Finely laminated sediments within Bainbridge Crater Lake, Galapagos, provide a record of El Nino-Southern Oscillation (ENSO) events over the Holocene. Despite the importance of this sediment record, hypotheses for how climate variability is preserved in the lake sediments have not been tested. Here we present results of long-term monitoring of the local climate and limnology and a revised interpretation of the sediment record. Brown-green, organic-rich, siliciclastic laminae reflect warm, wet conditions typical of El Nino events, whereas carbonate and gypsum precipitate during cool, dry La Nina events and persistent dry periods, respectively. Applying this new interpretation, we find that ENSO events of both phases were generally less frequent during the mid-Holocene (similar to 6100-4000 calendar years B.P.) relative to the last similar to 1500 calendar years. Abundant carbonate laminations between 3500 and 3000 calendar years B.P. imply that conditions in the Galapagos region were cool and dry during this period when the tropical Pacific E-W sea surface temperature (SST) gradient likely strengthened. The frequency of El Nino and La Nina events then intensified dramatically around 1750-2000 calendar years B.P., consistent with a weaker SST gradient and an increased frequency of ENSO events in other regional records. This strong interannual variability persisted until similar to 700 calendar years B.P., when ENSO-related variability at the lake decreased as the SST gradient strengthened. Persistent, dry conditions then dominated between 300 and 50 calendar years B.P. (A.D. 1650-1900, +/- similar to 100 years), whereas wetter conditions and frequent El Nino events dominated in the most recent century. Plain Language Summary Sediments accumulating at the bottom of Bainbridge Crater Lake have provided a record of Galapagos climate and the frequency of El Nino events over the past similar to 6000 years. Motivated by the importance of this lake for our understanding of climate in the tropical Pacific Ocean, we have been monitoring the link between climate, lake conditions, and the physical and chemical properties of the lake sediments since 2009. Based on this long-term monitoring, we find that the Bainbridge sediment record preserves both El Nino and La Nina events. This makes Bainbridge a particularly valuable archive of past climate, as most sediment-based records typically preserve only one or the other key phase of tropical Pacific climate.

Typically, multi-model ensemble studies show mixed responses of El Nino Southern Oscillation (ENSO) under global warming, so it is currently unknown how, or even if, global warming will impact ENSO and its teleconnections. ENSO is governed by various ocean-atmosphere interactions in the equatorial Pacific, which provide either positive amplifying or negative damping feedbacks and are not always accurate in models. This results in uncertainty in projected ENSO responses. In a flux adjusted HadCM3 perturbed physics ensemble, the Bjerknes' stability index (BJ index), a measure of ENSO stability, has been used to analyse the strength of ENSO feedbacks and their response under the SRES A1B warming scenario with respect to mean climate conditions. Despite mean sea surface temperature biases being minimised by flux adjustment, the important dominant feedbacks, namely the latent heat flux feedback, shortwave flux feedback, the thermocline feedback and the zonal advective feedback are found to be too weak in the ensemble. Common model biases cause weak ocean-atmosphere interactions such as a weak response of ocean currents to wind stress anomalies, a weak thermocline slope response to wind stress anomalies and weak thermodynamic dampings. These biases are linked to overly strong zonal surface ocean currents and convective response biases. Under global warming, a large increase in thermodynamic damping, caused by increasing shortwave damping, is found. This increase is linked to a strong convective response and overrides other feedback responses, resulting in a weakening BJ index in contrast to increasing ENSO amplitude. Positive feedback responses are also found but counteract each other, so have relatively little impact on total ENSO stability. Results here show that common model biases, such as the cold tongue bias, are linked to persistent ENSO feedback biases pointing to areas of improvement in future models. Results also suggest that caution must be exercised when using the BJ index to assess ENSO, as the BJ index is not always representative of ENSO amplitude. This may be caused by non-linearities in ENSO feedbacks which are not accounted for by the linear approximations used in the BJ index, or by ENSO feedbacks not being directly comparable in magnitude, as assumed by the BJ index calculation.

