Rigorous Derivation of Stochastic Conceptual Models for the El Ni\~no-Southern Oscillation from a Spatially-Extended Dynamical System

El Ni\~no-Southern Oscillation (ENSO) is the most predominant interannual variability in the tropics, significantly impacting global weather and climate. In this paper, a framework of low-order conceptual models for the ENSO is systematically derived from a spatially-extended stochastic dynamical system with full mathematical rigor. The spatially-extended stochastic dynamical system has a linear, deterministic, and stable dynamical core. It also exploits a simple stochastic process with multiplicative noise to parameterize the intraseasonal wind burst activities. A principal component analysis based on the eigenvalue decomposition method is applied to provide a low-order conceptual model that succeeds in characterizing the large-scale dynamical and non-Gaussian statistical features of the eastern Pacific El Ni\~no events. Despite the low dimensionality, the conceptual modeling framework contains outputs for all the atmosphere, ocean, and sea surface temperature components with detailed spatiotemporal patterns. This contrasts with many existing conceptual models focusing only on a small set of specified state variables. The stochastic versions of many state-of-the-art low-order models, such as the recharge-discharge and the delayed oscillators, become special cases within this framework. The rigorous derivation of such low-order models provides a unique way to connect models with different spatiotemporal complexities. The framework also facilitates understanding the instantaneous and memory effects of stochastic noise in contributing to the large-scale dynamics of the ENSO

Precipitation Variability and Predictability over the Arabian Peninsula, Central Southwest Asia, and Southern Africa

The Northern Hemisphere winter is the main rainy season for the Arabian Peninsula (AP), Central Southwest Asia (CSWA), and Southern Africa (SF), where precipitation predictability is limited or understudied. This dissertation research focuses on improving our understanding of these regions\u27 wet-season precipitation characteristics and predictability. First, I have identified the AP\u27s key moisture sources through a Lagrangian back-trajectory algorithm. Mid-latitude land and water bodies, such as the Mediterranean and Caspian Seas, are the primary moisture sources in the northern region. Areas further south rely on moisture transport from the Western Indian Ocean and the African continent. A significant drying trend in parts of the Peninsula is partly attributed to anomalies in moisture advection from the Congo Basin and South Atlantic Ocean. Next, I have identified key tropical and extratropical forcings that explain about three-quarters of winter precipitation variability in CSWA. Tropical forcing comes from an indirect ENSO forcing pathway, the dominant mode of precipitation variability in the Indian Ocean referred to as Indian Ocean Precipitation Dipole (IOPD). Extratropical forcing arises from a large-scale mode due to internal atmospheric variability. Seasonal forecasting systems effectively depict the characteristics of tropical forcing and its teleconnection with CSWA. Extratropical forcing spatial structure has also been skillfully represented. However, a lack of skill is noted in depicting its interannual seasonal variability and teleconnection with CSWA, which is the main driver of limited prediction skills in models. Lastly, I developed an empirical model using ENSO, Indian Ocean Dipole (IOD), and IOPD as precursors to investigate SF monsoon variability and predictability. I note that a reasonable skill in predicting SF monsoon precipitation can be achieved by preconditioning these modes as early as five months before the monsoon season. Seasonal forecasting systems that represent the interplay of these modes can achieve reasonable prediction skills over SF with a one to three months lead. However, ENSO forcing is overly strong in these models, making their predictions less skillful than the empirical model. These findings offer invaluable insight into the mechanisms of global teleconnections within the investigated regions, which should enhance the ability to predict wet season precipitation more accuratel

