夏季北太平洋における海洋前線帯周辺の下層雲と海面水温の相互作用に関する研究

Tohoku University須賀利雄課

Mechanismen und Vorhersagbarkeit des nordatlantisch-europäischen Klimas

This study investigates the mechanisms of North Atlantic-European climate using atmosphere general circulation models (AGCMs). Experiments with the AGCM ECHAM4, in which the sea surface temperature (SST) forcing is restricted to either the Atlantic or the Indo-Pacific oceans, show that both oceanic regions have an influence on North Atlantic-European climate in winter. In the experiment with SST forcing restricted to the Indo-Pacific oceans the atmospheric response projects on the North Atlantic Oscillation (NAO), while in the experiment with SST forcing restricted to the Atlantic Ocean the atmospheric response projects on the East-Atlantic Pattern. A multi-model intercomparison study shows that the region with the dominant influence on North Atlantic-European winter climate varies between different AGCMs. The dominant forcing of the atmospheric variability in the North Atlantic-European region in ECHAM4 is from the tropical eastern Pacific. However, in three other AGCMs the dominant forcing of this mode is from the tropical North Atlantic region. The importance of North Atlantic SST for North Atlantic-European climate is shown in another multi-model intercomparison study. The idealized North Atlantic SST anomaly pattern for this experiment has the structure of a tripole, which is believed to have the strongest impact on North Atlantic climate. Agreements between the responses in the different AGCMs are found concerning the NAO, Eurasian temperatures, rainfall over America and Africa, and the Asian monsoon. The results suggest that the extratropical North Atlantic region response is associated with remote Caribbean and tropical Atlantic SST anomalies, and with local forcing. All of these results support the conclusion that the ocean has a significant influence on North Atlantic-European climate. In addition to the mechanisms, this study investigates the predictability of North Atlantic-European climate. A control integration and ensemble experiments with the coupled atmosphere-ocean general circulation model (AOGCM) ECHAM5/MPI-OM are analyzed to investigate the decadal climate predictability. The ensemble experiments are realized with slightly perturbed atmospheric but the same oceanic initial conditions. The results show that the North Atlantic thermohaline circulation (THC) and SST are potentially predictable on multidecadal timescales. Over the ocean the predictability of surface air temperature (SAT) is very similar to that of SST, and this signal proceeds into the lower troposphere. Over land there is little evidence of decadal predictability of SAT with classical predictability methods. However, the estimation of the potential predictability over the European continent with probabilistic methods, commonly used in seasonal and medium-range forecasting, exhibits some limited success on decadal timescales. A multi-model comparison study with five AOGCMs confirms the potential predictability of North Atlantic THC and SST, albeit with skill levels dependent on the AOGCM. In general, models with greater decadal THC variability have higher levels of potential predictability

SUMMER ATMOSPHERIC HEAT SOURCES OVER THE TIBETAN PLATEAU

Ph.D

The signature of mesoscale eddies on sea surface temperature and its associated heat transport

This thesis aims at analysing the impact of oceanic turbulence and air-sea interactions on the sea surface temperature (SST) of the extra-tropical oceans on spatial scales of a few hundred kilometres (the so-called "mesoscale"). Using satellite-based measurements of SST and sea level, as well as surface tracks of mesoscale oceanic cyclones and anticyclones, it is shown that turbulence does not transport heat through systematic motions of cold cyclones and warm anticyclones, as was previously thought in regions of strong mean flows like the Gulf Stream. Rather, it is suggested that heat is transported as a result of the slight phase shift between temperature and pressure fluctuations developing on the mean flow. In addition, tentative estimates of the rate at which air-sea heat fluxes damp the SST signatures of cyclones and anticyclones are provided. The weak values obtained ( 20 W/m2K) contrast sharply with theoretical expectations, but are in agreement with the observed long-lived thermal heat content anomalies associated with the cyclones and anticyclones. These observations provide important benchmarks for high-resolution ocean models and may moreover guide the parametrization of subgrid-scale heat transport in climate models

Remote Sensing of Hydro-Meteorology

Flood/drought, risk management, and policy: decision-making under uncertainty. Hydrometeorological extremes and their impact on human–environment systems. Regional and nonstationary frequency analysis of extreme events. Detection and prediction of hydrometeorological extremes with observational and model-based approaches. Vulnerability and impact assessment for adaptation to climate change

