SR/SSD 97-37

08-01-97

Technical Attachment

A Climatology of Tropical Cyclones Affecting the Texas Coast

During El Niño/non-El Niño Years: 1900-1996

John A. Cole and Steven R. Pfaff

NWSO Corpus Christi, Texas

Introduction

Much speculation exists regarding effects of El Niño, abnormally warm surface water temperatures off the coasts of Ecuador and Peru, on North American weather patterns. Particular attention has been directed toward the potential effects of the El Niño phenomenon on hurricane frequency and the strength attained by tropical cyclones during El Niño years, in comparison to non-El Niño years. Studies show there is a lack of tropical cyclones in the Atlantic in the year following El Niño; however, historical records show that this may not be the case for the Texas coast and western Gulf of Mexico.

The purpose of this paper is to show that weaker tropical cyclones and relatively fewer hurricanes, characteristic of an El Niño's influence on Atlantic hurricanes (Gray 1984), cannot necessarily be assumed for the western Gulf of Mexico and the Texas coast. In fact, this climatological study suggests that Texas tropical cyclone activity and the potential for having a major hurricane (Category 3 or greater) may actually increase during El Niño events.

El Niño/El Viejo

The term El Niño, derived from the Spanish word for "boy child," is the cyclical warming of sea surface temperatures in the eastern equatorial Pacific waters. The typical El Niño cycle occurs within a two to seven year period. The area affected by significant increase in water temperatures typically lies between 10oN and 10oS latitude and within 90o longitude from the coasts of Ecuador and Peru. El Niño is caused by a relaxation of the trade winds across the eastern Pacific equatorial waters, which results in warmer water temperatures across this region due to the lack of upwelling. El Viejo is the opposite effect, with cooler than normal water temperatures across the eastern Pacific equatorial waters.

The effects of varying warmer water temperatures in the Pacific have significant impacts on short-term global climates, including weather patterns across North America. The increase in water temperatures (El Niño) directly translates to warmer air temperatures at the source of the warmer waters. The convection which results above the warm, moisture-laden sea surface acts as a block in the atmosphere, resulting in a reconfiguration of jet streams. An enhanced southern branch of the jet stream often results in increased precipitation across the southern tier of the United States, while drought conditions are frequently observed during El Viejo periods. Also, mid and upper-level westerly shearing winds increase over the tropical Atlantic as a result of El Niño, thus inhibiting tropical cyclone development across the Atlantic hurricane basin.

Data and Methodology

The number of tropical cyclones which directly or indirectly impacted the Texas coast from 1900 through 1996 were classified into several El Niño/non-El Niño categories. Data for this study were obtained from Neumann, et al. (1993) and other local records. A direct hit is the center or core of a tropical cyclone moving across the Texas coast, while an indirect hit is a significant impact along the Texas coast although the center or core did not move across the Texas coastline. For example, Hurricane Gilbert (1988), which made landfall well south of Brownsville, was considered an indirect hit, because hurricane force conditions were experienced across Deep South Texas.

For this study, a history of the El Niño versus Non-El Niño years (O'Sullivan, 1994) was used to classify the following phases:

El Niño Year: (Warm phase) Positive sea surface temperature anomalies throughout the year preceded by another complete El Niño year or any transition to an El Niño event.

El Viejo Year: (Cold phase) Negative sea surface temperature anomalies throughout the year either preceded by another complete El Viejo year or any transition to an El Viejo event.

Neutral Year: Little if any sea surface temperature anomaly throughout the year either preceded by another complete neutral year or any transition to a neutral year.

Transition Year: Sea surface temperature variability from an anomaly or neutral condition to a different anomaly. There are six phases of transition defined in this study: El Niño to El Viejo, El Niño to neutral, neutral to El Niño, neutral to El Viejo, El Viejo to El Niño, and El Viejo to neutral.

Relative frequency was calculated for comparison of the aforementioned categories. Relative frequency is the number of direct or indirect tropical cyclone hits per category, divided by the number of El Niño or non-El Niño classification years. There was a total of 17 direct and indirect strikes of major hurricanes from 1900-1996. Relative frequencies for these major hurricanes were calculated for comparison with individual El Niño and non-El Niño classifications with respect to the total number of direct and indirect tropical cyclone strikes along the Texas coast.

