Thirty-year time series of hindcast wave data were analysed for 10 coastal locations along the eastern Mexican coast to obtain information about storm events occurring in the region, with the goal of examining the possible presence of interannual trends in the number of storm-wave events and their main features (wave height, duration and energy content). The storms were defined according to their significant wave height and duration, and the events were classified as related to either tropical cyclones or Norte events. The occurrence and characteristics of both types of events were analysed independently. There is no statistically significant change in the number of storm-wave events related to Nortes or their characteristics during the study period. However, there is a subtle increase in the number of events related to tropical cyclones in the western Caribbean region and a more evident increase in wave height and energy content of these events.
Over the past decades, there has been increasing awareness of the effects of climate change in coastal regions, with numerous studies focused on possible implications of sea-level rise (Nicholls, 2002; Wong et al., 2014) and on the modification of the intensity, frequency and location of storms worldwide (Kossin et al., 2014), which represent a more immediate consequence of climate change. Despite these numerous studies, there is still low confidence in the results of large-scale trends in storminess over the last century (IPCC, 2013), mainly due to changes in the capabilities of observing techniques, which confound the possible presence of trends. One region with reliable data is the North Atlantic, where a robust increase in the frequency and intensity of the strongest storms has been observed since the 1970s, although there is still debate over the cause of this increase (IPCC, 2013; Webster et al., 2005).
Contrary to the meteorological effects of storms, where the main interest is focused on the location of the storm strike (e.g. Resio and Irish, 2015; Needham and Keim, 2014; Keim et al., 2007; Elsner et al., 1999; Simpson and Lawrence, 1971), wave conditions related to a storm event can be observed along distant coasts, well beyond the region of wind stress. Hence, a modification in storm characteristics will be associated with inherent changes in wave height and the storm surge reaching the coastline, which are the two main factors responsible for considerable economic losses in coastal and offshore areas (Mendelsohn et al., 2012; Neumann et al., 2014). This makes the investigation of changes in storminess trends a major concern for coastal management, even more so when taking into consideration that possible storm effects might be enhanced by the effect of sea-level rise and the increase in coastal development.
Wave conditions are commonly obtained from in situ observations (buoys, tide gauges, ships), satellite altimeter data or wave models. In the U.S. Gulf of Mexico (GoM) region, Komar and Allan (2008) analysed a 28-year register of measured buoy data in the central region of the Gulf of Mexico but did not find long-term changes in wave height during the summer months, which, according to the authors, would be attributable to an increase in wave heights caused by tropical cyclones (TCs). This dataset was also included in a study by Bromirski and Kossin (2008) which extended the analysed data to three deep-water (> 1000 m) buoy registers in the northern GoM, each covering a period of 28 years. Their study demonstrated the relationship of such long-term registers with shorter-term coastal registers, making their findings applicable to near-coastal regions. In their study, TC waves were separated from the rest of the register using the National Hurricane Center best-track record, and an increase in the number of TC-related events over the last decades was found. However, this increase was associated neither with increases in significant wave height (SWH) related to the TC events nor with the duration of the TC events. Their study also found a shift in the intra-annual distribution of TC-related events between the first and second parts of the register, with the majority of events taking place during August and September in the first half of the register and during September and October in the second half of the register.
Along the coast of the Mexican GoM, extreme storm-wave events are mainly caused by tropical cyclones and Nortes. Nortes are anticyclonic cold surges that enter the Gulf of Mexico from North America, generating strong northern winds and, therefore, presenting ideal conditions for fetch causing mature wind waves. The study of the occurrence and interannual trends of extreme storm-wave events related to both TCs and Nortes must be regarded separately, given that possible long-term changes in the behaviour of TCs and Nortes and their responses to climate change are not expected to be analogous (Komar and Allan, 2008).
