Ocean tides worldwide have exhibited secular changes in the past century, simultaneous with a global secular rise in mean sea level (MSL). The combination of these two factors contributes to higher water levels, and may increase threats to coastal regions and populations over the next century. Equally as important as these long-term changes are the short-term fluctuations in sea levels and tidal properties. These fluctuations may interact to yield locally extreme water level events, especially when combined with storm surge. This study, presented in three parts, examines the relationships between tidal anomalies and MSL anomalies on yearly and monthly timescales, with a goal of diagnosing dynamical factors that may influence the long-term evolution of tides in the Pacific Ocean. Correlations between yearly averaged properties are denoted tidal anomaly trends (TATs), and will be used to explore interannual behavior. Correlations of monthly averaged properties are denoted seasonal tidal anomaly trends (STATs), and are used to examine seasonal behavior. Four tidal constituents are analyzed: the two largest semidiurnal (twice daily) constituents, M2 and S2, and the two largest diurnal (once daily) constituents, K1 and O1. Part I surveys TATs and STATs at 153 Pacific Ocean tide gauges, and discusses regional patterns within the entire Pacific Ocean. TATs with statistically significant relations between MSL and amplitudes (A-TATs) are seen at 89% of all gauges; 92 gauges for M2, 66 for S2, 82 for K1, and 59 for O1. TATs with statistically significant relations between tidal phase (the relative timing of high water of the tide) and MSL (P-TATs) are observed at 55 gauges for M2, 47 for S2, 42 for K1, and 61 for O1. Significant seasonal variations (STATs) are observed at about a third of all gauges, with the largest concentration in Southeast Asia. The effect of combined A-TATs was also considered. At selected stations, observed tidal sensitivity with MSL was extrapolated forward in time to the predicted sea level in 2100. Results suggest that stations with large positive combined A-TATs produce total water levels that are greater than those predicted by an increase in MSL alone, increasing the chances of high-water events. Conversely, negative correlation between sea level and tidal properties may mitigate somewhat against sea level rise; changes in total water levels in 2100 at stations with a negative combined A-TAT are less than that predicted by MSL rise alone. Climate change scenarios that take into account greater increases in MSL due to increased Antarctic ice melt show larger changes in total water levels over the same time period. Part II examines the mechanisms behind the yearly (TAT) variability in the Western Tropical Pacific Ocean. Significant amplitude TATs are found at more than half of 26 gauges for each of the two strongest tidal constituents, K1 (diurnal) and M2 (semidiurnal). For the lesser constituents analyzed (O1 and S2), significant trends are observed at ten gauges. Frictional mechanisms related to the El Nino Southern Oscillation (ENSO) are found to be important in influencing tides in the Western Pacific, as well as resonant triad interactions, a nonlinear coupling that exchanges energy between the M2, K1, and O1 tides. Both of these factors contribute to the observed tidal variability in the Solomon Sea region. Part III analyzes the seasonal behavior of tides (STATs) at twenty tide gauges in the Southeast Asian waters, which exhibit variation by 10-30% of mean tidal amplitudes. A barotropic ocean tide model that considers the seasonal effects of MSL, stratification, and geostrophic and Ekman velocity is used to explain the observed seasonal variability in tides due to variations in monsoon-influenced climate forcing, with successful results at about half of all gauges. The observed changes in tides are best explained by the influence of non-tidal velocities (geostrophic and Ekman), though the effect of changing stratification is also an important secondary causative mechanism. From the results of these surveys and investigations, it is concluded that short-term fluctuations in MSL and tidal properties at multiple time scales may be as important in determining the state of future water levels as the long-term trends. Global explanations for the observed tidal behavior have not been found in this study; however, significant regional explanations are found at the yearly time scale in the Solomon Sea, and at the seasonal time scale in Southeast Asia. It is likely that tidal sensitivity to annual and seasonal variations in MSL at other locations also are driven by locally specific processes, rather than factors with basin-wide coherence.

The ENSO-monsoon system is known to have a biennial component. Here we show using high resolution satellite data, mainly daily rainfall and sea surface temperature (SST) from the Tropical Rainfall Measuring Mission (TRMM), and daily scatterometer surface winds from QuickSCAT, that there is a clear biennial oscillation (TBO) in summer monsoon rainfall over Central India – Bay of Bengal (Cl-BoB) and the far west Pacific in the period 1999-2005. Summer (JJAS) mean rainfall oscillates between high and low values in alternate years; the rainfall is high in the odd years 1999, 2001, 2003, and 2005, and low in even years 2000, 2002 and 2004. The amplitude of the oscillation is significant, as measured against the long term standard deviation of seasonal rain based on 1979-2005 Global Precipitation Climatology Project (GPCP) data. We find that the TBO in rainfall is associated with TBO of SST over the tropical Indian, west Pacific and Atlantic Oceans in different seasons. There is no TBO in east Pacific SST, and no strong El Nino in this period. The TBO of SST is related to change in evaporation due to TBO of surface wind speed. A TBO of the surface branch of the Walker circulation in the eastern Indian and western Pacific basins is clearest in the autumn season during 1999-2005. There is a clear relation between a large-amplitude TBO of winter surface air temperature over north Asia associated with TBO of the Arctic oscillation (AO), and the TBO of summer monsoon rainfall. High rainfall over CI-BoB lin summer is followed by a relatively high value of the AO Index, and warm air termperature over north Asia in the succeeding winter. The Inter Tropical Convergence Zone(ITCZ) over the central Pacific and Atlantic Oceans shift north by about two degrees when the northern hemisphere is warm, reminiscent of the behaviour of the climate system of ENSO, decadal and palaeoclimate time scales. In this thesis we document the biennial oscillation of monsoon rain and its spatial structure in the recent period, and its relation with biennial oscillation of surface climate over the global tropics and extratropical regions. The existence of TBO in the tropical Atlantic, and its relation with the monsoon, is a new finding. We demonstrate that the interannual variability of the summer monsoon during 1999-2005, including the drought of 2002, is part of a pervasive TBO of global surface climate.

