Hello, I am a PhD student in physical oceanography.
I downloaded sea surface temperature (SST) data from the ERA5 reanalysis products, which has a 3-hour temporal resolution (i.e., 8 times per day).
Naturally, I expect to see different values for different times of day (e.g., 00, 03, 06, 09, etc.), but that’s not the case. For example, the value at 00:00 is 301 K, and at 15:00 it is also 301 K. Why is that?
Moreover, I checked other variables like relative humidity, mean sea level pressure, geopotential, etc. For all of these variables, I do observe different values across different times of day — except for sea surface temperature.
(Just to mention, I am using Python.)
Thank you.
Hi, From ERA5: data documentation
The Guidelines (4) say:
"Sea surface temperature and sea-ice cover (sea ice area fraction), see Table 2 above, are available at the usual times, eg hourly for the HRES, but their content is only updated once daily. "
Hope that helps
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The reason for this is the source of the SST data, see Section 2b in Sea Surface Temperature Intercomparison in the Framework of the Copernicus Climate Change Service (C3S) in: Journal of Climate Volume 34 Issue 13 (2021).
The ERA5 SST dataset is produced by ECMWF to be used for ERA5 atmospheric reanalysis (Hirahara et al. 2016). It consists of hourly global gap-free SST data at a 0.25° × 0.25° latitude–longitude grid covering the period from 1979 to the present. ERA5 SST data are based on the HadISST2 (Titchner and Rayner 2014) product from 1979 to August 2007, and the daily operational OSTIA (Donlon et al. 2012) product from September 2007 to present. The HadISST1 version 2 was developed by the U.K. Met Office Hadley Centre, and its “pentad” dataset consists of spatially complete, 5-day mean fields on a 0.25° spatial resolution grid. OSTIA is a high-resolution (0.05° × 0.05°) operational daily product developed by the U.K. Met Office and distributed through the Copernicus Marine Environment Monitoring Service (CMEMS). These two SST datasets are aggregated into one continuous data record and interpolated onto the ERA5 model grid (Dee et al. 2011) to be used as boundary conditions for ERA5 atmospheric reanalysis. There are two types of sea surface temperature in ERA5: sea surface skin temperature and sea surface temperature. In this study we have used monthly ERA5 sea surface temperature. ERA5 SST is calculated as the SST from an ocean model with increment as the difference between OSTIA SST and the ocean analysis. Since the input of SST comes from both OSTIA and HadISST2, the ERA5 SST is a mixture of SST in the absence of diurnal variation, “foundation SST” (OSTIA), and SST at indeterminate depth, “SSTdepth” (HadISST2), following the SST definitions in Minnett and Kaiser-Weiss (2012). Here we give the SST type as SSTdepth for ERA5 SST.
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