Extratropical Research Articles

Unveiling the global influence of tropical cyclones on extreme waves approaching coastal areas

Tropical and extra-tropical storms generate extreme waves, impacting both nearby and remote regions through swell propagation. Despite their devastating effects in tropical areas, the contribution of tropical cyclones (TCs) to global wave-induced coastal risk remains unknown. Here, we enable a quantitative assessment of TC’s role in extreme waves approaching global coastlines, by designing twin oceanic wave simulations with and without realistic TC wind forcing. We find that TCs substantially contribute to extreme breaking heights in tropical regions (35-50% on average), reaching 100% in high-density TC areas like the North Pacific. TCs also impact remote TC-free regions, such as the equatorial Pacific experiencing in average 30% of its extreme wave events due to TCs. Interannual variability amplifies TC-induced wave hazards, notably during El Niño in the Central Pacific, and La Niña in the South China Sea, Caribbean Arc, and South Indian Ocean coastlines. This research offers critical insights for global risk management and preparedness.

A Machine Learning Bias Correction on Large‐Scale Environment of High‐Impact Weather Systems in E3SM Atmosphere Model

AbstractLarge‐scale dynamical and thermodynamical processes are common environmental drivers of high‐impact weather systems causing extreme weather events. However, such large‐scale environmental conditions often display systematic biases in climate simulations, posing challenges to evaluating high‐impact weather systems and extreme weather events. In this paper, a machine learning (ML) approach was employed to bias correct the large‐scale wind, temperature, and humidity simulated by the atmospheric component of the Energy Exascale Earth System Model (E3SM) at ∼1° resolution. The usefulness of the ML approach for extreme weather analysis was demonstrated with a focus on three high‐impact weather systems, including tropical cyclones (TCs), extratropical cyclones (ETCs), and atmospheric rivers (ARs). We show that the ML model can effectively reduce climate bias in large‐scale wind, temperature, and humidity while preserving their responses to imposed climate change perturbations. The bias correction is found to directly improve water vapor transport associated with ARs, and representations of thermodynamical flows associated with ETCs. When the bias‐corrected large‐scale winds are used to drive a synthetic TC track forecast model over the Atlantic basin, the resulting TC track density agrees better with that of the TC track model driven by observed winds. In addition, the ML model insignificantly interferes with the mean climate change signals of large‐scale storm environments as well as the occurrence and intensity of three weather systems. This study suggests that the proposed ML approach can be used to improve the downscaling of extreme weather events by providing more realistic large‐scale storm environments simulated by low‐resolution climate models.

Impact of Tropical and Extratropical Cyclones on Future U.S. Offshore Wind Energy

Impact of Tropical and Extratropical Cyclones on Future U.S. Offshore Wind Energy

Underestimates of Orographic Precipitation in Idealized Simulations. Part II: Underlying Causes

Abstract Warm-sector orographic precipitation in a midlatitude cyclone encountering a ridge is simulated in a “Cyc+Mtn” experiment. A second “Shear” simulation is conducted with horizontally uniform unidirectional flow over the same mountain having thermodynamic and cross-mountain wind profiles identical to those on the centerline in the “Cyc+Mtn” simulation. The relationship between integrated vapor transport (IVT) and orographic precipitation in the Mtn+Cyc case is consistent with observations, yet the same IVT in the Shear simulation produces far less precipitation. The difference between the precipitation rates in the Cyc+Mtn and Shear cases is traced to differences in the cross-mountain moisture-flux convergence and is further isolated to differences in the cross-mountain-velocity convergence over the windward slope. The winds at the ridge crest are stronger in the Shear case, leading to more velocity divergence and decreased moisture-flux convergence. The stronger ridge-crest winds in the Shear case are produced by a stronger mountain wave, which persists after being generated during the artificial startup of the Shear simulation. Initializing with a gradually ramped-up unidirectional flow and integrating to a quasi-steady state fails to adequately capture the processes regulating the leeside circulations. Even worse results are obtained if the shear flow is instantaneously accelerated from rest. An alternative microphysical explanation for the precipitation difference between the Cyc+Mtn and Shear simulations is examined using additional numerical experiments that enhance the seeder–feeder process. Although such enhancements increase precipitation, the increase is too small to account for the differences between the Cyc+Mtn and Shear simulations. Significance Statement Our results highlight the difficulty in conducting studies of mesoscale atmospheric circulations in isolation from their surrounding environment. This approach often allows for more computationally efficient and detailed analysis of the target phenomena by reducing the size of the computational domain. But, at least in the case of orographic precipitation, such isolation can significantly limit the realism of the simulated process.

