Moisture transport associated with atmospheric rivers and low level jets at global scale

Resumo

The research presented in this dissertation investigates the role of the two meteorological structures responsible for most moisture transport on a global scale. One is particularly important in tropical and subtropical regions, i.e., Low-Level Jets (LLJs), whereas the other is important in extratropical regions, i.e., Atmospheric Rivers (ARs). Both structures shape the atmospheric branch of the hydrological cycle, transporting most of the available moisture in the atmosphere to latitudes further than its origin, and they describe the link between evaporation from the ocean/continent and precipitation over the continents. This thesis is structured into five chapters: Chapter 1 presents a brief introduction to the hydrological cycle and describes the main mechanisms of moisture transport on a global scale, whereas Chapter 2 lists the main objectives of the thesis. Chapter 3 describes the methods and datasets used. Chapter 4 includes a compendium of five manuscripts published in scientific journals. Finally, the main conclusions and possible avenues of future research are presented in Chapter 5. The corpus of this thesis is the five research articles published in scientific journals included in the Science Citation Index (SCI), included in Chapter fourth: Set of Publications. The first article is a review article, which provides an update on recent advances in the source-sink moisture relationship in the distribution of rainfall. The role of the main moisture transport mechanisms, AR and LLJ, as triggers of hydrological extremes such as droughts and floods are highlighted. Finally, some of the fundamental challenges for future research are presented. The next four articles correspond to two studies examining LLJs and two more for ARs, with the same structure. The first article in each case focused on the global scale, and the second examined a particular structure or a specific region. In the global analysis of LLJs, 33 regions of occurrence were identified, with 20 and 13 regions in the northern and southern hemispheres, respectively. The global distribution of the high activity regions of LLJs is consistent with the potential moisture transport of these systems in tropical and subtropical regions. The analysis of the moisture source-sink relationship shows an essential modulation of the moisture transport of LLJs. The amount of moisture transported is highly asymmetric, with structures such as the Great Plains low-level jet (GPLLJ) or the South America low-level jet (SALLJ) or LLJs linked to monsoon regimens, transport a large amount of moisture. Nevertheless, LLJs that develop over Africa are more linked to the transport of dust. The comparison of jet and non-jet days showed significant differences in the sink of moisture, being more remote when an LLJ event occurs according to the intense moisture advection. For the particular case of LLJ, the focus was on the GPLLJ, which modulates the entry of moisture, and hence, the precipitation pattern in the south-eastern centre of the United States. Although it is one of the most studied LLJs, in this research was quantified the moisture transport. To address this question, a combination of Lagrangian and Eulerian techniques was used to identify and quantify the moisture transport associated with the GPLLJ objectively. The results show that the GPLLJ is responsible for more than 80% of the moisture transported to the southern United States. As the latitude increases, the proportion of moisture transported at the GPLLJ pole decreases, but the GPLLJ can explain more than half of the moisture that reaches the south of the Great Lakes. In the global analysis of ARs, 20 regions of high AR activity were identified, which corresponds to regions mainly located on the western coast of mid-latitudes. The anomalous moisture analysis shows that the primary region through which the ARs uptake moisture is the Western Hemisphere Warm Pool (WHWP). The study of global variability shows a significant increase, with an escalation close to Clausius-Clapeyron (CC), suggesting higher moisture transport by ARs under future global warming scenarios. For this particular study, the focus was on the ARs that reach the Arctic system, a region especially vulnerable to global warming. An increase in moisture transport to this region is associated with a decrease in the extent of sea ice. The ARs represents one of the main mechanisms involved in the transport of moisture; hence, latent heat from the tropics to higher latitudes. All ARs that exceed 60 ºN were seasonally identified to analyse their exceptional contribution of moisture. The North Atlantic and North Pacific sectors constitute the main entryways for the ARs that reach the Arctic; they are also the primary sources of moisture for ARs events. Intensification in the uptake of moisture is observed on the sources of moisture climatology during AR events. The sixth article focuses on examining the links between ARs and LLJs in terms of moisture transport from South America, which results in winter rainfall over South Africa. The results show that the particular phase of the SALLJ known as the No Chaco Jet Event (NCJE) transports moisture from the continental region of South America (particularly, from a region that includes the Paraná river basin and the southern basin of the Amazon River) towards the western and central South Atlantic Ocean basin. This moisture is then transported by ARs to the west coast of South Africa. Anomalous moisture sink for the NCJE days and IVT anomalies for ARs days have been detected, confirming the existence of this link. Thus, the two margins of the South Atlantic Ocean appear connected by a combination of two of the essential meteorological structures for the moisture transport, i.e., the LLJ and the ARs. This thesis has substantial scientific and socio-economic implications. LLJs and ARs can modulate extreme precipitation events, i.e., an intensification (or reduction) of moisture transport modifies precipitation anomalies, hence inducing flooding (or drought). The possible changes in the position and frequency of LLJs and ARs with their climatological behaviour can alter the rainfall patterns substantially. In the context of a changing climate, the high sensitivity of water vapour content to temperature leads to higher evaporation and precipitation rates in a warmer scenario, and hence, an intensification of the hydrological cycle, with implications in the large moisture transport via ARs and LLJs. Therefore, this analysis of the main moisture transport mechanisms helps to understand future changes in precipitation patterns.