Impact Of Global Warming On The North Atlantic Climate Variability

In this dissertation, the impact of global warming on the North Atlantic climate variability is studied, focusing on the North Atlantic Tripole (NAT) and the North Tropical Atlantic Mode (NTAM). The NAT, serving as the second Emperical Orthogonal Fuction (EOF) mode of the extratropical North Atlantic sea surface temperature (SST) in wintertime, is closely related to the North Atlantic Oscillation (NAO). It exerts great influence on the temperature and precipitation over the surrounding continents, as well as the Atlantic Meridional Overturning Circulation (AMOC), etc. The NTAM is the first EOF mode of the tropical Atlantic SST in spring. Through its tight association with the Inter Tropical Convergence Zone (ITCZ), the NTAM shows great impact on the Atlantic hurricane and the precipitation of the surrounding countries.The modulations of the NAT decadal variability from global warming are studied by conducting a series of coupled ocean-atmosphere experiments, using the Fast Ocean-Atmosphere Model version 1.5 (FOAM 1.5). The model reasonably captures the observed NAT decadal variability, with a preferred time scale of about 11 years. With the aid of partial-blocking and partial-coupling experiments, it is found that the NAT decadal cycle can be attributed to oceanic planetary wave adjustment in the subtropical basin and ocean-atmosphere coupling over the North Atlantic. In a doubled CO2 experiment, the spatial pattern of the NAT is preserved; however, the decadal cycle is significantly suppressed. This suppression appears to be associated with the acceleration of oceanic planetary waves due to an intensification of sratification in global warming. This shortens the time from a decadal to an interannual time scale for the first-mode baroclinic Rossby waves to cross the subtropical North Atlantic basin, which is the primary memory for the NAT decadal variability in the model. This acceleration of Rossby wave in response to global warming can also be seen in Geophysical Fluid Dynamics Laboratory coupled model version 2.0 (GFDL CM2.0). FOAM results also indicate that the global warming does not modulate the North Atlantic air-sea coupling significantly. However, the coupling is decreased sharply in GFDL CM2.0 model, suggesting that the modulation of air-sea coupling over the North Atlantic Ocean may be model dependent.To further investigate changes of air-sea coupling over the North Atlantic Ocean in global warming, the twentieth-Century Reanalysis version 2 (20CRv2), climate model simulations and observational data are used. It is found that the air-sea coupling over the North Atlantic Ocean is characterized by two coupling feedbacks: the atmosphere to ocean feedback and the ocean to atmosphere feedback, with the former being the dominant feedback. The atmosphere to ocean feedback is characterized by the tight relationship between wintertime NAO and NAT, which shows little change in global warming. However, the ocean to atmosphere feedback, characterized by the association between summer North Atlantic Horseshoe (NAH) SST anomalies and the following winter NAO, is significantly intensified in the second half of the 20th century. This intensification is likely associated with the enhancement of the North Atlantic storm tracks as well as the NAH SST anomalies, supported by Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) climate models and other reanalysis data. This study also reveals that most IPCC AR4 climate models fail to capture the observed NAO-NAH coupled feedback, with only 2 out of 21 models succeeded. Moreover, more than half of the IPCC models cannot reproduce the observed summer NAH.The extratropical North Atlantic Ocean is tightly associated with the tropical North Atlantic Ocean, by being an important factor for the internal variability of the Atlantic Meridional Mode (AMM), especially for the northern pole of the AMM- the NTAM. This study evaluates the relative contributions to the NTAM variability from the El Nino-Southern Oscillation (ENSO) forcing and ocean-atmosphere feedbacks internal to the tropical Atlantic Ocean. The ENSO forced and internal variability is extracted by conducting Pacific Ocean-Global Atmosphere (POGA) experiment, using GFDL CM2.1 model. POGA consists of 10-member coupled simulations for 1950-2012 where SST is restored to the observed anomalies over the eastern tropical Pacific but interactive with the atmosphere over the rest of the ocean. In these experiments, the ensemble mean is due to ENSO forcing and the inter-member difference arises from internal variability of the climate system independent of ENSO. It is found that both ENSO forcing and internal variability of the NTAM is attributed to wind-evaporation-SST (WES) feedback, and reaches its maximum in spring. However, the internal NTAM develops earlier and contributes more to the total variance. POGA reasonably captures these characteristics, but biases still exist. The NTAM simulated by POGA develops later than the observed and attributes more to the internal variability. Moreover, POGA simulated NTAM has a spurious high variance band to the north off the equator, which is associated with both WES feedback and ocean dynamics. This spurious band is found to be a common bias among climate models, with 15 out of 30 IPCC Coupled Model Intercomparison Project Phase 5 (CMIP5) models having this problem. In addition, POGA simulates a higher correlation between the NTAM and the Atlantic Nino than the observed, which can also be seen from other 9 IPCC models.