Effective Visualization of Radar Data for Users Impacted by Color Vision Deficiency

Abstract Color vision deficiency (CVD) is a decreased ability to discern between particular colors. Eight percent of genetic males and half a percent of genetic females have some form of CVD, with many in the radar community falling into this group. When presenting data on a two-dimensional plane, it is common to use colors to represent values via a colormap. Colormap choice in the radar community is influenced by the ability to highlight scientifically interesting features in data, institutional choices, and domain dominance of legacy colormaps. The problem with these current colormaps is that many do not project well for those with CVD (i.e., green next to red). In working with the CVD community to address this problem, multiple colormaps for moments such as equivalent reflectivity factor and Doppler velocity were created for users with forms of CVD such as deuteranomaly, protanomaly, protanopia, and deuteranopia using Python tools such as colorspacious and viscm. We show how these colormaps can improve interpretability for four cases: a mesoscale convective system, a pyrocumulonimbus storm, a wintertime midlatitude cyclone, and widespread storms with a large bird migration. These new radar equivalent reflectivity factor, Doppler velocity, and polarization colormaps are designed to highlight rain, frozen precipitation, nonmeteorological targets, and velocity-based items, are perceptually uniform, and are visually friendly for those with CVD.

A Diagnostic Analysis of the Mechanisms for Arctic Cyclone Intensity Evolution

Abstract The intensity evolution of Arctic cyclones (ACs) is examined via cyclone parameter space and composite analyses based on approximately 18 000 AC tracks during 1979–2021. Cyclone parameter spaces are defined by various parameters representing the cyclone structure and physical processes relevant to cyclone development. It is shown that intensifying ACs are associated with diabatic heating and characterized by a cold core in both the lower and upper troposphere, as well as a thermally asymmetric and vertically tilted structure. In contrast, the decay phase is associated with diabatic cooling and characterized by a vertically aligned cyclone with reduced horizontal asymmetry. The cyclone parameter space analysis also indicates a warm core in the lower troposphere for a subset of ACs, which may reflect a frontal occlusion. The transition from AC intensification to decay, on average, is marked by a sharp decrease in both upward motion and diabatic heating, along with the vertical alignment of the cyclone structure. Following this transition, an upright cyclone may persist for a long time due to the weak background vertical wind shear, diabatic cooling, and weak Rossby wave energy dispersion. The evolution of ACs can thus be regarded as a two-stage process: a baroclinic development stage aided by diabatic heating, during which the AC evolution may conform with the Norwegian model for midlatitude cyclones, and a slow decay stage of an equivalent barotropic cyclone, which may leave a remnant tropopause polar vortex after the erosion of the surface circulation. Significance Statement Arctic cyclones are the primary weather system in the Arctic and are also an important component in the Arctic climate system owing to their role in modulating Arctic sea ice variability and poleward moisture and energy transport. We examined the cyclone structural characteristics and physical processes associated with the Arctic cyclone intensity evolution and proposed a two-stage conceptual model. A better understanding of the Arctic cyclone intensity change mechanisms will help us better anticipate the changes in Arctic cyclone activity in a warmer climate.

Underestimates of Orographic Precipitation in Idealized Simulations. Part I: Evidence from Unidirectional Warm-Sector Environments

Abstract Heavy precipitation in midlatitude mountain ranges is largely driven by the episodic passage of weather systems. Previous studies have shown a high correlation between the integrated vapor transport (IVT) in the airstream striking a mountain and the precipitation rate. Using data collected during the Olympic Mountain Experiment (OLYMPEX) project from a pair of sounding stations and a dense precipitation network, we further document the tight relation between IVT and precipitation rate and obtain results consistent with earlier work. We also survey previous studies that simulated orographic precipitation forced by unidirectional shear flows. Some of these simulations were performed using models that produce reasonably accurate rainfall totals in nested simulations of actual events driven by large-scale flows. Nevertheless, the increase in precipitation with IVT in all the simulations with unidirectional upstream flows is far lower than what would be expected based on the observationally derived correlation between IVT and precipitation rate. As a first step toward explaining this discrepancy, we conduct idealized simulations of a midlatitude cyclone striking a north–south ridge. The relationship between IVT and rainfall rate in this “Cyc+Mtn” simulation matches that which would be expected from observations. In contrast, when the conditions upstream of the ridge in the Cyc+Mtn case were used as upstream forcing in a horizontally uniform unidirectional flow with the same IVT over the same mountain ridge, far less precipitation was produced. These idealized simulations will, therefore, be used to study the discrepancy in rainfall between simulations driven by unidirectional shear flows and observations in a companion paper. Significance Statement Idealized simulations of orographic precipitation using horizontally uniform environmental forcing fail to capture the observed relationship between integrated water vapor flux impinging on the mountain and the precipitation rate. This suggests we need to improve the design of such idealized simulations.

