The effects of climate change have been observed in many aspects of the Earth System, especially in the ocean. However, the response of the physical seasonal cycle to climate change in the North Sea and the northern (NE) Atlantic has not been well studied. We used a global sea surface temperature data set (HadISST) with a length of 140 years to describe the long-term variations in the targeted area. The seasonality of sea surface temperature (SST) was represented by the temperature difference between August and March. Nine locations were chosen to examine the differences between open ocean and shelf sea areas. Preliminary results show that in all regions, the seasonality has varied on a multi-decadal scale for about a hundred years after 1870. Then it increased rapidly in the period 1990-2010. The dramatic changes since late 1980s and early 1990s occurred when regime shifts were recorded in these areas. Open ocean regions display similar variations as the shelf sea areas, but with smaller amplitudes. The four seasons have distinct variation patterns, and winter pattern seem to be more related to NAO compared to other seasons. This is likely to be because wind plays a more dominant role in winter than in other seasons. Future work will be to examine the seasonal cycle of other physical parameters like salinity and mixed layer depth, to examine in detail the decadal variations in phytoplankton abundance and species composition, and ultimately to explain the influence of seasonally-varying physical processes on phytoplankton abundance.