Observed and modeled evolution of the tropical mean radiation budget at the top of the atmosphere since 1985

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95205/1/jgrd15401.pd

Surging of global surface temperature due to decadal legacy of ocean heat uptake

Global surface warming since 1850 consisted of a series of slowdowns (hiatus) followed by surges. Knowledge of a mechanism to explain how this occurs would aid development and testing of interannual to decadal climate forecasts. In this paper a global climate model is forced to adopt an ocean state corresponding to a hiatus (with negative Interdecadal Pacific Oscillation, IPO, and other surface features typical of a hiatus) by artificially increasing the background diffusivity for a decade before restoring it to its normal value and allowing the model to evolve freely. This causes the model to develop a decadal surge which overshoots equilibrium (resulting in a positive IPO state) leaving behind a modified, warmer climate for decades. Water mass transformation diagnostics indicate that the heat budget of the tropical Pacific is a balance between large opposite signed terms: surface heating/cooling due to air-sea heat flux is balanced by vertical mixing and ocean heat transport divergence. During the artificial hiatus, excess heat becomes trapped just above the thermocline and there is a weak vertical thermal gradient (due to the high artificial background mixing). When the hiatus is terminated, by returning the background diffusivity to normal, the thermal gradient strengthens to pre-hiatus values so that the mixing (diffusivity x thermal gradient) remains roughly constant. However, since the base layer just above the thermocline remains anomalously warm this implies a warming of the entire water column above the trapped heat which results in a surge followed by a prolonged period of elevated surface temperatures

The fluid dynamics of climate: General Circulation Models and applications to past, present and future climatic changes

La presente tesi di ricerca riguarda l’ottimizzazione e l’utilizzo di modelli climatici globali, ed in modo particolare del modello climatico globale ad alta risoluzione EC-Earth, per affrontare una serie di problemi di interesse nel contesto della dinamica del clima e del cambiamento climatico. In particolare, l’attività di ricerca ha riguardato il lavoro di ottimizzazione, ovvero di tuning, del modello climatico globale EC-Earth, appartenente alla categoria degli Earth System Models, e nello specifico della componente atmosferica del modello. Lo studio del cosiddetto “Equable Climate” dell’Eocene, un periodo caldo verificatosi circa 50 milioni di anni fa caratterizzato da una bassa differenza di temperatura tra equatore e poli e ridotto ciclo annuale alle alte latitudini. Gli “Equable Climates” sono un problema tutt’ora irrisolto nelle scienze del clima e la loro comprensione potrebbe avere importanti implicazioni circa la nostra comprensione ed interpretazione dei cambiamenti climatici in corso. L'analisi delle caratteristiche della precipitazione invernale nella regione montuosa dell’Hindu-Kush Karakoram, nell’Himalaya occidentale, e delle sue teleconnessioni, con particolare riferimento all’Oscillazione Nord Atlantica. Lo studio é stato condotto mediante l’utilizzo congiunto di dati osservativi, rianalisi atmosferiche e simulazioni climatiche realizzate con il modello EC-Earth. Lo studio del cambiamento climatico nelle regioni montane, ed in particolare della dipendenza dalla quota dell’aumento delle temperature superficiali terrestri registrato durante il corso del XX secolo e previsto per le prossime decadi (Elevation-Dependent Warming, EDW). Lo studio si é focalizzato principalmente sulla regione montuosa dell’Himalaya- Tibetan Plateau ed é stato condotto mediante l’utilizzo di un ensemble di modelli climatici globali che hanno partecipato al Coupled Model Intercomparison Project Phase 5 (CMIP5) e all’analisi dei dati osservativi disponibili

Dwindling relevance of large volcanic eruptions for global glacier changes in the anthropocene

Large volcanic eruptions impact climate through the injection of ash and sulfur-containing gases into the atmosphere. While the ash particles fall out rapidly, the gases are converted to sulfate aerosols that reflect solar radiation in the stratosphere and cause a lowering of the global mean surface temperature. Earlier studies have suggested that major volcanic eruptions resulted in positive mass balances and advances of glaciers. Here, we perform a multivariate analysis to decompose global glacier mass changes from 1961 to 2005 into components associated with anthropogenic influences, volcanic and solar activities, and the El Niño-Southern Oscillation. We find that the global glacier mass loss was mainly driven by the anthropogenic forcing, interrupted by a few years of intermittent mass gains following large volcanic eruptions. The relative impact of volcanic eruptions has dwindled due to strongly increasing greenhouse gas concentrations since the mid-20th century

Atmospheric circulation processes contributing to a multidecadal variation in reconstructed and modeled Indian monsoon precipitation

An analysis of the recently reconstructed gridded May–September total precipitation in the Indian monsoon region for the past half millennium discloses significant variations at multidecadal timescales. Meanwhile, paleo-climate modeling outputs from the National Center for Atmospheric Research Community Climate System Model 4.0 show similar multidecadal variations in the monsoon precipitation. One of those variations at the frequency of 40–50 years per cycle is examined in this study. Major results show that this variation is a product of the processes in that the meridional gradient of the atmospheric enthalpy is strengthened by radiation loss in the high-latitude and polar region. Driven by this gradient and associated baroclinicity in the atmosphere, more heat/energy is generated in the tropical and subtropical (monsoon) region and transported poleward. This transport relaxes the meridional enthalpy gradient and, subsequently, the need for heat production in the monsoon region. The multidecadal timescale of these processes results from atmospheric circulation-radiation interactions and the inefficiency in generation of kinetic energy from the potential energy in the atmosphere to drive the eddies that transport heat poleward. This inefficiency creates a time delay between the meridional gradient of the enthalpy and the poleward transport. The monsoon precipitation variation lags that in the meridional gradient of enthalpy but leads that of the poleward heat transport. This phase relationship, and underlining chasing process by the transport of heat to the need for it driven by the meridional enthalpy gradient, sustains this multidecadal variation. This mechanism suggests that atmospheric circulation processes can contribute to multidecadal timescale variations. Interactions of these processes with other forcing, such as sea surface temperature or solar irradiance anomalies, can result in resonant or suppressed variations in the Indian monsoon precipitation