As for the western Caribbean Sea (WCS), storminess trend studies based on wave datasets are absent to the authors' knowledge. Projected changes in wind shear suggest a decrease in the number and intensity of TCs in the region (Biasutti et al., 2012), although studies suggest an intensification of wind speed of higher-category events on a global scale (Holland and Bruyère, 2014). Wave hindcast results from Appendini et al. (2014) do not project increases in wave height, although a recent analysis indicates more intense waves in the future climate for the WCS as a result of tropical cyclones (Appendini et al., 2017).
Along the eastern coast of Mexico (Gulf of Mexico and western Caribbean Sea), in situ and satellite data are scarce and temporally discontinuous. This contribution analyses a 30-year time series of hindcast wave data, covering the period of 1979 to 2008 (Appendini et al., 2014). The selection of this time interval allows for comparison to previous work in the region and is considered the standard period by the World Meteorological Organization to characterize climate, although the use of shorter time periods has been suggested to characterize non-stationary climates, e.g. under climate change conditions (Arguez and Vose, 2011). Appendini et al. (2014) did not find any significant trend in time series of extreme wave heights (using the 99th percentile) along the eastern Mexican coast, but their research examined the entire time series without regard of the type of extreme event.
Study area with the analysed nodes (black dots) located in the GoM and the Mexican part of the western Caribbean Sea (Cancun and Tulum). Yellow dots indicate the location of nearby settlements.
The aim of this work is to determine possible interannual trends in the number of storm-wave events and their main features (wave height, duration and energy content). This was achieved by first identifying extreme storm-wave events for a number of near-coastal locations along the eastern Mexican coast and then classifying them according to the meteorological conditions that caused these events (TCs or Nortes).
The study area comprises the eastern coast of Mexico, which extends along
the GoM and the WCS. This area presents the highest significant wave height
correlation coefficient and the lowest bias of the hindcast dataset used in
the study done by Appendini et al. (2014). The current study comprises the
data from this hindcast at eight nodes located in the Mexican GoM at 50 m
depth and two nodes in the WCS within 100 m depth. Additionally, the nodes
were selected based on the proximity of coastal settlements: Matamoros,
Tampico, Veracruz, Coatzacoalcos, Paraiso, Campeche, Progreso, Holbox,
Cancun and Tulum (Fig. 1). Campeche, Puerto Progreso and Holbox are
characterized by an ubiquitous wide continental shelf of approximately 245 km with a slope of
Wave conditions are generally mild with mean SWH around 1 m, although wind waves with maximum individual wave heights of approximately 27 m have been measured by wave gauges in the northern GoM (Wang et al., 2005) during Hurricane Ivan. The main meteorological systems contributing to storm-wave conditions are mid-latitude anticyclonic meteorological systems that generate northerly cold fronts known as Nortes with 11 to 21 events crossing the GoM per season (Reding, 1992) and cyclonic systems that include tropical depressions, tropical storms and hurricanes. There is some overlap of the occurrence of either type of event during the year, and the two systems can interact, mainly in October. The North Atlantic tropical cyclone season runs from 1 June to 30 November, with September being the month with the highest number of tropical cyclones directly affecting the Mexican coasts (Rosengaus Moshinsky et al., 2002). Norte events usually occur from October to April, with the most intense events taking place from December to March (Appendini et al., 2014).
Three datasets are used in this study: (1) 30 years of hindcast wave data, (2) the International Best Track Archive for Climate Stewardship (IBTrACS) dataset for discerning TCs affecting the study area and (3) a Norte dataset designed for the GoM and WCS regions.
The hindcast wave data cover the period from 1979 through 2008 in 3 h intervals for the entire eastern Mexican coast (Appendini et al., 2014). We used the 30-year wave data in order to comply with the World Meteorological Organization's recommendation for characterizing non-stationary climates, i.e. under climate change conditions (Arguez and Vose, 2011).
As mentioned before, eight nodes located in the Mexican GoM at 50 m depth and two nodes in the WCS at 100 m depth were evaluated.