Hazard assessment of dynamic marine processes during two extreme weather systems near the Shandong Peninsula

Typhoons and extratropical cyclones both occurred frequently on the Shandong Peninsula and have induced severe dynamic marine disasters. In this study, the typhoons and extratropical cyclones that occurred from 1979 to 2018 were counted and classified according to their tracks. We simulated the storm surge and disastrous waves using a coupled ocean-wave model and assessed the hazard distribution under different types of typhoons and extratropical cyclones. Through comparison, it was found that the southern Yellow Sea steering type typhoons and Yangtze-Huai cyclones occurred most frequently and were most likely to cause disasters. Regarding the hazard posed by the storm surge, there was a clear demarcation between the northern and southern coasts of the peninsula, as well as between the eastern and western coasts. The western coast of the peninsula was more exposed to storm surges induced by typhoons, while the northern coast of the peninsula was more exposed to storm surges induced by extratropical cyclones. The hazard posed by waves exhibited a localized characteristic. The wave hazard was the greatest near the peninsula cape and the lowest surrounding the Laizhou Bay, which was contrary to the distribution of the storm surge hazard. The identification of key management areas for disasters provides basic data and a technical reference for coastal development planning and can play a positive role in disaster prevention.

Mesoscale weather influenced by auroral gravity waves contributing to conditional symmetric instability release?

Abstract. We consider possible influence on severe weather occurrence in the context of solar wind coupling to the magnetosphere–ionosphere–atmosphere system, mediated by aurorally excited atmospheric gravity waves. Solar wind high-speed streams from coronal holes cause intensifications of ionospheric currents at high latitudes launching gravity waves propagating in the upper and lower atmosphere. While these gravity waves reach the troposphere with much attenuated amplitudes, they can contribute to conditional symmetric instability release and intensification of storms. Severe weather events, including winter storms and heavy rainfall causing floods and flash floods, show a tendency to follow arrivals of solar wind high-speed streams from coronal holes. The ERA5 re-analysis is used to evaluate slantwise convective available potential energy and vertically integrated extent of realizable symmetric instability to assess the likelihood of slantwise convection in frontal zones of extratropical cyclones during severe snowstorms and flash floods. The observed low-level southerly winds and high wind shears in these regions are favorable conditions for over-reflection of down-going aurorally excited gravity waves potentially contributing to conditional symmetric instability release leading to slantwise convection and high-rate precipitation.

Sources of nutrients fuelling post-flood phytoplankton biomass in a subtropical bay

Extreme rainfall from an ex-tropical cyclone caused a major flood event in the Logan River system in southeast Queensland, Australia. This resulted in a significant flood plume, containing nutrients and sediment, being discharged into the adjacent estuary/Bay system. The spatial extent of higher phytoplankton biomass (Chl a) matched the distribution of higher nutrient and sediment concentrations post-flood, suggesting nutrients fuelled phytoplankton production. Particulate nitrogen (PN) constituted over 50 % of total nitrogen in floodwaters, with lower proportions of dissolved inorganic nitrogen (DIN) and phosphate (PO4-P). Phytoplankton utilised DIN rapidly but may have maintained growth due to the release of ammonia from suspended sediments and microbial mineralisation of particulate organic nitrogen. Ammonia release from intertidal sediments contributed minimally (0.85 %) to daily phytoplankton nitrogen demands. Our study highlights the need to understand the fate of particulate nitrogen in coastal systems and its role in stimulating phytoplankton growth.

Poleward migration of tropical cyclones over the western North Pacific in the CMIP6-HighResMIP models constrained by observations

Tropical cyclones (TCs) have experienced poleward migration in recent years, but whether this exists in future projections with high-resolution climate models remains unclear. This study investigates the poleward migration of TCs over the western North Pacific (WNP) using CMIP6-HighResMIP models. We first assess the model performance in TC genesis frequency and latitude, which differ greatly from the observations, especially in winter and spring due to misinterpretation of extratropical storms. In this study, we put forward a revised constrained detection method based on the sea surface temperature (SST) and the atmospheric conditions to resolve this bias. Results indicate that the revised detection method has good performance in capturing the annual cycle of TC genesis frequency and latitude. Future projections constrained by this method show that the latitude of TC genesis and lifetime maximum intensity (LMI) both undergo a poleward shift, with the former being more significant. Spatial changes in the dynamic potential genesis index and large-scale environment could explain this shift. The regional changes of Hadley circulation and the role of global warming and internal variability are also discussed.

