Identiﬁcation of Tropical Cyclones’ Critical Positions Associated with Extreme Precipitation Events in Central America

: Tropical cyclones are one of the most important causes of disasters in Central America. Using historical (1970–2010) tracks of cyclones in the Caribbean and Paciﬁc basin, we identify critical path locations where these low-pressure systems cause the highest number of ﬂoods in a set of 88 precipitation stations in the region. Results show that tropical cyclones from the Caribbean and Paciﬁc basin produce a large number of indirect impacts on the Paciﬁc slope of the Central American isthmus. Although the direct impact of a tropical cyclone usually results in devastation in the affected region, the indirect effects are more common and sometimes equally severe. In fact, the storm does not need to be an intense hurricane to cause considerable impacts and damage. The location of even a lower intensity storm in critical positions of the oceanic basin can result in destructive indirect impacts in Central America. The identiﬁcation of critical positions can be used for emergency agencies in the region to issue alerts of possible ﬂooding and catastrophic events.


Introduction
Among the main transient synoptic-scale hydrometeorological hazards that affect Central America are cold fronts, easterly waves, and tropical cyclones [1,2]. The first type of hazard occurs during boreal winter, especially from December through January [3], while the other two types occur usually from middle boreal spring to middle autumn, peaking during the August-October quarter [4]. Sometimes, a particular easterly wave becomes an atmospheric disturbance that evolves into a tropical cyclone [5]. Ref. [6] also discusses other systems associated with tropical-depression wave disturbances that could be associated with tropical cyclogenesis in the tropical Atlantic and in the Eastern Tropical Pacific. These systems are characterized by troughs or segregated low-pressure systems from the Inter Tropical Convergence Zone (ITCZ).
Central America is located between two very active cyclogenetic regions, the Atlantic basin and the Eastern Tropical Pacific [4]. Tropical cyclones in these two regions can affect Central America in   Gridded sea level pressure (SLP) and surface wind vectors from 1979 to 2010 at 0.75 by 0.75 degrees spatial resolution were obtained from the European Centre for Medium-Range Forecasts Interim Re-Analysis [28]. Daily means were based on basic synoptic hour observations. The data were used to determine the synoptic-scale atmospheric circulations related to extreme precipitation events in Central America associated with tropical cyclones.
Diverse information of the trajectories and other characteristics of tropical cyclones at hourly time steps from 1970 to 2010 were obtained from the combination of two databases: HURDAT [29] from the United States (US) National Hurricane Center and Central Pacific Hurricane Center part of the National Oceanographic and Atmospheric Administration and the final tracks from Unisys database from the repositories of the US National Aeronautics and Space Administration. The hourly data were concatenated from both data sources. The data includes the following information: hour, day, month, year, latitude, longitude, wind velocity, pressure, and instantaneous classification of the event (tropical depression, tropical storm, category 1 hurricane, category 2 hurricane, category 3 hurricane, category 4 hurricane, and category 5 hurricane). Tropical storms and hurricanes categories 1 to 5 represent named cyclones. Unless otherwise specified, extreme precipitation events (above the climatological 90th percentile) were associated with all the hourly cyclone data of that day to create composites and other subsequent analyses. The cyclone data were used as input to determine the relationship between extreme precipitation events and the locations of the tropical cyclones in the Caribbean Sea and Eastern Tropical Pacific Ocean. In Table 3, the classification of tropical cyclones according to their maximum wind speed is shown. A visualization tool was developed in C++ (openframeworks). The selection of this platform was due to the response features that are many times superior to other programming languages such as Java or JavaScript [30]. Besides, the openframeworks platform allows the multiplatform use of the resulting application (Apple OS X, Windows, and Linux); these two parameters, efficiency and universality of use, were the most important for the selection of the programming environment of the tool. The map was georeferenced with data from Natural Earth, a public domain map dataset.
The data of meteorological events, in turn, were uploaded locally through .csv files that were prepared ad hoc to increase the efficient use of the tool. The result is a tool that is capable of handling more than 40,000 events in real-time. The tool allows the user to select a particular group of meteorological stations. Selecting the stations allows the user to visualize a composite of the trajectories of the tropical cyclones that produced extreme precipitation events (daily precipitation above the 90th percentile of the precipitation climatology of the corresponding month). The tool allows the user to select all types of tropical cyclones or only certain type(s). The regions where there is a large accumulation of critical trajectories are shown with a dense shading.
The visualization tool was used to determine the critical positions of tropical cyclones that produce extreme precipitation events in each Central American region. The results are shown separately for tropical cyclones that originated in the Eastern Pacific and Caribbean/Atlantic oceanic basins. The data from the visualization tool was used to determine the foci of critical locations that produce a large number of extreme precipitation events in Central America. Note that this analysis of the critical regions was based only on their trajectories, regardless of the cyclone intensity (the only requirement is that the maps were constructed from named storms only).
The regions that produce extreme weather in Central America were identified from the trajectory density maps. For each critical region (where the density of trajectories was higher), an index was produced to analyze the associated atmospheric patterns and the time evolution of the presence of storms (of different ranges of categories) in the critical zones.
Contour maps of the number of stations in extreme precipitation events were plotted at the median daily position of the cyclone. This was used to identify critical cyclone locations associated with the largest quantity of impacts. To obtain a better geovisualization, the data were improved in a Geographic Information System Software (GISS) like ArcMap 10.7 from ArcGIS UCR license.

