Oceanography | Vol. 37, No. 3
RIP CURRENT – NEWS IN OCEANOGRAPHY
EARLY WARNING
OF A COLD WAVE IN THE GULF OF MAINE
By Nicholas R. Record, Andrew J. Pershing, and Douglas B. Rasher
Te Gulf of Maine was one of the frst
marine ecosystems to report a major
ocean heatwave, one that lasted for more
than a year in 2012–2013 and reached
temperatures projected for the end of the
twenty-frst century (Mills et al., 2013).
Tat heatwave drove major ecologi-
cal shifs, including historically low cod
stocks (Pershing et al., 2015), a multi-
year unusual mortality event for the
endangered North Atlantic right whale
(Record et al., 2019), the collapse of
northern shrimp (Richards and Hunter,
2021), the near disappearance of blue
mussels from the intertidal zone (Sorte
et al., 2017), and other efects (Reardon
et al., 2018; Scopel et al., 2019). While
these impacts constituted a major sur-
prise that drew a great deal of attention
(e.g., Woodard, 2020), the consequences
were documented only afer the fact,
highlighting the need to monitor rapid or
surprising oceanographic changes as they
are happening.
Subsequent research showed that the
heatwave was preceded by extremely
warm deep water entering the gulf
through the Northeast Channel a year
prior (Balch et al., 2022). Te Northeast
Channel is the only conduit for deep
water into this shallow sea, and because
of the sheer volume of water below
the thermocline, the water masses that
enter through this path have an out-
sized infuence on long-term dynamics.
Tis infuence is generally a refection of
the “Coupled Slope Water System” that
includes the Gulf of Maine (MERCINA
Working Group, 2001) and that is tightly
connected over long timescales to the
deep water in the Northeast Channel
(Sherwood et al., 2011). Changes in the
state of this system have coincided with
approximately decadal regime shifs in
the Gulf of Maine ecosystem (Greene
and Pershing 2007; Meyer-Gutbrod et al.,
2021). Oceanographers have suggested
the potential for early warnings of major
changes based on upstream dynamics
(Greene and Pershing, 2003; Gonçalves
Neto et al., 2021). Because of the impor-
tance of subsurface currents to this pro-
cess, focusing on surface conditions can
mask important dynamics.
Afer more than a decade of consis-
tently warming conditions, the deep
water in the Gulf of Maine has a thermo-
haline regime not detected since before
the heatwave signal appeared in 2011.
A buoy in Jordan Basin has recorded
water column temperatures and salinities
near-continuously since 2003 (Figure 1a).
Prior to 2011, mean annual tempera-
tures at 250 m depth had ranged from
approximately 7°C to 8.5°C. Since 2011,
these temperatures have ranged from
approximately 8.5°C to 10°C (Figure 1a).
Te lack of overlap between these two
regimes supports the idea that the supply
of water through the Northeast Channel
has undergone a major shif from sub-
arctic Labrador Slope Water to subtrop-
ical slope water associated with the Gulf
Stream. Te shif in salinity—a conserved
property—is consistent with a change in
source waters. Te fact that 2024 stands
out somewhat from the cluster of other
cold, fresh years suggests that the colder,
fresher Labrador source water, while sig-
nifcantly colder than it was in the past
decade, is running around 1°C warmer
ABSTRACT. Oceanographic changes are occurring more rapidly in recent decades,
with new implications for ocean ecosystems and adjacent human communities. It is
important to bring attention to these changes while they are unfolding rather than afer
they have occurred. Here we report on a rapid shif toward colder, fresher water in the
deep Gulf of Maine that, as of mid-June 2024, has persisted for at least six months.
Te shif likely represents an infux of Labrador Slope Water and resembles conditions
that predated a major warming shif that occurred in 2011–2012. Deep-water oceano-
graphic conditions in the Gulf of Maine have a strong infuence on ecosystem dynam-
ics, including the prey of critically endangered North Atlantic right whales, the sea-
sonal and disease dynamics of American lobster, and the distribution and abundance
of kelp forest communities, among others. Oceanographic surprises have an important
role in this system, and monitoring how this shif unfolds, oceanographically and eco-
logically, will give new insights into how oceanographic signals can inform our under-
standing of ecosystem responses.