Vegetation dynamics in northern south America on different time scales

The overarching goal of this doctoral thesis was to understand the dynamics of vegetation activity occurring across time scales globally and in a regional context. To achieve this, I took advantage of open data sets, novel mathematical approaches for time series analyses, and state-of-the-art technology to effectively manipulate and analyze time series data. Specifically, I disentangled the longest records of vegetation greenness (>30 years) in tandem with climate variables at 0.05° for a global scale analysis (Chapter 3). Later, I focused my analysis on a particular region, northern South America (NSA), to evaluate vegetation activity at seasonal (Chapter 4) and interannual scales (Chapter 5) using moderate spatial resolution (0.0083°). Two main approaches were used in this research; time series decomposition through the Fast Fourier Transformation (FFT), and dimensionality reduction analysis through Principal Component Analysis (PCA). Overall, assessing vegetation-climate dynamics at different temporal scales facilitates the observation and understanding of processes that are often obscured by one or few dominant processes. On the one hand, the global analysis showed the dominant seasonality of vegetation and temperature in northern latitudes in comparison with the heterogeneous patterns of the tropics, and the remarkable longer-term oscillations in the southern hemisphere. On the other hand, the regional analysis showed the complex and diverse land-atmosphere interactions in NSA when assessing seasonality and interannual variability of vegetation activity associated with ENSO. In conclusion, disentangling these processes and assessing them separately allows one to formulate new hypotheses of mechanisms in ecosystem functioning, reveal hidden patterns of climate-vegetation interactions, and inform about vegetation dynamics relevant for ecosystem conservation and management

