La Nina and Tropical Storms – Wully Davidson
La Nina has officially been declared. This is the cool counterpart of El Nino, which is a warming of the eastern tropical Pacific. La Ninas usually have a small global cooling effect, and the magnitude of this is dependent on whether the La Nina is classed as weak, moderate or strong. La Ninas usually last for a minimum of 5 months. The regional climatic effects around the globe include reduced rainfall in east Africa. During the strong 2010-11 event, between 50,000 and 100,000 people in Somalia and neighbouring countries died of famine. It would have been far worse in the past, without international aid.
‘In August, La Niña conditions were present, with below-average sea surface temperatures (SSTs) extending across the central and eastern equatorial Pacific Ocean (Fig. 1). In the last week, all Niño indices were negative, with the Niño-3.4 index at -0.9ºC and the Niño-1+2 and Niño-3 indices cooler than -1.0ºC (Fig. 2). Equatorial subsurface temperature anomalies averaged across 180°-100°W were negative (Fig. 3), with the largest departures observed in the east-central Pacific from the surface to 200m depth (Fig. 4). Atmospheric circulation anomalies over the tropical Pacific were also generally consistent with La Niña, despite sub-seasonal variability during the month. The low-level and upper-level winds were near average for the month as a whole, but enhanced low-level easterly winds were prominent across the equatorial Pacific Ocean during early and late August. Tropical convection remained suppressed over the western and central Pacific, and was near average over Indonesia (Fig. 5). Both the Southern Oscillation and Equatorial Southern Oscillation indices were positive. Overall, the coupled ocean-atmosphere system was consistent with La Niña conditions.’
10 September, 2020
Tropical Storm Sally
Tropical storm Sally has just formed over Florida, and is currently just about to enter the Gulf of Mexico. It has sustained winds of 40 mph, and is predicted by the National Hurricane Center to make landfall near New Orleans on Tuesday as a low end 80 mph category 1 hurricane. That would cause power outages and some structural damage, but nothing to write home about.
However, the NHC admits that there is ‘some uncertainty’ about the intensity, and many observers on the tropical weather forum I visit are suggesting that it could rapidly intensify up to a major hurricane category 3. There doesn’t seem to be any of the usual environmental factors that would hinder development, such as dry air, wind shear, low sea surface temperatures etc.
The NHC have a social responsibility not to cause unnecessary panic, but if Sally does rapidly intensify to a major, there won’t be much time for warnings to evacuate. Tropical storms have intensified to catastrophic category 4/5 hurricanes in less than 3 days in the past. Hopefully, Sally will do as the NHC predicts.
12 September, 2020
Record Simultaneous Named Storms
We’ve been warned for many years that global warming would result in more, and stronger tropical cyclones. Yesterday, there were five named storms in the Atlantic, equalling the record for simultaneous named storms. One of them has since dissipated, but there’s another system emerging from Africa to replace it. Sally will be making landfall near Mobile, Alabama tonight, as a category 1 hurricane. Tropical storm Vicky is the latest addition to the family – only the second time that ‘V’ has been used. Since we’re only 5 days past the peak of the Atlantic hurricane season, it’s almost certain that the National Hurricane Center will soon be using Greek alphabet names for the storms, only the second time they’ve had to do this.
At the start of the season, the Azores/Bermuda high pressure ridge was strong, and pushing storms west, threatening land. Now, it has weakened, and storms forming in the eastern Atlantic appear to be meandering harmlessly out to sea. A few may turn up on UK shores as ‘remnants’. The ridge could become strong again, and October has produced some monsters in the past. What we’re witnessing now is exactly what scientists predicted decades ago, and it’s only going to get worse.
15 September, 2020
Hurricane Sally has just made landfall (10 am) on the Alabama – Florida border, as a category 2 storm, with 105 mph sustained winds. It’s moving inland at a very slow 3 mph, which means it’ll hang around longer than most landfalling hurricanes, and cause more damage than would normally be expected for a 105 mph storm. The National Hurricane Center has warned of ‘historic and catastrophic’ flooding, due to the huge amount of rainfall. Pensacola, Florida, is getting the worst of it.
16 September, 2020
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