Indian summer monsoon (ISM) is an important component of the tropical climate system, known for its regular seasonality and abundance of rainfall over the country. The droughts and floods associated with the year-to-year variation of the average seasonal rainfall have devastating effect on people, agriculture and economy of this region. The demand for prediction of seasonal monsoon rainfall, therefore, is overwhelming. A number of attempts to predict the seasonal mean monsoon have been made over a century, but neither dynamical nor empirical models provide skillful forecasts of the extremes of the monsoon such as the unprecedented drought of 2002. This study investigates the problems and prospects of extended range monsoon prediction. An evaluation of the potential predictability of the ISM with the aid of an ensemble of Atmospheric General Circulation Model (AGCM) simulations indicates that the interannual variability (IAV) of ISM is contributed equally by the slow boundary forcing (‘externally’ forced variability) and the inherent climate noise (‘internal’ variability) in the atmosphere. Success in predicting the ISM would depend on our ability to extract the predictable signal from a background of noise of comparable amplitude. This would be possible only if the ‘external’ variability is separable from the ‘internal’ variability. A serious effort has been made to understand and isolate the sea surface temperature (SST) forced component of ISM variability that is not strongly influenced by the ‘internal’ variability. In addition, we have investigated to unravel the mechanism of generation of ‘internal’ IAV so that the method of isolating it from forced variability may be found. Since the primary forcing mechanism of the monsoon is the large-scale meridional gradient of deep tropospheric heat sources, large-scale changes in tropospheric temperature (TT) due to the boundary forcing can induce interannual variations of the timing and duration of the monsoon season. The concept of interannually varying monsoon season is introduced here, with the onset and withdrawal of monsoon definitions based on the reversal of meridional gradient of TT between north and south. This large scale definition of the monsoon season is representative of the planetary scale influence of the El Ni˜no Southern Oscillation (ENSO) on monsoon through the modification of TT and the cross equatorial pressure gradient over the ISM region. A sig- nificant relationship between ENSO and monsoon, that has remained steady over the decades, is discovered by which an El Ni˜no (La Ni˜na) delays (advances) the onset, advances (delays) the withdrawal and suppresses (enhances) the strength of the monsoon. The integral effect of the meridional gradient of TT from the onset to withdrawal proves to be a useful index of seasonal monsoon which isolates the boundary forced signal from the influence of internal variations that has remained steady even in the recent decades. However, consistent with the estimates of potential predictability, the boundary forced variability isolated with the above definitions explains only about 50% of the total interannual variability of ISM. Detailed diagnostics of the onset and withdrawal processes are performed to understand how the ENSO forcing modifies the onset and withdrawal, and thus the seasonal mean monsoon. It is found that during an El Ni˜no, the onset is delayed due to the enhanced adiabatic subsidence that inhibits vertical mixing of sensible heating from the warm landmass during pre-monsoon months, and the withdrawal is advanced due to the horizontal advective cooling. This link between ENSO and monsoon is realized through the advective processes associated with the stationary waves in the upper troposphere set up by the tropical ENSO heating. The remaining 50% of the monsoon IAV is governed by internal processes. To unravel the mechanism of the generation of internal IAV, we perform another set of AGCM simulations, forced with climatological monthly mean SSTs, to extract the pure internal IAV. We find that the spatial structure of the intraseasonal oscillations (ISOs) in these simulations has significant projection on the spatial structure of the seasonal mean and a common spatial mode governs both intraseasonal and interannual variability. Statistical average of ISO anomalies over the season (seasonal ISO bias) strengthens or weakens the seasonal mean. It is shown that interannual anomalies of seasonal mean are closely related to the seasonal mean of intraseasonal anomalies and explain about 50% of the IAV of the seasonal mean. The seasonal mean ISO bias arises partly due to the broadband nature of the ISO spectrum, allowing the intraseasonal time series to be aperiodic over the season and partly due to a non-linear process where the amplitude of ISO activity is proportional to the seasonal bias of ISO anomalies. The later relationship is a manifestation of the binomial character of the rainfall time series. The remaining part of IAV may arise due to the complex land-surface processes, scale interactions, etc. We also find that the ISOs over the ISM region are not significantly modulated by the Pacific and Indian Ocean SST variations. Thus, even with a perfect prediction of SST, only about 50% of the observed IAV of ISM could be predicted with the best model in forced mode. Even so, prediction of all India rainfall (AIR) representing the average conditions of the whole country and the season may not always serve the purposes of monsoon forecasting. One reason is the large inhomogeneities in the rainfall distribution during a normal seasonal monsoon. Agriculture and hydrological sector could benefit more if provided with regional scale forecasts of active/break spells 2-3 weeks ahead. Therefore, we advocate an alternative strategy to the seasonal prediction. Here, we present a method to estimate the potential predictability of active and break conditions from daily rainfall and circulation from observations for the recent 24 years. We discover that transitions from break to active conditions are much more chaotic than those from active to break, a fundamental property of the monsoon ISOs. The potential predictability limit of monsoon breaks (∼20 days) is significantly higher than that of the active conditions (∼10 days). An empirical real- time forecasting strategy to predict the sub-seasonal variations of monsoon up to 4 pentads (20 days) in advance is developed. The method is physically based, with the consideration that the large-scale spatial patterns and slow evolution of monsoon intraseasonal variations possess some similarity in their evolutions from one event to the other. This analog method is applied on NOAA outgoing longwave radiation (OLR) pentad mean data which is available on a near real time basis. The elimination of high frequency variability and the use of spatial and temporal analogs produces high and useful skill of predictions over the central and northern Indian region for a lead-time of 4-5 pentads. An important feature of this method is that, unlike other empirical methods to forecast monsoon ISOs, this uses minimal time filtering to avoid any possible end-point effects, and hence it has immense potential for real-time applications.