The IBTrACS (Knapp et al., 2010) record was used given that this platform
includes the most complete set of historical TCs available. The data were
downloaded from the NCDC website (
The Norte dataset includes the dates when Nortes entered and left the Gulf of Mexico. This dataset was derived from the Climate Forecast System Reanalysis (CFSR) data (Saha et al., 2010), and the Nortes are identified based on the pressure difference between Yucatan and Texas, as well as wind speed at different positions over the Gulf of Mexico.
In this study, the occurrence and interannual trends of extreme storm-wave events caused by TCs and Nortes were regarded separately, given that their behaviour and response to climate change are not expected to be analogous (Komar and Allan, 2008). The present study of the GoM and WCS wave data examines the use of a common tool for coastal management where the dataset is inspected looking for individual storm events based on the register of significant wave heights, establishing a minimal duration for storm events (Mendoza et al., 2011) and associated meteorological conditions based on the identification and separation of TC and Norte events. This approach not only provides improved insight into wave formation compared to using a summer and winter event distribution (Komar and Allan, 2008) but also allows further analyses of the number, energy content and duration of storm events, as well as possible trends associated with a given type of meteorological storm.
From an oceanographic point of view, a storm can be defined as an increase
in wave height and sea level (storm surge) exceeding a certain threshold
during a certain amount of time (e.g. Mendoza et al., 2011). A common way to
characterize extreme events is using a
Conventionally, the selection of the wave height threshold takes local
characteristics of the wave regime into account. In this study, a storm is
defined as an event reaching a SWH greater than
For each of the identified storms, the following main characteristics were
obtained: mean and maximum SWH, storm duration (
In order to separate TCs from Norte events, the IBTrACS database was used to
identify TCs, while Nortes were identified using the Norte identification
index. Regarding TCs, two regions of influence were defined: the WCS region
(7 to 23
For a considerable number of events (between 7 and 25, depending on the
node), a storm-wave event occurred during both types of events. To determine
the responsible event in these cases, the first approach was to compare the
number of hours during which the meteorological and wave events coincided. If
one of the meteorological events had a larger number of coincident hours, it
was determined to be the responsible event. If the hours were equal, the
second approach was to look at the date of occurrence; events occurring from
November to April were considered as Nortes, while those occurring from July
to September were considered as TCs. When the events occurred in the months
of May, June and October, the wind direction related to the maximum SWH of
the event was considered. Winds coming from 230 to
45
Time series of the event characteristics per season evaluated for the TCs and
Norte series are the number of storm events, the mean and maximum SWH
(SWH
A Mann–Kendall trend test (Kendall, 1975; Mann, 1945) was performed for the time series of the different evaluated characteristics of the storm events caused by TCs and Nortes. This non-parametric test is used to identify if there is a monotonic upward or downward trend through time, but it does not specify whether the detected trend is a linear or nonlinear trend. The null hypothesis assumes that the data are independent and identically distributed over time. Mann–Kendall tests were performed in order to confirm the occurrence of trends that were significantly different from zero at the 90 % confidence level or higher.
Following Casas-Prat and Sierra (2010), the
The largest SWH values in the region occur in Matamoros, with
Storms are defined for each node based on its SWH time series. The critical
SWH value (
As has been already mentioned, certain storm-wave events occurred during both a Norte and a TC meteorological event. These coincidences occurred a smaller number of times in Tulum and Cancun (7 and 12 events, respectively) and were more frequent in Progreso, Tampico and Veracruz (29, 21 and 21 events, respectively).
Monthly mean SWH for each studied node.
Number of extreme wave events per node (solid line) and their percentage of occurrence according to the considered classification (bars).
The classification into Nortes and TC-related events shows that between 5 and 13 % of the events were caused by TCs. Nortes were responsible for between 51 % (Matamoros) and 88 % (Coatzacoalcos) of the events in the GoM. These numbers are significantly lower in the WCS, where Nortes were responsible for up to 28 % of the events (Fig. 3).