The global ocean currently absorbs over a third of anthropogenic carbon dioxide (CO₂) emissions, slowing down the growth of atmospheric CO₂, and thus moderating climate change. However, there is significant variability in the strength of the ocean carbon sink on interannual to decadal timescales. There are also uncertainties in the ocean carbon sink, a source of which lies in the coastal ocean. Coastal carbon fluxes are globally relevant and highly variable, but due to the paucity of observations, the coastal ocean remains largely unconstrained. Quantifying and understanding the variability of the ocean carbon sink, and constraining its uncertainties, is essential for supporting climate policy and predicting how the ocean will continue to moderate climate change in the future. This is challenging due to the complex physical and biogeochemical processes in the ocean, as well as the limited observations of ocean carbon. The goal of this thesis is to contribute to the understanding of the ocean carbon cycle and its variability with observations of CO₂ fluxes in the coastal ocean (Chapter 2), a multi-model study of surface carbon interannual variability (Chapter 3), and a mechanistic investigation of decadal variability of air-sea CO₂ fluxes in the global ocean (Chapter 4). (Chapter 2) Jamaica Bay is a hypereutrophic coastal urban estuary within the land-ocean aquatic continuum. Anthropogenic perturbations to the carbon cycle of the continuum are often excluded from global carbon budgets. Studies have shown that not accounting for the lateral transport of anthropogenic carbon through the continuum can lead to an overestimation of land carbon sinks and an underestimation of ocean carbon sinks. In this study, we used the direct covariance method to make direct estimates of CO₂ fluxes in Jamaica Bay. Over a 587-day observational study, Jamaica Bay emitted CO₂ to the atmosphere at an average rate of 130 gC/m2/yr. However, we find that the waters within the estuary are a strong CO₂ sink (-170 gC/m2/yr). Thus, on average, air-water CO₂ fluxes damp estuary emissions. We find that the water CO₂ sink is strongest in the summer due to the growth of intense algal blooms which likely drawdown CO₂ via photosynthesis. Although the direction of air-water CO₂ flux is ultimately a function of surface carbon concentrations, we find that in the summer, sea-breeze is a near-daily forcing agent for air-water CO₂ fluxes, contributing up to 43% of the mean summer water CO₂ sink rate. (Chapter 3) The El Nino-Southern Oscillation (ENSO) in the equatorial Pacific is the dominant mode of global air-sea CO₂ flux interannual variability (IAV). Air-sea CO2 fluxes are driven by the difference between atmospheric and surface ocean pCO₂, with variability of the latter driving flux variability. Previous studies found that models in Coupled Model Intercomparison Project Phase 5 (CMIP5) failed to reproduce the observed ENSO-related pattern of CO₂ fluxes and had weak pCO₂ IAV, which were explained by both weak upwelling IAV and weak mean vertical DIC gradients. We assess whether the latest generation of CMIP6 models can reproduce equatorial Pacific pCO₂ IAV by validating models against observations-based data products. We decompose pCO₂ IAV into thermally and non-thermally driven anomalies to examine the balance between these competing anomalies, which explain the total pCO₂ IAV. The majority of CMIP6 models underestimate pCO₂ IAV, while they overestimate SST IAV. Insufficient compensation of non-thermal pCO₂ to thermal pCO₂ IAV in models results in weak total pCO₂ IAV. We compare the relative strengths of the vertical transport of temperature and DIC and evaluate their contributions to thermal and non-thermal pCO₂ anomalies. Model-to-observations-based product comparisons reveal that modeled mean vertical DIC gradients are biased weak relative to their mean vertical temperature gradients, but upwelling acting on these gradients is insufficient to explain the relative magnitudes of thermal and non-thermal pCO₂ anomalies. (Chapter 4) The ocean carbon sink has absorbed about 25% of anthropogenic emissions, thus mitigating the effects of climate change. Over time, the ocean carbon sink has grown almost proportionally with the growth of atmospheric CO₂ concentrations. However, natural variability in the ocean carbon sink combined with large uncertainties, makes it hard to distinguish changes in the ocean sink due to natural variability versus the forced-trend. Thus, there is a need to understand and quantify the variability in the ocean carbon sink. Using the LDEO-Hybrid Physics Data product (1959-2020), we assess the decadal variability of global air-sea CO₂ fluxes. Here, we compare regional contributions to the decadal variability of the global ocean carbon sink and evaluate global patterns of decadal changes to elucidate the mechanisms that drive the dominant mode of global air-sea CO₂ flux decadal variability. We find that the dominant mode of decadal air-sea CO₂ flux variability exhibits strong synchronous signals over the tropical Pacific and Southern Ocean. We suggest that the synchronicity between the tropical Pacific and the Southern Ocean is modulated by the Pacific Decadal Oscillation (PDO) index, which is connected to the Multivariate ENSO Index (MEI). The composite patterns over the tropical Pacific can be explained by ENSO-like mechanisms operating on the decadal timescale, while the composite patterns over the Southern Ocean show a different regime where the westerly winds weakened over the composite period, the mixed layer shoaled, and the Southern Ocean sink weakened. Using a box model, we show that this reduction in mixed layer entrainment drives an accumulation of DIC in the mixed layer, which, when amplified by the high Revelle factor in the Southern Ocean, results in a 14-fold amplification in the surface pCO₂, reducing the ocean's capacity to uptake CO₂.

