September 2024 | Oceanography
than the historical Labrador source water
of the early 2000s. Tis is consistent with
warming in the upstream source region.
At a time when North Atlantic sur-
face temperatures were broadly break-
ing record warming levels during 2023
(Hobday et al., 2023), Gulf of Maine sur-
face conditions were running just a little
above average. Meanwhile, a shif toward
an input of Labrador Slope Water that
began setting up during the fall of 2023
was detectable in the subsurface waters
(Figure 1c). Conditions in 2024 have been
consistently within the temperature and
salinity ranges that characterize the pre-
2011 period—conditions that had not
been recorded since before the heatwave.
Based on long-term climate projec-
tions, we expect the water mass input
to shif at some point back toward the
previous warm and salty conditions
(Brickman et al., 2021). However, we
do not know when or how quickly that
return will occur. Because of the large
volume of water and its separation from
the atmosphere, such deep-water condi-
tions carry a lot of thermal inertia. More
importantly, the winter and spring ocean-
ographic conditions at depth have histor-
ically had strong associations with how
the biology of the Gulf of Maine devel-
ops over the summer. Efects can con-
tinue into autumn, as vertical mixing
carries deep waters to the surface. Tus,
these conditions can serve as an early sig-
nal for potential ecological shifs in the
coming months whose possible efects are
described below.
NORTH ATLANTIC RIGHT WHALES. Te
primary prey of these critically endan-
gered whales, the copepod Calanus
fnmarchicus, is strongly infuenced by
the supply and conditions set up by these
same deep-water dynamics. During the
historic cold, fresh years, C. fnmarchicus
was abundant in the eastern Gulf of
Maine, and large numbers of right whales
would aggregate annually in late sum-
mer to feed. Following the heatwave, the
prey supply declined sharply and right
whales began foraging elsewhere, leading
to a complex chain of events that chal-
lenge our ability to manage this species
and the human activities that threaten
it (Davies and Brillant, 2019). Because
of the timeframe of oceanographic sup-
ply and C. fnmarchicus ontogeny, these
FIGURE 1. (a) Location of the Jordan Basin buoy (dot) and the general path of Labrador Slope Water into the Gulf of Maine superimposed on the winter
2024 sea surface temperature anomaly (January–March Optimum Interpolation Sea Surface Temperature [OISST] anomaly from the 30-year mean, °C).
(b) Annual mean temperature and salinity conditions at 250 m depth in Jordan Basin. Gray lines show the range of monthly mean values. (c) Monthly
temperature and salinity anomalies at 250 m depth in Jordan Basin. Missing values are times when no data were collected by the instrumentation.
°C