Tag Archives: SST

Why Was Typhoon Haiyan/Yolanda So Powerful, and is this a trend?

I’m sure the measurements are still being checked and adjusted but it is clear that Typhoon Haiyan/Yolanda was one of the most powerful tropical cyclones (termed “Typhoon” in the western Pacific) ever recorded. There are several ways to measure how big and bad a tropical cyclone is including it’s overall size from end to end, how low the barometric pressure gets, how high the sustained wind speed is, and how wide that wind field is. In addition, when a typhoon hits land details matter. The front right quadrant of a counter-clockwise spinning typhoon packs the maximum punch and if that part of the storm enters an embayment during high tide the storm surge can be immense. It seems that the storm surge for Haiyan/Yolanda was in the many tens of feet range, and quite possibly will be found responsible for the largest part of the still uncalculated death toll.

But here I want to look at one single factor that almost certainly contributed to the growth of Haiyan/Yolanda into a very powerful storm, a factor that probably doesn’t usually play into a storm’s strength. I refer to an anomaly in sea surface temperatures that was almost certainly caused by global warming, as part of a general warming of the ocean. But first a bit of background on the link between sea surface temperature and hurricanes. This is one of several factors that may be involved in climate change related effects on tropical storm intensity, a situation with which we should be concerned.

Tropical cyclones run on heat, and much of that heat comes from the sea surface. If the surface of the ocean is below a certain temperature, about 82 degrees F, about 28 degrees C, a hurricane or typhoon is very unlikely to form. Above that temperature, if other conditions are right, it may form. Warmer seas can make bigger or stronger storms, and as the storm passes over the ocean, the temperature of the sea surface has a strong influence on whether the storm increases or decreases in strength . As the storm moves over the sea, the interface between the windy storm and the roiling ocean becomes something of a mess, as though the surface of the ocean was in a blender, and there is a lot of exchange of heat across that interface. Also, deeper, cooler water is mixed with warmer surface water. A powerful storm moving across the ocean will leave in its wake a strip of cooler water. This sometimes causes subsequent storms moving along the same path to be weaker or to downgrade in strength more quickly.

This should indicate, one would think, that as sea surface temperatures (SST) have gone up with global warming, there should be more “hurricane” out there on the oceans. It has been hard to make the link between global warming and frequency of hurricanes, however. This may be because of the nature of hurricane formation. Once a hurricane forms in a given spot and gets big, it may reduce the chance of the next hurricane forming. Also, hurricanes are usually born as waves in a very large scale pattern of air masses. The total number of waves that form may not change with global warming, and the hurricane season is only a part of the year, and other factors have to come into play that are also ponderous in their timing to turn a wave into a major storm. An analogy might be this: Imagine that everyone in the working population of a downtown neighborhood becomes hungrier, perhaps because all the companies they work for insist on a two hour high intensity exercise program for everyone to lower their health insurance costs. Will this increase in hunger mean more lunches, snacks, and dinners consumed in the local restaurants? Or will the lunches, snacks, and dinners become larger, with people ordering more food with each sitting? Since there are only so many opportunities to go grab a bite to eat, there will probably be very few additional visits to the local eateries, but more food may well be consumed per event. Increased SST may be like increased hunger. There may not be very many more hurricanes, but among those that occur, some may be much stronger.

There is evidence for this. Kerry Emanuel did a study several years ago that linked sea surface temperatures in the Pacific with an index called PDI, which measures the overall energy involved in typhoon/hurricane activity. (Emanuel, K. (2005). Increasing destructiveness of tropical cyclones over the past 30 years, 436(August), 686–688. doi:10.1038/nature03906.) He came up with this graph:

Emanuel_2005_hurricane_sst_link

The graph shows that hurricanosity, as it were, goes up and down with sea surface temperature more or less. And, SST goes up and down with decadal oscillations like ENSO (El Nino) but with an overall upward trend caused by global warming.

Here’s the new part. If you look at a map of Sea Surface Temperature you are seeing a measurement of, well, the surface of the sea … the top of the water. As a hurricane chugs along on the surface of the sea, turning the top meter or so of ocean into spray and creating a very wavy situation, that heat is certainly directly affecting the storm, but the temperature of the water several meters down also matters. It turns out that sometimes this shallow-deep water (as opposed to deep deep water, way down farther) can be quite warm. When that happens, the dissipation of SST does not occur to the same degree. The leading edge of the hurricane gets a good dose of heat from the surface, but instead of the SST dropping as the top warm water is mixed with somewhat deeper cooler water, the heat supply is not attenuated, or at least not by much, as the massive storm moves along. More of the storm gets more heat, and the storm as a whole gets more heat. And there’s more heat left over for the next storm.

We think this happened with Haiyan. Have a look at the following map. It is sea surface temperature anomaly (how much more or less than expected the SST is) for the top 50 meters for the western Pacific at the time of the typhoon. The Philippines is down near the bottom of the map straddling the 10 and 15 degree N lines. (Maps are from here) Notice that the surface is not unusually warm.

PacificSST-top-50-meters-Anomoly

This does not mean that the sea surface was not warm. It was plenty warm as it is this time of year i that part of the ocean, just not warmer than expected. Here is the raw temperature (not anomaly) map so you can see that the tropical ocean is, well, tropically warm:

PacificSST-to-50-meters-TEMP

The purple area along the south is sufficiently warm to form typhoons. The ocean to the east of the Philippines is warm enough to form typhoons, but is there any source of extra heat to form a super typhoon? Have a look at this map. This is the water temperature at depth, here at 100 meters. This is an anomaly map, so its shows if the temperature is more (or less) than expected. Notice that east-west band of red indicating several degrees warmer than it usually is, at depth.

PacificSST-100_meters_ANOMALY

[Updated:] Here’s the same map with Haiyan/Yolanda’s track and history, graphic generated by Jeff Masters.

Haiyan_Path

So, it would appear that Haiyan/Yolanda passed over the usual very warm waters that allow the formation of typhoons, but also, over water that was warm at depth so as the top of the sea is churned up by the growing storm, there would be extra heat to feed that storm.

One final map. This is the actual temperature (not anomaly) at the 100 meter level. Notice the purple area.

Pacific_sst_100m_TEMP

At 100 meters depth, the sea was warm enough to form a typhoon. That, dear reader, is extreme.

The same thing happened with Katrina. According to a report from NOAA:

A number of factors contributed to making Katrina a strong Category 5 hurricane…Sea surface temperatures (SST) in the Gulf of Mexico were one to two degrees Celsius above normal …, and the warm temperatures extended to a considerable depth through the upper ocean layer. Also, Katrina
crossed the “loop current” (belt of even warmer water), during which time explosive
intensification occurred. The temperature of the ocean surface is a critical element in the
formation and strength of hurricanes.

We know that the ocean is absorbing a lot of the extra heat caused by global warming. Well, this is some of that heat, causing megastorms.

I’ve noticed that climate science denialists are very adamant about two things: Denying the importance of major storms like Haiyan, and denying the fact that heat is going into the oceans. Perhaps they see the link, and are frightened that people will believe that anthropogenic changes to our climate can kill thousands of people at a time, in a few hours, through the mechanism of anomalously high temperature at modest depth below the surface of the already tepid tropical sea.

It is time for action.