Ross Gyre variability modulates oceanic heat supply toward the West Antarctic continental shelf

ARTICLE

Ross Gyre variability modulates oceanic heat

supply toward the West Antarctic continental shelf

Channing J. Prend

1,2

✉

, Graeme A. MacGilchrist

3,4

, Georgy E. Manucharyan

, Rachel Q. Pang

Ruth Moorman

, Andrew F. Thompson

, Stephen M. Grif

3,5

, Matthew R. Mazloff

, Lynne D. Talley

Sarah T. Gille

West Antarctic Ice Sheet mass loss is a major source of uncertainty in sea level projections.

The primary driver of this melting is oceanic heat from Circumpolar Deep Water originating

offshore in the Antarctic Circumpolar Current. Yet, in assessing melt variability, open ocean

processes have received considerably less attention than those governing cross-shelf

exchange. Here, we use Lagrangian particle release experiments in an ocean model to

investigate the pathways by which Circumpolar Deep Water moves toward the continental

shelf across the Paci

c sector of the Southern Ocean. We show that Ross Gyre expansion,

linked to wind and sea ice variability, increases poleward heat transport along the gyre

’

eastern limb and the relative fraction of transport toward the Amundsen Sea. Ross Gyre

variability, therefore, in

uences oceanic heat supply toward the West Antarctic continental

slope. Understanding remote controls on basal melt is necessary to predict the ice sheet

response to anthropogenic forcing.

https://doi.org/10.1038/s43247-024-01207-y

OPEN

School of Oceanography, University of Washington, Seattle, WA, USA.

Environmental Science and Engineering, California Institute of Technology,

Pasadena, CA, USA.

Atmospheric and Oceanic Sciences Program, Princeton University, Princeton, NJ, USA.

School of Earth and Environmental Sciences,

University of St Andrews, St Andrews, UK.

NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA.

Scripps Institution of Oceanography,

University of California San Diego, La Jolla, CA, USA.

✉

email:

cprend@uw.edu

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1234567890():,;

A

ccelerated mass loss from the Antarctic Ice Sheet (AIS)

has been observed in recent years

, and could contribute

up to one meter of global sea-level rise by the end of the

century

. Determining its drivers is crucial to improve future sea-

level and climate projections; however, ice shelf melt exhibits

notable spatiotemporal variability

. AIS melt has long been linked

to oceanic heat from intrusions of warm Circumpolar Deep

Water (CDW) onto the continental shelf

–

. Consequently,

numerous studies have investigated on-shelf CDW transport

variability, which is typically attributed to climate-driven wind

anomalies

–

. However, cross-shelf exchange is only one factor

controlling the total heat supply to the ice shelf cavities. CDW

delivery to the AIS also depends on the offshore heat reservoir

adjacent to the shelf break, as well as the circulation and water-

mass transformations on the shelf

–

. The remote pathways that

transfer CDW from the core of the Antarctic Circumpolar Cur-

rent (ACC) to the continental slope, and their potential impact on

melt rate variability, remain relatively unexplored

Observational studies of CDW transport toward the AIS have

typically focused on speci

fi

c regions, dictated by data

availability

–

, while circumpolar investigations have relied on

models and often quantify the heat

fl

ux across a speci

fi

isobath

–

. These studies have emphasized the spatial variability

in continental shelf hydrography and its relationship to basal

melting. For example, in the Paci

fi

c sector, the Ross and

Amundsen/Bellingshausen Seas are often categorized as cold and

warm continental shelves, respectively

–

. This is re

fl

ected in

the minimal mass loss of the Ross Ice Shelf compared to the West

Antarctic Ice Sheet (WAIS)

. Therefore, determining the

processes that control changes in continental shelf heat content is

necessary to predict future AIS mass loss. For instance, melt rates

at Pine Island and Thwaites glaciers, as well as Dotson Ice Shelf,

have been shown to respond to wind-driven

fl

uctuations in CDW

supply to the Amundsen Sea Embayment at seasonal, inter-

annual, and decadal timescales

–

. More recently, it was sug-

gested that the Amundsen and Bellingshausen continental shelves

are sensitive to offshore CDW properties, which are in

fl

uenced by

the large-scale atmospheric and oceanic circulations

. However,

the mechanisms that connect open ocean variability with changes

in continental shelf properties and ice shelf mass loss are not well

constrained.

