UC Irvine

Remote forcing associated with El Niño and Arctic warming are two mechanisms capable of producing variability in the mid-latitude climate of the Northern Hemisphere. Such variability can give rise to abnormal and extreme events in mid-latitudes that can greatly influence agriculture and socioeconomics in highly populated regions. However, the impacts may have changed over the past few decades as global warming or natural decadal variability altered El Niño and amplified Arctic warming. This dissertation uses statistical analyses and numerical model experiments to examine the changing impacts of El Niño and Arctic warming on mid- latitude climate variability.

The center of the warm sea surface temperatures (SSTs) associated with El Niño has shifted from the tropical Eastern Pacific (EP) to tropical Central Pacific (CP) during the most recent two to three decades. This dissertation finds this shift has resulted in changes in the impacts of El Niño events on North American hydroclimate. CP and EP El Niño events exert influences on the Mississippi River discharge and the Great Plains Low-Level-Jet (GPLLJ) during boreal spring that are opposite in sign. Subsurface hydrological processes result in impacts on springtime Mississippi River discharge of the El Niño associated winter precipitation anomalies. The sea level pressure in the Gulf of Mexico and surface air temperature in the North America respond differently to the changing El Niño forcing, resulting in opposite impacts on the GPLLJ. The changing impacts of El Niño on the discharge from the world’s top thirty river basins and their underlying mechanisms are also examined. These findings offer information that can help to develop new water management strategies to cope with the changing El Niño impacts.

Arctic warming began to accelerate in the early 1990s, resulting in an increase in the importance of Arctic forcing on the mid-latitude climate variability. This dissertation also demonstrates how changing Arctic conditions affect the inter-basin connection between extreme marine events in the North Pacific and North Atlantic. The marine heatwave event in the North Pacific during 2014-2015 (referred to as the Pacific warm blob) is found to be synchronized with a marine cold spell event in the North Atlantic (referred to as the Atlantic cold blob) via an atmospheric circulation pattern. This pattern is identified as the Tropical Northern Hemisphere (TNH) pattern and is characterized by a cross-basin structure that enables it to simultaneously induce the warm and cold blob signatures in the North Pacific and North Atlantic respectively. Such co-occurring blob events have a tendency to occur more frequently since the early-1990s, coinciding with the time when the Arctic warming began to accelerate. This dissertation further finds that the amplified Arctic warming enhances the cross-basin structure of the TNH pattern to increase the inter-basin connectivity between the two oceans. The relative importance of El Niño and Arctic warming to the Pacific-Atlantic climate connectivity is also discussed.

The findings of this dissertation contribute to a better understanding of how the forcing of mid-latitude climate variability has changed in recent decades and of the relative importance of the tropical Pacific (i.e., El Niño) and Arctic (i.e., the amplified Arctic warming) forcings. The understanding gained has the potential to improve projections of climate variability in mid- latitudes, and in the development of new methodologies that use the tropical and Arctic conditions to predict mid-latitude extreme events.