Synoptic-scale Events Research Articles

Anatomy of Summertime Upslope Events in Northeastern Colorado: Ammonia (NH3) Transport to the Rocky Mountains.

The Transport and Transformation of Ammonia (TRANS2Am) airborne field campaign occurred over northeastern Colorado during the summers of 2021 and 2022. A subset of the TRANS2Am flights investigated easterly wind conditions capable of moving agricultural emissions of ammonia (NH3) through urban areas and into the Rocky Mountains. TRANS2Am captured 6 of these events, unveiling important commonalities. (1) NH3 enhancements are present over the mountains on summer afternoons when easterly winds are present in the foothills region. (2) The abundance of gas-phase NH3 is 1 and 2 orders of magnitude higher than particle-phase NH4+ over the mountains and major agricultural sources, respectively. (3) During thermally driven circulation periods, emissions from animal husbandry sources closer to the mountains likely contribute more to the NH3 observed over the mountains than sources located further east. (4) Transport of plumes from major animal husbandry sources in northeastern Colorado westward across the foothills requires ∼5 h. (5) Winds drive variability in the transport of NH3 into nearby mountain ecosystems, producing both direct plume transport and recirculation. A similar campaign in other seasons, including spring and autumn, when synoptic scale events can produce sustained upslope transport, would place these results in context.

An East Antarctic, sub-annual resolution water isotope record from the Mount Brown South Ice core

We report high resolution measurements of the stable water isotope ratios (δ18O, δD) from the Mount Brown South ice core (MBS, 69.11° S 86.31° E). The record covers the period 873 - 2009 CE with sub-annual temporal resolution. Preliminary analyses of surface cores have shown the Mount Brown South site has relatively high annual snowfall accumulation (0.3 metres ice equivalent) with a seasonal bias toward lower snowfall during austral summer. Precipitation at the site is frequently related to intense, short term synoptic scale events from the mid-latitudes of the southern Indian Ocean. Higher snowfall regimes are associated with easterly winds, while lower snowfall regimes are associated with south-easterly winds. Isotope ratios are measured with Infra-Red Cavity Ring Down Spectroscopy, calibrated on the VSMOW/SLAP scale and reported on the MBS2023 time scale interpolated accordingly. We provide estimates for measurement precision and internal accuracy for δ18O and δD.

Winter warm spells over Italy: Spatial–temporal variation and large‐scale atmospheric circulation

AbstractThis article analyses the winter warm spells (WWS) that occurred in central Mediterranean over the period 1993–2022, examining the daily maximum temperatures collected at eight airport sites located in the Italian Peninsula, belonging to different climate zones. According to the definition proposed in 1999 by the Expert Team on Climate Change Detection and Indices (ETCCDI), a WWS is a sequence of at least six consecutive days when daily maximum air temperature exceeds the calendar day 90th percentile centred on a 5‐day window for a base period. WWS occurring over the entire Italian territory or only over northern/central/southern Italy have been identified and related to the peculiar synoptic conditions. It was found that December is the month most prone to WWS and, on average, WWS last 9.4 days in northern Italy, 6.6 days in central Italy, and 8.5 days in southern Italy. Over the period under investigation, the Italian Peninsula experienced only one common event characterized by persistent high‐pressure systems associated with air subsidence over western Mediterranean and, therefore, with exceptional warming. Finally, it has been proven that the definition of WWS proposed by ETCCDI allows to capture synoptic scale events but, in orographically complex areas such as Italy, underestimates moderate spells, which generally might have a duration of at least 3 days. Consequently, it is important to consider the possibility of reducing the period length threshold used for the detection of WWS when orographically heterogeneous regions are studied.