Ο Δείκτης της Νότιας Κύμανσης (Southern Oscillation Index, SOI) περιγράφει την ατμοσφαιρική κυκλοφορία στον τροπικό Α. Ειρηνικό Ωκεανό σε σχέση με το παγκόσμιας κλιματικής σημασίας φαινόμενο ΕΝSO (El Niño-Southern Oscillation). Μετά τα τέλη της δεκαετίας του 1970, βρέθηκε ότι η ισχυρότερη αρνητική φάση του Δείκτη της Νότιας Κύμανσης των τελευταίων 150 ετών συμπίπτει με την ισχυρότερη θετική φάση του Δείκτη της Βορειοατλαντικής Κύμανσης (North Atlantic Oscillation Index, ΝΑΟΙ) των τελευταίων 180 ετών. Μάλιστα, η ανάλυση της συσχέτισης μεταξύ των Δεικτών της Νότιας και της Βορειοατλαντικής Κύμανσης την περίοδο 1950-2007 εμφάνισε, αλλά μόνο μετά το 1978, μια στατιστικά σημαντική σχέση ανάμεσα στις 20 ισχυρότερες αρνητικές φάσεις των τριμήνων Σεπτεμβρίου-Νοεμβρίου του Δείκτη της Νότιας Κύμανσης και των χειμερινών (Δεκεμβρίου-Φεβρουαρίου) φάσεων του Δείκτη της Βορειοατλαντικής Κύμανσης που χρονικά ακολούθησαν. Παρόμοια, οι 20 ισχυρότερες θετικές φάσεις των τριμήνων Οκτωβρίου-Δεκεμβρίου και Νοεμβρίου-Ιανουαρίου του Δείκτη της Νότιας Κύμανσης την περίοδο 1950-2007 παρουσίασαν, μετά τα τέλη της δεκαετίας του 1970, μια στατιστικά σημαντική συσχέτιση με τις χειμερινές (Δεκεμβρίου-Φεβρουαρίου) φάσεις του Δείκτη της Βορειοατλαντικής Κύμανσης που ακολούθησαν. Οι σχέσεις αυτές συνέπεσαν με μειωμένη χειμερινή βροχόπτωση στην Α. Μεσόγειο. Εάν οι σχέσεις μεταξύ των Δεικτών της Νότιας και της Βορειοατλαντικής Κύμανσης διατηρηθούν και στο μέλλον, επιτρέπουν, με βάση τη φθινοπωρινή φάση του Δείκτη της Νότιας Κύμανσης, τη δυνατότητα πρόγνωσης της χειμερινής βροχόπτωσης στις περιοχές που επηρεάζονται από τη Βορειοατλαντική Κύμανση (ΝΑΟ) (για περιοχές της Ελλάδας ακόμη και με πιθανότητα 90%). Αναλύθηκε η ετήσια κατά πάχος προσαύξηση του βλαστού σε δένδρα χαλεπίου (Pinus halepensis subsp. halepensis) και τραχείας (Pinus halepensis subsp. brutia) πεύκης από τη θερμο-μεσογειακή ζώνη βλάστησης των νήσων Ζακύνθου, Σκύρου, Σάμου και Κρήτης (Ιεράπετρα). Το πλάτος των αυξητικών δακτυλίων τους βρέθηκε πολύ ευαίσθητο στις μεταβολές της βροχόπτωσης και εμφάνισε στατιστικά σημαντική συσχέτιση με το μέσο όρο της μέσης ετήσιας βροχόπτωσης 37 μετεωρολογικών σταθμών της Α. Μεσογείου. Οι αυξητικοί δακτύλιοι κατέδειξαν, επίσης, ότι η ετήσια βροχόπτωση στην Α. Μεσόγειο μετά το 1970 εμφάνισε τη σημαντικότερη μείωση τουλάχιστον των τελευταίων 200 ετών. Η μείωση της αύξησης των δένδρων συμπίπτει χρονικά με την πρόσφατη αύξηση της θερμοκρασίας του πλανήτη. Συμφωνεί, δε, με την πρόβλεψη, μέσω των κλιματικών μοντέλων, της Διακυβερνητικής Επιτροπής για την Kλιματική Aλλαγή (Intergovernmental Panel on Climate Change, IPCC, 2007) ότι στη Μεσόγειο αναμένεται σημαντική μείωση των βροχοπτώσεων λόγω περαιτέρω αύξησης της θερμοκρασίας του πλανήτη, πράγμα που παρατηρήθηκε στο πρόσφατο παρελθόν. Τίθεται, επίσης, το ερώτημα εάν οι πρόσφατες διασυνδέσεις μεταξύ των Δεικτών της Νότιας και της Βορειοατλαντικής Κύμανσης (που σχετίζονται με την πρόσφατη μείωση της βροχόπτωσης στην Α. Μεσόγειο) συνδέονται και με την αύξηση της θερμοκρασίας του πλανήτη. Κατά τις υγρές περιόδους του 20ου αιώνα (620-760 mm μέση ετήσια βροχόπτωση) η ετήσια αύξηση των δένδρων στις περιοχές μελέτης καθορίστηκε σε μεγάλο βαθμό από βροχοπτώσεις που σημειώθηκαν εντός λίγων εβδομάδων ή μηνών πριν ή και κατά την έναρξη της αυξητικής περιόδου. Όμως, κατά την ξηρότερη περίοδο που καταγράφηκε (1990-96, 480 mm μέση ετήσια βροχόπτωση) η αύξηση εξαρτήθηκε από βροχόπτωση 3-4 ετών πριν, συμπεριλαμβανομένου του έτους σχηματισμού του δακτυλίου. Αυτό δείχνει ότι το νερό από βαθύτερα στρώματα του εδάφους, συσσωρευμένο από βροχοπτώσεις προηγούμενων ετών, κατέστη ιδιαιτέρως σημαντικό καθώς η ξηρασία εντάθηκε. Αυτή η διαδικασία πρέπει να σχετίζεται με το βαθύ ριζικό σύστημα των πεύκων. Όμως, μια σειρά ξηρών ετών μπορεί να εξαντλήσει τα υπόγεια «αποθέματα υγρασίας». Στην περίπτωση αυτή τα πεύκα μπορεί να φτάσουν πολύ κοντά στα όρια της επιβίωσης τους, ακόμη και να ξεραθούν. Τέτοια περιστατικά καταγράφηκαν στη νήσο Σάμο και στην Αχαΐα (Πελοπόννησο), όπου ακόμη και 80-χρονα πεύκα ξεράθηκαν στο τέλος των καλοκαιριών του 2000 και 2007. Με βάση το σενάριο Α1B-SRES (IPCC 2007), η μέση ετήσια βροχόπτωση στην περιοχή μελέτης προβλέπεται να μειωθεί την περίοδο 2090-2099 ακόμη και κατά 30% σε σχέση με τα επίπεδα