Average significant wave height,
Sum of the number of events per month for the entire study period
related to
A number of events were not classified as related to Nortes or TC events and, therefore, were not considered in this study. This number varies greatly between nodes, from 5 % in the southern GoM to > 58 % in the WCS (Fig. 3). An unexpected result is that a considerable number of events occurring in Matamoros are not related to any of the considered meteorological systems. This location is subject to the most restricted fetch among all analysed locations, so that storms are most likely related to other systems such as “suradas” or northerly winds that occur after the pass of a Norte. In Matamoros, suradas could occur while the Norte is still in the Gulf of Mexico, generating southerly winds at most of the other locations. This requires more detailed analysis, particularly to establish the duration of Norte events at particular locations instead of in the GoM as a whole. The large percentage of unclassified events in the WCS is an expected result given that wave conditions in the WCS are largely influenced by the trade winds, which is supported by the findings of Appendini et al. (2014).
A more detailed view of the annual distribution of both types of events (Fig. 4) shows that events resulting from the occurrence of Nortes are more abundant during January and December. Contrarily, the number of events resulting from the occurrence of TCs is highest during September and October, albeit considerably smaller in number than the number of events caused by Nortes.
The WCS nodes differ in the number and monthly distribution of storm events. Although Tulum has a larger number of registered events, fewer events are classified as caused by Nortes or TCs. This might be related to the location of Tulum, which is protected by Cozumel Island and, due to the orientation of the coastline, is characterized by a small fetch for events approaching from all directions except easterly waves, which make it prone to the occurrence of trade-wind-related events. Events resulting from Nortes in Cancun occur mostly from November to March, while in Tulum their number is highest from February to April. The distribution of events related to TCs is more similar, although the number of events is lower in Tulum.
Events caused by TCs start earlier in the WCS than in the GoM, with a few registered events in May and June. As already mentioned, in this region the majority of the extreme wave events are not related to Nortes or TCs, and their occurrence is distributed throughout the year, with the largest number of unclassified events taking place in March (not shown). These events are not studied herein, but they are most likely related to the intensification of the Caribbean Low-Level Jet (Appendini et al., 2015).
Trends found in the number of Norte events, estimated number of
Norte events, SWH
Trends found in the number of TC events, estimated number of TC,
estimated SWH
The results of the annual distribution of TC- and Norte-related events (Fig. 4) demonstrate that the definition of the winter and summer season given by Komar and Allan (2008) is not applicable to the present study because a large percentage of events would be excluded from the analysis. For example, during October both Norte- and TC-related storm events are common in the region. The effect of TCs in the WCS is mostly observed during what is currently defined as the hurricane season, i.e. from June to the end of November, with a similar behaviour at the two easternmost nodes of the GoM (Progreso and Holbox). The northernmost nodes (Matamoros and Tampico) also indicate a hurricane-season-type distribution of events, with TC-related events recorded during July and August but not in November or December, showing a pattern more similar to the U.S. GoM results presented by Bromirski and Kossin (2008). However, the nodes in the Bay of Campeche (Veracruz, Coatzacoalcos, Paraiso and Campeche) show that events related to TCs occur between August and December, with no registered events during June or July and the majority of events taking place in September and October.
Nortes and TCs are evaluated by season: from May to December for TCs and from September to the following August for Nortes. For this reason, there are 30 seasons for TCs but only 29 seasons for Nortes because the last season is only partially represented in the dataset.
As expected, the patterns of the number of events caused by Nortes and TCs differ (Figs. 5 and 6), as well as the trends and the Mann–Kendall test results obtained for each parameter, which are summarized in Tables 2 and 3 for the Nortes and TC events, respectively.
