The Wettest September for Antigua in Over Two Decades

31 10 2017

Dale C. S. Destin|

September 2017 was the wettest for Antigua in over two decades. The month yielded 270.3 mm (10.64 in). This is almost doubled the monthly of 144.0 mm (5.67 in). The last time September was wetter was in 1995, when 373.1 mm (14.69 in) of rain fell.

September 2017 now ranks sixth wettest of all Septembers on record, dating back to 1928. Only 5 other Septembers have seen more rainfall.

SeptemberRainfallTotals_RankedSeptember 2017 is also the wettest month for Antigua in nearly five years. No month has been wetter since October 2012.

The probability of such a high rainfall total for September is 8.6%. This means that this total occurs once in every 11 to 12 years, on average.

The vast majority – over 75% of the rain was due to Hurricanes Irma, Jose and Maria. Maria was the single greatest rain maker with over 30% of the rainfall total for the month.

Interestingly, the last time September was wetter, we had two hurricanes – Luis and Marilyn of 1995. This time around we had three hurricanes.

September is the third wettest month on average behind October and November.

The rainfall total for the year, thus far, is running on the high side of the near normal range, notwithstanding the well above normal rainfall for September.

The forecast for October and the rest of the year is for near to above normal rainfall. Meanwhile, it is likely (55% chance) that the year (2017) will end with above normal rainfall – 1329.0 mm (52.3 in) with a 70 confidence of it being in the range of 1022.3 to 1690.6 mm (40.2 to 66.6 in). This would be the highest total since 2011.

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Tropical Cyclone Frequency and Climate Change

17 10 2017

Dale C. S. Destin |

This is a continuation of our series – Tropical Cyclones and Climate Change – (TCs; tropical depressions, tropical storms, hurricanes). In our previous blog in this series, we looked at Tropical Cyclone Intensity and Climate Change. In this blog, we will look at whether climate change is having an impact on TC frequency.

Hurricanes Katia (left), Irma (middle) and Jose (right)

Hurricanes Katia (left), Irma (middle) and Jose (right) – Sep 8, 2017

To date, the 2017 Atlantic hurricane season has produced 15 named storms – three more than the average or 25% more than usual. This has equalled the number of storms for last year. Meanwhile, there have been 10 hurricanes – four more than the average or 67% more than usual. This tied with 2010 for the most hurricanes since 2012. Further, there have been six major hurricanes – three more than average or 100% more than usual, the most since 2005.

Have TCs become more frequent?

According to the Intergovernmental Panel on Climate Change (IPCC), it is unlikely that TCs have become more frequent. Recall that the IPCC is the United Nations’ body task with assessing climate change.

Here is what the IPCC Assessment Report Five (AR5) actually says: “…recent assessments indicate that it is unlikely that annual numbers of tropical storms, hurricanes and major hurricanes counts have increased over the past 100 years in the North Atlantic basin…”. “No robust trends” exist.

So, having examined hundreds of peer-reviewed scientific research papers on the subject, the IPCC concluded that there has been change in the frequency of tropical cyclones.

One of the many papers cited by the IPCC, explains not only that there has been no change in the annual number of TCs but that there have been previous active Atlantic TC eras similar to the present. These active eras alternate with inactive eras; hence, in the not too distant future, the Atlantic will return to an inactive phase, perhaps similar to the 1970-1994 interval, when fewer storms occurred.

Further, although there has been no change in the number of  TCs over the past 100 years, research suggest that relative to about a thousand years ago, the annual number of TCs have decreased significantly. This is supported by Michael E. Mann et al., Michael J. Burn and Suzanne E. Palmer and others.

The increased greenhouse gases, the cause of climate change, substantially explain the observed SST increases over such places as the Atlantic and North West Pacific during the last 50 years. Added to this, are the apparent significant increases in TCs over this period. However, when the raw dataset is adjusted for short-lived TCs i.e. TCs lasting less than two days, a different picture emerges.

