The Effects of El Niño and La Niña on Western Atlantic and Gulf of Mexico Tropical Storm and Hurricane Formation

Transcription

1 The Effects of El Niño and La Niña on Western Atlantic and Gulf of Mexico Tropical Storm and Hurricane Formation Joshua Mosser and Lawrence Wharton Loudoun County Public Schools November 18, 2011

2 Hurricanes, which historically peak in August and September of each year, can wreak havoc on the lives of millions of East Coast and Gulf Coast residents (Carey & Salazar, 2011). Of the over fifteen hundred tropical storms and hurricanes recorded from , three hundred fifty-six (23% of the total) and five hundred forty-eight (36% of the total) have occurred in August and September respectively (Carey & Salazar, 2011). El Niño and La Niña episodes of the ocean-atmosphere cycle known as El Niño-Southern Oscillation (ENSO), have a profound effect not only on United States weather patterns but on world weather patterns as well (Swanson & Lewis, 2009). Is there a link or correlation between the number of Western Atlantic and Gulf of Mexico tropical storms and hurricanes, where they make landfall, their intensity (leading to the life and property damage they cause), and whether or not these violent weather events occurred during a traditional El Niño or a La Niña episode? Does a La Niña episode lead to more Western Atlantic tropical storm and hurricane formation? Will a Western Atlantic tropical storm or hurricane cause more storm damage if it occurs during a La Niña episode? Does the relatively newly discovered second type of El Niño, El Niño Modoki, cause more landfalling hurricanes in the Western Atlantic or the Gulf of Mexico (Georgia Institute of Technology, 2009)? Research done by Joshua Mosser and Lawrence Wharton will endeavor to answer these questions. The research on tropical storm and hurricane formation initially centered on three different types ocean-atmospheric cycles: El Niño, La Niña and El Niño Modoki. El Niño Modoki is from the Japanese meaning similar, but different (Georgia Institute of Technology, 2009). The literature review performed for this project indicated a lack of general consensus among meteorologists and climatologists when El Niño Modoki episodes occurred. Despite this lack of consensus on the timing of Modoki events, research by Peter Webster, professor of the School of Earth and Atmospheric Sciences, at the Georgia Institute of Technology (Georgia Institute of

3 ) Technology, 2009) and Emilia Jin, Center for Ocean-Land-Atmospheric Studies, National Oceanic and Atmospheric Administration (NOAA) (Jin, 2009), was too compelling to exclude Modoki events in the opinion of these researchers. Therefore this project will compare hurricane formation in the Western Atlantic and Gulf of Mexico to normal oceanic-atmospheric conditions versus El Niño, La Niña, and El Niño Modoki episodes. Any discussion of normal ocean atmospheric conditions must first start with an understanding of the role played by the vast expanse of the Pacific Ocean which makes up threefifths of all water masses (Caviedes, 2001). The Pacific at the widest span is 11, 447 miles representing 45 percent of the earth s circumference in contrast to the Atlantic Ocean s widest span of 4,200 miles (Caviedes, 2001). # Normal Ocean-Atmospheric Cycle $% &' ('!" In normal or neutral El Niño- La Niña years strong trade winds blow from east (South America) to west (towards Australia and Indonesia) along the Equator and push warm surface

4 * water into the Western Pacific-see Figure 1. In fact the Indonesian sea level is about 30 centimeters higher than the sea level off the coast of Peru (Chavez, 2004). This coupled air and ocean movement along the coast of South America creates an upwelling of cooler but nutrient rich bottom water to replace the well mixed and warmer surface water heading west (Chavez, 2004). The cool bottom water and warmer surface water form a distinct line or boundary, known as a thermocline, and its relative position or tilt has a profound effect on Pacific meteorology and climate (Mitchell, 1997). In the western Pacific the warm water layer lowers or tilts down to about 200 meters below the surface while the warm water in the eastern Pacific may only lower to 50 meters or less below the surface (Chavez, 2004). The Coriolis Effect-Figure 2- is caused by the earth s rotation and affects the movement of both winds and ocean currents (Briney, 2008). Winds and ocean currents tend to deflect to the right or in a northerly direction in the Northern Hemisphere and in the opposite, to the left or in a southerly direction, in the Southern Hemisphere (Mitchell, 1997). As the Coriolis effect becomes more pronounced as latitude increases, hurricanes typically must form more than plus or minus five degrees from the Equator to begin to rotate and strengthen (Briney, 2008). Figure 2- Coriolis effect (Pidwirny, Global Circulation of the Atmosphere, 2006) (Briney, 2008)