του 1980-1999, φτάνοντας τα 390 mm. Τα επίπεδα αυτά βροχόπτωσης είναι πολύ χαμηλότερα από την οριακή τιμή των 480 mm που προσδιορίστηκε ως κρίσιμη για τους μελετηθέντες πληθυσμούς πεύκων (Pinus) στο να επιβιώσουν υπό ξηρασία στηριζόμενοι σε βαθύτερα αποθέματα υγρασίας. Συνεπώς, εάν τέτοιες συνθήκες επικρατήσουν κατά τον 21ο αιώνα, θα αυξηθεί σημαντικά ο κίνδυνος καταστροφής των δασών της θερμο-μεσογειακής ζώνης βλάστησης. Επίσης, με τη συνδυασμένη επίδραση αυξημένων θερινών θερμοκρασιών και πυρκαγιών θα μεγαλώσει και ο κίνδυνος ερημοποίησης. / The Southern Oscillation Index (SOI) describes atmospheric circulation in the eastern tropical pacific related to ENSO (El Niño-Southern Oscillation), a phenomenon of global climatic significance. After the late 1970s, SOI’s strongest negative phase in 150 year was found to coincide with the strongest positive phase of the North Atlantic Oscillation Index (NAOI) in 180 years. Correlation analysis between SOI and NAOI during 1950-2007 revealed a statistically significant connection between the 20 strongest negative phases of the Sept.-Nov. SOI and the following winter’s (Dec.-Feb.) NAOI phases only after 1978. Similarly, the 20 strongest positive phases of the Oct.-Dec. and Nov.-Jan. SOI of 1950-2007 produced a statistically significant correlation with the following winter’s (Dec.-Feb.) NAOI phases after the late 1970s. Such relationships coincided with reduced winter precipitation in the eastern Mediterranean. If these SOI-NAOI connections hold, the possibility exists to forecast winter precipitation conditions in regions effected by NAO from the previous autumn’s SOI state (with even a 90% accuracy for regions of Greece). Annual radial stem increment was analysed in Pinus halepensis subsp. halepensis and Pinus halepensis subsp. brutia trees for Thermo-Mediterranean vegetation zones of the Greek islands of Zakinthos, Skiros, Samos and Crete (Ierapetra). Tree-ring width was found to be very sensitive to precipitation and produced a statistically significant correlation with annual rainfall from mean of 37 meteorological stations of the eastern Mediterranean. Tree-rings also indicated that annual rainfall reached its lowest values in nearly 200 years after the 1970s. This reduction in growth coincides with recent global warming. Thus, it is in line with IPCC (2007) climate model projections’ that the Mediterranean will experience a significant decline in precipitation as global warming progresses, as was the case in the recent past. It also raises the question whether recent SOI-NAOI links (involved in the recent decline in precipitation in the eastern Mediterranean) are also connected to global warming. During moist periods of the 20th century (ca. 620-720 mm average annual precipitation) annual tree growth in the regions under investigation was largely controlled by rainfall during a few weeks or months before or during the beginning of the growing season. In contrast, during the driest period on record (1990-1996; 480 mm average annual precipitation) growth depended on rainfall of 3-4 years before, including the year of tree ring formation. This suggests that water from deeper ground, accumulated during rainfall of previous years and became increasingly important as drought intensified. Deep rooting must be involved in such a process. However, a series of dry years may exhaust deeper ground “moisture reserves”. In this case pines may be pushed very close to their survival limits and can even be desiccated. Such incidents were recorded in Samos and Achaia (Peloponnesus) of Greece where pines died in late summer 2000 and 2007, including some 80-year-old trees. Mean annual precipitation for the studied area in 2090-2099 is projected to decrease by even 30% compared to 1980-99 levels, based on Α1B-SRES (IPCC 2007), reaching 390 mm. These levels of rainfall are far bellow the threshold of 480 mm determined as critical for the investigated populations of Pinus to survive drought by relying on deeper moisture reserves. Thus, if such conditions persist during the 21st century they will contribute to the risk of devastation for Thermo-Mediterranean zone forests. Combined with higher summer temperatures and fire outbreaks the risk of desertification will also increase.