Storm Gust Prediction with the Integration of Machine Learning Algorithms and WRF Model Variables for the Northeast United States

Abstract Wind gusts are often associated with severe hazards and can cause structural and environmental damages, making gust prediction a crucial element of weather forecasting services. In this study, we explored the utilization of machine learning (ML) algorithms integrated with numerical weather prediction outputs from the Weather Research and Forecasting (WRF) Model, to align the estimation of wind gust potential with observed gusts. We have used two ML algorithms, namely, random forest (RF) and extreme gradient boosting (XGB), along with two statistical techniques: generalized linear model with identity link function (GLM-Identity) and generalized linear model with log link function (GLM-Log), to predict storm wind gusts for the northeast (NE) United States. We used 61 simulated extratropical and tropical storms that occurred between 2005 and 2020 to develop and validate the ML and statistical models. To assess the ML model performance, we compared our results with postprocessed gust potential from WRF. Our findings showed that ML models, especially XGB, performed significantly better than statistical models and Unified Post Processor for the WRF (WRF-UPP) Model and were able to better align predicted with observed gusts across all storms. The ML models faced challenges capturing the upper tail of the gust distribution, and the learning curves suggested that XGB was more effective than RF in generating better predictions with fewer storms.

Dynamical Tests of a Deep Learning Weather Prediction Model

Abstract Global deep learning weather prediction models have recently been shown to produce forecasts that rival those from physics-based models run at operational centers. It is unclear whether these models have encoded atmospheric dynamics or simply pattern matching that produces the smallest forecast error. Answering this question is crucial to establishing the utility of these models as tools for basic science. Here, we subject one such model, Pangu-Weather, to a set of four classical dynamical experiments that do not resemble the model training data. Localized perturbations to the model output and the initial conditions are added to steady time-averaged conditions, to assess the propagation speed and structural evolution of signals away from the local source. Perturbing the model physics by adding a steady tropical heat source results in a classical Matsuno–Gill response near the heating and planetary waves that radiate into the extratropics. A localized disturbance on the winter-averaged North Pacific jet stream produces realistic extratropical cyclones and fronts, including the spontaneous emergence of polar lows. Perturbing the 500-hPa height field alone yields adjustment from a state of rest to one of wind–pressure balance over ∼6 h. Localized subtropical low pressure systems produce Atlantic hurricanes, provided the initial amplitude exceeds about 4 hPa, and setting the initial humidity to zero eliminates hurricane development. We conclude that the model encodes realistic physics in all experiments and suggest that it can be used as a tool for rapidly testing a wide range of hypotheses.

Improving the Representation of Moisture and Convective Instability in Baroclinic-Wave Channel Simulations

Abstract We present an improved approach to generating moist baroclinically unstable background states for f-plane-channel simulations via potential vorticity (PV) inversion. Previous studies specified PV distributions with constant values in the troposphere and the stratosphere, but this produces unrealistic static-stability profiles that decrease sharply with height in the troposphere. Adding moisture to such environments can yield unrealistically large values of convective available potential energy (CAPE) even for reasonable relative humidity (RH) distributions. In our modified approach, we specify a PV distribution that increases with height in the troposphere and the stratosphere, yielding background states with more realistic values of static stability and CAPE. This modification produces environments that are better suited for representing moist processes, namely, deep convection, in idealized extratropical-cyclone simulations. Also, we present a method for introducing moisture that preserves a specified RH distribution while maintaining hydrostatic balance. Our approach allows for a large degree of control over the initial conditions, as background states with different jet strengths and shapes, average temperatures, moisture contents, or horizontal shears can easily be obtained without changing the underlying PV formula and inadvertently producing unreasonable values of static stability or CAPE. We demonstrate the characteristics of idealized extratropical cyclones developing in our background states by adding localized perturbations that represent an upper-level trough passing over a low-level frontal zone. In particular, we illustrate the impacts of horizontal shear, moisture, and grid spacing on baroclinic-wave development.