5 + Ecuadorian and Peruvian fishermen observed from pre-colonial times a periodic movement and influx of warm surface water into their fishing areas in December, the start of summer in the Southern Hemisphere (Mitchell, 1997). Coincidental with the arrival of these warmer waters, these coastal residents of Peru also experienced unusual amounts of rain causing nearby deserts to transform into grasslands (Mitchell, 1997). This event became known as an El Niño, the little one or the Child Jesus (Caviedes, 2001). Sir Gilbert Walker, a British scientist, provided scientists with a first clue linking far-reaching climatic effects and El Nino which he called the Southern Oscillation (SO) (Mitchell, 1997) and documented his findings in 1924 (Katz, 2002). Walker observed large scale air movement and circulation with highs or sinking air masses in the eastern Pacific accompanied by low or rising air masses in the western Pacific (Katz, 2002). The two air masses or centers of action had a tendency to seesaw or oscillate with reversals of areas of atmospheric pressure over time (Henson & Trenberth, 2001). In 1969 a University of California professor, Jacob Bjerknes, named the seesawing of Pacific airflows the Walker Circulation and recognized the connection between unusually warm waters (the ocean component) and weakening Pacific trade winds (the atmospheric component) (Henson & Trenberth, 2001). This El Niño and Walker circulation linkage is now referred to as the El Niño-Southern Oscillation or ENSO (Mitchell, 1997). A critical development in understanding ENSO came with the Tropical Atmosphere Ocean Project (TAO) conducted between 1985 and 1994 jointly by NOAA and the Pacific Marine Environmental Laboratory (PMEL) (Henson & Trenberth, 2001). The TOA project consisted of placing 70 deep ocean moored buoys capable of measuring winds, sea surface temperatures, relative humidity and sea temperatures at ten different levels down to 500 meters in depth

6 ! (National Oceanic and Atmospheric Administration (NOAA), 2000). These buoys (Figure 4) were placed in the tropical Pacific Ocean as shown in Figure 3 (National Oceanic and Atmospheric Administration (NOAA), 2000). Figure 3-TAO Buoy station map locations Figure 4-TAO moored buoy (National Oceanic and Atmospheric Administration (NOAA), 2000)

7 In 2000 the TAO buoys were upgraded to the TAO ATLAS buoy as shown below which provided more precise data measurements (National Oceanic and Atmospheric Administration (NOAA), 2000). Figure 5- Next Generation of TAO ATLAS (Autonomous Temperature Line Acquisition System) buoys (National Oceanicc and Atmospheric Administration (NOAA), 2000) Meteorologists and climatologists did not reach consensus on a uniform definition as to what constituted an El Niño versus a La Niña episode or event until 2005 (NOAA News OnLine, 2005). The NOAA National Weather Service, the Meteorological Service of Canada and the National Meteorological Service of Mexico came to agreement on February 23, 2005 for the definitions of a measurement index, the Oceanic Niño Index (ONI), an El Niño event and a La Niña event (NOAA News OnLine, 2005).