Influencia de la variabilidad decadal de la temperatura de la superficie del mar en la precipitación tropical: Monzón de África Occidental y Sudamérica

Tesis de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de la Tierra y Astrofísica, leída el 12/02/2018The Sahel is the semiaridWest African region between the Sahara desert and the wet tropical savanna. The Sahel rainfall depends on the West African Monsoon (WAM) system and peaks between July and September. The rainfall regimes of the Amazonia and Northeast regions, located in northern Brazil, depend on the South American Monsoon system. The Amazonia is the region covered by the Amazon River basin, where heavy rains occur throughout the year but with a rainier season extending from December to May. The Northeast is a semiarid region with a short rainy season between March and May. Precipitation regimes in these three regions have undergone changes over time with important humanitarian, environmental and economic consequences and have been a major topic of study (e.g. Rodríguez-Fonseca et al., 2015; Zhou and Lau, 2001; Marengo et al., 2016). At decadal-to-multidecadal time scales, these changes have been mainly associated with the global sea surface temperature (SST) variability. Particularly, the Sahel precipitation has been associated with the global warming (GW), the Atlantic Multidecadal Variability (AMV) and the Interdecadal Pacific Oscillation (IPO) modes of decadal-to-multidecadal SST variability (e.g. Mohino et al., 2011a). The Amazonia and Northeast rainfall changes have been related to the Pacific and the Atlantic SST variability at decadal time scales (e.g. Grimm and Saboia, 2015), which is led by the AMV and IPO. Climate study through Global Circulation Models (GCMs) is crucial to understand climate changes and assessing its effects. So, in the first part of this Thesis a multi-model analysis is done addressing the influence of the main decadal-to-multidecadal modes of SST variability on precipitation in the Sahel, Amazonia and Northeast using different GCMs simulations from the 5th phase of the Coupled Model Intercomparison Project (CMIP5) (Taylor et al., 2012)...El Sahel es la región semiárida de África Occidental entre el desierto del Sáhara y la húmeda sabana tropical. La precipitación del Sahel depende del sistema del Monzón de África Occidental (WAM1) y alcanza su punto máximo entre julio y septiembre. Los regímenes de precipitación de la Amazonía y Nordeste, ubicadas en el norte de Brasil, dependen del sistema del Monzón Sudamericano. La Amazonía es la región cubierta por la cuenca del río Amazonas, donde se producen fuertes lluvias durante todo el año, pero con una temporada más lluviosa que se extiende desde diciembre a mayo. El Nordeste es una región semiárida con una estación lluviosa corta entre marzo y mayo. Los regímenes de precipitación en estas tres regiones han experimentado cambios a lo largo del tiempo con importantes consecuencias humanitarias, ambientales y económicas y han sido objeto de diversos estudios (Rodríguez- Fonseca et al., 2015; Zhou and Lau, 2001; Marengo et al., 2016). A escalas de tiempo decadales a multidecadales, estos cambios se han relacionado principalmente con la variabilidad de la temperatura de la superficie del mar (TSM). Particularmente, la precipitación del Sahel se ha asociado con el calentamiento global (GW2 ), la Variabilidad Multidecadal del Atlántico (AMV3) y la Oscilación Interdecadal del Pacífico (IPO4) (Mohino et al., 2011a). Los cambios en la lluvia de la Amazonía y el Nordeste se han relacionado con la variabilidad de la TSM del Pacífico y el Atlántico a escalas de tiempo decadal (Grimm and Saboia, 2015), que está modulada por la AMV y la IPO. Estudiar el clima a través de Modelos de Circulación Global (GCMs5) es crucial para comprender los cambios climáticos y evaluar sus efectos. Por eso, en la primera parte de esta Tesis se aborda la influencia de los principales modos de variabilidad decadal a multidecadal de la TSM sobre la precipitación en el Sahel, la Amazonía y el Nordeste mediante un análisis multimodelo utilizando diferentes simulaciones de los GCMs de la quinta fase de la Proyecto de Intercomparación de Modelos Acoplados (CMIP56) (Taylor et al., 2012)...Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEunpu

Moisture and Thermal Characteristics of Southern Plains Ice Storms: Insights from a regional climatology and high-resolution WRF-ARW sensitivity study

