Aircurrents La Niña and Cool PDO Deliver a One-Two Punch to the 2010 Pacific Typhoon Season
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AIRCURRENTS LA NIÑA AND COOL PDO DELIVER A ONE-TWO PUNCH TO THE 2010 PACIFIC TYPHOON SEASON 01.2011 By Dr. Peter Sousounis INTRODUCTION Indeed, the 2010 typhoon season in the Northwest Pacific Since records were first kept by the Japan Meteorological generated only about half of its normal activity. Furthermore, Agency (JMA) in 1951, no typhoon season in the Northwest for only the second time ever1, it was surpassed by tropical Pacific has been as benign as last year’s; only 14 named cyclone activity in the North Atlantic, where 19 named storms formed in 2010. Despite the low number, three storms formed. storms affected South Korea—which is typically impacted by just one landfalling typhoon annually—and in October, a ENSO, WHILE POWERFUL, DOES NOT ACT Super Typhoon ravaged the Philippines. ALONE The 2010 year morphed quickly from El Niño to La Niña THE NORTHWEST PACIFIC BASIN FALLS SHORT conditions. Thus, a knee-jerk explanation for the season’s IN 2010 anemic activity might be, “La Niña did it.” During the La The Northwest Pacific basin ocean spawns more tropical Niña phase of the El Niño/La Niña-Southern Oscillation, or cyclones each year than any other basin in the world. This ENSO, typhoon activity from August to November tends to is in part because this basin is one of the world’s largest by be below normal. While it is true that the 2010 season was area. Additionally, it contains some of the warmest water on under the growing influence of a powerful La Niña—in fact, earth. On average, 25-26 named storms are generated here the second strongest transition since the La Niña of 1998, each year. This past year, however, the basin fell far short which produced only 16 named storms—that explains only of its reputation as the world’s most fertile water body for part of the story. Witness the developing La Niña of 1983, typhoon genesis. which was the third strongest since recordkeeping began, yet that season yielded 23 named storms. AIRCURRENTS 01.11| LA NIÑA AND COOL PDO PROVIDE A ONE-TWO PUNCH FOR THE 2010 PACIFIC TYPHOON SEASON BY DR. PETER SOUSOUNIS A previous AIR Current—Warm and Cold Year Impacts THE PACIFIC DECADAL OSCILLATION on Typhoon Activity in the Pacific Ocean, (Sousounis and Though ENSO garners much attention, it is not the only Desflots, 2008)—explored the contrast in typhoon activity climate signal impacting the basin. In fact, ENSO has an in the Northwest Pacific basin during the seven strongest older cousin: the Pacific Decadal Oscillation (PDO). The El Niño events versus during the seven strongest La Niña PDO cycle is in many ways similar to the ENSO cycle. For events. That study was restricted to the strongest El example, during the positive phase of PDO, the western Niño-Southern Oscillation signals because other climate Pacific becomes cool and part of the eastern ocean oscillations can partially or completely mask ENSO’s effects becomes warm. During a cool or negative phase, the when ENSO signals are weak. opposite pattern occurs. Furthermore, the sea surface temperature (SST) anomalies for PDO are similar to those of But when other climate oscillations don’t mask ENSO, ENSO. Figure 1 highlights the warm and cool phases of PDO the impacts on typhoon activity are clear: during an El and ENSO from a sea surface temperature and surface wind Niño event—which is the positive, or warm, phase of the perspective. ENSO cycle (La Niña is the negative, or cold, phase)—the anomalously warm waters over the central and eastern Pacific expand the area where typhoons can form. The expanded area translates to a higher number of storms generated. The aforementioned AIR Current discussed how the seven most intense El Niño events of all time yielded an annual average of 27 named storms, while the seven most intense La Niña events yielded fewer: only 22 named storms each year on average. More striking than the difference in the number of storms between warm and cold years, however, was the number of storm days—153 for warm years versus 74 for cold ones. The increased longevity of the typhoon season during warm Figure 1 Sea surface temperature (shaded) and surface wind stress (arrows) anomalies for positive (warm) and negative (cool) phases of the El Niño / years is a direct result of the greater expanse of warm water Southern Oscillation and Pacific Decadal Oscillation. (Source: Joint Institute for available for storms together with an eastward extension of the Study of the Atmosphere and Ocean) the genesis region. While the ENSO cycle may seem to be merely a slightly different form of PDO (Gershunov and Barnett 1998), more Differences in storm tracks were also evident