Sensitivity of Simulated Conditions to Different Parameterization Choices Over Complex Terrain in Central Chile

This study evaluates the performance of fourteen high-resolution WRF runs with different combinations of parameterizations in simulating the atmospheric conditions over the complex terrain of central Chile during austral winter and spring. We focus on the validation of results for coastal, interior valleys, and mountainous areas independently, and also present an in-depth analysis of two synoptic-scale events that occurred during the study period: a frontal system and a cut-off low. The performance of the simulations decreases from the coast to higher altitudes, even though the differences are not very clear between the coast and interior valleys for 10 m wind speeds and precipitation. The simulated vertical profiles show a warmer and drier boundary layer and a cooler and moister free atmosphere than observed. The choice of the land-surface model has the largest positive impact on near-surface variables with the five-layer thermal diffusion scheme showing the smallest errors. Precipitation is more sensitive to the choice of cumulus parameterizations, with the simplified Arakawa–Schubert scheme generally providing the best performance for absolute errors. When examining the performance of the model simulating rain/no-rain events for different thresholds, also the cumulus parameterizations better represented the false alarm ratio (FAR) and the bias score (BS). However, the Morrison microphysics scheme resulted in the best critical success index (CSI), while the probability of detection (POD) was better in the simulation without analysis nudging. Overall, these results provide guidance to other researchers and help to identify the best WRF configuration for a specific research or operational goal.

Calibrated forecasts of quasi‐periodic climate processes with deep echo state networks and penalized quantile regression

AbstractAmong the most relevant processes in the Earth system for human habitability are quasi‐periodic, ocean‐driven multi‐year events whose dynamics are currently incompletely characterized by physical models, and hence poorly predictable. This work aims at showing how (1) data‐driven, stochastic machine learning approaches provide an affordable yet flexible means to forecast these processes; (2) the associated uncertainty can be properly calibrated with fast ensemble‐based approaches. While the methodology introduced and discussed in this work pertains to synoptic scale events, the principle of augmenting incomplete or highly sensitive physical systems with data‐driven models to improve predictability is far more general and can be extended to environmental problems of any scale in time or space.

Measurement report: Assessment of Asian emissions of ethane and propane with a chemistry transport model based on observations from the island of Hateruma

Abstract. The island of Hateruma is the southernmost inhabited island of Japan. Here we interpret observations of ethane (C2H6) and propane (C3H8) together with carbon monoxide (CO), nitrogen oxides (NOx and NOy) and ozone (O3) carried out in the island in 2018 with the GEOS-Chem atmospheric chemistry transport model. We simulated the mixing ratios of these species within a nested grid centred over the site, with a model resolution of 0.5∘ × 0.625∘. We use the Community Emissions Data System (CEDS) dataset for anthropogenic emissions and add a geological source of C2H6 and C3H8. The model captured the seasonality of primary pollutants (CO, C2H6, C3H8) at the site – high mixing ratios in the winter months when oxidation rates are low and flow is from the north and low mixing ratios in the summer months when oxidation rates are higher and flow is from the south. It also simulates many of the synoptic-scale events with Pearson's correlation coefficients (r) of 0.74, 0.88 and 0.89 for CO, C2H6 and C3H8, respectively. Mixing ratios of CO are simulated well by the model (slope of the linear fit between model results and measurements is 0.91), but simulated mixing ratios of C2H6 and C3H8 are significantly lower than the observations (slopes of the linear fit between model results and measurements are 0.57 and 0.41, respectively), most noticeably in the winter months. Simulated NOx mixing ratios were underestimated, but NOy appears to be overestimated. The mixing ratio of O3 is moderately well simulated (slope of the linear fit between model results and observations is 0.76, with an r of 0.87), but there is a tendency to underestimate mixing ratios in the winter months. By switching off the model's biomass burning emissions we show that during winter, biomass burning has limited influence on the mixing ratios of compounds but can represent a more sizeable fraction in the summer. We also show that increasing the anthropogenic emissions of C2H6 and C3H8 within the domain by factors of 2.22 and 3.17 increases the model's ability to simulate these species in the winter months, consistent with previous studies.