Nas últimas décadas, particularmente desde a década de 1970, testemunhou-se um rápido recuo das geleiras em várias partes dos Andes tropicais. Uma tendência de aquecimento foi observada na região durante o mesmo período, com um hiato recente desde no início de 2010. No entanto, este hiato pode não ser o principal fator a influenciar as observações de aquecimento e recuo das geleiras em altitudes elevadas nos Andes tropicais. Com o surgimento de imagens de alta resolução espacial e espectral, e de modelos digitais de elevação (MDE) de alta resolução, agora é possível compreender as mudanças multitemporais das geleiras, o que era difícil de realizar utilizando as técnicas tradicionais e os dados de baixa resolução. Neste trabalho foram calculadas as variações da linha de neve das geleiras selecionadas ao longo dos Andes tropicais desde o início de 1980. A linha de neve máxima observada durante a estação seca (inverno austral) nos trópicos pode ser considerada como equivalente à linha de equilíbrio que separa a zona de acumulação da zona de ablação. A fim de reduzir o erro na estimativa da linha de neve foram consideradas somente as geleiras com declividades menores que 20o. Dependendo da região estudada e da presença de cobertura de nuvens, foram selecionadas imagens de várias fontes. As imagens da série Landsat (MSS, TM, ETM+ e OLI), EO1 OLI, ASTER e IRS LISS III foram usadas junto com MDE do ASTER GDEM-v2. Três bandas espectrais (TM5 - infravermelho médio, TM4- infravermelho próximo e TM2 - verde) foram utilizadas para calcular a linha de neve durante a estação seca, aplicando limiares adequados para TM4 e TM2. Os conjuntos de dados meteorológicos de várias fontes também foram analisados para observar as mudanças na precipitação, na temperatura e na umidade que influenciam os parâmetros glaciológicos como: o balanço de massa e a linha de equilíbrio. Geleiras representativas nos trópicos internos e trópicos externos foram consideradas separadamente dentro de um novo quadro, que foi baseado na precipitação, umidade e condições de temperatura ao longo da América do Sul. Neste âmbito, os Andes tropicais são classificados em trópicos internos, trópicos externos úmidos do norte, trópicos externos úmidos do sul e os trópicos externos secos. O Vulcão Cotopaxi no Equador (trópicos internos), o Nevado Caullaraju-Pastoruri que é uma geleira na Cordilheira Branca no Peru (trópicos externos úmidos do norte), o Nevado Cololo na Cordilheira Apolobamba na Bolívia (trópicos externos úmidos do sul), o Nevado Coropuna na Cordilheira Ampato no Peru e o Nevado Sajama na Cordilheira Ocidental da Bolívia (trópicos externos secos) são as geleiras representativas de cada grupo consideradas neste estudo. As geleiras tropicais nos trópicos internos, especialmente as situadas perto da Zona de Convergência Intertropicais (ZCIT), são mais vulneráveis a aumentos na temperatura e menos sensíveis a variações na precipitação. Em contraste, as geleiras nos trópicos externos respondem à variabilidade de precipitação muito rapidamente em comparação com a variação de temperatura, particularmente quando se deslocam para as regiões subtropicais. A dependência do balanço de massa sobre as características de sublimação também aumenta a partir dos trópicos internos para os trópicos externos. As condições de aquecimento, com maior umidade, tendem a aumentar a perda de massa por causa do derretimento em vez da sublimação. A elevação da umidade nos trópicos externos pode alterar as geleiras dominadas pela sublimação (nos trópicos externos e subtrópicos) e para as geleiras dominadas por derretimento. Observa-se que as geleiras próximas da ZCIT (trópicos internos e trópicosexternos úmidos do sul) estão recuando mais rapidamente como uma resposta ao aquecimento global, enquanto que as geleiras nos trópicos externos úmidos do norte e trópicos externos secos mostraram recuo relativamente mais lento. Possivelmente isso pode ser devido à ocorrência de fases frias do El Niño - Oscilação Sul (ENOS) conjuntamente com a Oscilação Decenal do Pacífico (ODP). As anomalias observadas nas variáveis meteorológicas seguem os padrões de ODP e as variações anuais de linha de neve seguem eventos de El Niño particularmente na fase ODP quente. No entanto, uma forte correlação entre as variações da linha de neve e dos fenômenos ENOS (e ODP) não está estabelecida. As geleiras do Equador mostram menos retração em resposta à tendência de aquecimento se comparadas às observações feitas por outros pesquisadores na Colômbia e na Venezuela, provavelmente devido à grande altitude das geleiras equatorianas. Em poucas palavras, as geleiras menores e em baixas altitudes nos trópicos internos e trópicos externos úmidos do sul estão desaparecendo mais rapidamente do que outras geleiras nos Andes tropicais. Também se observou neste estudo a existência de uma propriedade direcional no recuo das geleiras, o que não se observou em quaisquer outros estudos recentes. As geleiras nas cordilheiras leste do Peru e da Bolívia, que alimentam muitos rios nos lados leste das cordilheiras orientais, estão recuando do que aquelas geleiras situadas nas encostas ocidentais dos Andes tropicais. / Recent decades, particularly since the late 1970s, witnessed a rapid retreat of glaciers in many parts of the tropical Andes. A warming trend is observed in this region during the same period, with a recent hiatus since the early 2010s. However, this hiatus is observed to have not influenced the retreat of high elevation glaciers in the tropical Andes. Due to the emergence of high spatial and spectral resolution images and high quality digital elevation models (DEM), it is now possible to understand the multi-temporal glacier changes compared with the techniques that existed a few decades before. We calculated the snowline variations of selected glaciers along the tropical Andes since the early 1980s. The maximum snowline observed during the dry season (austral winter) in the tropics can be considered as nearly equivalent to the equilibrium line that separates the accumulation zone from the ablation zone. In order to reduce the error in the estimated snowline, glaciers with slopes < 20o only were considered in this research. Depending on the study region and the presence of cloud cover, images from multiple sources were selected. Landsat series (MSS, TM, ETM+, and OLI), EO1 OLI, ASTER, and IRS LISS III images were used along with digital elevation models (DEM) from ASTER GDEM-v2. Three wavebands (TM5 - Middle Infrared, TM4 - Near Infrared, and TM2 - Green) were used to calculate the dry season