Two sets of novel techniques are developed to study the atmospheric response to an SST anomaly. Firstly, a model surgery method (“Masking Technique”) is developed to be incorporated into a General Circulation Model (GCM), in which an interactive “mask” is used to systematically isolate and study the air-sea interaction over the warm and cold sectors of extratropical cyclones independently to each other. This method is applied to an idealised aquaplanet GCM (AFES) with a 150 km resolution forced with a fixed SST anomaly comparable to a tightening of the SST gradient over a Western Boundary Current. Results show that the atmospheric response via the cold sector (CSA) alone was able to reproduce the response when both cold and warm sectors interact with the SST anomaly.
The same experiment with an identical set-up but with increased resolution (50km grid size) was performed. An opposite result was produced where the warm sector response (WSA) was more successful in reproducing the full response, suggesting that Low Resolution (LR) models often used in climate simulations suppress the processes occurring in the warm sector. In both LR and HR experiments, the Pressure Adjustment Mechanism was observed in the CSA case and not in the WSA case. However, in the HR model, response in the WSA showed regions of high baroclinicity and Eddy Kinetic Energy as well as strong upper-level response comparable to the full response, highlighting the role of the warm sector in restoring the baroclinicity in the storm track regions, as well as its ability to perturb the upper troposphere. This is in contrast to the cold sector response in which interaction was largely confined to shallow layers of the atmosphere.

Secondly, a new frontal tracking method is developed that identifies atmospheric fronts objectively from gridded datasets. This diagnostic is applied to the aquaplanet simulation previously described to study the interaction between fronts and the SST anomaly. Applying the tracking method on the aquaplanet with the prescribed SST anomaly showed an increase in the frequency of fronts near the centre of the tightened SST gradient and a poleward shift of the trajectory of the fronts. Combining the tracking method and composite analysis showed that, on average, transverse circulation of individual fronts was dampened by crossing the SST anomaly.