No description available.

Cette thèse analyse (i) la variabilité pluviométrique contemporaine en Nouvelle-Calédonie et ses téléconnexions avec les températures de surface océanique (TSO) du Pacifique tropical et (ii) l'impact des anomalies atmosphériques sur l'activité des incendies estimés par satellites. L'objectif est de construire un modèle permettant de prévoir l'intensité de la saison des feux entre septembre et décembre (SOND). Le croisement de trois bases de données de feux détectés par satellites avec le réseau des stations météorologiques montre de forts déficits pluviométriques jusqu'à trois mois avant les feux. Ces déficits pluviométriques sont partiellement liés aux phases chaudes du phénomène El Niño Southern Oscillation (ENSO) et plus particulièrement à celles durant lesquelles les anomalies thermiques se situent à proximité de la ligne de changement de date équatoriale lors du printemps austral. Ces anomalies renforcent la circulation moyenne de Hadley et la subsidence au niveau des latitudes néo-calédoniennes. La téléconnexion entre les TSO du Pacifique central et les précipitations du Pacifique SW s'affaiblit à partir du mois de décembre au moment où l'ENSO atteint, paradoxalement, son intensité maximale. Cette modulation saisonnière est le produit d'une interaction entre (i) le cycle saisonnier des TSO brutes dans le Pacifique central, (ii) le cycle de vie des anomalies thermiques des épisodes chauds et (iii) l'intensité du gradient zonal des TSO le long de l'équateur. Une analyse en ondelettes montre que les pluies néo-calédoniennes sont également sensibles à des modes de variations plus lents (> 8 ans) du Pacifique central entre septembre et novembre. / This PhD analyses (i) New Caledonian rainfall variability and its relationships with sea surface temperature (SST) in the tropical Pacific ocean and (ii) the impacts of atmospheric variability on fire activity. Our main goal is to build an empirical statistical scheme for predicting the September to December fires. We examined the relationships between fires detected by ATSR and MODIS sensors and local-scale atmospheric conditions. While the signal in maximum temperature is weak and not robust among the fire records, the local-scale anomalies of rainfall are always clearly negative for at least 3 months before the fires. These rainfall anomalies are related to warm El Niño Southern Oscillation (ENSO) events and specially to those exhibiting highest SST anomalies in the central Pacific during the austral spring. The warm central Pacific events strengthen the southern Hadley cell around New Caledonian longitudes, with positive rainfall anomalies in the equatorial Pacific leading to an anomalous release of latent heat in the upper troposphere and an increased subsidence in the SW Pacific. Atmospheric anomalies are strongest in September–November because of a combination of a rather strong zonal SST gradient with the warmest SST in the equatorial Pacific just west of the dateline. Squared wavelet coherence between New Caledonia rainfall and Niño 4 SST index shows that their negative correlations are mostly carried by two distinct timescales : the classical ENSO variability and a quasi-decadal one, mainly during the September-November season.