NA_TCs_Raw&Adjusted

Green-shaded curves depict global mean temperature (HadCRUT3 data set) and August–October main development region (MDR; 10° N–20° N, 80° W–20° W) SST anomalies (HadISST data set). Blue-shaded curves represent unadjusted tropical storm counts. Red-shaded curves include time-dependent adjustments for missing storms based on ship-track density. The curve labelled ‘>2-day’ depicts storms with a duration greater than 2.0 days. Orange-shaded curves depict US landfalling tropical storms and hurricanes (no adjustments). Solid black lines are five-year means (1878–2008); dashed black lines are linear trends. Vertical axis ticks represent one standard deviation. Series normalized to unit standard deviation. Only the top three series have significant linear trends (p = 0.05). Source: Knutson et al. 2010

Before the satellite era – pre-1966, a number of storms went undetected due to the fact that they never made landfall and occurred in unfrequented parts of the Atlantic Ocean by ships – the only source of TC detection over open waters before there were satellites. Thus, when the raw data is adjusted for these missing cyclones, no long-term trend is detected.

The trend in major hurricanes is not (statistically) significance. Notwithstanding, the current trend is unreliable since studies have shown that the wind speeds for TCs, over the period 1851 to 1920, were systematically underestimated, a fact supported by paleoclimatology.

To a great extent, the numbers of TCs on record, pre-1966, are there only because they made landfall. Counting landfalling TCs is not a robust method for determining overall trend; nevertheless, the method shows no trend in various regions of the world.

Will TCs become more frequent in the future?

To this question, the IPCC says no! Here are the exact words from the IPCC AR5, “…it is likely that the global frequency…of tropical cyclones will either decrease or remain essentially unchanged…. However, “substantial increases in the frequency of the most intense cyclones and it is more likely than not that this increase will be larger than 10% in some basins”.

One of the main scientific papers used by the IPCC to come to this conclusion is one done by Knutson TR, McBride JL, Chan J, et al. They say that late in this century, there will be a 6 to 34% decrease in TC frequency. However, on the other hand, the same paper projects a 2 to 11% increase in TC winds; hence, by virtue of this, predicts an increased frequency of major hurricanes being more likely than not by year 2100. Notable TC researchers Kevin J.E. Walsh, John L. McBride, Philip J. Klotzbach et al. endorsed this conclusion.

TrendOfTCs

General consensus assessment of the numerical experiments described in Supplementary Material Tables 14.SM.1 to 14.SM.4. All values represent expected percent change in the average over period 2081–2100 relative to 2000–2019, under an A1B-like scenario, based on expert judgement after subjective normalization of the model projections. Four metrics were considered: the percent change in (I) the total annual frequency of tropical storms, (II) the annual frequency of Category 4 and 5 storms, (III) the mean Lifetime Maximum Intensity (LMI; the maximum intensity achieved during a storm’s lifetime) and (IV) the precipitation rate within 200 km of storm centre at the time of LMI. For each metric plotted, the solid blue line is the best guess of the expected percent change, and the coloured bar provides the 67% (likely) confidence interval for this value (note that this interval ranges across –100% to +200% for the annual frequency of Category 4 and 5 storms in the North Atlantic). Where a metric is not plotted, there are insufficient data (denoted ‘insf.d.’) available to complete an assessment. A randomly drawn (and coloured) selection of historical storm tracks are underlain to identify regions of tropical cyclone activity. Source: IPCC AR5

One of the fallacious arguments being advanced for the increase in the frequency of TCs is increasingly warmer SSTs being caused by climate change. But quite obviously, warm SSTs are not the only parameters required for increased frequency. SSTs are very necessary but far from being sufficient for causing increased TC frequency.

A recently issued paper by Jeffrey P. Donnelly & Jonathan D. Woodruff indicates that such high SSTs as at present are not even necessary to support periods of frequent major hurricane activity. There have been eras with similar or higher number of TCs with significantly lower SSTs.