The time series of the number of storm events related to Nortes per season
are given in Fig. 5. The trends obtained from simple linear regression show
almost null or positive slopes of less than 0.03 events yr
Number of events caused by Nortes per season. Simple linear
regressions (LRs) and estimated temporal evolution,
There is more consistency in the time series of the number of events caused
by TCs (Table 3). Although, according to the Mann–Kendall test results,
significant positive trends in the number of storm events caused by TCs are
only found at the Holbox, Cancun and Tulum nodes, the entire study area
shows a certain organization, with Paraiso and Coatzacoalcos showing trends
close to zero and increasing slopes of the regression lines with increasing
distance from these nodes. This pattern is maintained when the variations in
the number of events per season are calculated using Eq. (4), and in this
manner the meaningless results obtained by simple linear regression for the
Cancun and Tulum datasets (where negative values were found at the beginning
of the study period) are corrected. Furthermore, it allows an
increase to be observed in the slope of the obtained positive trends in the number of
TC-related events (Fig. 6) during the two halves of the study period: an
augmentation of 0.04 and 0.03 events yr
Time series of the number of events caused by TCs each season and
trends shown as simple linear regressions (LRs) and as the estimated trend
(ET) obtained from the log-transformed data and the probability of storm
occurrence,
According to the Mann–Kendall test results at the 90 % confidence level,
none of the nodes – with the exception of Matamoros, Tampico and Progreso –
show significant trends for the intensity or the duration of the Nortes. The
significant trends that were found are as follows: (i) Matamoros shows a decrease
in SWH
Despite the Mann–Kendall test results, it can be observed that, along the GoM
region, SWH
The negative trends in SWH
In general, the result of the analyses of energy content and duration of
events caused by Nortes do not show significant trends for the majority of
the nodes. There is no consistency at the regional scale (i.e. several
proximal nodes showing similar results) nor in the presence of significant
trends for a single node (e.g. Matamoros shows significant trends for the
SWH
Previous research by Pérez et al. (2014) found a decrease in the
duration and an increase in the intensity of Norte events for future climate
scenarios using the TL959L60-AGC model of the Meteorological Research
Institute of Japan under the A1B scenario. A decrease in duration is
observed in our results for the northernmost nodes of the GoM and in the
WCS but is not statistically significant based on the Mann–Kendall test
performed on the results. However, this might be related to the definition
of a storm event used in this work, which requires more than 12 h with
SWH
Probability of storm occurrence (
The TC time series show a considerable number of years without the presence of storm events occurring at the different nodes. This number ranges between 43 and 57 % of the analysed years, depending on the node (Fig. 6). This result is corroborated by Ramírez (1998), who investigated TCs arriving at the Yucatan Peninsula during the 1970–1995 period. In general, the percentage of years with no events related to TCs is larger during the first half of the study period, a circumstance more obvious for the WCS nodes, and in agreement with Bromirski and Kossin (2008), who found a general tendency for more significant wave events related to TCs since 1995. This is also consistent with an increasing overall count of named storms during recent years. Klotzbach et al. (2015) attributed the decrease in the number of TCs from 1972 to 1992 and the following positive trend in the number of events to the Atlantic Multidecadal Oscillation.
The trends in the number of storm events shown in Fig. 6 reveal that, in
addition to an increase in the probability of occurrence of a storm event
(
The estimated trends in the SWH
In the GoM, the Mann–Kendall test only corroborates the presence of a trend
in a low percentage of the time series (mainly those with analogous signals
in the probability of occurrence and the trend related to the occurrence of
events). Veracruz, Coatzacoalcos, Paraiso, Campeche and Progreso do not show
any trends according to the Mann–Kendall test. The Mann–Kendall test
corroborates the presence of a trend in the SWH
Trends obtained for the time series, excluding years with no events related to TCs.
Estimated temporal evolution found in the TC-related storm time
series of
Total amount of time (in days) with TC-related storm conditions per season.
In contrast, the WCS nodes show significant trends according to the Mann–Kendal test results for all analysed parameters, and the effect of the increase in the number of TC events since 1994 is also more obvious than for the GoM nodes (Fig. 10).