Here, we investigate remote controls on oceanic heat delivery

to the AIS by characterizing CDW pathways from the ACC to the

continental slope, which precede on-shelf heat transport

Using Lagrangian particle release experiments in a state estimate

with active mesoscale eddies

, we show that changes in the

extent and strength of the Ross Gyre mediate the magnitude and

distribution of CDW transport toward the continental shelf

throughout the Paci

fi

c sector of the Southern Ocean. Gyre

expansion is associated with an increased southward

fl

ux of CDW

via the gyre

’

s eastern limb and a greater fraction of the heat

associated with this layer moving toward the Amundsen and

Bellingshausen Seas. The off-shelf heat availability to the WAIS is,

therefore, regulated by interannual variations in the gyre circu-

lation, which are caused by large-scale changes in the winds and

sea ice state. Ultimately, heat delivery to the ice shelf cavities will

also depend on the cross-shelf exchange and shelf

circulation

. Still, the Ross Gyre, despite its distance from

the WAIS, may be important to understanding the ice sheet

’

response to changes in forcing

Results

Ross Gyre variability

. The cyclonic Ross Gyre is forced by a

clockwise (negative) wind stress curl and dominates the circula-

tion south of the ACC in the western Paci

fi

c sector of the

Southern Ocean

. The gyre acts as a conduit between the open

ocean and the Antarctic margins, carrying CDW poleward to the

continental slope on its eastern edge and exporting shelf waters

equatorward along its western

fl

ank

. Given the dif

fi

culty of

collecting in situ observations in this vast, seasonally ice-covered

region, our understanding of the gyre dynamics stems primarily

from satellite altimetry and numerical models

–

. In this work,

we combine available observations with a dynamically consistent

model by analyzing the Biogeochemical Southern Ocean State

Estimate (B-SOSE)

, a data-assimilating simulation that con-

strains a general circulation model with satellite and hydro-

graphic measurements. The con

fi

guration used here has 1/6

∘

nominal resolution ( ~ 8 km) and covers the period from 2013 to

2018 (Methods). Earlier iterations of this model have been used to

investigate Ross Gyre transport and its relationship to the ther-

mohaline structure of the region

. As such, we have con-

fi

dence in the model

’

s representation of the gyre circulation. For

example, the expression of the gyre is visible from its relatively

cold temperature on CDW isopycnal surfaces

(Fig.

a).

fi

ning a precise gyre boundary is nontrivial. From altimetry,

the boundary is typically derived from dynamic ocean

topography

, or implicitly the surface geostrophic

fl

ow. Here,

we leverage the model

’

s additional subsurface information by

determining the gyre extent from a speci

fi

c contour (

−

Sverdrup) of the barotropic streamfunction

, which represents

the depth-integrated volume transport and is calculated from the

model velocity

fi

eld (Methods). The position of the gyre is

bounded to the south by the continental slope and to the north

and west by the Paci

fi

c-Antarctic Ridge (Fig.

b). In fact, the

interaction of the ACC with bathymetry both enables the

formation of a gyre and in

fl

uences its circulation and transport

properties

. Bottom pressure torques associated with bathy-

metric features, for example, are thought to be necessary to close

the barotropic vorticity budget in the region

. While the gyre is

topographically constrained for much of its course, the separation

of the

fl

ow from the Paci

fi

c-Antarctic Ridge permits much greater

variability in the gyre

’

s eastern limb until the

fl

ow meets the

Marie Byrd seamounts around 68

∘

S, 130

∘

W. (Fig.

b). This

eastern boundary steers warm CDW southward toward the

continental shelf. Therefore,

fl

uctuations in its position could

alter oceanic heat delivery to the shelf break.

The strength and structure of the Ross Gyre respond to

changes in the ocean surface stress

, which is the force applied

to the sea surface by winds and sea ice (Methods). Satellite data

suggest that changes in the ocean surface stress curl cause the gyre

fl

uctuate at both seasonal and interannual timescales, with the

leading mode of variability being linked to atmospheric forcing by

the Antarctic Oscillation (or Southern Annular Mode)

. B-SOSE

shows similar amplitude variations in gyre transport and area,

which are highly correlated to the ocean surface stress curl over

the central gyre (Fig.

a; Supplementary Figs. 1, 2). The high

correlation (r

=

0.73) at monthly timescales indicates that the

barotropic response to the ocean surface stress curl is nearly

instantaneous. While the relationship between the strength of the

forcing and the gyre transport is straightforward, the mechanisms

that set the location of the gyre

’

s eastern boundary, and associated

variations in southward CDW transport, are less clear.

To characterize the impact of interannual variations in Ross

Gyre dynamics on poleward heat

fl

ux, we select 2014 and 2018

(hereafter referred to as end-member years) as case studies of

contracted and expanded gyre states, respectively. The contrast

between these years is re

fl

ected in the barotropic streamfunction,

which exhibits substantial differences in magnitude and extent

(Fig.

b, c). The gyre

’

s northern and southern boundaries track

contours, where

is the Coriolis parameter and

is depth.

Greater cyclonic vorticity input by the winds in 2018, which is

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enabled by anomalously low sea ice concentrations in that year

(Supplementary Fig. 3), spins up the gyre and shifts its outer

bound to an

value that is larger in magnitude (black contours

in Fig.

b, c). This corresponds to a more northward separation of

the

fl

ow from the Paci

fi

c-Antarctic Ridge and an eastward shift of

the gyre

’

s eastern boundary (blue shading in Fig.

c). Down-

stream of the separation, the current crosses

contours, similar

to standing meander regions in the ACC

. Just as meander

curvature is related to the forcing strength

, we suggest that the

length of the gyre

’

s barotropic streamlines should scale with the

ocean surface stress curl. In other words, the eastern limb of the

gyre can extend further when there is greater vorticity input by

the winds. This, in turn, affects the location where heat moves

toward the continental shelf.

