Multi-Decadal Climate Variability: A Pacific Mechanism With Global Implications

Abstract

Over the instrumental record, global surface temperatures have increased in a step-like pattern. Two periods of rapid temperature rise (1910 to 1945 and 1976 to roughly 2000) are separated by periods of relative temperature stability (1945 to 1976 and roughly 2000 to present). There is no consensus as to the relative roles of internal variability and external forcing in generating this multi-decadal variability. Both mechanisms could cause decadal changes in the rate of warming. Changes in external forcings can modulate earth’s net energy imbalance; during periods where negative forcings are large enough to mask greenhouse gas forcings, temperatures can stabilize. In contrast, internal variability does not necessarily change the net energy uptake of the earth system. Rather it shifts the distribution of heat uptake between components of the climate system, modifying the rate of global surface temperature rise. In this dissertation, we use a combination of observation, model, and theory to explore the sources of decadal variability in the historical record. We find that atmospheric aerosols cannot reproduce the circulation patterns tied to the most recent period of temperature stability. These results suggest that this warming plateau is not externally forced and also call to question the attribution of the mid-century plateau to aerosol forcing. We also investigate the sources of internal variability in the equatorial Pacific, a region critical to modulating global climate. By combining temperature and salinity measurements from the Argo array with current measurements from the Tropical Atmosphere Ocean (TAO) buoys, we characterize the waters that upwell from the Equatorial Undercurrent (EUC) into the eastern equatorial Pacific. We find a large hemispheric asymmetry in the water mass contribution to the EUC, with up to 90% of waters originating from the Southern Hemisphere. This suggests that variability in the EUC and eastern equatorial Pacific should originate in the South Pacific. Focusing on variability in ventilated thermocline of the South Pacific, we propose a new mechanism to explain decadal variability in the tropical Pacific. Data, model, and theory point to decadal changes in the structure of the ventilated thermocline that modify EUC strength and temperatures in the eastern equatorial Pacific. Approximating this mechanism of variability in a climate model creates a staircase pattern of warming, similar to the pattern observed over the historical temperature record. This suggests the proposed mode of internal variability can explain some of the decadal variability in global surface temperatures. Data reveal the a thickness anomaly in the ventilated thermocline of the South Pacific propagating westward and equatorward starting in 2010. Our hypothesis predicts that when this anomaly reaches the equator, it will end the current period of temperature stability, and also suggests the step-wise warming pattern observed over the 20th century will continue into the future.