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For Weather-Junkies only.....

Posted by deanna on July 28, 2009, 10:07 am, in reply to "Overnight rain!!"

..from one of the Weather Underground bloggers:

Posted by: Weather456, 10:26 AM GMT on July 28, 2009

Record Low Global Activity

July 2009 will likely go down as one of the most inactive months in the past 30 years, not just in the Atlantic but the entire Northern Hemispheric tropics. The Accumulated cyclone energy (ACE), which measures the activity of individual cyclones and cyclone seasons, was about 15 as of Typhoon Molave on July 19, well below the average of 70 for the month of July!

Most tropical cyclones that develop in the Northern Hemispheric tropics normally occur between May and November when sea surface temperatures and convective activity are at their climatological peak. Normally, activity increases from West to East, that is, the Western Pacific basin is more active than the Eastern Pacific Basin, which is more active than the Atlantic Basin. According to the Joint Typhoon Warning Center (JTWC) and the National Hurricane Center (NHC), total tropical storms as of July 27 were 7, 4 and 0 respectively.

When was the last time we had such low activity?

The last time we had so little activity was in 1977, more than 30 years ago and ironically, the May-June-July period for that year is about the same as the May-June-July period for this year. This is what Wikipedia said about the year 1977 and I quote:

“The 1977 season was very inactive, with only 6 named storms. The Atlantic basin was not alone in this inactivity, though; the 1977 Pacific hurricane season was also inactive as was the 1977 Pacific typhoon season. The cause is unknown.”

Figure 1: Total Accumulated Tropical Cyclone (ACE) energy for the Northern Hemisphere for May-June-July. This three-month ACE sum is the lowest since 1977. Credit: Ryan N Maue, Florida State University.

Which basin is currently the most inactive thus far?

Table 1 shows the current ACE, the yearly average, the average through June 30, and July 31 and July average. The largest difference between actual and what is expected for July 31 occurred in the Western Pacific and this is no surprise since it is normally an active basin and anything less is noticeable. The Eastern Pacific is also well below the normal ACE through July 31 with the Atlantic having the smallest difference due to the fact that thus far the basin is normally quiet.

Table 1. Global Tropical Cyclone ACE valid July 28, 2009 00Z, showing most basins where below their climatological values. Credit: Ryan N Maue, Florida State University.

What is causing such low activity?

The causes are just as unknown as in 1977, but we have a bit more tools to give us an idea of what is occurring in 2009. We often here that cyclogenesis depends on several features such as warm sea surface temperatures, low vertical shear and high relative humidity and instability. However, we often neglect the last ingredient, a pre-existing disturbance with sufficient deep convection. In a publication titled “Introduction to Tropical Meteorology”, it stated that this last factor has proven over previous years to be the most important. Have these pre-existing disturbances been relatively absent or weak this year? The answer is somewhat and the culprit is none other than the Madden Julian Oscillation (MJO).

The Madden Julian Oscillation (MJO)

The MJO is a wave-like planetary disturbance that travels across the globe enhancing tropical rainfall. During the positive (negative phase), tropical convection is enhanced (suppressed) and the likelihood of tropical cyclone activity is increase (decreased). According to some recent MJO updates by the Climate Prediction Center (CPC), the MJO signals have been weaker than normal over the past 2-3 months. The suppressive states are also lasting longer and more pronounce that the enhancing state. This is likely causing the lack of convective activity in the tropics, which is the first step in tropical cyclogenesis.

Figure 2: Outgoing long-wave radiation (OLR) which is a proxy used to measure tropical cloudiness. High (low) values of OLR signifies less (more) cloudiness. High OLR values were observed across much of the tropical Atlantic and Pacific basin with the exception of near the Philippines, which was a result of multiple tropical cyclone tracks. Credit: NOAA Climate Prediction Center

Vertical shear and sea surface temperatures have become somewhat corporative across all basins but this downward motion in the MJO has continued to preclude any tropical development. This interesting discussion came up in the San Juan Puerto Rico area forecast discussion at 2:05 PM AST 27 July:

“Moreover, the latest 24-month running tropical cyclone accumulated cyclone energy /ace/ index for the northern hemisphere shows the lowest sums in nearly 30 years. What this means is that while the local environment may allow for a tropical cyclone to sustain itself...the overall tropical environment has precluded one from actually forming at this time.”

This is only one theory to describe the inactivity in the Northern Hemispheric Basin as other tropical disturbances and cyclones has fell to other environmental variables – Hurricane Carlos (cool SSTs), Tropical Storm Dolores (high shear) and numerous tropical Atlantic waves, African dust. However, the suppression of tropical rainfall over Africa is likely causing this increase in African dust and subsidence is increasing the high pressure, which is driving the dust west.

La Nina

The second cause was proposed in an article entitled The Great Depression! Tropical Cyclone Energy at 30-year lows by Ryan N. Maue at the Florida State University which stated that the tropical atmosphere cooled during the past 2 years due to the 2007-2009 La Nina. This is not surprising since the La Nina of winter 2007-2008 brought some of the coldest temperatures to Saint Kitts, first snow in Iraq and one of China’s largest snowstorms in almost 40 years. This has led to a suppression of tropical activity worldwide during 2009.

Solar Cycles

On 2 January 2008, the Space and Science Research Center (SSRC) issued a press release stating that the sun’s surface flow has slowed and we are entering a solar minimum. They stated the records of sunspot counts over the past 6000 years show that anything lower than 50 means an intense cold climate. This could potentially be one of the causes of cooler tropical oceans but since there is little known about the relationship between hurricane frequency and sun spot counts, this theory unfounded.

What does this mean for the Atlantic Hurricane Season?

It is hard to say, but if the current trend continues, the season might be similar to 1977, which had 6 named storms. However, we have no idea of whether this trend will continue and in 1977, 5 of those storms became hurricanes with Anita the most notable. In addition, 1977 lay in the decadal period of low inactivity whereas 2009 lies in a period of high activity for the Atlantic basin. In other hurricane seasons where the May-June-July period have been low was 1988, 1998 and most recently, 2007. All these years and 1977 featured a retired category 5 hurricane (Anita, Gilbert, Mitch, Dean and Felix). This is due to the fact that accumulated potential energy that builds in the atmosphere is not being evenly distributed.

In Conclusion, for the period May-June-July 2009, record low activity has been recorded for all basins in the Northern Hemispheric tropics. There are currently three theories put forward to explain this current inactivity but we have no idea if it will continue, especially into the peak of the Atlantic Hurricane Season.



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