snowline, after applying suitable threshold values to TM4 and TM2. Meteorological datasets from multiple sources were also analysed to observe the changes in precipitation, temperature, and humidity that influence key glaciological parameters such as the mass balance and the equilibrium line. Representative glaciers in the inner and the outer tropical Andes were considered separately within a new framework, which is based on the precipitation, humidity, and temperature conditions along the South America. In this framework, tropical Andes are classified in to inner tropics, northern wet outer tropics, southern wet outer tropics, and dry outer tropics. Cotopaxi ice-covered volcano, Ecuador (inner tropics), Nevado Caullaraju-Pastoruri Glacier, Cordillera Blanca, Peru (northern wet outer tropics), Nevado Cololo, Cordillera Apolobamba, Bolivia (southern wet outer tropics), and Nevado Coropuna, Cordillera Ampato Peru and Nevado Sajama, Cordillera Occidental, Bolivia (dry outer tropics) are the representative glaciers in each group considered in this study. Inner tropical glaciers, particularly those situated near the January Intertropical Convergence Zone (ITCZ), are more vulnerable to increases in temperature and these glaciers are less sensitive to variations in precipitation. In contrast, outer tropical glaciers respond to precipitation variability very rapidly in comparison with the temperature variability, particularly when moving towards the subtropics. Mass balance dependency on sublimation characteristics also increases from the inner tropics to the outer tropics. Warming conditions with higher humidity tends to enhance mass loss due to melting rather than sublimation. Increased humidity observed in the outer tropics may change the sublimation dominated glaciers in the outer tropics and subtropics to melting dominated ones in the future. It is observed that the glaciers above and near the January ITCZ (inner tropics and southern wet outer tropics) are retreating faster as a response to global warming, whereas the glaciers in the northern wet outer tropics and dry outer tropics show relatively slower retreat. This can be possibly due to the occurrence of cold phases of El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) together. The observed anomalies in the meteorological variables slightly follow PDO patterns and the variations in annual snowlines follows El Niño events, particularly when in phase with warm PDO. However, a strong correlation between snowline variations and ENSO (and PDO) is not established. Mountain glaciers in Ecuador show less retreat in response to the warming trend compared with observations done by other researchers in Colombia and Venezuela, probably due to very high altitude of the Ecuadorean glaciers. In a nutshell, smaller glaciers at lower altitudes in the inner tropics and the southern wet outer tropics are disappearing faster than other glaciers in the tropical Andes. Another observation made in this study is the directional property of glacier retreat, which was not covered in any other recent studies. Those glaciers on the eastern cordilleras of Peru and Bolivia, which feed many rivers on the eastern sides of the eastern cordilleras, are retreating faster than those glaciers situated on the western sides.

Nas últimas décadas, particularmente desde a década de 1970, testemunhou-se um rápido recuo das geleiras em várias partes dos Andes tropicais. Uma tendência de aquecimento foi observada na região durante o mesmo período, com um hiato recente desde no início de 2010. No entanto, este hiato pode não ser o principal fator a influenciar as observações de aquecimento e recuo das geleiras em altitudes elevadas nos Andes tropicais. Com o surgimento de imagens de alta resolução espacial e espectral, e de modelos digitais de elevação (MDE) de alta resolução, agora é possível compreender as mudanças multitemporais das geleiras, o que era difícil de realizar utilizando as técnicas tradicionais e os dados de baixa resolução. Neste trabalho foram calculadas as variações da linha de neve das geleiras selecionadas ao longo dos Andes tropicais desde o início de 1980. A linha de neve máxima observada durante a estação seca (inverno austral) nos trópicos pode ser considerada como equivalente à linha de equilíbrio que separa a zona de acumulação da zona de ablação. A fim de reduzir o erro na estimativa da linha de neve foram consideradas somente as geleiras com declividades menores que 20o. Dependendo da região estudada e da presença de cobertura de nuvens, foram selecionadas imagens de várias fontes. As imagens da série Landsat (MSS, TM, ETM+ e OLI), EO1 OLI, ASTER e IRS LISS III foram usadas junto com MDE do ASTER GDEM-v2. Três bandas espectrais (TM5 - infravermelho médio, TM4- infravermelho próximo e TM2 - verde) foram utilizadas para calcular a linha de neve durante a estação seca, aplicando limiares adequados para TM4 e TM2. Os conjuntos de dados meteorológicos de várias fontes também foram analisados para observar as mudanças na precipitação, na temperatura e na umidade que influenciam os parâmetros glaciológicos como: o balanço de massa e a linha de equilíbrio. Geleiras representativas nos trópicos internos e trópicos externos foram consideradas separadamente dentro de um novo quadro, que foi baseado na precipitação, umidade e condições de temperatura ao longo da América do Sul. Neste âmbito, os Andes tropicais são classificados em trópicos internos, trópicos externos úmidos do norte, trópicos externos úmidos do sul e os trópicos externos secos. O Vulcão Cotopaxi no Equador (trópicos internos), o Nevado Caullaraju-Pastoruri que é uma geleira na Cordilheira Branca no Peru (trópicos externos úmidos do norte), o Nevado Cololo na Cordilheira Apolobamba na Bolívia (trópicos externos úmidos do sul), o Nevado Coropuna na Cordilheira Ampato no Peru e o Nevado Sajama na Cordilheira Ocidental da Bolívia (trópicos externos secos) são as geleiras representativas de cada grupo consideradas neste estudo. As geleiras tropicais nos trópicos internos, especialmente as situadas perto da Zona de Convergência Intertropicais (ZCIT), são mais vulneráveis a aumentos na temperatura e menos sensíveis a variações na precipitação. Em contraste, as geleiras nos trópicos externos respondem à variabilidade de precipitação muito rapidamente em comparação com a variação de temperatura, particularmente quando se deslocam para as regiões subtropicais. A dependência do balanço de massa sobre as características de sublimação também aumenta a partir dos trópicos internos para os trópicos