Quiet Before the Storms

 

 

 
It's that time of year. Early to mid spring likely sees the lowest tropical cyclone activity all year. Each basin seems to have an excuse for their silence; it is not quite time for the northern hemisphere basins to start up and the southern basins tend to end earlier in their "autumn" than the northern basins do. Below are some of the specific reasons for the near lack of activity, basin by basin.


Indian Ocean (SInd and NInd)


Indian Ocean, the circle is usually a "hot spot" this time of year

The circled region on the image above is somewhat of a hot spot of  activity, especially later in the season. While there have been a large number of Invest areas there, very little has actually developed. In fact, the last storm in the area, category one Cyclone Victoria, lasted much shorter than was originally forecasted. The northern portion of the ocean is quiet, but that is rather typical this time of year.


South Pacific (SPac)


South Pacific, the insert is a Aqua/MODIS image of the highlighted system

There is a small circulation in this basin, circled above, that represents what is currently the most likely system to develop, and that is not saying much. The rest of South Pacific is rapidly becoming unfavorable for tropical cyclone development as the mid-latitude cyclone tracks begin to move closer to the equator with the approach of the austral winter. Even the passage of one of these storms' fronts will tear a tropical cyclone to pieces.


North Atlantic (NAtl)


North Atlantic; currently a very hostile region for tropical cyclones

This basin officially begins its season June,1. However, the first named storms here often occur significantly later. Right now, a handful of non-tropical weather systems occupy the basin, so the development of any tropical system is almost impossible.


Northeast Pacific (NEPac)


Northeast Pacific; water vapor image with 500mb heights and wind barbs

May, 15 is the official start of this basin's season and it is often sees its first named storm not long after that. Currently, any possibility of development is being hindered by a large and stubborn upper-level feature called an omega block, named after the Greek letter the height contours somewhat resemble. In this case, the block means a strong upper-level high pressure center is anchored right over the basin, which seriously inhibits the growth of tropical cyclones. In the above image, the colored lines are the 500mb height contours, with red being the highest, the wind barbs show the direction and intensity of the wind, also at the 500mb level, and the satellite image shows the water vapor at the same time as the overlays. Notice the dark zone right in the middle of the omega block, this signifies very dry air, which is also a hindrance to storm development. 


Northwest Pacific (NWPac)


Northwest Pacific; a cold front is making its way into the tropics

Technically this basin has begun its season with Sonamu and Shanshan. However, it has been quiet for awhile. One factor that has prevented storm development are the frontal zones associated with powerful mid-latitude cyclones further north that plow through the tropics. As with the storms in the South Pacific, a single frontal passage will wipe out almost any developing system. During the height of the season a strong subtropical high pressure ridge sets up between the tropics and the mid-latitudes, effectively protecting the developing storms. Some signs of a persistent ridge have occurred recently, but for the meantime, another front is poised to pass through the region (labeled on the above image) and disrupt any development for a little while.