8 , The ONI is measured over a Pacific equatorial region (designated as El Niño 3.4 and known as the equatorial cold tongue ) that extends from 5 degrees north latitude to 5 degrees south latitude and from 120 degrees west longitude to 170 degrees west longitude (NOAA News OnLine, 2005). The index is a three-month average of surface sea temperatures (SST) that vary plus or minus 0.5 degrees Celsius from normal sea surface temperatures (NOAA News OnLine, 2005). NOAA s North America operational definitions for El Niño and La Niña are as follows: El Niño is a phenomenon in the equatorial Pacific Ocean characterized by a positive sea surface temperature departure from normal (for the base period) in the Niño 3.4 region greater than or equal in magnitude to 0.5 degrees C (0.9 degrees Fahrenheit), averaged over three consecutive months (NOAA News OnLine, 2005). La Niña is a phenomenon in the equatorial Pacific Ocean characterized by a negative sea surface temperature departure from normal (for the base period) in the Niño 3.4 region greater than or equal in magnitude to 0.5 degrees C (0.9 degrees Fahrenheit), averaged over three consecutive months (NOAA News OnLine, 2005). An El Niño warm water episode occurs when the easterly trade winds weaken and the water surface layers warm up in the eastern and central part of the Pacific Ocean around the equator (STORMFAX Inc., 2011). The thermocline near the South American coast is flattened and deepwater upwelling is suppressed (Henson & Trenberth, 2001)- see Figure 6. El Niño events occur irregularly, roughly every three to seven years with varied intensity (Chavez, 2004) see Figure 11 (Golden Gate Weather Services, 2011). The upwelling that is suppressed has a profound effect on marine life and the harvesting of cold water fish off the coast of Ecuador and Peru during an El Niño episode is reduced significantly -see Figure 7 (Chavez, 2004).

9 - Figure 6- El Niño Ocean-Atmospheric Cycle (Pidwirny, El Nino, La Nina and the Southern Oscillation, 2006; Pidwirny, Global Circulation of the Atmosphere, 2006) The easterly trade winds as shown in Figure 6 weaken in the western and central Pacific and an early warning sign of the onset of an El Niño event is when the winds along the equator in the western Pacific between the International Date Line and Indonesia reverse direction, start heading west and strengthen (Chavez, 2004). As the eastern trade winds weaken, the thermocline along the equator flattens out, rising in the western Pacific and dropping in the eastern Pacific-see Figure 9, which in turn sends warm surface water towards South America where it must turn northward and southward along the coast (Mitchell, 1997). The effects of this influx of warm water coming in from an El Niño have been felt as far north as Canada and as far south as Chile (Mitchell, 1997). Other El Niño effects include heavy rains in the central Pacific, drought conditions in Indonesia and Australia, wide-

10 spread flooding in the southern United States and mild winters on the northeast United States (Mitchell, 1997). The additional supply of warm water from an El Niño affects Atlantic Ocean hurricane activity by influencing winds that shear off the tops of developing tropical storms which in turn reduces the number of hurricanes that can develop and strengthen (National Oceanic and Atmospheric Adminstration, 2008). Research done by Chunzai Wang from NOAA s Miami based Atlantic Oceanographic and Meteorological Laboratory and Sang-Ki Lee of the Cooperative Institute for Marine and Atmospheric Studies at the University of Miami found that a long-term increase of vertical wind shear caused by warm ocean waters coincided with a downward trend in the number of Atlantic hurricanes that reached land (National Oceanic and Atmospheric Adminstration, 2008). More recent research by Peter Webster also shows that El Niño episodes result in the formation of fewer Atlantic hurricanes (Georgia Institute of Technology, 2009). Figure 7-Upwelling comparison of a normal year versus an El Niño episode (Chavez, 2004)

11 Just as an El Niño represents a warm water episode or event, La Niña represents the opposite, a cold water or cold phase ENSO episode or event (Swanson & Lewis, 2009). As shown in Figure 8 and Figure 9 the thermocline tilts deeper in the western Pacific and is closer to the surface in the eastern Pacific producing a much stronger upwelling than in normal or neutral years (Swanson & Lewis, 2009) and (Wall, 2011).,#(Niña#0 1. ###2####,, $/ " During a, La Niña episode clouds and thunderstorms shift from the central Pacific as shown in Figure 6 to the western Pacific as shown in Figure 8 which in turn directly influence the path of west-to-east traveling jet streamss high above the earth as shown in Figure 10 (Climate Prediction Center/NCEP, 2011) and (Swanson & Lewis, 2009).