Retroflections in the tropical Atlantic Ocean

The upper ocean circulation is the area of confluence of two major vertical cells: the large-scale overturning Atlantic Meridional Overturning Circulation (AMOC) and the regional relatively shallow Subtropical Cells (STCs). Both meridional cells interact in a complex system of zonal currents driven by fluctuations of the trade winds. The thermocline waters that subduct in the subtropical South Atlantic join the northward AMOC limb in the western boundary North Brazil Current and Undercurrent system (NBC/NBUC). When the NBC crosses the equator the Coriolis force vanishes and it becomes unstable, therefore it retroflects forced by fluctuations in the easterly winds and associated zonal sea surface height (SSH) gradient . These changes are part of coupled ocean-atmosphere variations that cover a wide range of timescales. A total net transport of 20.6 Sv and 1.0 PW are effectively exported equatorward through the NBC/NBUC system as a result transfer pathways between tropical and subtropical gyres in the South Atlantic. Once at the equator, 11.4 Sv of South Atlantic waters retroflect from the NBC into the eastward Equatorial Undercurrent (EUC) at thermocline layer which join 2.8 Sv arriving from the northern hemisphere. The NBC-EUC retroflection takes place at different latitudinal bands between 3 degrees south and 12 degrees north. A boreal fall maximum is only present in the northernmost retroflection coinciding with the seasonal cycle of the North Equatorial Countercurrent (NECC). At interannual scales the EUC reflects a major increase in the contribution from the South Atlantic tropical waters during 2008-2016 as compared with 1997-2007. The NBC-EUC retroflection occurs within an area where changes in SSH anomalies respond to the two main modes of tropical Atlantic variability: the Meridional Mode and Equatorial Mode. During the development of both events the zonal SSH gradient is modified by the activation of the Rosbby wave (RW) reflected mechanism. Consequently, the zonal current system is altered. Concretely, during a positive Meridional Mode NECC, EUC and north South Equatorial Current (nSEC) intensify, or weaken during negative phase. The associated RW-reflected mechanism results to a Kelvin wave (KW) which reverses the equatorial zonal gradients developing an Equatorial Mode-like pattern. The ocean currents response is not than robust compared with the Meridional Mode. Concretely, the EUC displays a non-linear response probably as a result of external forcings and/or the intrinsic diversity of the Equatorial Mode. The diversity of Equatorial Mode has been studied by applying an empirical orthogonal function (EOF) analysis of a sample of 22 time series of warm Equatorial Mode (or Atlantic Niño) events. This technique reveals four different types of spatio-temporal distributions of SST anomalies (SSTA) in the equatorial Atlantic in the period 1982-2019. These four different types are preceded by different local and remote forcings (i.e: ENSO). Moreover, they are associated with different climatic response on areas in the surrounding continents: northeast region in South America and Western Africa.Dos importants sistemes de circulació meridional conflueixen a l'Atlàntic tropical. La circulació meridional de l'Atlàntic (AMOC, per les seves sigles en anglès) i el sistema regional de cèl·lula subtropicals (STCs). Ambdues cèl·lules circulatòries interactuen en un complex sistema de corrents zonals governat pel patró de vents alisis. Les aigües de la termoclina que s'enfonsen als subtròpics per bombament d'Ekman retornen cap a l'equador mentre es troben amb la branca de retorn de la AMOC al marge occidental, concretament al sistema de la corrent Nord del Brasil (NBC/NBUC). Quan la NBC creua l'equador esdevé inestable per l'absència de Coriolis i dona lloc a un sistema de retroflexió forçat per canvis en els gradients zonals d'altura dinàmica del mar (SSH) associat a fluctuacions de vents dominants de l'est. Aquestes oscil·lacions formen part d'un complex sistema acoblat entre la atmosfera i oceà que es presenta a diferents escales temporals. Un transport net de 20.6 Sv i 1.0 PW, com a resultat de la transferència entre el gir tropical i subtropical de l'Atlàntic Sud, arriba a l'equador a través del sistema NBC/NBUC. Un cop a l'equador, la retroflexió de la NBC alimenta amb 11.4 Sv d'aigües de l'Atlàntic Sud a la corrent equatorial situada a la capa de la termoclina (EUC) que es sumen a 2.8 Sv provinents de l'hemisferi nord. El transport de la retroflexió NBC-EUC es troba repartit en diferents bandes latitudinals entre 3 graus sud i 12 nord que presenten diferents patrons estacionals. Les aigües més septentrionals que tomben cap a l'est mostren un màxim a la tardor boreal que coincideix amb la formació de la contracorrent nord-equatorial (NECC). A escala interanual, el transport la EUC mostra un creixent transport entre el període 2008 al 2016 en comparació amb el període 1997-2007 que es manifesta bàsicament en un increment de les aigües de procedència tropical de l'Atlàntic Sud. La retroflexió de la NBC-EUC succeeix en una àrea on canvis en anomalies de SSH responen als dos modes principals de variabilitat climàtica interanual de l'Atlàntic tropical, el Mode Meridional i el Mode Equatorial. Durant el desenvolupament dels dos modes, el gradient zonal de SSH es veu modificat per l'activitat de la reflexió d'ones Rossby (RW) al marge occidental i en conseqüència el sistema de corrents zonal. Concretament, en situacions de Mode Meridional positiu, la NECC, la EUC i la branca nord de la corrent sud-equatorial (nSEC) s'intensifiquen mostrant una resposta contraria durant el mode negatiu. El sistema de reflexió RW es propaga cap a l'est en forma d'ona Kelvin provocant un canvi de signe en el gradient anòmal zonal de la SSH. El resultat és el desenvolupament d'una situació de mode equatorial del mateix signe. Tanmateix, durant el mode equatorial la resposta de les corrents zonals és menys clara, sobretot per la EUC al marge oriental, indicant la presència d'altres forçaments o la diversitat intrínseca del Mode Equatorial. Mitjançant la tècnica d'anàlisi de funcions ortogonals en mapes de temperatures anòmales de la superfície del mar (SSTA) en l'Atlàntic equatorial s'obté diferents tipus de Mode Equatorial positiu, conegut també com Atlàntic Niño. Concretament entre 1982 i 2019 se'n deriven quatre tipus d'Atlàntic Niño. Cada un d'aquests tipus presenta diferents precondicions que intervenen en el seu desenvolupament, com per exemple teleconnexions atmosfèriques provinents de situacions de Niño en el Pacífic. Addicionalment, els diferents tipus d'Atlàntic Niño estan associats a diferents respostes en el règim de precipitacions tant en el marge nord-est d'Amèrica del Sud com a l'Àfrica Occidental.Postprint (published version