Extreme Surface Energy Budget Anomalies in the High Arctic in Winter

Abstract In recent decades, the Arctic has warmed faster than the global mean, especially during winter. This has been attributed to various causes, with recent studies highlighting the importance of enhanced downward infrared radiation associated with anomalous inflow of warm, moist air from lower latitudes. Here, we study wintertime surface energy budget (SEB) anomalies over Arctic sea ice on synoptic time scales, using ERA5 (1979–2020). We introduce a new algorithm to identify areas with extreme, positive daily mean SEB anomalies and connect them to form spatiotemporal life cycle events. Most of these events are associated with large-scale inflow from the Atlantic and Pacific Oceans, driven by poleward deflection of the storm track and blocks over northern Eurasia and Alaska. Events originate near the ice edge, where they have roughly equal contributions of net longwave radiation and turbulent fluxes to the positive SEB anomaly. As the events move farther into the Arctic, SEB anomalies decrease due to weakening sensible and latent heat-flux anomalies, while the surface temperature anomaly increases toward the peak of the events along with the downward longwave radiation anomaly. Due to these temporal and spatial differences, the largest SEB anomalies are not always related to strongest surface warming. Thus, studying temperature anomalies alone might not be sufficient to determine sea ice changes. This study highlights the importance of turbulent fluxes in driving SEB anomalies and downward longwave radiation in determining local surface warming. Therefore, both processes need to be accurately represented in climate models. Significance Statement Mechanisms behind wintertime rapid Arctic warming and sea ice growth changes are not well understood. While much is known about the impact of radiative fluxes on both sea ice variability and surface warming, the relative importance of radiative and turbulent fluxes remains unclear. The purpose of this study is to clarify what controls surface energy budget (SEB) anomalies over sea ice. Along the life cycle of synoptic-scale events, positive SEB anomalies are shown to decrease and surface temperature anomalies increase after their onset. Additionally, variations in SEB anomalies are primarily controlled by turbulent fluxes, while downward longwave radiative fluxes are mainly responsible for surface temperature variations. These results highlight the need for accurate representations of these fluxes for predicting future Arctic climate.

Identifying an Evaporative Thermal Refugium for the Preservation of Coral Reefs in a Warming World—The Gulf of Eilat (Aqaba)

AbstractCoral bleaching events are more frequent and severe as global temperatures rise. However, analysis of the surface energy fluxes that substantially affect the thermoregulation of shallow reef environments is rare. Previous work on evaporative cooling in a humid reef environment led to the hypothesis that in proximity to desert regions, drier air enhances evaporative cooling of water overlying coral reefs, reducing the risk of extreme high‐water temperatures. Eddy Covariance measurements made over a shallow desert fringing coral reef in the Gulf of Eilat (Israel) during summer measured evaporation of 10.3 mm d−1 which is maintaining water temperature much lower than the overlying air temperature. In addition, measurements of the most severe marine heatwave that was ever documented in the region are presented as a case study for future reference to the processes that lead to and proceed a marine heatwave, which is induced by synoptic scale events that cause a ∼50% reduction in evaporation rate. Results are compared to similar measurements from a tropical coral reef, where a lower evaporation rate suppressed by higher humidity is unable to offset the heating of water overlying the reef. We conclude that evaporative cooling is a key mechanism protecting coral reefs located in deserts from extreme high‐water temperatures, thereby representing possible thermal refugium for corals against background global warming.

Isotopic Composition of Precipitation in a Southeastern Region of Brazil during the Action of the South Atlantic Convergence Zone

The use of stable isotopes of hydrogen and oxygen is a tool widely used to trace water paths along the hydrological cycle, providing support for understanding climatic conditions in different spatial scales. One of the main synoptic scale events acting in southeastern Brazil is the South Atlantic Convergence Zone (SACZ), which causes a large amount of precipitation from southern Amazonia to southeastern Brazil during the southern summer. In order to determine the isotopic composition of precipitation during the action of SACZ in São Francisco Xavier in southeastern Brazil, information from the Weather Forecasting and Climate Studies Center of the National Institute for Space Research (CPTEC) was used regarding SACZ performance days, the retrograde trajectories of the HYSPLIT model, and images from the GOES-16 satellite, in addition to the non-parametric statistical tests by Spearman and Kruskal–Wallis. A high frequency of air mass trajectories from the Amazon to southeastern Brazil was observed when the SACZ was operating. During the SACZ events, the average isotopic composition of precipitation was more depleted, with a δ18O of −9.9‰ (±2.1‰), a δ2H of −69.3‰ (±17.9‰), and d-excess of 10.1‰ (±4.0‰). When disregarding the SACZ performance, the annual isotopic composition can present an enrichment of 1.0‰ for δ18O and 8.8‰ for the δ2H. The long-term monitoring of trends in the isotopic composition of precipitation during the SACZ events can assist in indicating the evapotranspiration contribution of the Amazon rainforest to the water supply of southeastern Brazil.