The occurrence of wildfire is influenced by a suite of factors ranging from "top-down" influences (e. g., climate) to "bottom-up" localized influences (e. g., ignitions, fuels, and land use). We carried out the first broad-scale assessment of wildland fire patterns in northern Mexico to assess the relative influence of top-down and bottom-up drivers of fire in a region where frequent fire regimes continued well into the 20th century. Using a network of 67 sites, we assessed (1) fire synchrony and the scales at which synchrony is evident, (2) climate drivers of fire, and (3) asynchrony in fire regime changes. We found high fire synchrony across northern Mexico between 1750 and 2008, with synchrony highest at distances < 400 km. Climate oscillations, especially El Nino-Southern Oscillation, were important drivers of fire synchrony. However, bottom-up factors modified fire occurrence at smaller spatial scales, with variable local influence on the timing of abrupt, unusually long fire-free periods starting between 1887 and 1979 CE. Thirty sites lacked these fire-free periods. In contrast to the neighboring southwestern United States, many ecosystems in northern Mexico maintain frequent fire regimes and intact fire-climate relationships that are useful in understanding climate influences on disturbance across scales of space and time.

Central America is a region regularly affected by natural disasters, with most of them having a hydro-meteorological origin. Therefore, the understanding of annual changes of precipitation upon the region is relevant for planning and mitigation of natural disasters. This thesis focuses on studying the precipitation variability at annual scales in Central America within the framework of the Swedish Centre for Natural Disaster Science. The aims of this thesis are: i) to establish the main climate variability sources during the boreal winter, spring and summer by using different statistical techniques, and ii) to study the connection of sea surface temperature anomalies of the neighbouring oceans with extreme precipitation events in the region. Composites analysis is used to establish the variability sources during winter. Canonical correlation analysis is employed to explore the connection between the SST anomalies and extreme rainfall events during May-June and August-October. In addition, a global circulation model is used to replicate the results found with canonical correlation analysis, but also to study the relationship between the Caribbean Sea surface temperature and the Caribbean low-level jet. The results show that during winter both El Niño Southern Oscillation and the Pacific Decadal Oscillation, are associated with changes of the sea level pressure near the North Atlantic Subtropical High and the Aleutian low. In addition, the El Niño Southern Oscillation signal is intensified (destroyed) when El Niño and the Pacific Decadal Oscillation have the same (opposite) sign. Sea surface temperature anomalies have been related to changes in both the amount and temporal distribution of rainfall. Precipitation anomalies during May-June are associated with sea surface temperature anomalies over the Tropical North Atlantic region. Whereas, precipitation anomalies during August-September-October are associated with the sea surface temperature anomalies contrast between the Pacific Ocean and the Tropical North Atlantic region. Model outputs show no association between sea surface temperature gradients and the Caribbean low-level jet intensification. Canonical correlation analysis shows potential for prediction of extreme precipitation events, however, forecast validation shows that socio-economic variables must be included for more comprehensive natural disaster assessments.

Esta tese apresenta um estudo hidroclimático para a bacia hidrográfica do rio Piquiri-Pr, o trabalho desenvolve-se sob a égide sistêmica e a bacia hidrográfica entendida como unidade de análise da paisagem é a delimitação espacial escolhida para fornecer respostas à hipótese norteadora da pesquisa. O objetivo da pesquisa é compreender como ocorre a dinâmica hidroclimática na bacia em questão, para isso considera-se a precipitação pluviométrica e a vazão fluvial como elementos dinamizadores do sistema e o El Niño Oscilação Sul (ENOS) como o responsável por influenciar a variabilidade temporal desses elementos. Para isso buscou-se descrever estatisticamente a precipitação e a vazão fluvial, os dados de 41 postos pluviométricos foram fornecidos pelo Instituto das Águas do Paraná e os dados de três postos fluviométricos pela Agência Nacional de Águas (ANA) no período de 1976 a 2010. Por meio da correlação linear procurou-se explicar as relações existentes entre a precipitação pluviométrica e a vazão fluvial, calculou-se a evapotranspiração e o coeficiente de escoamento da bacia hidrográfica. Determinou-se a influência do ENOS