142 research outputs found

    Can rogue waves be predicted using characteristic wave parameters?

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    Rogue waves are ocean surface waves larger than the surrounding sea that can pose a danger to ships and offshore structures. They are often deemed unpredictable without complex measurement of the wave field and computationally intensive calculation which is infeasible in most applications, consequently there a need for fast predictors. Here we collate, quality control, and analyse the largest dataset of single‐point field measurements from surface following wave buoys to search for predictors of rogue wave occurrence. We find that analysis of the sea state parameters in bulk yields no predictors, as the subset of seas containing rogue waves sits within the set of seas without. However, spectral bandwidth parameters of rogue seas display different probability distributions to normal seas, but these parameters are rarely provided in wave forecasts. When location is accounted for, trends can be identified in the occurrence of rogue waves as a function of the average seas state characteristics at that location. These trends follow a power law relationship with the characteristic sea state parameters: mean significant wave height and mean zero up‐crossing wave period. We find that frequency of occurrence of rogue waves and their generating mechanism is not spatially uniform, and each location is likely to have its own unique sensitivities which increase in the coastal seas. We conclude that forecastable predictors of rogue wave occurrence will need to be location specific and reflective of their generation mechanism. Therefore, given location and a sufficiently long historical record of sea state characteristics, the likelihood of occurrence can be obtained for mariners and offshore operators. Plain Language Summary Rogue waves are waves much larger than expected for the surrounding sea state and their size and unexpected nature can pose a danger to ships and offshore structures. They are often thought to be unpredictable without complex computational calculation. Here we try to find the relationship between rogue wave occurrence and the characteristics of the sea state they occur in to circumnavigate this and allow prediction. Here we find that when all the data is analysed in bulk only weak relationships can be seen; however, when the data is analysed spatially relationships can be found between wave height and wave period and rogue wave occurrence. We find that the number of rogue waves and their cause differs spatially and note that each location is likely to have its own unique sensitivities which increase in the coastal seas. We conclude that forecastable predictors of rogue wave occurrence will need to be location specific, reflecting their cause. Therefore, given location and a sufficiently long historical record of sea state characteristics, the likelihood of occurrence can be obtained for mariners and offshore operators

    Seasonal intensification and trends of rogue wave events on the US western seaboard

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    Studies of changes in wave climate typically consider trends in sea state statistics, such as the significant wave height. However, the temporal variability of individual rogue waves, which pose a hazard to users of the sea and coastal environment has not been investigated. We use time series of continuous surface elevation over 124–270 months (spanning 1994–2016), from 15 wave buoys along the US western seaboard, to investigate regional trends in significant wave height and individual rogue waves. We find high spatial variability in trends in significant wave height and rogue waves across the region. Rogue wave occurrence displays a mostly decreasing trend, but the relative height – or severity – of the waves is increasing. We also identify seasonal intensification in rogue waves with increased rogue wave occurrence, of higher severity, in the winter than in the summer. Therefore, the common practice of stating a single occurrence likelihood for an ocean basin is not valid. In addition, the buoy data show that the magnitude and significance of trends in significant wave height increases towards higher percentiles, supporting previous findings

    Insights into decadal North Atlantic sea surface temperature and ocean heat content variability from an eddy-permitting coupled climate model

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    An ocean mixed layer heat budget methodology is used to investigate the physical processes determining subpolar North Atlantic (SPNA) sea surface temperature (SST) and ocean heat content (OHC) variability on decadal-multidecadal timescales using the state-of-the-art climate model HadGEM3-GC2. New elements include development of an equation for evolution of anomalous SST for interannual and longer timescales in a form analogous to that for OHC, parameterization of the diffusive heat flux at the base of the mixed layer and analysis of a composite AMOC event. Contributions to OHC and SST variability from two sources are evaluated i) net ocean-atmosphere heat flux and ii) all other processes, including advection, diffusion and entrainment for SST. Anomalies in OHC tendency propagate anticlockwise around the SPNA on multidecadal timescales with a clear relationship to the phase of the Atlantic meridional overturning circulation (AMOC). AMOC anomalies lead SST tendencies which in turn lead OHC tendencies in both the eastern and western SPNA. OHC and SST variations in the SPNA on decadal timescales are dominated by AMOC variability because it controls variability of advection which is shown to be the dominant term in the OHC budget. Lags between OHC and SST is traced to differences between the advection term for OHC and the advection-entrainment term for SST. The new results have implications for interpretation of variations in Atlantic heat uptake in the CMIP6 climate model assessment