Caribbean Oceanic Basin Results
The density maps of the trajectories of hurricanes categories 1 to 5 in the Caribbean basin, associated with extreme precipitation events in the countries of Central America are shown in Figure 1.
Although it is evident that hurricanes can produce impacts when they are located anywhere around the Caribbean basin, there are indications of several poles of the high density of trajectories associated with extreme precipitation events. To illustrate the atmospheric circulation associated with one pole of high accumulation of trajectories located off the Yucatan Peninsula and the Gulf of Honduras (which is present in several of the country maps), we produced SLP and wind composites during extreme precipitation events concurrent to days when the hurricanes were located in that pole (positions inside the box bounded by 14.5 • N to 23 • N and 84 • W to 90 • W) ( Figure 2). As expected, there is a low-pressure anomaly near the composited average location of the hurricanes inside the box. The low-pressure system implies a gradient between the pressure anomalies in the Pacific and the Caribbean, and this configuration drives anomalous winds from the Pacific Ocean to the west coast, which causes indirect impacts by orographic lifting in the slope opposite to the location of the hurricane. This type of hurricane impact is well known to the emergency response authorities in the Central American countries and mentioned in impact studies literature [1], but here a better identification of critical zones will be provided below. The mechanism was also previously illustrated in the schematic shown in Figure 3 of [7], but here we used actual data for analyzing the circulation patterns.
Two aspects are interesting to note from Figure 2: first, the coloring of the meteorological stations evidences the relative difference in the number of extreme precipitation events on both slopes of the isthmus (also consistent with the difference of impacts in both slopes reported by [1]). That is, the largest number of extreme precipitation events are reported on the Pacific slope of Nicaragua, a moderate number on the Pacific slope of Guatemala and on the Pacific northwestern coast of Costa Rica with fewer reports observed in the Caribbean slopes, southern Costa Rica, and Panama. This is interesting, as it confirms that the atmospheric circulation patterns of Figure 2 are consistent with the precipitation response and that emergency response resources associated with hurricane indirect effects in Central America should be focused on the Pacific slope. Indirect effects are an important threat in southern Central American countries, as a direct hit of a hurricane in these countries is rare considering the normal patterns of Caribbean cyclone tracks. Colon (1969) cited by [18], determined that the probability of a Caribbean hurricane landing in Costa Rica is less than 5 %. Along with indirect effects, northern countries experience occasional direct hits of hurricanes from the Caribbean, and several large events (e.g., hurricanes Mitch and Stan) have resulted in catastrophes of large proportions. The second issue that is interesting to note is that although in Figure 1 the analyzed pole of higher track density is located in the Gulf of Honduras, the SLP pattern depicted in Figure 2a is located further north within the interest box, just offshore the Yucatan Peninsula. A possible reason is that Figure 1 does not take into consideration the hurricane intensity, while in Figure 2a the composite is constructed by averaging the SLP anomalies during days of extreme precipitation events, and therefore the larger SLP anomalies from stronger hurricanes dominate the composite. This may result in the difference in the critical hurricane position shown. Nevertheless, trajectories that end up in the critical box identified before, may result in significant indirect impacts on the Pacific slope of the Central American countries. This is an important observation, as the forecasts of storm trajectories (regardless of the intensity of the cyclone) that may contain this zone, should be considered for issuing suitable emergency alerts. The use of the software tool mentioned before can be used in an interactive manner to determine possible critical regions, associated with extreme precipitation events in one or more user-selected meteorological stations. This tool is a valuable asset for emergency response institutions in the region.  We counted the number of trajectories from 1948 to 2010 reaching the critical zone defined before. The results are shown in Figure 3. As can be seen, although the trends are positive in all cases, none of them are significant at the 95 % confidence level according to the Mann-Kendall test [31,32] applied to the data, in agreement with the previous results of [16,17,33], and partially with the results of [34]. Note that although there are extreme precipitation events in the Caribbean coast near the low-pressure critical zone, probably associated with the direct impacts of the hurricanes, the number of stations reporting extreme precipitation events (red dots in Figure 2) is greater on the Pacific slope, where the effects are indirect. The direct hit of a hurricane usually brings destruction and a large number of impacts to the affected region, but the indirect effects are more frequently reported in the precipitation stations of the Pacific slope. The prevalence of impacts in this subregion is also shown in the spatial distribution of tropical cyclone impacts analyzed by [1]. Another important issue is that even a relatively lower intensity storm located in a critical position can result in a catastrophe. For example, that was the case of Tropical Depression 16 of 2008, an unnamed storm that caused a disaster on the Costa Rican Pacific slope [35]. Around 490 communities were affected by flooding, landslides, and infrastructure damages [35]. In Costa Rica, there were around 1850 people directly affected by flooding and landslides and approximately 92,600 people, directly and indirectly, impacted [35]. The trajectory and municipalities impacted by the storm are shown in Figure in the Appendix A Figure A1.
In Figure 4, we identified the critical cyclone locations that produce the largest number of stations reporting extreme precipitation events. One of the most consistent critical regions in the different classification of events used in Figure 4 is the zone of the Gulf of Honduras. It is evident that the location of the cyclones in this position directly affects the Caribbean coast and a large number of indirect effects in the stations of the Pacific slope.

