Response of Pacific Subtropical-Tropical Thermocline Water Pathways and Transports to Global Warming and its Implications for ENSO

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Intellectual Merit:The subtropical-tropical thermocline water pathways are the subsurface branches of the Subtropical Cells (STCs). The STCs are shallow meridional circulation cells in which water flows out of the tropics within the surface layer, subducts in the subtropics, flows equatorward within the thermocline, and upwells in the eastern equatorial ocean. The changes in the Pacific STC transports have profound implications on global climate through its impact on tropical thermocline variability and sea surface temperature (SST) anomalies associated with the El Nio-Southern Oscillation (ENSO). The ENSO is a climate phenomenon that affects weather, ecosystems, and societies across much of the globe. Observations and understanding of ENSO have rapidly progressed over the last few decades; however its considerably temporal irregularity is not well understood. Recent studies have demonstrated that thermocline variability along the equator can be a leading candidate responsible for this irregularity. Observations show that the thermocline water transport from the subtropics to the tropics in the Pacific has considerably been reduced from the 1970s to the 1990s, causing an appreciable SST increase in the tropics. Coupled climate models attribute the observed warming to the anthropogenic forcing. Under the future anthropogenic forcing, global warming may also have significant influence on the Pacific subtropical-tropical thermocline water pathways and transports, and thereby on ENSO. The objectives of this project are to investigate the changes in the large-scale circulation around the subtropics and the tropics in the Pacific under future anthropogenic forcing or global warming, and to explore the role of these changes in the subtropical-tropical thermocline water pathways and transports for modulating ENSO. The hypotheses are that (1) buoyancy forcing plays as important a role as wind forcing in the changes in the large-scale ocean circulation associated with the STCs, and (2) changes in the water pathways and transports from the subtropics to the tropics modulate ENSO properties through shifting the thermocline structure along the equator. Broader Impacts:Understanding ENSO temporal irregularity remains an outstanding issue in the climate variability and predictability with broad societal implications across many scientific disciplines. This research will identify a way of connecting the Pacific STCs with ENSO irregularity and provide a basis for understanding how does global warming affect subtropical-tropical inter-gyre ocean circulation in the Pacific. The results will be applied to future model improvements and be shared with the IPCC climate model community. The results will be disseminated through presentations at scientific meetings/workshops and publications in scientific journals.