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Churning away in the Atlantic, Hurricane Joaquin has reached Category 4 strength as of Thursday afternoon, and appears to have significant rainfall in store for the US east coast. How did this storm develop when so much was stacked up against it?
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How Joaquin fought off El Niño to emerge as a hurricane

Scott Sutherland
Meteorologist/Science Writer

Thursday, October 1, 2015, 3:35 PM - Churning away in the Atlantic, Hurricane Joaquin reached Category 4 strength as of Thursday afternoon and nearly reached Category 5 strength as of Saturday. How did this storm develop when so much was stacked up against it?

El Niño 2015 is going strong, revving up conditions over the Pacific Ocean and contributing to the development of no less than 5 super typhoons and 8 major hurricanes in the Pacific so far. Meanwhile, the influence of this El Niño has stretched over the tropical Atlantic Ocean as well, resulting in strong wind shear - with stronger winds above and weaker winds near the surface - thus making it very difficult for tropical storms and hurricane to develop there.

NOAA/NESDIS sea surface temperature anomaly map shows El Niño's influence on the Pacific. Inset: NOAA's mid-August prediction for the development and strength of El Niño 2015.

NOAA forecasters predicted these conditions in the Atlantic fairly early on, issuing a below-average hurricane season outlook in May - calling for 6-11 named storms, 3-6 of which would develop into hurricanes, and between 0-2 of those becoming major hurricanes, ranking as Category 3 or greater. Based on how the season was developing, the US National Hurricane Center even lowered those numbers as of their August update - to 6-10 storms, 1-4 hurricanes and 0-1 major hurricanes - and with good reason. As of mid-August, only three storms had spun up in the Atlantic - Ana, Bill and Claudette - none of which developed beyond Tropical Storm strength (although Bill still managed to cause millions in damages as it made landfall, and ultimately resulted in 4 deaths).

Since then, however, seven more storms have popped up, three of which developed into hurricanes, and Joaquin is the second of those to strengthen to Category 3 or higher.

The tracks of all 10 Atlantic storms, as well as one depression, in the 2015 Atlantic Hurricane Season so far. Credit: WikiProject Tropical cyclones/Tracks.

What's going on here?

So, what's going on in the Atlantic now to cause this sudden surge in tropical storm and hurricane activity?

It's generally not unusual for the Atlantic hurricane season to ramp up in late August, with most of the activity ending up concentrated in September and October. Even in 1987, which saw an El Niño develop along very similar lines to the current one, there was this same concentration of storms, with six named storms in August and September, two of which developed into hurricanes, and one made it to Category 3 strength.

This year, though, there's something in the Atlantic that's been providing a significant boost to storms as of the month of August.

August 2015 global land and ocean temperature percentiles. Credit: NOAA/NCEI Global Analysis

Although the North Atlantic is showing a large blob of cold temperatures (likely from melting Greenland glaciers), the tropical Atlantic is at the other end of the spectrum, with temperatures ranked as "Record Warmest" in NOAA's August 2015 global analysis report.

It typically only takes ocean water at 26.5oC to kick off a tropical cyclone and maintain its strength. Temperatures about a degree warmer than that will cause the storm to strengthen significantly, and the warmer the water, the stronger the storm.

Currently, the ocean water that Joaquin formed over is near or even above 30 degrees Celsius. That's comparable to the sea surface temperatures that Hurricane Katrina passed over in the Gulf of Mexico, just as it was reaching peak strength.

Thus, even with a strong El Niño doing its best to suppress storm formation in the Atlantic, the record ocean temperatures underneath Joaquin were "pushing back" against the conditions produced by the El Niño with the potential to produce a Category 5 Hurricane. With that environment in place, all it took was a localized region of lower wind shear - enough for the storm to develop a vertical structure and sustain it - for Joaquin to spin up and strengthen.

RELATED: How will Hurricane Joaquin impact Canada? Details here

Sources: NOAA | National Hurricane Center | National Weather ServiceHurricane Science

RELATED VIDEO: Hurricane Joaquin strengthens, east coast on high alert

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