A Manifestation of Climate Change? A Look at Typhoon Yolanda in Relation to the Historical Tropical Cyclone Archive

By Carlos Primo David, Bernard Alan Racoma, Jonathan Gonzales and Mark Vincent Clutario

Environment Monitoring Laboratory, National Institute of Geological Sciences

University of the Philippines Diliman

Published in U.P. Diliman Journals Online SCIENCE DILIMAN Vol 25, No 2 (JULY-DECEMBER 2013)

Abstract

The IBTRACS world database of tropical cyclone (TC) tracks was analysed to determine potential historical trends in TC characteristics for the west Pacific basin. Trends are then related to the characteristics of Typhoon Yolanda to see if this individual event constitutes as a data outlier or is part of a trend that can be related to climate change. In terms of TC frequency, it is deduced that there is a decreasing pattern in tropical cyclone formation starting in 1970. It is also noted that while there is no trend in the annual mean maximum wind speed observed, a decrease in the number of high wind speed TCs is measured for the months of November and December. The location of TC formation has also been changing towards a higher latitude but closer to the Philippines in terms of longitude. Lastly, typhoons making landfall in the Visayas and Mindanao region have also become slightly more frequent in the last decade. Except for the last finding, the 2013 typhoon season does not fit in these general trends.  This year may be the start of a new trend or shift in TC characteristics (which we will only know after a few more years) but is most likely part of the inherent annual variability of typhoon characteristics. Yolanda goes against perceived trends but its occurrence signifies that there is still much to learn about tropical cyclones and the impending impacts of climate change in general.

Introduction

On November 8, 2013, Super Typhoon Yolanda (International Name: Haiyan) made landfall in Guiuan, Eastern Samar. It was, by many accounts, the most powerful tropical cyclone (TC) that made landfall ever recorded in history. According to the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) the recorded maximum 10-minute sustained winds of Yolanda was 230 kilometers per hour (kph) with gustiness reaching 250kph shortly after landfall. The Joint Typhoon Warning Center (JTWC) calculated 1-minute maximum sustained wind speed of 315kph with gustiness reaching 380kph. The National Disaster Risk Reduction and Management Council (NDRRMC) report dated December 16, 2013 reported 6,069 individuals perished (5,087 from the province of Leyte) while 1,779 are still missing because of Super Typhoon Yolanda. More than 5,000 casualties came from the province of Leyte and this was mainly due to the storm surge that affected its coastline. The estimated total worth of damages is pegged at PhP 35.5 billion.

One question that often arises during discussions in the aftermath of the disaster is whether this extreme event can be considered to be the “new normal” and therefore attributable to climate change. A consensus from meteorologists have deduced that the warming of ocean waters due to climate change will theoretically influence cyclogenesis but the high level of uncertainty in results preclude any definitive cause-effect relationship (WMO, 2006). The intergovernmental panel on climate change (IPCC) report conceded that the resolution of coupled ocean and atmospheric modelling is still too coarse to completely resolve climate change-related changes to tropical cyclone characteristics (IPCC, 2007). Still, based on various modelling studies, the IPCC report projected a decrease in mid-latitude storms globally per year but an increase in average wind intensity. This statement was slightly revised in its Fifth Assessment Report (IPCC, 2013) wherein it said that current datasets indicate no significant observed trends in global tropical cyclone frequency over the past century. Conflicting results were provided by McDonald et al. (2005) wherein they report an almost insignificant decrease in the number of typhoons (6% decrease) but an increase in wind intensity. Still, Emori et al. (2005) reported a projected decrease in both the number and intensity of cyclones in the northern Pacific basin but related precipitation will increase. One of the more recent works on tropical cyclone modelling was done by Knutson et al. (2010) wherein they project a 6-34% global decrease in the number of tropical cyclones but an increase by about 20% in the number of very intense cyclones by 2100. The same paper suggested a poleward shift in tropical cyclone formation.

Considering the apparent uncertainty of TC frequency and intensity trends from global climate models, the other technique that can be used in figuring out tropical cyclone trends is to analyse historical archives and looking at a single event, such as Yolanda, in relation to a typhoon database. This work aims to contribute to this form of analysis by looking at not only annual frequency and intensity trends but also other typhoon metrics by dissecting typhoon characteristics on a month by month scale.