na precipitação e na vazão fluvial utilizando-se dos dados do Índice de Oscilação Sul (IOS) disponibilizados pelo Bureau of Meteorology National Climate Centre da Austrália, a partir da correlação entre as variáveis e a aplicação do modelo GAMLSS (Modelos aditivos generalizados para posição, escala e forma) buscou-se explicar a participação do ENOS na variabilidade temporal da precipitação pluviométrica e da vazão. Com o desenvolvimento da pesquisa conclui-se que a vazão não é apenas resultante da precipitação, mas os processos de evapotranspiração e infiltração também influenciam na sua variabilidade. Com relação à influência do fenômeno ENOS, as correlações mostraram que o IOS influencia a precipitação e a vazão na bacia hidrográfica do rio Piquiri, com maior confiança, de um a três meses de defasagem, o modelo GAMLSS mostrou-se satisfatório para a determinação da influência do IOS nas variáveis precipitação e vazão. / This thesis presents a hydroclimatic study for the Piriqui watershed, Paraná State, Brazil. The study was developed under a systemic approach, and the watershed, understood as the unit of analysis of the landscape, was the spatial limit chosen to provide responses to the hypothesis guiding the research. The objective was to investigate the hydroclimatic dynamics of the watershed in question, considering precipitation and river flow as elements dynamizing the system and the El Niño Southern Oscillation (ENSO) as responsible for influencing the temporal variability of these elements. To this end, precipitation and river flow were described statistically. Data from 41 rain gauge stations in the period from 1976 to 2010 were provided by Instituto das Águas do Paraná and the river flow datas were provided by National Water Agency (Agência Nacional de Águas - ANA). Using linear correlation analysis, we explained the relationships existing between precipitation and river flow and calculated the evapotranspiration and the runoff coefficient of the drainage basin. The influence of ENSO on precipitation and river flow was determined using data from the Southern Oscillation Index (SOI) provided by the Bureau of Meteorology - National Climate Centre of Australia. The correlation between the variables and the application of the GAMLSS model allowed us to explain the participation of ENOS in the temporal variability of precipitation and flow. Through the development of the research, we concluded that the river flow results not only from precipitation, but the processes of evapotranspiration and infiltration may interfere with its variability. With respect to the influence of the ENSO phenomenon, the correlations revealed that SOI influences the precipitation and flow in the Piquiri watershed, with greater reliability with a delay of one to three months, and the GAMLSS model showed to be satisfactory for determining the influence of SOI on the precipitation and river flow variables.

Cette thèse analyse (i) la variabilité pluviométrique contemporaine en Nouvelle-Calédonie et ses téléconnexions avec les températures de surface océanique (TSO) du Pacifique tropical et (ii) l'impact des anomalies atmosphériques sur l'activité des incendies estimés par satellites. La Nouvelle-Calédonie figure aujourd'hui parmi les 34 " points chauds " de la biodiversité à l'échelle planétaire. A ce titre, l'un de nos objectifs est de construire un modèle permettant de prévoir l'intensité de la saison des feux entre septembre et décembre, qui correspond au maximum annuel. Le croisement de trois bases de données de feux détectés par satellites (MODIS, ATSR et LANDSAT) avec le réseau des stations météorologiques de Météo-France a montré qu'aucune anomalie thermique significative n'est détectée avant un feu, tandis que de forts déficits pluviométriques par rapport à la normale sont enregistrés jusqu'à trois mois avant le départ des feux. Ces déficits pluviométriques sont partiellement liés aux phases chaudes du phénomène El Niño Southern Oscillation (ENSO) et plus particulièrement à celles durant lesquelles les anomalies thermiques se situent à proximité de la ligne de changement de date équatoriale lors du printemps austral. Ces anomalies renforcent la circulation moyenne de Hadley (via le dégagement de chaleur latente dans la moyenne et haute troposphère) et la subsidence au niveau des latitudes néo-calédoniennes, tandis que les épisodes les plus intenses du siècle dernier (i.e. 1982-83 et 1997-98) dont les anomalies thermiques les plus importantes sont localisées dans le Pacifique oriental, sont associés à des conditions pluviométriques proches de la normale dans le Pacifique sud-ouest (SW). La téléconnexion entre les TSO du Pacifique central et les précipitations du Pacifique SW s'affaiblit à partir du mois de décembre au moment où l'ENSO atteint, paradoxalement, son intensité maximale. Cette modulation saisonnière est le produit d'une