Post by Sara Ganetis on Aug 31, 2013 19:46:47 GMT
Our first Friday Weather Discussion at SBU was held yesterday, 30 August. It was led by Dr. Brian Colle and titled, "Why has the Atlantic hurricane season been so quiet and how long will it last?" We only hold discussion for a half-hour so we try to only focus on one thing (sorry for ignoring the horrible fires out west!) Here are the highlights--
Dr. Colle began his discussion by providing some statistics to put the current Atlantic hurricane season into perspective. He showed NOAA’s official forecast from May for 13-20 named storms with 11 hurricanes and 3-6 major storms. Their latest updated forecast called for 18 named storms with 8 hurricanes and 3 major storms. Despite the relatively “slow start” they are still calling for a lot of activity. The climatological peak for TC genesis in the North Atlantic is about the week of September 10th (NOAA/NHC), so it’s not like we already missed the expected peak period of activity. Dr. Colle discussed the quantity called accumulated cyclone energy (ACE) which measures the relative intensity of each storm by estimating the energy used by each storm and is proportional to the square of the wind velocity (more info here). On a per-month basis, ACE also peaks in September in the Atlantic (from Dr. Ryan Maue’s page). Given this information, perhaps this Atlantic hurricane season shouldn’t yet be written off but it looks like the date of the first hurricane formation may break records (McNoldy, CWG). ACE on a globally integrated scale has been decreasing since 2005 (Maue 2011) but that may be a result of there being less intense storms during the later period (Maue’s page).
What was the deal with the start of this Atlantic hurricane season? To date there have been 6 storms. ENSO isn’t a major player this season, so Dr. Colle showed some plots to explain why the activity had been low starting on 1 July. The first point that he made was that there was anomalously dry air above the boundary layer (-12% RH anomaly) stretching westward from Africa all the way across the Atlantic into the Caribbean especially during the period 1-15 August. The Saharan Air Layer (SAL) provided a harsh environment for TC development and growth. In analyzing vertical stability, the Tropics are more stable this year compared to climatology (McNoldy, CWG) so that would act to discourage convection. SST anomaly maps didn’t show too much of an explanation for why activity has been weak. Maps of 500-hPa shear anomalies did show that during the period 1-15 August there was a 2-3 m/s westerly shear anomaly in the tropical Atlantic basin. Therefore, the weak activity was shown to be likely tied to the dry air and westerly shear that created an unfavorable environment for TC development.
How long will this stifled activity last? Likely not long, Dr. Colle explained. Climatological shear values are back in place and the dry anomalies are starting to weaken. At the time of the discussion, there was an invest area with a 40% probability for the chance of development that should move westward into an area with weaker shear but still some residual dry air. If it makes it past the subtropical high, according to the GFS, it may encounter a trough that may recurve it and keep it away from the Caribbean and East Coast. We’ll see what happens. Looking beyond this one system and to the rest of the Atlantic hurricane season as a whole, most students at the discussion agreed that they were not ready to give up on the season yet. Another factor that was discussed was the MJO moving into Phase 8 and then Phase 1 which has been shown to increase precipitation in the tropical Atlantic (NOAA/CPC).
So that's what we chatted about at SBU; do you have any comments? Did you have a much cooler discussion this week at your university that you'd like to share? Reply below.
Dr. Colle began his discussion by providing some statistics to put the current Atlantic hurricane season into perspective. He showed NOAA’s official forecast from May for 13-20 named storms with 11 hurricanes and 3-6 major storms. Their latest updated forecast called for 18 named storms with 8 hurricanes and 3 major storms. Despite the relatively “slow start” they are still calling for a lot of activity. The climatological peak for TC genesis in the North Atlantic is about the week of September 10th (NOAA/NHC), so it’s not like we already missed the expected peak period of activity. Dr. Colle discussed the quantity called accumulated cyclone energy (ACE) which measures the relative intensity of each storm by estimating the energy used by each storm and is proportional to the square of the wind velocity (more info here). On a per-month basis, ACE also peaks in September in the Atlantic (from Dr. Ryan Maue’s page). Given this information, perhaps this Atlantic hurricane season shouldn’t yet be written off but it looks like the date of the first hurricane formation may break records (McNoldy, CWG). ACE on a globally integrated scale has been decreasing since 2005 (Maue 2011) but that may be a result of there being less intense storms during the later period (Maue’s page).
What was the deal with the start of this Atlantic hurricane season? To date there have been 6 storms. ENSO isn’t a major player this season, so Dr. Colle showed some plots to explain why the activity had been low starting on 1 July. The first point that he made was that there was anomalously dry air above the boundary layer (-12% RH anomaly) stretching westward from Africa all the way across the Atlantic into the Caribbean especially during the period 1-15 August. The Saharan Air Layer (SAL) provided a harsh environment for TC development and growth. In analyzing vertical stability, the Tropics are more stable this year compared to climatology (McNoldy, CWG) so that would act to discourage convection. SST anomaly maps didn’t show too much of an explanation for why activity has been weak. Maps of 500-hPa shear anomalies did show that during the period 1-15 August there was a 2-3 m/s westerly shear anomaly in the tropical Atlantic basin. Therefore, the weak activity was shown to be likely tied to the dry air and westerly shear that created an unfavorable environment for TC development.
How long will this stifled activity last? Likely not long, Dr. Colle explained. Climatological shear values are back in place and the dry anomalies are starting to weaken. At the time of the discussion, there was an invest area with a 40% probability for the chance of development that should move westward into an area with weaker shear but still some residual dry air. If it makes it past the subtropical high, according to the GFS, it may encounter a trough that may recurve it and keep it away from the Caribbean and East Coast. We’ll see what happens. Looking beyond this one system and to the rest of the Atlantic hurricane season as a whole, most students at the discussion agreed that they were not ready to give up on the season yet. Another factor that was discussed was the MJO moving into Phase 8 and then Phase 1 which has been shown to increase precipitation in the tropical Atlantic (NOAA/CPC).
So that's what we chatted about at SBU; do you have any comments? Did you have a much cooler discussion this week at your university that you'd like to share? Reply below.