externos. As condições de aquecimento, com maior umidade, tendem a aumentar a perda de massa por causa do derretimento em vez da sublimação. A elevação da umidade nos trópicos externos pode alterar as geleiras dominadas pela sublimação (nos trópicos externos e subtrópicos) e para as geleiras dominadas por derretimento. Observa-se que as geleiras próximas da ZCIT (trópicos internos e trópicosexternos úmidos do sul) estão recuando mais rapidamente como uma resposta ao aquecimento global, enquanto que as geleiras nos trópicos externos úmidos do norte e trópicos externos secos mostraram recuo relativamente mais lento. Possivelmente isso pode ser devido à ocorrência de fases frias do El Niño - Oscilação Sul (ENOS) conjuntamente com a Oscilação Decenal do Pacífico (ODP). As anomalias observadas nas variáveis meteorológicas seguem os padrões de ODP e as variações anuais de linha de neve seguem eventos de El Niño particularmente na fase ODP quente. No entanto, uma forte correlação entre as variações da linha de neve e dos fenômenos ENOS (e ODP) não está estabelecida. As geleiras do Equador mostram menos retração em resposta à tendência de aquecimento se comparadas às observações feitas por outros pesquisadores na Colômbia e na Venezuela, provavelmente devido à grande altitude das geleiras equatorianas. Em poucas palavras, as geleiras menores e em baixas altitudes nos trópicos internos e trópicos externos úmidos do sul estão desaparecendo mais rapidamente do que outras geleiras nos Andes tropicais. Também se observou neste estudo a existência de uma propriedade direcional no recuo das geleiras, o que não se observou em quaisquer outros estudos recentes. As geleiras nas cordilheiras leste do Peru e da Bolívia, que alimentam muitos rios nos lados leste das cordilheiras orientais, estão recuando do que aquelas geleiras situadas nas encostas ocidentais dos Andes tropicais. / Recent decades, particularly since the late 1970s, witnessed a rapid retreat of glaciers in many parts of the tropical Andes. A warming trend is observed in this region during the same period, with a recent hiatus since the early 2010s. However, this hiatus is observed to have not influenced the retreat of high elevation glaciers in the tropical Andes. Due to the emergence of high spatial and spectral resolution images and high quality digital elevation models (DEM), it is now possible to understand the multi-temporal glacier changes compared with the techniques that existed a few decades before. We calculated the snowline variations of selected glaciers along the tropical Andes since the early 1980s. The maximum snowline observed during the dry season (austral winter) in the tropics can be considered as nearly equivalent to the equilibrium line that separates the accumulation zone from the ablation zone. In order to reduce the error in the estimated snowline, glaciers with slopes < 20o only were considered in this research. Depending on the study region and the presence of cloud cover, images from multiple sources were selected. Landsat series (MSS, TM, ETM+, and OLI), EO1 OLI, ASTER, and IRS LISS III images were used along with digital elevation models (DEM) from ASTER GDEM-v2. Three wavebands (TM5 - Middle Infrared, TM4 - Near Infrared, and TM2 - Green) were used to calculate the dry season snowline, after applying suitable threshold values to TM4 and TM2. Meteorological datasets from multiple sources were also analysed to observe the changes in precipitation, temperature, and humidity that influence key glaciological parameters such as the mass balance and the equilibrium line. Representative glaciers in the inner and the outer tropical Andes were considered separately within a new framework, which is based on the precipitation, humidity, and temperature conditions along the South America. In this framework, tropical Andes are classified in to inner tropics, northern wet outer tropics, southern wet outer tropics, and dry outer tropics. Cotopaxi ice-covered volcano, Ecuador (inner tropics), Nevado Caullaraju-Pastoruri Glacier, Cordillera Blanca, Peru (northern wet outer tropics), Nevado Cololo, Cordillera Apolobamba, Bolivia (southern wet outer tropics), and Nevado Coropuna, Cordillera Ampato Peru and Nevado Sajama, Cordillera Occidental, Bolivia (dry outer tropics) are the representative glaciers in each group considered in this study. Inner tropical glaciers, particularly those situated near the January Intertropical Convergence Zone (ITCZ), are more vulnerable to increases in temperature and these glaciers are less sensitive to variations in precipitation. In contrast, outer tropical glaciers respond to precipitation variability very rapidly in comparison with the temperature variability, particularly when moving towards the subtropics. Mass balance dependency on sublimation characteristics also increases from the inner tropics to the outer tropics. Warming conditions with higher humidity tends to enhance mass loss due to melting rather than sublimation. Increased humidity observed in the outer tropics may change the sublimation dominated glaciers in the outer tropics and subtropics to melting dominated ones in the future. It is observed that the glaciers above and near the January ITCZ (inner tropics and southern wet outer tropics) are retreating faster as a response to global warming, whereas the glaciers in the northern wet outer tropics and dry outer tropics show relatively slower retreat. This can be possibly due to the occurrence of cold phases of El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) together. The observed anomalies in the meteorological variables slightly follow PDO patterns and the variations in annual snowlines follows El Niño events, particularly when in phase with warm PDO. However, a strong correlation between snowline variations and ENSO (and PDO) is not established. Mountain glaciers in Ecuador show less retreat in response to the warming trend compared with observations done by other researchers in Colombia and Venezuela, probably due to very high altitude of the Ecuadorean glaciers. In a nutshell, smaller glaciers at lower altitudes in the inner tropics and the southern wet outer tropics are disappearing faster than other glaciers in the tropical Andes. Another observation made in this study is the directional property of glacier retreat, which was not covered in any other recent studies. Those glaciers on the eastern cordilleras of Peru and Bolivia, which feed many rivers on the eastern sides of the eastern cordilleras, are retreating faster than those glaciers situated on the western sides.