Processes and principles for producing credible climate change attribution messages: lessons from Australia and New Zealand

Extreme event attribution (EEA) information is increasingly in demand from climate services. EEA messages can: raise awareness about the effect climate change has already imposed, inform climate change liability conversations, and be combined with climate projections to inform adaptation. However, due to limitations in observations, models and methods, there are barriers towards operationalising EEA in practice. Operational services will need EEA to be done transparently and using preset formats. Here we review recent experience and practice in EEA in Australia and New Zealand with a view to inform the design of an EEA component of climate services. We present a flow chart of the processes involved, noting particular care is needed on the trigger, event definition, and climate model evaluation, with effective stage gates. We also promote the use of tailored causal network diagrams as a standard tool to inform an EEA study and communicate results, with particular care needed for messages on events with lower confidence or complex sets of influences, including tropical cyclones and extratropical cyclones. We suggest that extending EEA to impact attribution is essential for making EEA messages salient but requires an uplift in forming interdisciplinary teams and in granular exposure and vulnerability datasets and is likely to raise new interdisciplinary methodological questions. Finally, we suggest communication of EEA messages can learn more from its origins in medical epidemiology.

New perspectives on South Atlantic storm track through an automatic method for detecting extratropical cyclones' lifecycle

AbstractThis study introduces new insights into the climatology of South Atlantic (SAt) cyclones by employing a novel cyclone life cycle detection method, the CycloPhaser. Utilizing the minimum relative vorticity series and its derivative at the cyclone centre, the program effectively identifies distinct phases in the cyclone life cycle. Cyclone tracks are obtained through the analysis of relative vorticity at 850 hPa, using the ERA5 dataset. The study identified six main cyclone life cycle patterns from the analysis of 28,458 systems. The predominant cyclone type, accounting for approximately 60% of the analysed systems, exhibited a four‐phase configuration: incipient, intensification, mature and decay. Detailed statistics for each developmental phase and the overall life cycle are presented, offering valuable comparisons and new insights while corroborating previous research findings. Key genesis regions in the SAt are identified, along with track density maps that reveal distinct preferences in cyclone developmental cycle. The main outcome of this study is the demonstration that the automated classification procedure enables the analysis of cyclones' life cycles to be conducted promptly and with low computing costs, facilitating the comprehensive study of cyclone behaviour with high efficiency.

The performance of the CoMorph‐A convection package in global simulations with the Met Office Unified Model

AbstractThe impact on global simulations of a new package of physical parametrizations in the Met Office Unified Model is documented. The main component of the package is an entirely new convection scheme, CoMorph. This has a mass‐flux structure that allows initiation of buoyant ascent from any level and the ability for plumes of differing originating levels to coexist in a grid box. It has a different form of closure, where the mass flux of initiation is dependent on local instability, and an implicit numerical solution for detrainment that yields smooth timestep behaviour. The scheme is coupled more consistently to the cloud, microphysics, and boundary‐layer parametrizations and, as a result, significant changes to these have also been made. The package, called CoMorph‐A, has been tested in a variety of single‐column and idealized regimes. Here we test it in global configurations and evaluate it against observations using a range of standard metrics. Overall it is found to perform well against the control. Biases in the climatologies of the radiative fluxes are significantly reduced across the Tropics and subtropics, tropical and extratropical cyclone statistics are improved, and the Madden–Julian oscillation and other propagating tropical waves are strengthened. It also improves overall scores in numerical weather prediction trials, without revisions to the data assimilation. There is still work to do to improve the diurnal cycle of precipitation over land, where the peak remains too close to the middle of the day.

Response of the Southern Hemisphere extratropical cyclone climatology to climate intervention with stratospheric aerosol injection

Little is known about how climate intervention through stratospheric aerosol injection (SAI) may affect the climatology of the Southern Hemisphere extratropical cyclones under warming scenarios. To address this knowledge gap, we tracked extratropical cyclones from 2015 to 2099 in a set of projections of three international projects: the Assessing Responses and Impacts of Solar Climate Intervention on the Earth System with Stratospheric Aerosol Injection (ARISE), the Stratospheric Aerosol Geoengineering Large Ensemble (GLENS), and the Geoengineering Model Intercomparison Project (GeoMIP/G6sulfur). Comparisons were performed between no-SAI and SAI scenarios as well as between different timeslices and their reference period (2015–2024). Among the findings, both no-SAI and SAI project a decrease in cyclone frequency towards the end of the century although weaker under SAI scenarios. On the other hand, cyclones tend to be stronger under no-SAI scenarios while keeping their intensity more similar to the reference period under SAI scenarios. This means that under SAI scenarios the climatology of cyclones is less affected by global warming than under no-SAI. Other features of these systems, such as travelling distance, lifetime, and mean velocity show small differences between no-SAI and SAI scenarios and between reference and future periods.

