NATO navies converged in the North Sea in late June for this year's Dynamic Mongoose anti-submarine-warfare exercise, reflecting a growing focus on countering enemy submarines amid "great-power competition" with Russia and China.

During the exercise, US Navy destroyer USS Roosevelt and four other surface ships took turns hunting and being hunted by Navy fast-attack sub USS Indiana and four other subs in the waters off Iceland.

"It was fantastic because we would have such a small, confined area that it forced interaction between the submarine and the surface ships," Cmdr. Ryan Kendall, commanding officer of the Roosevelt, said in an interview.

Royal Navy frigate HMS Kent with Allied warships, including USS Roosevelt, left, at the end of Dynamic Mongoose, July 9, 2020. British Royal Navy/LPhot Dan Rosenbaum

Roosevelt and its counterparts brought helicopters with dipping sonar and torpedoes. Five maritime patrol aircraft, including a US Navy P-8 Poseidon, also took part.

"We'd all take turns controlling the aircraft and controlling the helicopters in the air. So we would have the P-8 come out, drop some sonobuoys, and help us localize" the sub, Kendall said. "Surface ships would come in to get closer to see if we could get our towed array or our whole active sonar on the submarines."

The Roosevelt's AN/SQQ 89A(V) 15 sonar "is one of the most advanced in the Navy," Kendall said. "We have an MH-60R helicopter with an active dipper, so we're able to use the tag team of Roosevelt and our active sonar and passive sonar, as well as our multifunctional towed array, to localize the submarine and then use our helicopter to pounce."

Subs like Indiana "mesh with the other parts of the team who can provide fast response and broad area coverage," said Cmdr. David Grogan, commanding officer of USS Indiana.

"We represent the persistent, in-stratum asset who can effectively use the underwater environment to maximize detection and engagement possibilities of an adversary submarine," Grogan added.

NATO ships and subs during exercise Dynamic Mongoose, July 2020. British Royal Navy/LPhot Dan Rosenbaum

Dynamic Mongoose allows ships and subs to exercise in real-world conditions for an extended period, Kendall said.

US and French aircraft operated out of Iceland while British planes flew from Scotland. Ships dealt with rough seas, and helicopters practiced staying aloft for extended periods.

"Normally when they execute a training scenario, it could be anywhere from two to three hours," Kendall said. "In this case, we were doing it for 12 or 22 hours straight, and you'd have watch-team turnovers."

Coordinating with aircraft and ships "can be complicated for a number of reasons," Grogan said. "Exposing my team to that, as well as learning to best employ each submarine, surface, and air asset ... was vital to expanding my team's ability within the greater ASW effort."

Canadian frigate HMCS Fredericton, left, British frigate HMS Kent, right and a German diesel submarine, center, during Dynamic Mongoose, July 2020. British Royal Navy/LPhot Dan Rosenbaum

Subs were also able to use "the environment and ... water temperature, salinity, depth, [and] bottom contours to evade us or to hide so that they can into an advantageous position to attack us," Kendall added.

Roosevelt analyzed the water "hour by hour, day by day," Kendall said, because changes in it mean "we'll get different ranges on our sonar" though Grogan said the "only tangible difference onboard in colder water is the presence of an extra sweater here or there."

"You can't do synthetic training pier-side with a computer-generated model for ... tracking real-world submarines," Kendall added. "You have real people making real decisions, and you have consequences for those decisions."

NATO ships during Dynamic Mongoose, July 2020. British Royal Navy/LPhot Dan Rosenbaum

Russian and Chinese submarine fleets have gotten bigger and better, and the US Navy has dedicated more time and resources to ASW.

"They finished fielding the P-8, they're putting the new AN/SQQ 89 sonar system onto all the destroyers, and they've been investing in some unmanned systems for anti-submarine warfare," said Bryan Clark, a senior fellow at the Hudson Institute and former special assistant to the Navy's chief of naval operations.

They're also practicing more. "Dynamic Mongoose is one example," Clark said, citing recent exercises above the Arctic Circle.

"The most important mission for those deployments was anti-submarine warfare basically looking for Russian submarines deploying out of their bases in the Kola Peninsula," where the powerful Northern Fleet is based, Clark added.

Sailors man the rails on the flight deck of USS Roosevelt at the end of Dynamic Mongoose 2020, July 9, 2020. US Navy/MCS Seaman Austin G. Collins

During the Cold War, the Greenland-Iceland-UK Gap was an important chokepoint between that Russian fleet and the Atlantic. "We still rely on these chokepoints as kind of the line in the sand," Clark said.

Russia subs passing through that gap could threaten Europe's links to North America in a war. (A Russian ship monitored Dynamic Mongoose but "didn't affect the exercise at all," Kendall said.)

"There's a big concern on the US part about even a couple Russian nuclear submarines making it past the GIUK gap, because they can then just drift around in the Atlantic until they can track down a US [ballistic missile sub] or threaten the US coast," Clark added.

Subs and aircraft assumed much of the ASW mission after the Cold War, but the Navy has invested in more surface-ship ASW because that sub fleet which will shrink in coming decades as older Los Angeles-class subs retire now has many other missions.

Surface ships are also now the primary missile-defense platform and are responsible for maritime security operations, meaning ASW "is another mission on top of all the other missions they've got to do," Clark said.

A US Navy MH-60R Seahawk helicopter prepares to land on the guided-missile destroyer USS Roosevelt in the Atlantic Ocean, August 4, 2013. US Navy/MCS2 Samantha Thorpe

Dynamic Mongoose illustrates the mismatch, Clark said. "Every time the Russians deploy a ... nuclear submarine down through the GIUK gap, we deploy a dozen airplanes or ships to go up there and track it. So the Navy just isn't set up to be able to do ASW at any scale."

Dynamic Mongoose involved five submarines, but Russia can muster more.In October, Russia's Northern Fleet reportedly sent 10 subs toward the Atlantic to test NATO's detection abilities and show it could threaten the US.

"These submarines are really hard to track unless you want to put one of our submarines on it," Clark said, "and we don't have enough submarines to do that."

Clark argues more unmanned vehicles should be acquired and used to suppress some subs while manned assets to pursue the subs that need to be eliminated.

"You have to decide which submarines are ... ones you have to sink and which ones can you accept just harassing and suppressing, because you don't have enough submarines to go try and kill every opponent submarine," Clark said.

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