Floods

After several posts which talked about tropical cyclones, now let’s turn to another type of natural disasters Floods. Although it is a totally different type of natural disasters compared with tropical cyclones, it sometimes has close relationships with storms. So, I choose to explore secrets about Floods as the following part. 

But I should make it clear here that it does not mean all floods caused by tropical cyclones. Tropical cyclones is one of the reasons, sometimes, but not the only or necessary reason. Floods also come along with continuous heavy rains, large sea tidal surges, incompletely freeze/ thaw during winter/ spring or sudden destruction of some infrastructures such as dikes and dams. Furthermore, floods often lead to huge losses both in human life and economy. Let’s watch the general introduction video below to feel the tremendous power of floods.

A Big Hit----Hurricane Katrina

Although my hometown meet typhoon every year in summer, the aftermath from Hurricane Katrina still scared me. What a strong natural power!

Tropical cyclones and climate change

With the figure below, we will begin today’s journey about tropical cyclones and climate change. We can see clearly from this figure that western pacific has a high frequency of storms. The intensity (measured by minimum pressure) is lower over cooler water and close to equator, and higher over warmer water. (Mendelsohn et al., 2012)

Figure1 Storm tracks and minimum pressure for a sample of synthetic storms 

Knutson et al., 2010 indicates that Atlantic tropical cyclone power dissipation is statistical related to SST over the past 50 years. Figure2 shows this correlation between tropical atlantic ocean SST which  has more obvious growth than tropical mean SST and Atlantic power dissipation. The statistical correlation between power dissipation with low frequency variability and local SSTs in northwest Pacific Ocean is much weaker than for Atlantic Ocean. (Knutson et al., 2010)

Figure2 Past and extrapolated changes in Atlantic hurricane power dissipation index (PDI)

The relationship between western north Pacific Ocean and North Atlantic Oscillation (NAO) is a significant interdecadal change. Before the 1970s, they are in weak correlation which gradually change to positive and stronger, especially after 1980, the positive correlation is apparent at the 95% level. 1948-1977 is a weak phase of relationship between western north Pacific Ocean and NAO and 1984-2008 is a strong phase which of 0.48 coefficient is obvious at the 99% level. (Zhou & Cui, 2014)

Figure3 Running correlations between the NAOI and the WNPTCF in summer with a 25-year window width

Although the IPCC AR5 indicates that there is no significant trend of tropical cyclone activities in long term. And some researches even argued that global tropical cyclone activities decrease by 12.4 hurricanes per century with different decrease rates in northern hemisphere and southern hemisphere. Tropical cyclones attract lots of attentions from the public, because the huge damage of tropical cyclones influences human beings’ lives deeply. From figure4 (modeling future damage of tropical cyclones in 2100) we can see that the damage of tropical cyclones in different regions almost all increase, especially North America and East Asia. These regions occupy nearly 88% baseline global damage, because of more storms and lots in harm’s way. (Mendelsohn et al., 2012) And the rapid projected growth of economy in Asia and Central Amarica leads to significant growth of damage. Figure5 adds the impact from climate change, whose highest damage point is also North America with 26 billion dollars per year. And East Asia is the second highest damage region of 15 billion dollars per year, and Central America- Caribbean is the third one of 5 billion less than East Asia. (Mendelsohn et al., 2012) The damage is caused by various factors, not only natural factors, but also social factors. For example, the region in harm’s way or hit by tropical cyclones with high intensity are natural factors, whereas the rapid growth in economy or shift of population to coastal areas are social factors. (Mendelsohn et al., 2012)

Figure4 Present and future baseline tropical cyclone damage by region 

Figure5 Climate change impacts on tropical cyclone damage by region

Does the hurricane get worse than before?

It seems like that we hear about hurricane news more often than before. (Well, Hurricane Gonzalo has just gone, nearly two weeks ago.) Maybe many people hold the thought that the environment gets worse, and then extend the scope of this perspective to all branches such as the climate system, atmosphere and water cycle. How about hurricanes? Does it get worse than before?

Frequency of hurricanes

Global frequency
According to the conclusion of the latest IPCC report, it is in low confidence that changes of tropical cyclone activities are obvious in long term. From global time series pictures below, although the data set is limited, it can be seen that there is no significant trend of hurricanes’ occurrences from 1970 to 2004. Based on the scenario from Knutson et al. 2010, the projection shows that the global frequency of tropical cyclones maybe will keep generally still or decrease when average intensity increases by 2~11% and rainfall rates of tropical cyclones increase by nearly 20% within 100km of the cyclone centre. (IPCC chapter 14)

Global time series for 1970-2004 of number of storms

Regional frequency

But since 1970, some places have significant increasing trends of intense tropical cyclone activities, North Atlantic should be mentioned here. Webster et al. 2005 indicates that North Atlantic Ocean has an obvious increasing frequency in comparison with regional time series of other oceans. And this trend has a very high confidence level, which reaches at 99%. Based on time series of various cyclone indices including the duration, the power dissipation, the intensity and a compound of tropical cyclone frequency, since the late 1970s, the rising trend is presented in North Atlantic and the slight rising trend is found in western North Pacific. (IPCC chapter 2) What should be noted is that the trend of annual numbers of tropical cyclones is not robust, which has been observed over past one hundred years, as well as in North Atlantic. The annual regional frequency of tropical cyclones projection has similar trends with the annual global frequency, but has lower confidence level.(IPCC chapter 14)

Factors

The tropical cyclone is a complicated system, so it is influenced by a mix of natural and anthropogenic factors. IPCC AR4 (chapter 10) points out that anthropogenic factors more likely than not result in an upward trend in the intensity of tropical cyclones. Greenhouse gases increased by humanity affect the potential intensity of tropical cyclones, via leading to the change of Sea Surface Temperatures (SST). the SST is also influenced by the change of the radiation. Aerosols (including lots of types) also is a factor which influences different areas in different ways such as kinematics and thermodynamics. For example, in North Atlantic, the emission of pollution aerosols decreases which cause the increase of tropical Sea Surface Temperatures (SST) by changing cloud albedo. However, in northern Indian Ocean, the rising of aerosols lead to reduced vertical wind shear. It is still uncertain of the link between tropical cyclones and these factors.(IPCC chapter 14)

From above, the phenomenon, at least on global scale, may be not that terrible like some people think. Even though, we also need pay attention to hurricanes, because of the variability characteristics of them.