Tropical Pacific observing system

This paper reviews the design of the Tropical Pacific Observing System (TPOS) and its governance and takes a forward look at prospective change. The initial findings of the TPOS 2020 Project embrace new strategic approaches and technologies in a user-driven design and the variable focus of the Framework for Ocean Observing. User requirements arise from climate prediction and research, climate change and the climate record, and coupled modeling and data assimilation more generally. Requirements include focus on the upper ocean and air-sea interactions, sampling of diurnal variations, finer spatial scales and emerging demands related to biogeochemistry and ecosystems. One aim is to sample a diversity of climatic regimes in addition to the equatorial zone. The status and outlook for meeting the requirements of the design are discussed. This is accomplished through integrated and complementary capabilities of networks, including satellites, moorings, profiling floats and autonomous vehicles. Emerging technologies and methods are also discussed. The outlook highlights a few new foci of the design: biogeochemistry and ecosystems, low-latitude western boundary currents and the eastern Pacific. Low latitude western boundary currents are conduits of tropical-subtropical interactions, supplying waters of mid to high latitude origin to the western equatorial Pacific and into the Indonesian Throughflow. They are an essential part of the recharge/discharge of equatorial warm water volume at interannual timescales and play crucial roles in climate variability on regional and global scales. The tropical eastern Pacific, where extreme El Niño events develop, requires tailored approaches owing to the complex of processes at work there involving coastal upwelling, and equatorial cold tongue dynamics, the oxygen minimum zone and the seasonal double Intertropical Convergence Zone. A pilot program building on existing networks is envisaged, complemented by a process study of the East Pacific ITCZ/warm pool/cold tongue/stratus coupled system. The sustainability of TPOS depends on effective and strong collaborative partnerships and governance arrangements. Revisiting regional mechanisms and engaging new partners in the context of a planned and systematic design will ensure a multi-purpose, multi-faceted integrated approach that is sustainable and responsive to changing needs

Advances in tropical climatology–a review

Understanding tropical climatology is essential to comprehending the atmospheric connections between the tropics and extratropical latitudes weather and climate events. In this review paper, we emphasize the advances in key areas of tropical climatology knowledge since the end of the 20th century and offer a summary, assessment, and discussion of previously published literature. Among the key areas analyzed here, we explore the advances in tropical oceanic and atmospheric variability, such as El Niño–Southern Oscillation and the Madden-Julian Oscillation, and how those teleconnection events have helped us to better understand variabilities in tropical monsoons, tropical cyclones, and drought events. We also discuss new concepts incorporated into the study of tropical cyclones, such as rapid intensification, and how those studies are evolving and helping scientists to better prepare and predict hurricanes. Regarding tropical aerosols, we discuss how satellite-based dust detection has improved the comprehension of Saharan dust as a driver of drought in locations far from the dust source region while simultaneously altering tropical cyclone variability. Finally, our review shows that there have been significant advances in tropical hydroclimatic studies in order to better investigate monsoons, flooding, and drought, helping scholars of tropical climatology to better understand its extreme events

Study of Climate Variability Patterns at Different Scales – A Complex Network Approach

