Thursday, September 11, 2014

Tropical Storms in the Southwest

The remnants of Hurricane Norbert hit the Southwestern U.S. this week and caused havoc. Record rainfall in the region caused flooding, closed roads and led to at least two deaths. To really add injury to injury, the flooding is not likely to help with the drought. The system missed the areas hardest hit by the drought and didn't help at all. Even in areas that got rain, the rain came so hard and fast that it ran off rather than soak in. Some, of course, will be caught in the reservoirs and will provide some help, but most will lost.

A tropical storm system hitting this region is very unusual. Just look at the fact that it was not only the single day record rain, but it really shattered the old record. Norbert dumped 3.29 inches on Phoenix on Monday. The old single day record was 2.91 inches and was 81-years old. The new record is more than 13% higher than the old one. (There was also a 4.98 inch rainfall in 1911 that occurred over two days, but within a 24-hour period, making it the 24-hour record holder.) What isn't revealed in that statistic is that the 3.29 inches fell in just seven hours. You don't see this kind of record history very often in places that get tropical storms on a regular basis.

According to Wikipedia, there have been four tropical storms that have hit the Southwester U.S. with gale force winds since 1900. Norbert was not the fifth. Storm organization had degraded quickly when the storm hit Mexico and wind speeds dropped precipitously.

So, why do so few storms hit this region? After all, there are lots of Pacific storms that form in this region and tropical cyclones in the Northern Hemisphere tend to hook towards the Northeast. You would think this would create a situation conducive for West Coast storm strikes.

The answer lies in the ocean temperature. The California Current along the West Coast originates in the north and moves southwards along the continental margin.  Take a look at this plot from Climate Reanalyzer showing the ocean temperature for September 11, 2014:

Source: Climate Reanalyzer
The light blue band along the coast line clearly shows how the waters in this region are cooler than ocean waters farther out to sea (to the left, or west). There are two things that are needed for tropical cyclone development: warm ocean water and a lack of high-altitude wind shear. As it turns out, the California Current not only cools the water, it also leads to high-altitude wind shear. Storms that form and move into this region run right into a hurricane killer. The U.S. West Coast has it very own hurricane-barrier.

But, what is going to happen as the ocean gets warmer as a result of climate change? We don't need to speculate, we can look at the record and we can see what is already happening.

While Norbert didn't bring gale force winds, it did fall in the category of tropical storm remnant affecting the Southwest. Again, according to Wikipedia (not a research quality source, but adequate for our purpose here), there have been 67 storms (not including Norbert) that are recorded to have had an impact on California.
Number of recorded storms impacting California
Period Number of storms
Adding Norbert, this list indicates that there have been 39 of the 68 storms since the beginning of the 1970s, over 57% of all of the storms on this list. Why did I use that time span? Take a look at this graph for the answer:

Global Ocean Heat Content 1955-present 0-700 m
Source: NOAA

This graph shows the ocean temperature started a significant increase in the late-1960s. Remember, tropical storms need warm water. We would expect, if all things remain equal (a bad assumption, but a good starting point) that the California hurricane barrier would start to break down and that is what the data seems to be indicating.

Take a look at this plot, also from Climate Reanalyzer:

Source: Climate Reanalyzer

This plot shows the temperature anomaly for September 11, 2014, the same day as the other ocean temperature plot. The difference between the two is the first plot showed the actual water temperature and this one shows the difference between the actual temperature and the average temperature. Just as the above plot clearly showed the California Current to provide cooler waters along the West Coast, this one shows, equally clearly, that the waters along the West Coast are getting warmer. Quite a bit so, in fact.

Does this mean California and the Southwest can expect more tropical storm remnants? I believe the data shows that is already happening. Does this mean there is a possible relief for the long-term drought? Again, I believe the data already shows this is not the case. Droughts are relieved, and caused, by long-term changes in the precipitation pattern. Flash floods with large amounts of runoff are not going to provide the relief needed. That will take long, soaking rains and heavy snowfall in the mountains. In fact, there is evidence the warming oceans is what is actually causing the drought.

The conclusion I reach from this is that this region is already experiencing severe effects from global warming and will continue to see them into the future.

No comments:

Post a Comment