    The Effects of Twitter Sentiment on Stock Price Returns

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    Social media are increasingly reflecting and influencing behavior of other complex systems. In this paper we investigate the relations between a well-know micro-blogging platform Twitter and financial markets. In particular, we consider, in a period of 15 months, the Twitter volume and sentiment about the 30 stock companies that form the Dow Jones Industrial Average (DJIA) index. We find a relatively low Pearson correlation and Granger causality between the corresponding time series over the entire time period. However, we find a significant dependence between the Twitter sentiment and abnormal returns during the peaks of Twitter volume. This is valid not only for the expected Twitter volume peaks (e.g., quarterly announcements), but also for peaks corresponding to less obvious events. We formalize the procedure by adapting the well-known "event study" from economics and finance to the analysis of Twitter data. The procedure allows to automatically identify events as Twitter volume peaks, to compute the prevailing sentiment (positive or negative) expressed in tweets at these peaks, and finally to apply the "event study" methodology to relate them to stock returns. We show that sentiment polarity of Twitter peaks implies the direction of cumulative abnormal returns. The amount of cumulative abnormal returns is relatively low (about 1-2%), but the dependence is statistically significant for several days after the events

    Ocean heat convergence and North Atlantic multidecadal heat content variability

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    We construct an upper ocean (0-1000m) North Atlantic heat budget (26°-67°N) for the period 1950-2020 using multiple observational datasets and an eddy-permitting global ocean model. On multidecadal timescales ocean heat transport convergence controls ocean heat content (OHC) tendency in most regions of the North Atlantic with little role for diffusive processes. In the subpolar North Atlantic (45°N-67°N) heat transport convergence is explained by geostrophic currents whereas ageostrophic currents make a significant contribution in the subtropics (26°N-45°N). The geostrophic contribution in all regions is dominated by anomalous advection across the time-mean temperature gradient although other processes make a significant contribution particularly in the subtropics. The timescale and spatial distribution of the anomalous geostrophic currents are consistent with a simple model of basin scale thermal Rossby waves propagating westwards/northwestwards in the subpolar gyre and multidecadal variations in regional OHC are explained by geostrophic currents periodically coming into alignment with the mean temperature gradient as the Rossby wave passes through. The global ocean model simulation shows that multidecadal variations in the Atlantic Meridional Overturning Circulation are synchronized with the ocean heat transport convergence consistent with modulation of the west-east pressure gradient by the propagating Rossby wave

    Community-Based Climate Change Adaptation Action Plans to Support Climate-Resilient Development in the Eastern African Highlands

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    Smallholder farmers in the Eastern African Highlands depend on rain-fed agriculture for their livelihoods. Climate adaptation and sustainable development goals must be targeted in an integrated way to better match farmers’ realities and address local priorities and vulnerabilities in these areas. To support climate-resilient development in the Eastern African Highlands, 224 local stakeholders were engaged in the development of community-based climate change adaptation action plans for the Jimma Highlands in Ethiopia, Taita Hills in Kenya and Mount Kilimanjaro in Tanzania. Participatory methods, high-resolution climate projections and the United Nations Development Programme’s (UNDP’s) guidelines were used in the design of these climate action plans with specific objectives to: 1) engage stakeholders to increase understanding of climate change impacts, adaptation options and their potential trade-offs, 2) build their capacities to design climate change adaptation projects, 3) empower stakeholders to identify existing vulnerabilities and enhance climate resilience and 4) strengthen networks to facilitate information access and sharing. Increased risk of water stress and reduction of agricultural productivity were the most frequently identified climate-change-induced problems in the three areas. The developed action plans target the underlying causes of these problems and describe sector-specific responses, activities, critical barriers and opportunities and support the National Adaptation Programmes of Action.Peer reviewe

    Full-depth temperature trends in the Northeastern Atlantic through the early 21st century

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    The vertical structure of temperature trends in the northeastern Atlantic (NEA) is investigated from a blend of Argo and hydrography data. The representativeness of sparse hydrography sampling in the basin-mean is assessed using a numerical model. Between 2003 and 2013, the NEA underwent a strong surface cooling (0-450?m) and a significant warming at intermediate and deep levels (1000?m-3000?m) that followed a strong cooling trend observed between 1988 and 2003. During 2003-2013, gyre-specific changes are found in the upper 1000?m (warming and cooling of the subtropical and subpolar gyres, respectively) whilst the intermediate and deep warming primarily occurred in the subpolar gyre, with important contributions from isopycnal heave and water mass property changes. The full-depth temperature change requires a local downward heat flux of 0.53?±?0.06?W?m?2 through the sea-surface, and its vertical distribution highlights the likely important role of the NEA in the recent global warming hiatus
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