Das Klimasystem der Erde besteht aus zahlreichen interagierenden Teilsystemen, die sich über verschiedene Zeitskalen hinweg verändern, was zu einer äußerst komplizierten räumlich-zeitlichen Klimavariabilität führt. Das Verständnis von Prozessen, die auf verschiedenen räumlichen und zeitlichen Skalen ablaufen, ist ein entscheidender Aspekt bei der numerischen Wettervorhersage. Die Variabilität des Klimas, ein sich selbst konstituierendes System, scheint in Mustern auf großen Skalen organisiert zu sein. Die Verwendung von Klimanetzwerken hat sich als erfolgreicher Ansatz für die Erkennung der räumlichen Ausbreitung dieser großräumigen Muster in der Variabilität des Klimasystems erwiesen. In dieser Arbeit wird mit Hilfe von Klimanetzwerken gezeigt, dass die Klimavariabilität nicht nur auf größeren Skalen (Asiatischer Sommermonsun, El Niño/Southern Oscillation), sondern auch auf kleineren Skalen, z.B. auf Wetterzeitskalen, in Mustern organisiert ist. Dies findet Anwendung bei der Erkennung einzelner tropischer Wirbelstürme, bei der Charakterisierung binärer Wirbelsturm-Interaktionen, die zu einer vollständigen Verschmelzung führen, und bei der Untersuchung der intrasaisonalen und interannuellen Variabilität des Asiatischen Sommermonsuns. Schließlich wird die Anwendbarkeit von Klimanetzwerken zur Analyse von Vorhersagefehlern demonstriert, was für die Verbesserung von Vorhersagen von immenser Bedeutung ist. Da korrelierte Fehler durch vorhersagbare Beziehungen zwischen Fehlern verschiedener Regionen aufgrund von zugrunde liegenden systematischen oder zufälligen Prozessen auftreten können, wird gezeigt, dass Fehler-Netzwerke helfen können, die räumlich kohärenten Strukturen von Vorhersagefehlern zu untersuchen. Die Analyse der Fehler-Netzwerk-Topologie von Klimavariablen liefert ein erstes Verständnis der vorherrschenden Fehlerquelle und veranschaulicht das Potenzial von Klimanetzwerken als vielversprechendes Diagnoseinstrument zur Untersuchung von Fehlerkorrelationen.The Earth’s climate system consists of numerous interacting subsystems varying over a multitude of time scales giving rise to highly complicated spatio-temporal climate variability. Understanding processes occurring at different scales, both spatial and temporal, has been a very crucial problem in numerical weather prediction. The variability of climate, a self-constituting system, appears to be organized in patterns on large scales. The climate networks approach has been very successful in detecting the spatial propagation of these large scale patterns of variability in the climate system. In this thesis, it is demonstrated using climate network approach that climate variability is organized in patterns not only at larger scales (Asian Summer Monsoon, El Niño-Southern Oscillation) but also at shorter scales, e.g., weather time scales. This finds application in detecting individual tropical cyclones, characterizing binary cyclone interaction leading to a complete merger, and studying the intraseasonal and interannual variability of the Asian Summer Monsoon. Finally, the applicability of the climate network framework to understand forecast error properties is demonstrated, which is crucial for improvement of forecasts. As correlated errors can arise due to the presence of a predictable relationship between errors of different regions because of some underlying systematic or random process, it is shown that error networks can help to analyze the spatially coherent structures of forecast errors. The analysis of the error network topology of a climate variable provides a preliminary understanding of the dominant source of error, which shows the potential of climate networks as a very promising diagnostic tool to study error correlations

Analysis of Land and Sea Temperatures Trend During 1985-2021 Period to Understand Local or Global Warming Effect in Bengkulu City

Global warming is a phenomenon where the earth's temperature rises drastically. The temperature increase causes negative impacts on the environment globally. Bengkulu City, Indonesia, is situated with a growing population and land-use change that may cause temperature rise. This research aimed to analyze the temperature change in the land and sea area of Bengkulu City. To understand the local or global factors influencing temperature changes in Bengkulu City, we also studied the correlation between land and sea temperatures. The temperature data were obtained from BMKG and NOAA PSL. Firstly, we analyzed the temperature trendlines for the last 36 years. Then we evaluated the coefficient determination (R2) value to determine the correlation between sea and land temperatures. The results show that during the last 36 years, the sea temperature is increased by 0.40 °C, while the land temperature is increased by 1.07 °C. Moreover, we found a relatively weak correlation between sea and land temperature, with a 10.7% correlation. We argued that the increased temperature in Bengkulu City land is associated with land change use and rising population in the last few decades, which means the local factor affected the land temperature changes. On the other hand, global phenomena (IOD and ENSO) influenced sea temperature changes, which means the global factor affected the sea temperature changes. The rising land temperature is relatively high; hence it is necessary to understand better what parameters are causing temperature changes that may affect the physical environment in Bengkulu, Indonesia

林野火災における大気・海洋変動指標と水文気象要因の定量的分析

Tohoku University博士（工学）thesi