Based on the intra-annual distribution of storm events and the trends found for TC- and Norte-related events, the studied nodes can be separated into four regions:
The northernmost nodes (Matamoros and Tampico) are characterized by a TC
season starting earlier than at the other nodes, with a majority of TC
events taking place in August and September and no registered TC events in
November and December. The nodes show a similar monthly distribution of the
occurrence of Norte- and TC-related events, similar values regarding the
probability of TC occurrence with time and a certain consistency in the
trends of TC-related events. However, there are also significant differences
between these two nodes, the most noticeable being the large number of
unclassified events occurring in Matamoros. The Bay of Campeche nodes (Veracruz, Coatzacoalcos, Paraiso and
Campeche) show no events (of any type) occurring during June and July during
the study period. The majority of events occur during the months of January,
February and December, mostly associated with Nortes. Most of the TC-related
events take place during September and October, but events have been
recorded from August to December (with only one recorded event in December,
which corresponds to Tropical Storm Olga). These nodes do not show
significant trends for any of the evaluated parameters, neither for Nortes
nor for TCs. The Progreso and Holbox nodes show a behaviour partly comparable to the
WCS nodes, mainly Holbox, which is closer to the WCS. In general, they
record a smaller number of storm events than the rest of the GoM nodes, with a
slightly smaller number of Norte-related events and a slightly larger number
of TC-related events than the rest of the GoM region. The trends found for
events caused by TCs present steeper slopes than (ii), but the Mann–Kendall
test does not support the presence of a statistically significant trend. The WCS nodes (Cancun and Tulum) show a majority of events that cannot
be attributed to TCs or Nortes. In this region, the number of events related
to Nortes is considerably smaller, and the majority of events related to TCs
occurred between July and October. Trends found for the Norte events are not
significant according to the Mann–Kendall test, but trends for events
related to TCs show an increase in number, duration and intensity during the
study period.
Time series of the number of TC events, SWH
In the GoM region, Nortes are responsible for the majority of extreme wave events occurring from 1 November to 30 April, while TCs are responsible for the majority of extreme wave events occurring during August. During the months of September and October, both Nortes and TCs can be responsible for extreme wave events. The TC season in the Mexican GoM starts and ends later than in the North Atlantic; it lasts from August to December instead of June to October.
There is not a general statistically significant change in the number of storm-wave events related to Nortes or their characteristics in the GoM and the WCS during the study period. The time series of events related to Nortes and their characteristics do not show consistent behaviour in the study area. Although there are a few time series where the presence of a trend in the data is corroborated by the Mann–Kendall test at the 90 % significance level, the available evidence is not sufficient to conclude that there is variation in storminess related to Nortes.
For most of the GoM time series of events related to TCs and their characteristics, the presence of a trend in the data cannot be corroborated by the Mann–Kendall test at the 90 % significance level. However, the basin shows a certain consistency, with no trends (or trends close to zero) for the Coatzacoalcos and Paraiso nodes and increasing trends with increasing distance from these nodes. Overall, with the exception of the Bay of Campeche, the results for TC-related events show an increase in wave height in the WCS and the GoM.
In the WCS, the data confirm the presence of positive trends in the number, SWH, duration and energy of storm events. There is a subtle increase in the number of storms related to TCs, which will result in an increase of one TC-related event every 10 years in Cancun and every 20 years in Tulum. Considering the mean SWH, the estimated trends show an increase of 2.2 m in both Cancun and Tulum during the entire study period. The obtained trends result in a more evident increase in mean SWH associated with TC events in the second half of the study period.
The IBTrACS record (Knapp et al., 2010) was downloaded from
the NCDC website (
The authors declare that they have no conflict of interest.
The Mexican National Council for Science and Technology (CONACYT) and the Universidad Nacional Autónoma de México provided financial support through projects INFR-2014-01-225561 and Proyectos Internos Instituto de Ingeniería 5341 and 6602. Elena Ojeda is a Cátedras CONACYT researcher under project 1146, Observatorio costero para estudios de resiliencia al cambio climático. The authors would like to thank the technical support from José López-Gonzaléz, Gonzalo Martín-Ruiz, Edgar Escalante Mancera and Iván Adrián Moreno. Edited by: Nadia Pinardi Reviewed by: three anonymous referees