Thursday, 29 September 2016

Hot oceans boost extreme weather globally

From Maryland to the Caribbean to Asia, Record-Hot Ocean Waters Give Extreme Weather Potentials a Big Boost

27 September, 2016

The forecasts began coming in this morning: Heavy rainfall expected over the next two days. Possible flash flooding. Turn around, don’t drown.
These advisories buzzed up from local news media for the DC, Maryland, and Northern Virginia metro areas as a crazy, wavy Jet Stream spawned an upper-level low that’s predicted to gorge on an insane amount of moisture spewing up off the record-hot Atlantic Ocean.
Forecast GFS model guidance shows an upper-level low-pressure system situated over the Great Lakes region in association with a big trough dipping down from the Arctic. Over the next 24 to 48 hours, the low is expected to shift south and east. Becoming cut off from the upper-level flow, the low is then predicted to set up a persistent rainfall pattern over DC, Maryland and Northern Virginia.
(NOAA’s precipitation forecast model shows extreme rainfall predicted for the DC area over the next seven days. Note that record global heat and, in particular, excessively hot sea-surface temperature anomalies off the U.S. east coast are providing an unprecedented amount of fuel for storms. Should such storms fire off, they could produce rainfall totals in excess of those currently predicted. Image source: NOAA.)

Easterly winds are expected to be drawn into the low from a record-hot Atlantic Ocean. These winds will bear upon them an extraordinary burden of atmospheric moisture which has been continuously evaporating up from a very warm Gulf Stream. Such moisture is fuel for powerful rainstorms. Given the destabilizing kick provided by the upper-level low, it is expected to deliver some pretty intense downpours on Wednesday, Thursday and Friday.
NOAA model guidance shows rainfall amounts of 3 to 6 inches over the area for the next five days. However, given the high atmospheric moisture content and the record atmospheric and ocean heat that’s spiking storm energy potentials, there is a possibility for locally higher amounts.
Extreme Ocean Heat Contributes to Severe Weather

As the DC area prepares for what could be another record or near-record rainfall event, various other regions over the Atlantic and on the other side of the world are also facing the possibility of intense weather. Very warm sea-surface temperatures are the common thread that links all these events.
(Hot sea surfaces are loading up the atmosphere with moisture and helping to produce convective updrafts that heighten storm tops. Such are the results of climate change, which has now dumped an extraordinary amount of heat energy into the Earth’s ocean and atmosphere — energy that now provides fuel for both extreme rainfall events and more-intense hurricanes. Image source: Earth Nullschool.)

In the Atlantic Ocean, just off Maryland and DC, sea-surface temperatures running at an amazing 5.7 degrees Celsius above average are helping fuel this week’s possible extreme rainfall event. For reference, ocean temperatures over the course of the 20th century tended to range between 2 C above average to 2 C below average. Any deviation beyond a 2 C departure for any significant length of time was considered pretty out of the ordinary. But off the U.S. east coast over the past couple of years and concurrent with record-hot global temperatures, sea surfaces have regularly hit such high ranges. The heat bleeding off those waters has contributed to a growing number of intense precipitation events.

Possible Strong Cyclone to Form in Caribbean, Track Toward U.S. Coast
Farther south, the Caribbean is also quite hot. Ranging from 1-2 C above average, this region of warmer-than-normal ocean water is about to receive a strong tropical wave running in from the east. Over the next week, this wave is expected to gorge on these hot waters, firing off intense thunderstorms with rising tops around a tightening center of circulation, and developing into a tropical cyclone that could reach hurricane strength by late this week or early next week. Long-range model runs predict all kinds of possible rough weather related to this potential storm for the U.S. east or Gulf coasts or even for the Canadian Maritimes.

(Most recent ECMWF model run shows an extremely powerful 938-millibar hurricane threatening southeast Florida on October 7. If such a storm does form, it will be fueled by hotter-than-normal ocean conditions brought about by human-caused climate change. Image source: Tropical Tidbits.)

One of the most accurate forecast models, the European Centre for Medium-range Weather Forecasting (ECMWF), is pointing toward the possibility of a major hurricane of 938-mb intensity threatening the southeast coast of Florida by October 7. Though such long-range forecasts are highly uncertain so far out, the underlying models are obviously picking up on the potential energy provided by all that ocean heat. The result is a kind of climate roulette — with a few extra bullets in the chambers. In other words, with all that ocean heat laying around, the potential for a big storm is just sitting there, waiting for something big to come along and suck it up. This ongoing, worsening situation could result in some serious added weather consequences over the next ten days or so.
Megi is Third Tropical Cyclone to Impact Taiwan

All across the world this year, big rainfall and related storm events have been popping up. Louisiana alone saw two 30-inch plus rainfall events while nearby Texas got hit again and again and again. This week, major floods in Iowa spurred officials to urge thousands to evacuate. Meanwhile, the recent Ellicott City, Maryland flood has people in the DC area on edge over this week’s potential for very heavy rainfall.

It’s worth noting that rainfall hasn’t stopped over Taiwan yet, even as the massive circulation of Megi plows toward China — which is likely to receive heavy rainfall from the storm as well.
Like the possible extreme weather events related to very-hot surface waters in the North Atlantic and Caribbean, Megi formed over waters that are 1-3 C hotter than normal. But what’s a bit odd about Megi is that she followed almost in the exact tracks of two other cyclones — one which brushed by just to the north of Taiwan, and another which skirted the island’s south side. Typically, upwelling of cooler waters caused by hurricanes and tropical storms is enough to prevent an immediate follow-on by powerful systems, due to the fact that surface waters tend to be warmer than waters below the surface. But Megi followed these two systems and was able to tap enough ocean heat to reach Category 4 intensity even as it supported a massive outflow.
(Typhoon Megi dwarfs Taiwan. Image source: Earth Observatory.)

How did this happen? Well, considering the fact that ocean surfaces in Megi’s path are still 1-2 C hotter than normal, it’s likely that the waters at depth were also much warmer than usual, meaning storm-related upwelling wasn’t able to limit the strength of follow-on storms. This possible new feature of the Western Pacific raises the strange potential for regions to be hit by a train of tropical storms and cyclones, as happened with Taiwan over the past couple of weeks.
Conditions in Context — Record-Hot Ocean, Atmosphere Fuels More Severe Storms

The common link between the forthcoming potential severe weather along the U.S. east coast and the intensity of Typhoon Megi upon following behind two other storms is increased ocean heat. Such heat acts as a kind of energy and moisture engine for more, and more powerful, storms, such as the aforementioned extreme rainfall events and powerful, peak-intensity cyclones. In short, these are aspects of a world undergoing fundamental climate shifts — shifts that continue to ramp up due to the great and ongoing emission of greenhouse gasses into the Earth’s atmosphere.
Scientific Hat tip to Dr. Jeff Masters
Hat tip to DT Lange
Hat tip to Colorado Bob

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