A new marine heat wave spreading across a portion of the Pacific Ocean off the coast of British Columbia has so far grown into one of the largest of its kind in the last four decades, officials say, second only to the infamous "blob" that disrupted marine life five years ago.

The swath of unusually warm water stretches roughly from Alaska down to California, according to the National Oceanic and Atmospheric Administration (NOAA) in the United States. The marine phenomenon began in the Gulf of Alaska sometime around June 15 and ballooned over the summer.

A marine heat wave happens when sea surface temperatures are higher than normal for at least five consecutive days.

Officials tracking the system said it is already the second-largest experts have seen since 1981 — the first year for which satellite data used to track marine heat waves is available.

"Already, on its own, it is one of the most significant events that we've seen," Andrew Leising, a research scientist at the NOAA's Southwest Fisheries Science Center in La Jolla, Calif., said in a statement Thursday.

Above average water temperature

Leising said this year's heat wave resembles a similar West Coast heat wave that upset marine life in 2014 and 2015. Nicknamed "the Blob," the system, which stretched from Mexico to the Bering Sea, was blamed for warmer weather on land, abysmal feeding conditions for salmon and the sudden deaths of two dozen whales in the Pacific.

The Blob saw temperatures in the water peak at 3.9 C above average. The NOAA said the water this year has already reached temperatures of more than 2.7 C above average off the coast of Washington state.

"It's on a trajectory to be as strong as the prior event," said Leising, who developed a system for tracking and measuring heat waves in the Pacific Ocean using satellite data.

"It's really only time that will tell if this feature is going to persist and then rival [the Blob]."

The NOAA said its staff is monitoring this year's system to see whether it will last long enough to impact the marine ecosystem, though some biologists suggest it already has based on its sheer size.

The agency blamed the recent marine heat wave on a persistent weather pattern that began in June: weaker-than-normal winds and a weaker high-pressure system over the wedge of warm ocean between B.C., Hawaii and Washington state.

Officials say a formal analysis to try to pinpoint the reasons for the unusual weather pattern will take "some months" to complete. During the previous "blob" event, a number of studies suggested long-term ocean warming due to climate change made the heat wave stronger than it otherwise would have been.

Cold water rising along the coast from the ocean depths has held the warmer water offshore thus far, but experts said the chilled surge usually peters out in the fall. The heat wave in the water could move onshore and affect coastal temperatures if that happens, Leising said in the statement.

Officials also noted the marine heat wave is still new enough to break up if the weather shifts.

"It looks bad, but it could also go away pretty quickly if the unusually persistent weather patterns that caused it change," wrote NOAA research scientist Nate Mantua.