Asymmetric air-sea heat flux response and ocean impact to synoptic-scale atmospheric disturbances observed at JKEO and KEO buoys

This study aims to identify patterns of surface heat fluxes, and corresponding surface ocean responses, associated with synoptic-scale atmospheric events and their modulation on seasonal time scales. In particular, northerly and southerly wind events associated with atmospheric disturbances were analyzed using high-temporal resolution time-series data from two moored buoys (JKEO: 2007–2010 and KEO: 2004–2019) north and south of the Kuroshio Extension current. Although each synoptic-scale wind event generally impacted both sites, the composite surface heat flux was larger at the northern site, especially for northerly events. Both types of wind events were observed throughout the year, with a minimum during June-July–August. Northerly wind events tended to be accompanied by lowered air-temperature, while southerly events tended to have elevated air-temperature relative to the previous three days. The resulting anomalous surface heat loss was asymmetric, with larger changes in northerly events compared to the southerly events. A large and significant ocean response of − 0.28 to − 0.46 K (p-value < 0.05) in SST was confirmed only for northerly events in spring–summer at the northern site, while smaller changes were found at the southern site. The results of this study suggest that sub-monthly air-sea interactions may affect seasonal variability and potentially climate change over longer timescales.

Future Antarctic snow accumulation trend is dominated by atmospheric synoptic-scale events

Over the last century, the increase in snow accumulation has partly mitigated the total dynamic Antarctic Ice Sheet mass loss. However, the mechanisms behind this increase are poorly understood. Here we analyze the Antarctic Ice Sheet atmospheric moisture budget based on climate reanalysis and model simulations to reveal that the interannual variability of regional snow accumulation is controlled by both the large-scale atmospheric circulation and short-lived synoptic-scale events (i.e. storm systems). Yet, when considering the entire continent at the multi-decadal scale, only the synoptic-scale events can explain the recent and expected future snow accumulation increase. In a warmer climate induced by climate change, these synoptic-scale events transport air that can contain more humidity due to the increasing temperatures leading to more precipitation on the continent. Our findings highlight that the multi-decadal and interannual snow accumulation variability is governed by different processes, and that we thus cannot rely directly on the mechanisms driving interannual variations to predict long-term changes in snow accumulation in the future.

Response of the Bay of Bengal to 3‐7‐Day Synoptic Oscillations During the Southwest Monsoon of 2019

AbstractThe satellite‐derived precipitation over central India (73–78°E, 20–25°N) during the 2019 southwest monsoon season reveals that high‐amplitude 3‐7‐day synoptic oscillations dominated the other intraseasonal oscillations and contributed to the excess seasonal monsoon rainfall. In this paper, we address the contribution of 3‐7‐day synoptic oscillations in atmospheric and oceanic parameters during the 2019 southwest monsoon season over the Bay of Bengal (BoB). Our study reveals that both satellite observations and the Nucleus for European Modeling of the Ocean (NEMO) model simulations show high‐amplitude 3‐7‐day synoptic scale events during the 2019 monsoon. The upper 300 m of the water column responds to the basin‐scale 3‐7‐day synoptic oscillations as revealed in the vertical structures of NEMO model simulated temperature, salinity, and currents in the BoB. The synoptic scale wind stress drives strong currents in the shallow mixed layer due to salinity stratification in the northern BoB. The northward propagation of the synoptic scale oscillations is evident across the BoB but is capped to the base of the mixed layer, above which the propagation is affected by wind stress and the spread of freshwater flux. The 3‐7‐day synoptic scale oscillations in NEMO sea surface height appear to be related to synoptic scale wind‐driven convergence and divergence of waters through mesoscale eddy circulations that in turn contribute to thermocline variability, rather than the temperature and salinity in the mixed layer.