Winter storms, including snowstorms and ice storms, are infrequent in the Southern Great Plains of the United States (SGP), but can produce significant hazard and socioeconomic disruption. During 2000-2010, several severe ice storms impacted the region. These events combined resulted in nearly $800 million in damages, over 30 fatalities, and power disruption to over 3 million homes and businesses. Hitherto, basic climatological information for winter storms in this region remain understudied. This dissertation examines the characteristics of freezing precipitation events for the SGP by developing a regional spatial and synoptic climatology (1993-2011). Thermal profiles conducive to winter precipitation of varying types and intensities are also examined and compared with past literature. A combination of sounding analysis, and Principal Component (PC)/composite techniques are used to derive this climatology. Results identified that the SGP experiences freezing precipitation of varying intensity, but that ice storms to the region are notable for their large above-freezing inversion layer (‘warm layer’) temperatures/depths and mixing ratio. Freezing precipitation occurs most often over the central and eastern domain during December-February, while snowfall maximizes northwest of this zone with broader seasonal occurrence. The synoptic analysis showed that patterns conductive to storms with a pronounced mixed-phase region typically involved topographically aided ageostropic down-gradient advection of cold stable air in the lee of the Rocky Mountains, with an arctic high pressure over the northern/central Great Plains. A mid-level trough and low-level warm air advection provided ascent, and anomalously warm air to the south provided sufficient support for a warm layer. Long-duration ice storms were observed with a slow-moving high-amplitude western trough, direct moisture transport from the Gulf of Mexico, and a ridge over the southeastern U.S. Based on the climatology and past literature, a hypothesis is proposed that the Gulf of Mexico, as the proximal basin and major moisture source, may impact ice storm severity by modulation of the warm layer profile associated with strongly positive or negative SST anomalies. This hypothesis is tested using high-resolution nested WRF-ARW sensitivity studies with six representations of SST, including the 30-year climatology, a uniform ±2 degrees K perturbation to the control, and a physical upper and lower limit using the SST field for the warmest and coolest basin-average anomalies 1981-2011. Two case studies were utilized corresponding to different synoptic types. The simulations revealed discernible influence of SST on freezing precipitation, including its temporal evolution and intensity. For the December 9-11 2007 case study, the warm layer formed well prior to the event, associated with persistent southerly flow and a warm anomaly over the southern U.S. The impact of SST on the warm layer intensity was weak in comparison to its existing magnitude, however the atmospheric stability profile was altered such that strongly negative SST produced stabilization above the maximum inversion temperature and markedly reduced precipitation on the first day of the ice storm. A dynamical weakening of the low-level jet and moisture transport in the strongly positive SST case counteracted observed increases in mixing ratio to yield weaker accumulation differences during the second precipitation episode. For the January 28-30 2010 case study, the impact of SST was more pronounced on the warm layer, which had formed in association with return flow from the Gulf. Warmer SST, especially strongly positive localized anomalies within the fetch of the impacted area, lead to both a moisture induced intensification of precipitation, and increased peak warm layer temperature, leading to changes in the location of freezing precipitation versus rain/snow, especially for Arkansas. Dynamical intensification (weakening) of precipitation occurred as increased (decreased) baroclinicity, warm air advection and latent heat release promoted a stronger geopotential low at 850 hPa, and a strengthened (weakened) low-level jet yielding greater (less) moisture transport. Despite the differing thermal and dynamical responses, both case studies displayed potential for enhanced icing conditions with warmer SST, while cooler SST produced a marked reduction in severity. The January 2010 event showed greater sensitivity in the location and amount of icing due to the warm layer evolution being more directly connected to diabatic processes over the Gulf of Mexico 24-48 hours prior. Results showed discernible impact even with comparatively small SST perturbations (e.g., climatology versus control) indicating that winter precipitation is sensitive to basin SST anomalies. This work may be of use to forecasters and regional climatologists in gaining situational awareness and recognizing the role of both large-scale synoptic and regional thermodynamic drivers of phase type and intensity. Furthermore, given the observed increases in SST resulting from global climate change, this work provides physical understanding of processes that may impact ice storm evolutions in a warming climate, particularly with respect to the warm layer

Emerging Hydro-Climatic Patterns, Teleconnections and Extreme Events in Changing World at Different Timescales

This Special Issue is expected to advance our understanding of these emerging patterns, teleconnections, and extreme events in a changing world for more accurate prediction or projection of their changes especially on different spatial–time scales