12 Figure 9 Comparison of La Niña, normal and El Niño conditions Figure 10-Diagram of El Niño and La Niña effect on Pacific jet streams $1 %1 '1&% "

13 ) La Niña episodes can trigger cold air outbreaks in the northern United States and fewer coastal winter storms in the eastern United States (Swanson & Lewis, 2009). More hurricanes form in the Atlantic Ocean during La Niña episodes according to Webster (Georgia Institute of Technology, 2009) because there is less vertical wind shear present due to the change in jet stream travel to reduce tropical storm and hurricane formation (National Oceanic and Atmospheric Adminstration, 2008). A study in 2009 by Peter Webster concluded that El Niño episodes were changing and intensifying into a new type of event which he called (Science Daily, 2010) El Niño Modoki from the Japanese meaning similar, but different (Georgia Institute of Technology, 2009). Studies by Emilia Jin concluded similarly that there was a new form of El Niño as well (Jin, 2009). The significance of a Modoki episode is that in addition to the increase of Atlantic Ocean hurricanes from La Niña episodes, Modoki events may now contribute to the formation of not only more hurricanes but also more hurricanes making eastern United States landfall (Georgia Institute of Technology, 2009). Modoki differs from a typical El Niño episode in that the warmest waters are found in central equatorial Pacific Ocean rather than in the eastern Pacific along the South American coast (Science Daily, 2010). El Niño Modoki, this central Pacific warming episode (CPW), is also referred to as central-pacific El Niño, warm-pool El Niño, or dateline El Niño (Science Daily, 2010). Modoki events have been observed in , , , and (Science Daily, 2010). In the literature review conducted for this project there was not consensus for when Modoki years have actually occurred and NOAA has not issued a statement at this time clarifying what the difference in operational definition is between a Modoki, El Niño or La Niña episode. In fact Henson and Tremberth have suggested that the original 2005 NOAA baseline used in the ONI

14 * calculation may itself need to change as the result of the increase in average sea level temperatures in the tropical Pacific over the past decade (Henson & Trenberth, 2001). Figure 11 El Niño and La Niña Years and Intensities El Niño La Niña Weak Moderate Strong Weak Moderate Strong Based on Oceanic Nino Index (ONI) Jan Null, CCM Updated September 8, (Golden Gate Weather Services, 2011) Tropical cyclones are large-scale storms that form over warm ocean waters in tropical regions, and their outer circulations can extend more than 1000 km from the center of the storm. Tropical cyclones are characterized by extremely high winds, torrential rainfall, and high storm surge; cyclones are therefore some of the most life threatening and destructive natural phenomena on Earth (Montgomery & Farrell, 1993). The less intense storms, called tropical depressions and tropical storms, carry destructive potential, but are primarily associated with very heavy rainfall. Cyclones form in moist, unstable environments in association with lowlevel easterly waves or tropical cloud clusters and warm sea surface temperatures (Montgomery & Farrell, 1993). Hurricane activity occurs over oceans in regions where sea surface temperatures exceed 26 C. Higher sea surface temperatures are associated with an increase in