The Kevin J.E. Walsh et al. paper indicates the models used to simulate TCs, produce an increased frequency of TCs as temperature decreases and decreased frequency when temperature increases. Thus, increased SSTs do not necessarily mean increased TC frequency, by consensus, the opposite seems to be true.

Also required for increased number of TCs are conducive atmospheric conditions – unstable Atmosphere, moist middle Atmosphere and the overturning of the tropical Atmosphere caused by the Hadley cell. All three are needed for the formation of tropical disturbances – the precursors of TCs. However, fortunate for us, climate change is causing these things to go in the negative direction; hence, the projected decrease in the frequency of TCs by the end of this century.

Based on the IPCC and the vast majority of the TC researchers, TC frequency has not changed over the last century. By the preponderance of research papers, climate change has NOT caused TCs to become more frequent. However, by late this century the frequency of TCs is projected to decrease or perhaps remain unchanged relative to present. However, the number of major hurricanes is more likely than not to increase late in the current century. This is the conclusion of the IPCC AR5 – we either accept the full report or none at all. And if we accept all of it, we would not be blaming current TC and hurricane frequencies on climate change, at this time.

Our next blog in this series will look at the impact, if any, of climate change on tropical cyclone rainfall.





Tropical Cyclone Intensity and Climate Change

5 10 2017

Dale C. S. Destin|

This is a continuation of our series – Tropical Cyclones and Climate Change – TCs (tropical depressions, tropical storms and hurricanes). In this blog, we will look at whether climate change is having an impact on TC intensity/strength, especially with respect to wind speeds and provide you scientifically based answers.

As many Caribbean islands rebuild after the havoc caused by Hurricanes Irma and Maria, which killed over 185 persons and caused over US$150 billion in damage, many – including political leaders, have declared that these hurricanes were caused by climate change.

Even the United Nations (UN) Secretary-General seems to be peddling the notion that climate change caused Irma, Maria and the hyperactive 2017 Atlantic hurricane season.

But is climate change really to be blamed? Are these statements in harmony with the findings of the Intergovernmental Panel on Climate Change (IPCC)? What is the consensus of TC researchers?

The 2017 Atlantic hurricane season

Thus far, the season has produced 14 named storms, 8 of which became hurricanes and 5 major hurricanes – Category 3 and over. Of the 5 major hurricanes, 2 (Irma and Maria) became Category 5 – the highest category on the Saffir-Simpson Hurricane Wind Scale.2017 Atl Hurricane Season TracksRecord shows that Hurricane Irma was no ordinary Category 5 Hurricane. Far from it – it was more like a Super Category 5 Hurricane. If there were a Category 6 – it would have easily been so categorised.

Hurricanes Irma and Maria are among the strongest hurricanes to ever form over the Atlantic Basin. Irma had peak sustained winds of 295 km/h (185 mph), which makes her joint holder with three other hurricanes for the second strongest Atlantic hurricane on record, dating back to 1851. Only Hurricane Allen of 1980 was stronger with 305 km/h (190 mph) winds. As for Maria – her peak sustained winds of 280 km/h (175 mph) tied her with seven other hurricanes for the eighth strongest on record.

Super Category 5 Hurricane Irma on our doorsteps

Super Category 5 Hurricane Irma on our doorsteps – Sep 5, 2017

Have TCs become more intense?

According to the IPCC, “unlikely“. The IPCC is the UN international body designated to assess the science of climate change. It was set up in 1988 by World Meteorological Organization (WMO) and UN Environment Programme (UNEP) to provide policymakers with rigorous and balanced scientific information on climate change.

This is what the IPCC Assessment Report Five (AR5) – the latest report, says about the impact of climate change on TCs: “In summary…recent assessments indicate that it is unlikely that annual numbers of tropical storms, hurricanes and major hurricanes counts have increased over the past 100 years in the North Atlantic basin…”

So, based on the examination of many peer-reviewed scientific research papers, the IPCC concluded that there is no trend in the intensity of tropical cyclones – there is no robust data to support the notion that climate change has caused TCs to be “stronger and bigger” “with each passing hurricane season”.