Methodology

The US National Oceanic and Atmospheric Administration (NOAA) maintains a database that assimilates all recorded tropical cyclones by various weather agencies. The International Best Track Archive for Climate Stewardship (IBTRACS) boasts of more than 300,000 tropical cyclone-related entries, one-third of which are western Pacific cyclones (Knapp et al., 2010). It includes TC tracks (6-hourly), calculated wind speed and barometric pressure among other information and that the dataset covers the years 1884-2012. The IBTRACS has been endorsed by the World Meteorological Organization as an official archiving program for tropical cyclone information. The 2013 TC data from JTWC whenever applicable is included for completeness.

The IBTRACS data is parsed using the Python programming language and Microsoft Excel and plotted using ESRI’s ArcGIS. In many of the interpreted data, a 10-year moving average is employed to reduce annual variability and highlight the longer term changes in the parameters measured. Statistical analysis is performed to confirm any possible trends from the dataset.

Results and Discussion

Tropical cyclone frequency

Figure 1 shows tropical cyclone formation in the west Pacific basin on an annual basis. Evident in this plot is the increase in TCs recorded from the start of the dataset until around the 1970s wherein the highest total number of typhoons recorded was 61 in 1971; the 10-yr moving average in 1971 was 47.7 typhoons per year. The apparent 300% increase over the 86-year period is partly due to incomplete tropical cyclone reporting as these were based on data dependent on the density of shipping vessels reporting such weather disturbances (Knuttson et al., 2010). Higher ship density started in the 1960s and satellite-based reporting only became operational in 1966. Starting in 1970, a decreasing trend spanning 43 years is recorded in the 10-year moving average. The current 10-year average stands at 28.4 typhoons per year.

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Figure 1. Number of tropical cyclones recorded annually in the west Pacific Basin. The black line is the 10-yr moving average.

 Typhoon Yolanda was the 34th of 35 tropical cyclones that formed in the west Pacific basin in 2013, seven more than the 10-year average. This is the highest recorded number of TCs in the last 12 years and constitutes a 3-year increasing trend starting in 2010.

Tropical cyclone intensity

Tropical cyclone intensity is measured via the maximum wind speed each TC system has attained during its lifetime. The IBTRACS data has a record of historical wind speeds between the years 1977 and 2012. It is based on the 10-minute maximum sustained winds which is similar to PAGASA’s signal system but different from the Storm category system used in the United States which is based on a 1-minute maximum sustained winds measurement. Figure 2 shows the annual maximum wind dataset.  Average annual maximum wind speeds do not show any definite trend; if at all there is a slight decrease in mean typhoon intensity in the 25th to 75th percentile of annual typhoons since 2007.  Typhoon Yolanda’s 230kph matches 2010’s Typhoon Juan (International Name: Megi) wind speed. The 2013 maximum wind speed average falls within historical range despite recording five typhoons that exceeded 185kph maximum wind speeds (two made landfall in the Philippines: Odette and Yolanda).

 12

Figure 2. Maximum wind speed data. The dashed line shows the range of wind speeds per year, the box denotes the range of typhoons falling within the 25th to 75th percentile (interquartile range) and the horizontal line inside the box represents the mean of the annual maximum wind speeds.

The IBTRACS dataset is further analysed to look at monthly trends for typhoon intensity. Table 1 shows the average number of significant typhoons (>150kph) that formed in the west Pacific basin for each month per decade. August and September record the most number of significant typhoons and that these are increasing throughout the decades. Significant typhoons in November and December show a decreasing trend.

years

jan

feb

mar

apr

may

jun

jul

aug

sep

oct

nov

dec

tot

1977-1979

0.3

0.0

0.0

0.0

0.0

0.7

1.0

1.7

1.7

1.7

2.0

1.0

10

1980-1989

0.2

0.2

0.3

0.5

0.4

1.1

2.1

2.2

3.0

2.9

1.8

0.8

16

1990 – 1999

0.0

0.0

0.1

0.3

0.5

1.0

1.4

2.4

3.2

2.4

1.4

0.3

13

2000-2009

0.0

0.1

0.4

1.2

1.3

1.5

2.4

3.3

3.3

2.0

1.1

0.4

17

2010-2013

0.0

0.0

0.0

0.3

0.5

1.0

1.5

2.8

3.8

2.5

0.5

0.3

13

average

0.1

0.1

0.2

0.5

0.5

1.1

1.7

2.5

3.0

2.3

1.4

0.6

14

Table 1. Average frequency of significant typhoons per decade (>150kph).