Snapshot atmospheric parameters for the Istanbul metropolitan area during the course of two major wind regimes

At present scientific community is greatly concerned with extreme weather events due to their potential adverse effects that is associated with mesoscale weather events which can often be embedded in synoptic scale events that may be influenced by climatic variabilities. Extreme weather events can also change associated chemical compositions of the atmospheric parameters over metropolitan region. In this paper, we have used two major wind directions (N and SW) to characterize the upper and lower limits of atmospheric pollutants (PM10 and SO2) over Istanbul/Turkey rather than the wind or precipitation events that are associated with such events. In contrast to expectations, northerly air mass that originates from polar regions and traverses the industrialized or semi-industrialized countries and the Black Sea sets the lower limits for both PM10 and SO2 as 13 and 2.9 μg/m3, while southwesterly winds with its Saharan dust load set upper limits for PM10 as an average 300 μg/m3 reaching to 800 μg/m3 as peak hourly averages. SO2 levels during SW winds were also below 15 μg/m3. It has been further shown that the pristine northerly air mass can be used to delineate anthropogenic hot spots that can otherwise impossible to identify due to local perturbations.

Springtime Onset of Isolated Convection Precipitation across the Southeastern United States: Framework and Regional Evolution

Abstract This study examines the geographic and temporal characteristics of the springtime transition to the summer precipitation regime of isolated convection in the southeastern (SE) United States during 2009–12, using a high-resolution surface radar-based precipitation dataset. Isolated convection refers herein to isolated elements or small clusters of precipitation in radar imagery less than 100 km in horizontal dimension. Though the SE United States does not have a monsoon climate, it is useful to apply the established framework of monsoon onset to study the timing and regional variation of the onset of the summer isolated convection regime. Overall, isolated convection rain onset in the SE U.S. domain occurs in late May. Onset begins in south Florida in mid-April, continuing nearly simultaneously across the southeastern coastal plain in early to mid-May. In the northern domain, from Virginia to the Ohio Valley, onset generally occurs much later (mid-June to early July) with more variable onset timing. The sharpness of onset timing is most evident in the coastal plain and Florida. Results suggest the hypothesis, to be examined in a forthcoming study, that the timing of isolated convection onset in the spring may be triggered by specific synoptic-scale events within gradual seasonal changes in atmospheric conditions including extratropical cyclone tracks, convective instability, and the westward migration of the North Atlantic subtropical high. This approach may offer a useful framework for evaluating long-term changes in precipitation for subtropical regimes in an observational and modeling context.

Assessing the Impact of GNSS ZTD Data Assimilation into the WRF Modeling System during High-Impact Rainfall Events over Greece

The derivation of global navigation satellite systems (GNSSs) tropospheric products is nowadays a state-of-the-art technique that serves both research and operational needs in a broad range of applications in meteorology. In particular, GNSS zenith tropospheric delay (ZTD) data assimilation is widely applied in Europe to enhance numerical weather predictions (NWPs). The current study presents the first attempt at introducing assimilation of ZTDs, derived from more than 48 stations of the Hellenic GNSS network, into the operational NWP system of the National Observatory of Athens (NOA) in Greece, which is based on the mesoscale Weather Research and Forecasting (WRF) model. WRF was applied during seven high-impact precipitation events covering the dry and wet season of 2018. The simulation employing the ZTD data assimilation reproduces more accurately, compared to the control experiment, the observed heavy rainfall (especially for high precipitation events, exceeding 20 mm in 24h) during both dry and wet periods. Assimilating ZTDs also improves the simulation of intense (&gt;20 mm) convective precipitation during the time window of its occurrence in the dry season, and provides a beneficial influence during synoptic-scale events in the wet period. The above results, which are statistically significant, highlight an important positive impact of ZTD assimilation on the model’s precipitation forecast skill over Greece. Overall, the modelling system’s configuration, including the assimilation of ZTD observations, satisfactorily captures the spatial and temporal distribution of the observed rainfall and can therefore be used as the basis for examining further improvements in the future.