15 + water vapor in the lower levels of the troposphere. The warm, moist air near the equator rises upward from near the surface, creating a depression of pressure. This area of lower pressure creates a local pressure gradient where air from surrounding areas with higher pressure pushes into the low pressure area. This intruding air becomes warm and moist and also rises. As the warm air continues to rise, the air surrounding it swirls in to take its place, creating circulation and rotation (Shapiro & Goldenberg, 1998). As the storm system rotates faster and faster, an eye forms in the center. Hurricanes that affect the United States begin as an easterly wave of heat energy from the Sahara Desert during the period of June-November. This column of hot air swirls upward and spreads as it moves to the west coast of Africa. The air then picks up moisture from the warm waters of the Atlantic Ocean, and the combination of the flow of the ocean and the Coriolis Effect lead to the rapid growth of the storm system (Trenberth, 2005). For this project, a metadata study of cyclonic storms affecting the Western Atlantic basin and the Gulf of Mexico was conducted. We charted the tropical storms and hurricanes that were present in either basin from Using Figure 11, we identified each year as an El Niño, La Niña, El Niño Modoki, or neutral year. For each type of ocean event, we averaged the number of storms that were present during those time periods. This data can be found below in Tables 1 and 2.

16 ! Tropical Table 1: Western Atlantic Cyclones, Storms Category 1 Category 2 Category 3 Category 4 Category 5 Total Avg/EN Avg/LN Avg/Mod Avg/Neu Tropical Table 2: Gulf of Mexico Cyclones, Storms Category 1 Category 2 Category 3 Category 4 Category 5 Total Avg/EN Avg/LN Avg/Mod Avg/Neu

17 The first question that must be addressed is whether there is a difference in the number of storms in each different year type. The data is primarily count data and the overall numbers are small. The statistical analysis that was run puts all of the years in order of increasing number of storms and then gives each a numerical rank. Those years with the same number of storms would share a rank. The logic of the test is that if the type of event that occurred during a year is influencing the number of storms, then it is expected that average ranks for all years for a given event would be very similar and therefore higher or lower than the other events. Therefore, the alternative hypothesis that the ranks for each storm event are different would be offered. If there is no influence from oceanic event, then there would be no difference in the ranks. Therefore, the question that is being asked is what is the probability that the results would occur if there were no difference in ranks? If the probability is 0.05 or less, then that would suggest that they are statistically different. A Kruskal Wallis Test was used to determine if there was a difference among El Niño, La Niña, El Niño Modoki, and neutral events. The resulting probability was p= We are able to reject the hypothesis of no difference in ranks and therefore state that there is a difference between some pairs. Examining each pair was done with the Mann Whitney U test. El Niño vs. La Niña: p=0.025 (the number of storms is significantly different) El Niño vs. El Niño Modoki: p=0.028 (the number of storms is significantly different) El Niño vs. Neutral: p=0.275 (the number of storms is not significantly different) La Niña vs. El Niño Modoki: p=0.178 (the number of storms is not significantly different) La Niña vs. Neutral: p=0.563 (the number of storms is not significantly different) El Niño Modoki vs. Neutral: p=0.088 (the number of storms is not significantly different)

18 , There are significantly fewer storms that occur during El Niño years vs. La Niña years or El Niño Modoki years. It is not expected that there would be significantly fewer storms than Neutral years, as it is expected that Neutral years are midway between El Niño and La Niña events. As Neutral years are non-event years, it is the baseline from which the events vary. El Niño Modoki years are believed to act more like a La Niña years than El Niño years in their storm-generating capacity. Modoki years have significantly more storms than El Niño years, thus supporting that something is happening during those years that make them more similar to a La Niña year. The Kruskal Wallis test was run individually on both the Western Atlantic (p=0.032) and the Gulf of Mexico (p=0.266). The p values indicate that there is a difference among the different events in the Western Atlantic basin, but no apparent difference in the Gulf of Mexico. The lack of difference is likely to be caused by the low number of storms in the Gulf of Mexico.

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21 Swanson, B., & Lewis, A. (2009, June 15). How El Nino and LaNina affect U.S. storms. USA Today. Trenberth, K. (2005, June 17). Uncertainty in Hurricanes and Global Warming. Science, 308. Wall, T. (2011, August 8). Discovery News. Retrieved October 30, 2011, from East Africa Drought Linked to La Nina: html