Location of formation

The location of tropical cyclone formation will indirectly have bearing on whether typhoons will make landfall in the Philippines. With a general west-northwest typhoon track, the higher the formation latitude and further east longitude, the lower the chance of the typhoon passing by our country.  The Philippines is located from latitude 5⁰N to 20⁰N and longitude 117⁰E to 127⁰E. This latitude range is roughly the same range as western Pacific typhoon formation. Figure 3a shows a definite shift in annual mean latitude of formation from about 17⁰N to as low as 12⁰N in the mid-1990s. This is coupled with an increase in annual mean formation longitude (farther east from the Philippines) from 125⁰E to 145⁰E (Figure 3b). Since then, however, a shift back to higher latitudes but closer longitude of formation is recorded in the last 18 years. This means that on average, typhoons have been more recently forming nearer the Philippines but at a higher latitude equivalent to Metro Manila. This is despite the fact that significant typhoons within the recent past have originated from very near the equator. Typhoon Sendong (International Name: Washi) became a tropical depression at 6⁰ north, Typhoon Pablo (International Name: Bopha) at 5⁰ north and that Typhoon Yolanda formed at 7⁰N of the equator.

 13

Figure 3a and 3b. Mean latitude and longitude of typhoon formation per year.

Number and location of landfall

Figure 4a shows that annually a range of 8-76% of TCs that form in the west Pacific basin make landfall in the Philippines. The historical average is 30.3% with the highest recorded percentage happening in 1991 and culminating to the highest 10-yr average of 37.7% in 1995. However, since then, this percentage has gone down to 28% with 2013 recording only 31.4% of the TCs making landfall in the country. To determine whether typhoon tracks are changing through time, the number of TCs making landfall in Northern Luzon, Southern Luzon-Bicol, and Visayas-Mindanao are plotted (Figures 4b-d). Northern Luzon still accounts for most TCs making landfall (20.2% of all TCs formed), followed by Visayas-Mindanao (9.8%) and Southern Luzon-Bicol (8.4%). However, noticeable in these plots is the slight decrease in TCs entering Luzon and Bicol and increase in the percent of TCs entering Visayas-Mindanao which is up by 0.8% to 10.6%. The peak occurred in the 1990s when 12-15% of TCs passed by Visayas-Mindanao. The 2013 typhoon season recorded 7 of 11 tropical cyclones made landfall in Visayas or Mindanao. This also constitutes 20% of all TCs that formed in the west Pacific basin, double the 10.6% average for the region.

14

Figures 4a-d. Percent of tropical cyclones formed that made landfall in the Philippines. The dark line shows the 10-year moving average for the dataset.

Conclusions

There are evident tropical cyclone trends as shown by the analyses of the IBTRACS database. This includes: the decreasing number of TCs forming in the west Pacific basin, the increase in latitude (and decrease in longitude) of mean TC formation, the decrease in the number of significant storms in November and December, and the increase in TCs entering Visayas and Mindanao. Further analyses of the IBTRACS database are already underway including the separation of apparent linear trends relatable to climate change with possible inter annual cyclical occurrences such as the El Nino Southern Oscillation.

Acknowledgment

This study is funded through a research project of the Philippine Council for Industry, Energy and Emerging Technology Research and Development (PCIEERD-DOST) and a professorial chair award from the Oscar M. Lopez (OML) Center for Climate Change.

References

Emori,S., Hasegawa, A., Suzuki, T. and Dairaku, K. 2005. Validation, parameterization dependence, and future projection of daily precipitation simulated with a high-resolution atmospheric GCM. Geophys. Res. Letters. DOI: 10.1029/2004GL022306

IPCC, 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

IPCC, 2013. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.

Knapp, K. R., M. C. Kruk, D. H. Levinson, H. J. Diamond, and C. J. Neumann, 2010. The International Best Track Archive for Climate Stewardship (IBTrACS): Unifying tropical cyclone best track data. Bulletin American Meteor. Society, 91, 363-376.

Knutson, T.R., McBride, J.L., Chan, J., Emmanuel, K., Holland, G., Landsea, C., Held, I., Kossin, J.P., Srivastava, A.K., and Sugi, M. 2010. Tropical cyclones and climate change. Nature Geosci.  DOI 10.1038/NGEO779.

World Meteorological Organization, 2006. WMO International Workshop on Tropical Cyclones Statement on Tropical Cyclones and Climate Change.

Dr. Carlos Primo David, Bernard Alan Racoma, and Mark Vincent Clutario are the project leader, science research analyst, and project staff respectively of FloodNET, a component of Project NOAH. This project aims to gather all flooding related data and bridge the gaps between data, modelling, and information output. They are also behind the rainfall prediction website ClimateX, which provides the rainfall probability data in the Project NOAH website

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