interaction entre (i) le cycle saisonnier des TSO brutes dans le Pacifique central, (ii) le cycle de vie des anomalies thermiques des épisodes chauds et (iii) l'intensité du gradient zonal des TSO le long de l'équateur. Par ailleurs, la convection profonde semble particulièrement sensible à la propagation vers l'est des anomalies de TSO faibles-à-modérées au niveau de la ligne de changement de date équatoriale (soit légèrement à l'ouest de la boîte Niño 3.4), modifiant significativement la position et l'intensité de la courroie de transmission des téléconnexions. Une analyse en ondelettes montre que les pluies néo-calédoniennes sont également sensibles à des modes de variations plus lents (> 8 ans) du Pacifique central entre septembre et novembre. La synchronisation entre la saisonnalité des feux et la prévisibilité saisonnière liée à l'ENSO permet d'estimer la surface brûlée totale en septembre-décembre à partir des états thermiques de l'océan Pacifique en juin-août, dont la polarité est clairement établie plusieurs mois à l'avance. La corrélation entre l'observation et la simulation du logarithme du total des surfaces brûlées en Nouvelle-Calédonie est de 0.87 sur la période 2000-2010 selon un modèle linéaire en validation croisée.

The background state and changes associated with the El Niño-Southern Oscillation (ENSO) in the tropical Pacific Ocean influence climate patterns all over the world. Understanding how the tropical Pacific will be impacted by climate change is therefore critical to accurate regional climate projections. However, sparse historical data coverage and strong natural variability in the basin make it difficult to assess the response of the tropical Pacific to anthropogenic climate change. Further, climate models disagree regarding the response of the basin to continued anthropogenic forcing into the future. Building off of the limited instrumental record, high-resolution records from coral and lake sediment archives can be used to assess the response of the tropical Pacific to past climate changes and to compare and assess climate model projections. In the present study, I use high-resolution coral and lake records from the equatorial Pacific to assess climate model projections and the response of the coupled ocean-atmospheric climate system in the basin (ocean temperature, salinity, winds, precipitation) to natural and anthropogenic forcing. Using a simple model of how climate is recorded by corals, we compare historical climate data and climate model simulations with coral paleoclimate records to assess climate model projections and address uncertainties in the historical data, models and paleoclimate records. We demonstrate that this simple model is able to capture variability and trend observed in the coral records, and show that the both sea surface temperature and salinity contribute to the observed coral trend. However, we find major discrepancies in the observed and climate model simulated trends in the tropical Pacific that may be attributed to uncertainties in model simulated salinity. We then assess 20th-century variability and trends in SST and salinity in the central tropical Pacific using replicated coral δ¹⁸O and Sr/Ca records from the Republic of Kiribati and the central Line Islands. We find that the coral records from these sites display a warming and freshening trend superimposed on strong interannual and low-frequency variability. Further, we demonstrate an apparent strengthening of the E-W SST gradient across the dateline (between 173°E and 160°W) and a slight weakening of the N-S SST gradient due to enhanced warming along the equator and west of the dateline relative to other sites. However, we find no evidence of increased variability in the central Pacific, suggesting that there has not been an increase in central Pacific style ENSO events. Finally, we show that the salinity response to climate change may be very patchy within the basin. Using a new ~90 year coral Mn/Ca record from the central Pacific, we investigate variability and trends in tropical Pacific trade winds. First, we demonstrate a strong association between westerly wind anomalies and coral skeletal Mn/Ca, which recorded all of the major historical El Niño events of the 20th century. In this new long Mn/Ca record, we find a reduction in the amplitude and frequency of Mn/Ca pulses between 1893 and 1982, suggesting a decrease in westerly wind anomalies in the western equatorial Pacific Ocean. Finally, we use a sediment record from Bainbridge Crater Lake, Galápagos Archipelago to assess variability in the eastern tropical Pacific over the past ~6 thousand years. Based on results from long-term monitoring of the lake, we propose a new climate interpretation of the sediment record and find further evidence reduced mid-Holocene ENSO variability and a ramp up of ENSO variability starting around 1775 cal. years BP.