The impact of Atlantic Ocean variability on North Atlantic storminess and the Northern Hemisphere Jet Stream

Tropical cyclones (TC)s are a major natural hazard and the jet stream is closely linked to mid-latitude storm activity. Both TCs and the jet stream can therefore have a significant impact on society, yet the extent to which ocean variability impacts on TCs and the jet stream is far from understood. This thesis aims to improve understanding of when and how the ocean modulates storm activity. The emphasis is on how airsea interactions and variability of the Atlantic Ocean circulation impact on the frequency and intensity of tropical cyclones, together with the ocean influence on the northern hemisphere jet stream. Ocean drivers of recent active Atlantic hurricane seasons are found to take two forms: late winter changes in the ocean circulation related to a reduced Atlantic Meridional Overturning Circulation and late spring/early summer changes in the airsea heat flux. Over the Atlantic, the TC rainfall rate (mm/hr) increases by 6% for a 1°C SST rise in the Main Development Region (MDR). Over land, however, the rainfall rate increases by over 30% for a 1°C rise in SST in the MDR and appears linked mainly to the increase in TC wind speed. In the subtropical Atlantic, around Bermuda, average TC intensity is found to be increasing at 5kts per decade linked to rising ocean temperatures in the region. The prediction of TC potential intensity is also found to be closer to actual intensity using the average temperature through the top 50m layer (?) as opposed to SST, with the improvement proportional to the SST - ? temperature difference. For the northern hemisphere jet stream, the ocean acts to reduce the seasonal range of jet latitude variability, particularly over the North Atlantic where the oceanic Meridional Heat Transport (MHT) is greatest. Interannual to decadal variability in jet latitude and speed is most evident in the North Pacific in winter where the Pacific Decadal Oscillation explains 50% of the variance in jet latitude since 1940. On multidecadal timescales trends vary significantly on a regional basis. The largest increasing trends in jet latitude and jet speed are observed in the North Atlantic, with increases in winter of 3° and 4.5ms-1, respectively. There are no trends in jet latitude or speed over the North Pacific.