We used long-term observations of grassland aboveground net plant production (ANPP, 19392016), growing seasonal advanced very-high-resolution radiometer remote sensing normalized difference vegetation index (NDVI) data (1982-2016), and simulations of actual evapotranspiration (1912-2016) to evaluate the impact of Pacific Decadal Oscillation (PDO) and El Nino-Southern Oscillation (ENSO) sea surface temperature (SST) anomalies on a semiarid grassland in northeastern Colorado. Because ANPP was well correlated (R-2 = 0.58) to cumulative April to July actual evapotranspiration (iAET) and cumulative growing season NDVI (iNDVI) was well correlated to iAET and ANPP (R-2 = 0.62 [quadratic model] and 0.59, respectively), we were able to quantify interactions between the long-duration (15-30 yr) PDO temperature cycles and annual-duration ENSO SST phases on ANPP. We found that during cold-phase PDOs, mean ANPP and iNDVI were lower, and the frequency of low ANPP years (drought years) was much higher, compared to warm-phase PDO years. In addition, ANPP, iNDVI, and iAET were highly variable during the cold-phase PDOs. When NINO-3 (ENSO index) values were negative, there was a higher frequency of droughts and lower frequency of wet years regardless of the PDO phase. PDO and NINO-3 anomalies reinforced each other resulting in a high frequency of above-normal iAET (52%) and low frequency of drought (20%) when both PDO and NINO-3 values were positive and the opposite pattern when both PDO and NINO-3 values were negative (24% frequency of above normal and 48% frequency of drought). Precipitation variability and subsequent ANPP dynamics in this grassland were dampened when PDO and NINO-3 SSTs had opposing signs. Thus, primary signatures of these SSTs in this semiarid grassland are (1) increased interannual variability in ANPP during cold-phase PDOs, (2) drought with low ANPP occurring in almost half of those years with negative values of PDO and NINO-3, and (3) high precipitation and ANPP common in years with positive PDO and NINO-3 values.

Porites lobata is an important reef building coral in the tropical eastern Pacific and the dominant Porites species in the Galápagos archipelago. Following the 1982-83 El Niño-Southern Oscillation the Galápagos Islands experienced 97-99% coral mortality, leaving many areas throughout the archipelago denuded of corals. Because very few long term assessments have been conducted on the growth and resilience of P. lobata to natural disturbances in the Galápagos Islands (Glynn et al., 2001; Glynn et al., 2009), benthic surveys were performed on a uniquely dense aggregation of P. lobata colonies at Devil’s Crown, Floreana Island between 1993 and 2011. Annual changes in live tissue area were calculated for the majority of the population (n=17) using Coral Point Count with Excel extensions (CPCe 3.6) software to determine growth and recovery trends for this aggregation. Total live tissue area (n=10) increased from 1993 to 2011, however due to high interannual variability this increase was not significant. Within this overall pattern, a general trend of decline was observed in live tissue cover from 1993 to 2000, with increases in tissue area observed from 2000 to 2011. Severe bleaching (85-100%) was observed during the 1998 survey, followed by 42% tissue loss (n=10), coinciding with sea water warming associated with the very strong 1997-1998 El Niño-Southern Oscillation event. Subsequent regrowth of coral tissue was observed during the 2001 survey with continued recovery through 2009. Multiple comparison testing revealed a significant difference between the impacted state (1999) and the recovered state (2009), (p = 0.002, Dunn’s method, n=17), suggesting this aggregation required a period of ten years to recover from this disturbance. During this recovery period the moderately strong 2007-2008 La Niña, with accompanying stressful low temperatures, occurred but did not interrupt tissue regrowth. Warmer than average sea surface temperatures occurred during the warm months from 2008 to 2011, during which time a cool period occurred from 2010 to 2011. While the magnitude and duration of temperature anomalies during warming were not as great as those observed during the 1997-98 ENSO, low temperatures observed during the cool period were similar to those experienced throughout the 2007-08 La Niña. During this time total live tissue cover was reduced by 19% (n=10); however it is unknown whether this was due to warming or the following cool period. Based on results from the 1997-98 El Niño and 2007-08 La Niña, this reduction in live tissue was most likely caused by elevated sea surface temperatures. Data on the growth and resilience of P. lobata populations at Devil’s Crown will be used for conservation and management of this important resource.

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