A mechanism of stratospheric O3 intrusion into the atmospheric environment: a case study of the North China Plain

Abstract. Stratosphere-to-troposphere transport results in the stratospheric intrusion (SI) of O3 into the free troposphere through the folding of the tropopause. However, the mechanism of SI that influences the atmospheric environment through the cross-layer transport of O3 from the stratosphere and free troposphere to the atmospheric boundary layer has not been elucidated thoroughly. In this study, an SI event over the North China Plain (NCP; 33–40° N, 114–121° E) during 19–20 May 2019 was chosen to investigate the mechanism of the cross-layer transport of stratospheric O3 and its impact on the near-surface O3 based on multi-source reanalysis, observation data, and air quality modeling. The results revealed a mechanism of stratospheric O3 intrusion into the atmospheric environment induced by an extratropical cyclone system. The SI with downward transport of stratospheric O3 to the near-surface layer was driven by the extratropical cyclone system, with vertical coupling of the upper westerly trough, the middle of the northeast cold vortex (NECV), and the lower extratropical cyclone, in the troposphere. The deep trough in the westerly jet aroused the tropopause folding, and the lower-stratospheric O3 penetrated the folded tropopause into the upper and middle troposphere; the westerly trough was cut off to form a typical cold vortex in the upper and middle troposphere. The compensating downdrafts of the NECV further pushed the downward transport of stratospheric O3 in the free troposphere; the NECV activated an extratropical cyclone in the lower troposphere; and the vertical cyclonic circulation governed the stratospheric O3 from the free troposphere across the boundary layer top, invading the near-surface atmosphere. In this SI event, the average contribution of stratospheric O3 to near-surface O3 was accounted for at 26.77 %. The proposed meteorological mechanism of the vertical transport of stratospheric O3 into the near-surface atmosphere, driven by an extratropical cyclone system, could improve the understanding of the influence of stratospheric O3 on the atmospheric environment, with implications for the coordinated control of atmospheric pollution.

The bimodal regime of ocean waves and winds over the continental shelf of Maranhão

This study analyzed surface waves and winds from ERA5 reanalysis dataset in the Western Equatorial Atlantic (WEA) Ocean in order to properly understand the long-term regime of oceanic waves over the Continental Shelf of Maranhão (CSMA), pointing to the changes that occur at the western and eastern sectors as the waves propagate to the coastal zone. A data validation with observational data collected in situ by the PNBOIA and PIRATA projects has been conducted. The parameters analyzed were significant wave height (Hs), peak period (Tp), mean wave direction (Dm), and the zonal (u10) and meridional (v10) components of wind at 10 m. The analysis covered a period of 43 years (1979–2023). The monthly climatology of the Hs, Tp and Dm found has revealed a semi-annual cycle in the waves pattern in the region. The wave direction variability executes a clockwise motion from northeast-east-southeast throughout a year. The annual climatology analysis reveals that 59 % of sea states in the region are characterized by swell conditions in the region, while wind-sea dominates in 41 % of cases, with seasonal variability. During the austral summer (autumn), swells prevail over wind-seas along the WEA, accounting for 77 % (84 %) of the cases. This coincides with the boreal winter, characterized by high storm activity, whose storms generate swells that traverse the ocean, reaching Maranhão's waters and resulting in the highest waves in the region (3.54 m in summer). In contrast, during the spring season, sea states are dominated by wind-sea conditions (70 %) due to the increase in southeast trade winds. The highest Hs observed from wind-sea conditions is 2.94 m. The largest contribution for the maxima Hs in the region is given by swells in the summer/autumn and by wind-seas in the winter/spring seasons, with swells propagating more energy and power to cause erosional processes on the eastern sector of Maranhão coast. Oppositely, in the western sector of the littoral, whose geomorphology is dominated by tidal flats, cheniers plains and mangroves, the Hs is small, indicating a depositional environment. The algorithm indicated that swells generated by tropical (extratropical) storms typically take approximately 3.5–4.5 (6−7) days to travel the distance to the CSMA, while in the equatorial zone, the arrival of swells at the CSMA averages between 3.5 and 7 days. In this study we demonstrated the CSMA region is dominated by a bimodal wave regime with a strong semi-annual cycle, which is driven by the tropical/extratropical storms propagating from the North Atlantic Ocean toward the study area and by trade winds variability.