect to other physicochemical features, such as salinity, transparency and other optical properties.

Mesoscale eddies are disturbances in the current field resulting from anomalies of temperature, salinity and sea level on spatial scales from 10 to 100 km and durations from a few days to a month. Local structures with dimensions smaller than 10 km are defined as sub-mesoscale eddies. Eddies of various sizes in the Baltic are often seen on satellite radiometric images (e.g. in distributions of SST, chlorophyll a levels etc.) and on radar images, as sea surface disturbances [3-4].

References [1] Kowalewski M., Ostrowski M., 2005, Coastal up- and downwelling in the southern Baltic, Oceanologia, 47 (4), 453–475 [2] Kahru M., Håkansson B., Rud O., 1995, Distributions of the sea-surface temperature fronts in the Baltic Sea as derived from satellite imagery, Continental Shelf Research, 15, 663–679 [3] Gurova E., Chubarenko B., 2012, Remote-sensing observations of coastal sub-mesoscale eddies in the south-eastern Baltic,  Oceanologia, 54 94), 631-654 [4] Karimova S.S., Lavrova O.Yu., Solov’ev D.M., 2012, Observation of eddy structures in the Baltic Sea with the use of radiolocation and radiometric satellite data, Izvestiya, Atmospheric and Oceanic Physics, 48, 1006–1013

Links to the parameter in SatBaltic System:

Sea surface temperature (AVHRR, PM3D) Sea surface temperature (model 3D CEMBS) Sea surface temperature (PM3D) (for registered users)