Winter storms accelerate the demise of sea ice in the Atlantic sector of the Arctic Ocean

A large retreat of sea-ice in the ‘stormy’ Atlantic Sector of the Arctic Ocean has become evident through a series of record minima for the winter maximum sea-ice extent since 2015. Results from the Norwegian young sea ICE (N-ICE2015) expedition, a five-month-long (Jan-Jun) drifting ice station in first and second year pack-ice north of Svalbard, showcase how sea-ice in this region is frequently affected by passing winter storms. Here we synthesise the interdisciplinary N-ICE2015 dataset, including independent observations of the atmosphere, snow, sea-ice, ocean, and ecosystem. We build upon recent results and illustrate the different mechanisms through which winter storms impact the coupled Arctic sea-ice system. These short-lived and episodic synoptic-scale events transport pulses of heat and moisture into the Arctic, which temporarily reduce radiative cooling and henceforth ice growth. Cumulative snowfall from each sequential storm deepens the snow pack and insulates the sea-ice, further inhibiting ice growth throughout the remaining winter season. Strong winds fracture the ice cover, enhance ocean-ice-atmosphere heat fluxes, and make the ice more susceptible to lateral melt. In conclusion, the legacy of Arctic winter storms for sea-ice and the ice-associated ecosystem in the Atlantic Sector lasts far beyond their short lifespan.

Argo Floats as a Novel Part of the Monitoring the Hydrography of the Bothnian Sea

We made an assessment of the hydrography in the Bothnian Sea based on data collected by the Argo floats during the first six years of operation in the Bothnian Sea (2012–2017). We evaluated the added value of Argo data related to the pre-existing monitoring data. The optimal usage and profiling frequency of Argo floats was also evaluated and the horizontal and vertical coverage of the profiles were assessed. For now we lose 4m of data from the surface due to sensor design and some meters from the bottom because of the low resolution of available bathymetry data that is used to avoid bottom collisions. Mean monthly temperature and salinity close to surface and below halocline from the float data were within the boundaries given in literature, although some variation was lost due to scarcity of winter profiles. The temporal coverage of the Argo data is much better than that of ship monitoring, but some spatial variability is lost since the floats are confined in the over 100m deep area of the Bothnian Sea. The possibility to adjust the float profiling frequency according to weather forecasts was successfully demonstrated and found a feasible way to get measurements from storms and other short term phenomena unreachable with research vessels. First six years of operation have shown that Argo floats can be successfully operated in the challenging conditions of the Bothnian Sea and they are shown to be an excellent addition to the monitoring network there. With multiple floats spread in the basin we can increase our general knowledge of the hydrographic conditions and occasionally get interesting data related to intrusions and mixing during high wind events and other synoptic scale events.

Sea surface temperature and rainfall anomaly over the Bay of Bengal during the El Niño-Southern Oscillation and the extreme Indian Ocean Dipole events between 2002 and 2016