There is, however, a line in the IPCC report that some may point to as evidence of climate change causing TCs to be stronger. It says: “Evidence, however, is for a virtually certain increase in the…intensity of the strongest tropical cyclones since the 1970s in that [North Atlantic] region.” Is this an endorsement of the view that climate chance is causing TCs to be stronger? Certainly not.

TC activity in the North Atlantic, like most places, go through phases – inactive and active periods that last for multiple of decades at a time, which are closely linked to the Atlantic Multidecadal Oscillation (AMO). Active phases are marked by above normal number and strength of TCs and the opposite for inactive phases.

3-Year-Average ACE

A plot of three-year-averaged Accumulated Cyclone Energy (ACE). It is a measure of the activity of a hurricane season based on strength, duration and the number of TCs. It shows the active and inactive phases of the Atlantic hurricane season. These phases are natural – NOT caused by human-induced climate change.

So, from the 1970s to the mid-1990s, the Atlantic went through an inactive phase, where there was a decline in the frequency and intensity of the number of TCs from the mid-1920s to 1960s – the previous active period before our current one.

The Atlantic is currently in an active phase which began around the mid-1990s. So, certainly, the record shows an increase in the frequency and intensity of TCs since the 1970s. But this does not mark an overall increase – it is just a part of the decadal cycle. Further, according to the IPCC’s latest report, there is low confidence that this increase is due to human-induced climate change. In other words, there is an 80% chance that this increase is not due to climate change.

So, the IPCC, and by extension, the UN is clear about the impact of climate change on TCs – it is currently “unlikely” having any effect on TC intensity. The same IPCC indicates that it is unequivocal that man is changing the climate. This is a position shared by over 99% of climate scientists and people in general. If we believe this part of the report, we must believe the part that speaks about the impact or lack thereof, at this time, of climate change on TCs.

This research position also shared by WMO and the TC research community.

Will TCs become more intense?

According the IPCC AR5, “more likely than not”. Going forward, IPCC AR5 says that climate change will “more likely than not” cause changes in TC intensities late in this current century – near year 2100.

This conclusion by the IPCC is consistent with one of the most authoritative scientific papers on the subject written by Knutson TR, McBride JL, Chan J, et al.. It says that late in this century, not now, based on models, there will be a 2 to 11% increase in wind speeds of TCs.

Empirical Data and the impact of climate change on TCs

Let’s look at the empirical data to see what they say regarding the impact of climate change on TCs. Based on the https://coast.noaa.gov/hurricanes/ database, the following is true:

CountOfHurricanes1924-1969VS1970-2016

A count of the number of hurricanes – 1924 to 1969 vs. 1970 to 2016. The bracketed numbers are the yearly averages.

From the above, clearly, there is no significant difference between numbers for the period 1924-1969, when climate change was not an issue, and the period 1970-2016, when climate change became an issue. What is interesting is that the numbers for 1924-1969 would have been higher, if not for a number of TCs being missed, due to a lack of satellite technology prior to the mid-1960s.

Further, some of the most powerful hurricanes to form across the Atlantic Basin occurred when climate change was not an issue – before the mid-1980s. Hurricane Allen which still holds the record for the highest maximum sustained wind speed ever on record over the Atlantic occurred in 1980. Of the top 12 strongest hurricanes, in terms of maximum sustained winds, six occurred before the mid-1980s.

Based on the IPCC and the vast majority of the TC researchers, TCs are an unworthy poster child for climate change. By the preponderance of research papers, climate change has NOT caused TCs to be stronger. Climate change had nothing to do with the strength of Hurricanes Irma and Maria or the activity of the 2017 Atlantic hurricane season. It is all due to natural variability.

However, by late this century “more likely than not” climate change WILL cause an intensification of TCs. But this is not yet evident. This is the conclusion of IPCC AR5 – we either accept the full report or none at all. And if we accept all of it, we would not be blaming hurricane intensities on climate change, at this time.

Our next blog in this series will look at the impact, if any, of climate change on tropical cyclone frequencies.








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