Eastern Tropical Pacific Oceanic Basin Results
In the Eastern Tropical Pacific Ocean, the spatial distribution of trajectories associated with heavy rainfall in Central America is scattered in a large region ( Figure 5). In a similar fashion as for the Caribbean, we selected a critical region of high trajectory density. In this case the box defined by 10 • N to 25 • N and 101 • W to 121 • W was selected for studying the associated circulation, which is shown in Figure 6. As can be seen, the box is now significantly larger and contains a large number of dispersed events. The contours of Figure 6a are more closely plotted, but even so, the low-pressure anomaly is significantly weaker than for the Caribbean. The streamlines in the Pacific are widely spaced and there is a vortex in the Caribbean associated with curled low velocities, that may be marginally affecting the Pacific moisture carrying mechanism pattern.  In Figure 7 the locations of the critical zones are identified. As can be seen, there are a lot of dispersed locations in the Pacific that cause extreme precipitation events, but in a smaller number of stations with respect to the Caribbean. The exception is when lower intensity cyclones (Figure 7) are located near and on the coast of Nicaragua and Costa Rica, for example, when the Tropical Storm Alma landed in Nicaragua in 2008. However, comparing Figure 7 to Figure 4 it is evident that these types of cyclone locations are rare. Another important issue is that as in the Atlantic basin, even a relatively lower intensity storm located in a critical position at the Eastern Tropical Pacific can induce a circulation over Central America that could result in important impacts, like the low-pressure system located offshore Guanacaste and causing damage on the Costa Rican slope from 22 to 28 May at this early cyclone stage. This system evolved later into Tropical Storm Agatha [36]. The trajectory and municipalities impacted by Tropical Storm Alma and Tropical Storm Agatha are shown in Figure A2.
The number of storms in the Pacific's critical zone appear to be decreasing from 1970 to 2010 (Figure 8).The trends are significant when all cyclones and only named cyclones are considered. In general, the impacts of Pacific oceanic basin cyclones are also located on the Pacific slope of the isthmus. These results are in partial agreement with those of [34,37], who found no evidence of positive trends in the annual time series of Tropical Cyclone occurrences in the Eastern Tropical Pacific basin.

Conclusions
Tropical cyclones from the Caribbean and Pacific basin produce a large number of indirect impacts on the Pacific slope of the Central American isthmus. Although the direct impact of a tropical cyclone usually results in devastation in the affected region, the indirect effects are more common and sometimes equally severe. In fact, the storm does not need to be an intense hurricane to cause considerable impacts and damage. The location of even a lower intensity storm in critical positions of the oceanic basin can result in destructive indirect impacts in Central America.
Storms' critical locations in the Caribbean oceanic basin are identified with more precision compared to the Pacific basin. One important critical zone is located in the Gulf of Honduras and surrounding areas. The Pacific basin location is more widespread, and the largest number of extreme precipitation events were observed when the cyclone was located off the Pacific coast of Nicaragua and Costa Rica and inside the continent (i.e., Tropical Storm Alma in 2008). For other positions, the number of impacted stations is smaller, and the locations of critical regions are not very well defined.
Positive (but not significant) trends  in the number of incursions of tropical cyclones in the Caribbean critical region were observed. There is great variability from year to year in these data. In the Pacific, a larger area of influence showed significant decreasing trends (1970-2010) when summing the incursions of all cyclones and when only named storms were considered. Although the influence of low-frequency climate modes can not be discarded, attention should be given by the authorities to the possible increase of indirect effects from Caribbean cyclones entering critical zones.
Identifying these critical locations is important for emergency preparedness. Forecasts issued that contain cyclones' projected trajectories into those critical zones must result in adequate alerts in potentially affected communities. The information from this work will be shared with the emergency agencies of the region.   Figure A1, but for the Pacific.