Phenomena like the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) regulate the climate of several nations situated around the Indian Ocean rim as well as the entire globe. The present study focused on the Bay of Bengal (BoB) in the northern Indian Ocean, which is known for its crucial role in the Indian summer monsoon. The sea surface temperature (SST) and rainfall (RF) anomalies in the central, eastern, western and northern BoB were thoroughly analyzed with respect to all the El Niño, La Niña and extreme IOD events between the years 2002 and 2016. Most of the El Niño (La Niña) phases were accompanied by a positive (negative) SST anomaly and a negative (positive) RF anomaly. However, the magnitude of the SST and the RF anomalies did not co-vary with Oceanic Niño index observed in the Niño 3.4 region. The SST and RF anomalies in the BoB lagged the onset of El Niño by 1–7 months and 3 months, respectively. During La Niña, a negative SST anomaly was accompanied by a positive RF anomaly; however, the magnitude of the anomaly in case of both parameters was larger during El Niño phases when compared to the La Niña phases. Longer El Niño periods corresponded to larger reductions in rainfall in BoB. In general, El Niño is accompanied by positive IOD phase and La Niña by negative IOD phase. We observed a negative rainfall anomaly in the BoB during a positive IOD and vice versa. However, this trend reversed whenever El Niño coincided with negative IOD and La Niña with positive IOD. This showed that whenever an El Niño or La Niña coincided with the conventionally opposite IODMI phases, the effect of ENSO phases overruled the IODMI phases in BoB. Many previous studies have documented that SST and RF anomalies can potentially regulate as well as helps to forecast both droughts and floods in landmasses, not only in the vicinity to oceans but also inner continental regions. Since BoB plays a crucial role in regulating the monsoon and India being a monsoon dependent agricultural nation, the present observations may be of help to climate modelers to study the effect of SST anomalies in BoB (with respect to synoptic scale events like ENSO or IOD) upon the regional climate of India.

An Analysis of Anomalous Winter and Spring Tornado Frequency by Phase of the El Niño/Southern Oscillation, the Global Wind Oscillation, and the Madden-Julian Oscillation

Winter and spring tornado activity tends to be heightened during the La Niña phase of the El Niño/Southern Oscillation and suppressed during the El Niño phase. Despite these tendencies, some La Niña seasons have fewer tornadoes than expected and some El Niño seasons have more than expected. To gain insight into such anomalous seasons, the two La Niña winters and springs with the fewest tornadoes and the two El Niño winters and springs with the most tornadoes between 1979 and 2016 are identified and analyzed in this study. The relationships between daily tornado count and the Global Wind Oscillation and Madden-Julian Oscillation in these anomalous seasons are also explored. Lastly, seasonal and daily composites of upper-level flow, low-level flow and humidity, and atmospheric instability are generated to describe the environmental conditions in the anomalous seasons. The results of this study highlight the potential for large numbers of tornadoes to occur in a season if favorable conditions emerge in association with individual synoptic-scale events, even during phases of the El Niño/Southern Oscillation, Global Wind Oscillation, and Madden-Julian Oscillation that seem to be unfavorable for tornadoes. They also highlight the potential for anomalously few tornadoes in a season even when the oscillations are in favorable phases.

Nearshore circulation on a sea breeze dominated beach during intense wind events

A field experiment was conducted on the northern Yucatan coast from April 1 to April 12, 2014 to investigate the role of intense wind events on coastal circulation from the inner shelf to the swash zone. The study area is characterized by a micro-tidal environment, low-energy wave conditions, and a wide and shallow continental shelf. Furthermore, easterly trade winds, local breezes, and synoptic-scale events, associated with the passage of cold-fronts known as Nortes, are ubiquitous in this region. Currents were measured concurrently at different cross-shore locations during both local and synoptic-scale intense wind events to investigate the influence of different forcing mechanisms (i.e., large-scale currents, winds, tides, and waves) on the nearshore circulation. Field observations revealed that nearshore circulation across the shelf is predominantly alongshore-directed (westward) during intense winds. However, the mechanisms responsible for driving instantaneous spatial and temporal current variability depend on the weather conditions and the across-shelf location. During local strong sea breeze events (W > 10ms-1 from the NE) occurring during spring tide, westward circulation is controlled by the tides, wind, and waves at the inner-shelf, shallow waters, and inside the surf/swash zone, respectively. The nearshore circulation is relaxed during intense land breeze events (W ≈ 9ms-1 from the SE) associated with the low atmospheric pressure system that preceded a Norte event. During the Norte event (Wmax≈ 15ms-1 from the NNW), westward circulation dominated outside the surf zone and was correlated to the Yucatan Current, whereas wave breaking forces eastward currents inside the surf/swash zone. The latter finding implies the existence of large alongshore velocity shear at the offshore edge of the surf zone during the Norte event, which enhances mixing between the surf zone and the inner shelf. These findings suggest that both sea breezes and Nortes play an important role in sediment and pollutant transport along/across the nearshore of the Yucatan shelf.