I am often amazed at the Earth. It is such an amazing planet. All aspects of weather fascinate me, and my sheer awe of it defined my career pathway early on. When I see something extraordinary, it captures my attention even more. Meteorologists and weather enthusiasts marveled at something quite amazing this week. A tropical cyclone-like feature formed off the coast of Chile. Yep, that one in South America. Technically, it is what we call a subtropical cyclone. What does that mean and just how rare is this?
I want to deal with the second question first. It is very rare. This part of the Pacific Ocean is typically too cold to support the development of tropical cyclones. One of the reasons is that there is a process called upwelling that brings deep, cold water up near the Chilean coast (see diagram below). Chile is also located in a global circulation pattern in which the air typically sinks. Any basic meteorology course describes how latitudes near 20 to 30 deg North (and South) are situated in sinking branches of something called the Hadley Cell (see below). If you look at the location of most of the global deserts, they are found near these latitudes because sinking air warms and is not conducive for convection. Jonathan Belles writes at weather.com,
if there was a cyclone season in that part of the world, this system would have nearly fallen outside of it. A cyclone in the southern Pacific occurring in May is like a tropical storm occurring in November in the Atlantic.....How Rare Is This? It may be one of a kind. No other recognized subtropical or tropical storm has been documented in that part of the world. Unofficially, that is up to some debate. In 2015, a similar subtropical cyclone formed near Easter Island, which is located nearly 2,000 miles farther west of this week's cyclone. Researchers called it Subtropical Storm Katie.
If you look closely at the climatology of tropical cyclone tracks over a 150-year period (through 2006), you can see that there have been no tropical storms in this part of the Pacific Ocean. Oh by the way, look at that little squiggly line off the Brazilian coast. That "Brazil cane" was rare too and happened only in the past 15 years.
Even with such meteorological, oceanographic, and geographical adversity, a subtropical cyclone formed between May 7-9, 2018 a few hundred miles off the coast of Chile. NOAA's website says,
The southeastern Pacific Ocean is normally not conducive to tropical cyclone development. Sea surface temperatures off the west coast of South America are normally far too cold and the region is located in a semi-permanent high pressure zone, characterized by dry, sinking air. NOAA satellite data show sea surface temperatures at the site of the storm just under 20 degrees Celsius (68 degrees Fahrenheit). While these temperatures are not usually warm enough for convective activity, the right atmospheric conditions in the vicinity of this storm allowed thunderstorms to form, with wind speeds attaining the strength of a weak tropical storm.
The storm was captured by NOAA's remarkable new GOES East geosynchronous satellite.
So what is a subtropical cyclone? NOAA's Hurricane Research Division website defines it as a "low-pressure system existing in the tropical or subtropical latitudes (anywhere from the equator to about 50°N) that has characteristics of both tropical cyclones and mid-latitude (or extratropical) cyclones." Other characteristics include:
- Energy from temperature differences in air masses (like a typical mid-latitude cyclone in U.S. during winter) and from convective clouds as water evaporates from the ocean and condenses.
- Radius of maximum winds are farther from the center than typical tropical systems, and they rarely exceed 74 mph, which is the threshold for a hurricane.
- They can often develop into full blown tropical cyclones as Hurricane Otto did in 2010. Allison even transition from a tropical cyclone to a subtropical storm in 2001.
- Before 2002, they were not given names. Gustav was the first named subtropical storm.
Tropical weather expert Dr. Ryan Maue of Weather.us sums up the rarity and possible causation in a series of tweets on May 7th,
Weak area of low pressure west of Santiago, Chile in the Southeastern Pacific may have developed/sustained convection near the center sufficient for classification as a Tropical Depression......I'm not aware of historical tropical storms off coast of Chile (or Peru) but this one that transitioned from a cut-off low seems to fit the pattern of high-latitude systems mirrored in the North Atlantic or Kona lows NE of Hawaii.
Maue also tweeted that the best weather prediction models were also picking up on this "weird weather" system.