Results

Seventy-three tropical cyclones affected the Texas coast from 1900 to 1996. Seventeen storms were classified as major hurricanes, fifteen of which directly made landfall on the Texas coast (Hebert, et al. 1997). Figure 1 shows that most of the years were attributed to the neutral category (26), followed by the El Niño to neutral transition category (14), with the fewest number of years in the El Viejo category (4).

As seen in Fig. 2, the highest percentage of tropical cyclones affecting the Texas coast was associated with the neutral phase (30.1%); however, as previously mentioned, most of the last 97 years were also associated with neutral conditions. The second highest percentage of tropical cyclones (24.7%) occurred during the El Niño to neutral phase, followed by complete El Niño (14.7%). There were no Texas coast landfalls of tropical cyclones during El Viejo events, however only four years were associated with complete El Viejo conditions. Surprisingly, the highest percentage of major hurricane landfalls occurred during the El Niño to neutral phase (35.3%), followed by El Niño, neutral, and El Viejo to El Niño phases (all three 17.7%).

Combining the years in which El Niño, El Viejo, and neutral conditions occurred for at least a part of a given year yields interesting results, as seen in Fig. 3. Approximately 77% of all major tropical cyclones affecting the Texas coast occurred when El Niño was present for at least a part of the year, while 59% occurred with neutral conditions, and 29% with El Viejo conditions present for at least part of the year. One would expect the highest percentage to be associated with the neutral phase, since there were more years with neutral conditions present.

Interesting results have been found when analyzing the relative frequencies for El Niño and non-El Niño phases (Fig. 4). The highest relative frequencies of tropical cyclones affecting the Texas coast occurred during El Niño to neutral phases (an average of 1.29 storms per year) and El Niño phases (0.90 storms per year). Regarding major hurricanes, high relative frequency values were shown for El Niño (0.30 storms per year) and El Viejo to El Niño (0.38 storms per year) phases, but the highest relative frequency of major hurricanes occurred during the El Niño to neutral transition (0.43 storms per year).

Conclusions

Although there is a correlation between weaker and fewer tropical cyclones during El Niño events for the Atlantic hurricane basin in general, this may not be the case for the western Gulf of Mexico and Texas. Records show that the relative frequency of tropical cyclones affecting the Texas coast is greatest during the El Niño to neutral phase, and is closely followed by the El Niño phase, suggesting there may be an increased threat of land-falling tropical cyclones during these periods.

The highest percentage of all major hurricanes which have affected the Texas coast occurred when El Niño was present for at least part of a given year. Thirteen of the seventeen major hurricanes which affected the Texas coast occurred during El Nino or partial El Nino years. Some of the most significant hurricanes ever to strike Texas occurred during strong or very strong El Niño years, including the 1900 Galveston hurricane which killed over 8000 people. Some other notable storms which occurred during strong or very strong El Niño years include the 1915, 1919 (the strongest hurricane on record to strike the Texas coast), and 1941 hurricanes. The last major hurricane to strike the Texas coast occurred in 1983 (Alicia) and was associated with the strongest El Niño event on record.

Although this study appears to indicate a positive correlation between both an increased frequency and intensity of land-falling tropical cyclones on the Texas coast during El Niño/partial El Niño years, the results should be used with caution because of the relatively small data set which was investigated. A more comprehensive study would be needed to analyze the potential effects of El Niño and non-El Niño phases on hurricane climatology along the entire United States Gulf Coast. Further investigation may reveal meteorological factors responsible for the results found in this research.

References

Gray, William M., 1984: Atlantic seasonal hurricane frequency: Part I: El Niño and 30 mb quasi-biennial oscillation influences. Mon. Wea. Rev., 112, 1649-1668.

Hebert, P.J., J.D. Jarrell, and M. Mayfield, 1997: The deadliest, costliest, and most intense United States hurricanes of this century (and other frequently requested hurricane facts). NOAA Technical Memorandum NWS TPC-1. 40 pp.

Neumann, C.J., B.R. Jarvinen, C.J. McAdie, and J.D. Elms, 1993: Tropical Cyclones of the North Atlantic Ocean, 1871- 1992. Historical Climatology Series 6-2, National Climatic Data Center, Asheville, NC. 4th rev., 193 pp.

O'Sullivan, D., 1994: El Nino and the southern oscillation, Queensland, Department of Primary Industries (QDPI), compiled from QDPI publication Q194015: Will it Rain?, 2nd Edition - The Effects of the Southern Oscillation and El Nino on Australia, edited by I. J. Partridge.