The warm hemisphere

I have been inspired to start the writing process again. This time inspired by the latest events. A couple of weeks ago I flew to east Asia for the first time for this amazing opportunity to present my work.  Another reason that I wanted to talk about this region is the recent devastation caused by Typhoon Haiyan, one of the strongest tropical cyclones ever recorded. At the time of this writing, Haiyan has claimed over 3600 lives. For information about how to help the victims of the typhoon, follow this link.

This sparked some curiosity to learn about the weather and climate of southeast Asia, Oceania (informally called here the Maritime Continent), and northern Australia. Some people have asked me: "Can a storm like Haiyan hit the U.S. coast?", and "How can storms like this form?". While the details of these questions can be left to tropical cyclone experts, there are some aspects about the region where Haiyan formed that are well known by many.  I will talk a little about these salient features through several blog posts. 

The area that encompasses the Maritime Continent, Southeast Asia, India  and Northern Australia is colloquially called the Indo-Pacific Warm Pool. This is a special region of the globe, where a combination of factors, including having the Maritime Continent near the equator, the nearly isolated Indian Ocean,  as well as even the planet's rotation combine to create some of the warmest sea surface temperatures (SSTs) in the world. Below is an animation of SST throughout the year. Notice how the warmest temperatures are in the Western Pacific Ocean throughout the year, with some wobbling north and south with changes in season.

Animation of climatological sea surface temperatures.

For these, and many other reasons, the Eastern Hemisphere is often described as the Warm Hemisphere (note that the western hemisphere tropics aren't cold,  they're  just not as warm as the Eastern Hemisphere tropics). Perhaps the signature of the warmer temperatures are even more obvious in a product called column integrated water vapor, which describes the total amount of water vapor in the troposphere.  Below is the column water vapor obtained from the ECMWF 40-year Reanalysis (ERA-40) Atlas, a nice open source tool that shows nice climatological plots. 

For boreal summer (June-August, JJA), it is clear that the Warm Pool is the region where the largest amounts of water vapor are.  This feature has profound impacts regionally,  in the form of the monsoon system, and globally. 

Column integrated water vapor JJA climatology from the ERA-40 Atlas

One of the many interesting results of all this is that this region has a lot more diabatic heating from condensation than anywhere else in the world, especially during the boreal summer months (JJA). Just see where the deepest shades of red are in the plot below. There are some regions of strong heating outside the Warm pool, corresponding to the Intertropical Convergence Zone, and a signature of the Gulf stream over the east coast of the US, which is also the signature of disturbances that curve to the northeast over that location.

Diabatic heating JJA climatology from the ERA-40 Atlas

This massive area of heating is crucial to the climate system, and an important contributor to the transport of heat towards higher (colder) latitudes. It also influnences significantly the circulation of the atmosphere by generating what we call stationary waves. Below is a map I made showing the difference of SST over the hemispheres by removing the mean SST from each latitude. This plot makes it clear where the warmest SSTs are found. Additionally, I show geopotential height anomalies, which can be thought as pressure. Over the tropical western Pacific, the atmosphere is "thicker", which means pressure is lower near the surface, but higher upper portions of the troposphere. It is the opposite over the eastern Pacific and the Atlantic.  Why so? I will cover the reasons why in a future blog entry.