between careful analysis of the locations and intensities of the SST strong El Niño and La Niña years; on average, only 12 and wind anomalies reveals that the impact of PDO is more storms managed to cross north of 30 degrees latitude north pronounced over the North Pacific region, while ENSO’s during cold years, while 17 crossed this threshold in warm effects are more pronounced over the Equatorial Pacific. ones. Typhoon tracks are suppressed during cold years, and this is a result of stronger northeasterly trade winds , which Despite these differences, there is evidence that when like- steer the storms westward, rather than allowing them to signs from each phenomenon combine, the net result is a curve northward, and then again to the northeast. version that is stronger than either of the individual signals. It should be noted that La Niña years can result in significant landfall activity in the southwestern portion of the basin, even in the face of reduced basinwide activity; indeed, the Philippines, Vietnam, Guangdong (China), Hong Kong, and even Taiwan can see more than their usual number of storms during a La Niña year (Goh and Chan, 2010). 2 AIRCURRENTS 01.11| LA NIÑA AND COOL PDO PROVIDE A ONE-TWO PUNCH FOR THE 2010 PACIFIC TYPHOON SEASON BY DR. PETER SOUSOUNIS ENSO AND PDO: THE BENEFIT OF A the PDO index at -0.17 was slightly negative—indicating COMBINED INDEX that perhaps there was not unanimous support for an To illustrate the combined effects of ENSO and PDO on above-average number of storms to form. typhoon activity, we consider the correlation between the annual number of named storms from the JMA record The 1998 season provides another example of the and the ENSO Index , the PDO Index, and the two indices combined index offering a clearer picture of typhoon combined, respectively. The ENSO Index in this analysis numbers. That year was the previous record-holder for is defined as the difference between the average index fewest storms—before 2010, that is—with 16 named values for the second and first halves of the year; averages events. The ENSO index recorded a respectable -2.22 and done this way better account for La Niñas developing late the PDO index was -0.33; that is, both were negative, so in the season after strong El Niños in the early part of the the two combined are consistent with a very low storm season—and vice versa—and they yield a higher correlation count. than that achieved when seasonally averaged ENSO Indices are used alone. The analysis period in this exercise is As a last example, we look at the 1988 season where 31 restricted to the years since 1981 in order to better capture named storms—an above average number—formed when the true and total number of storms over open water—as the ENSO index was -1.40 (e.g., strongly negative, and obtained with the assistance of satellite imagery. While suggesting a reduced number of storms). In that same consideration of the magnitude and sign of both the ENSO year, the PDO index was 0.33—a weakly positive value and PDO indices during the same time period will not surrounded by, and in the middle of, a very extended provide a perfect explanation for the annual typhoon totals positive cycle (Figure 2). Again, the combined index proved in the northwest Pacific, such a combined approach will more telling of the season’s activity than the ENSO Index provide further insight as to those totals. alone. The correlation between ENSO alone and the JMA annual storm count since 1981 is 0.37; the correlation between PDO alone and the JMA is 0.26; and the correlation between ENSO + PDO combined and the JMA is 0.46. (It is worth nothing that the correlation between the ENSO and PDO indices is -0.02, or nearly zero. The fact that the correlation is practically nonexistent indicates that these indices are providing value to typhoon counts in a mutually independent way. That is, there is very little overlap in terms of the benefit they provide. Plots of the time series of these indices are shown in Figure 2.) A higher correlation indicates a stronger casual relation between the combined oscillation and the resulting impact on typhoon frequency. While the result of 0.46 by itself is not sufficient to say that the combined effect of ENSO and PDO is a better predictor of typhoon activity than either of the two separately, we can look at key years in which an individual index alone did not correlate with the JMA seasonal storm total, but Figure 2: The upper graph shows values for the ENSO and PDO indices, respectively, for each year since 1981. The lower graph shows annual values for the combined index did. In fact, we need not go back any the ENSO+PDO index combined (right axis), compared to the JMA’s storm count further than 2009 to see the benefit of a combined index.