Temporal and spatial variations of the bias and standard deviation of the Advanced Microwave Scanning Radiometer - EOS  (AMSR-E) Level-2 (L2) sea surface temperature (SST) fields relative to L2  MODerate-resolution Imaging Spectroradiometer (MODIS) SST  fields have been determined for a long term co-located data sets. MODIS SST  observations were co-located at full AMSR-E  grid resolution and analyzed as a function of space and time. After pre-processing of the  individual satellite data, the highest quality L2 MODIS SST s at 1 km resolution were averaged to the  56 km AMSR-E  footprint for comparisonwith co-located rain-free AMSR-E SST s. Daytime and nighttime  fields were analyzed separately. The best agreement occurred for nighttime fields. Biases found in  the daytime fields are thought to result from diurnal warming caused by solar heating. Under the  clear-sky conditions, themean biaseswere found to be 0.03 C and 0.05 C and standard deviations to  be 0.43 C and 0.48 C for nighttime and daytime, respectively. Of particular interest is the significant  increase both in the bias and the standard deviation as cloud fraction in the MODIS  fields increased. This it thought to result from cloud contaminated MODIS  pixels being misclassified as clear – this analysis was performed using only the ‘best’ MODIS  values; i.e., pixels not meeting this criterion  were not used in the calculations. However, it is also possible that the AMSR-E  retrieval algorithm introduces these errors as part of the retrieval process.