Dynamics of Tropically Forced Zonally Symmetric Climate Variability

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Recent observational and modeling studies have revealed the ubiquitous presence of zonally symmetric and hemispherically symmetric climate anomalies in the tropics and extratropics that are not well-understood. These anomalies, occurring on interannual to interdecadal time scales, are strong enough to leave an imprint on the precipitation anomalies in midlatitudes, influencing multi-year droughts and interdecadal shifts in precipitation over densely populated land areas. A hypothesis is put forth that the coherent, zonally symmetric, climate anomalies are forced primarily by tropical sea surface temperature (SST) anomalies. A positive SST forcing first produces a homogenized band of tropical tropospheric warmth. The latter then induces a meridional shift of the zonally symmetric eddy-driven circulation, causing cooling and anomalous ascent in midlatitudes. Through numerical modeling and theoretical investigations, the goals of this project are to verify and refine this hypothesis and to clarify the mechanisms that facilitate the tropics-extratropics interaction and zonalization of extratropical climate anomalies. The principal tools of this work are a hierarchy of dynamical models including atmospheric general circulation models (GCMs) and linear 3-D and 2-D models with and without transient eddies, all driven by tropical forcings. Extensive GCM simulations will be performed to attribute the statistically significant signals in the zonally symmetric variability to tropical SSTs in different ocean basins. Linear transient-free models will be used to deduce the direct zonally symmetric response to tropical forcing, and linear storm track models used to further clarify the role of transient eddies in creating and maintaining the extratropical zonally symmetric variability. Synthesizing the results from these models will reveal a road map from a localized tropical SST forcing to extratropical zonally symmetric climate anomalies. Intellectual merit: Classical work on the tropical influence of extratropical climate variability has made substantial progresses on the aspect of forced stationary waves but overlooked the equally important zonally symmetric climate signals. The results from this project will advance our understanding of the latter, making the theory of the tropical influences on midlatitude climate variability complete. To achieve that goal, this study will expand and integrate the current knowledge on tropical dynamics, tropics-extratropics interaction, and extratropical wave-zonal mean interaction, subjects that the PIs have been intensively investigating in the last decade. The expected scientific gains of the research and the PIs' readiness to explore them highlight the intellectual merit of this work. Broader impacts: The work will help provide a full understanding of the atmospheric dynamics that link tropical ocean temperatures to hydroclimatic changes in the extratropics. This understanding will form the basis for efforts to predict droughts and wet periods months to decades in advance. A student will be trained and mentored and a young scientist will gain research experience through this grant.