Interannual

fl

uctuations in gyre size co-occur with temperature

and pressure anomalies on CDW isopycnal surfaces (Fig.

). The

annual mean temperature anomaly (relative to the 2013

–

2018

mean) on the 27.9 neutral density (

) surface displays opposing

patterns between the end-member years (Fig.

a, c). The

temperature properties of the CDW water mass itself do not

vary on these timescales. Therefore, the isopycnic temperature

anomalies indicate redistribution of heat within the system. For

example, a stronger gyre is associated with cold anomalies in the

gyre interior and warm anomalies on the Amundsen continental

shelf (Fig.

a, c). This warming of the Amundsen Sea Embayment

connects smoothly to offshore anomalies (Fig.

b, d). Indeed, the

offshore heat reservoir adjacent to the Amundsen undergoes a

multi-year warming that is driven by a zonal heat transport

convergence consistent with enhanced supply of gyre-sourced

waters (Supplementary Fig. 4). We note that gyre anomalies

cannot be instantaneously communicated to the shelf, and the

temperature changes on the shelf will necessarily also re

fl

ect

variations in onshore CDW transport. Still,

fl

uctuations in the

offshore temperature could in

fl

uence the continental shelf heat

content, but have received little attention previously

. To probe

the gyre-shelf connection further, we conducted a series of

Lagrangian particle release experiments to test how changes in

gyre state modify the pathways of CDW transport from the ACC

to the continental slope.

Particle pathways

. The

fi

rst set of experiments involves releasing

particles on CDW density surfaces at different locations along the

continental slope and running them backward in time over the

full 6-year model run to diagnose where the waters originated

Fig. 1 Circumpolar Deep Water temperature and time-varying Ross Gyre boundary.

2013

–

2018 mean

temperature (shading) and velocity (pink arrows)

on the

γ

=

27.9 kg m

−

neutral density surface, which corresponds to the core of the Circumpolar Deep Water layer in the model (the white region in the

central Ross shelf is where the density surface outcrops), and

sea surface height (shading) and Ross Gyre boundary (green contour), as well as monthly

realizations of the gyre boundary (white contours), which is de

ned by the

−

8 Sverdrup contour of the barotropic streamfunction. In both panels, black

contours mark bathymetry in 1000 m intervals with the 1000 m isobath plotted thicker to indicate the approximate location of the Antarctic Slope Front

Brown lines denote the Polar Front (PF) and Southern Boundary of the Antarctic Circumpolar Current (Sbdy)

, a brown star marks the Marie Byrd

seamounts, and a brown triangle and square mark the locations of Thwaites and Pine Island glaciers, respectively.

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(Methods). This is intended to contextualize past studies on

cross-shelf exchange, which have focused on transport across the

1000 m isobath

. Comparing release locations in the Ross and

Amundsen Seas (Supplementary Fig. 5) illustrates the gyre

’

s role

in poleward heat transfer. CDW moves south along the eastern

fl

ank of the gyre and separates into two distinct streams where the

fl

ow bifurcates around the Marie Byrd seamounts. One of these

streams follows the gyre

’

s westward near-slope

fl

ow, while the

other proceeds to the southeast toward the Amundsen con-

tinental shelf. These two routes are associated with different

timescales and degrees of watermass transformation. Particles

that end up in the Ross Sea have cooled substantially after the

fl

ow bifurcates, while particles that enter the Amundsen Sea

contain relatively unmodi

fi

ed CDW (Fig.

d). These results

support the role of far-

fi

eld processes in setting the distinct

hydrographic characteristics of the Ross and Amundsen con-

tinental shelves

To assess how particle pathways are affected by gyre variability,

we conducted two additional experiments based around the

strong and weak end-member years. Namely, we used the same

release locations and 6-year backward integration, but looped the

model velocity output from 2014 and 2018 to represent cases

where the gyre is

fi

xed in a contracted and expanded state. It

should be noted that particles take several years to move from the

ACC to the continental slope, so these looped scenarios do not

fl

ect realistic trajectories. Nevertheless, they provide

Fig. 2 Ross Gyre structure, variability, and forcing. a

Monthly time series of gyre strength (blue), de

ned as the mean value of the barotropic

streamfunction within the

−

8 Sverdrup contour (which is outlined in purple in

), and ocean surface stress curl (black) averaged over the central gyre

(140 to 152

∘

W, 70 to 73

∘

S, see Supplementary Fig. 1). A dashed horizontal line marks the mean gyre strength over the full record and error bars denote the

standard error.

2014 (weak gyre) and

2018 (strong gyre) mean barotropic streamfunction (shading), with

contours overlaid (black). A pink star

shows the location of the Marie Byrd seamounts.

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mechanistic insight into how gyre extent affects the particle

pathways. An expanded and strengthened gyre (Fig.

c) provides

a closer connection between the gyre

’

s eastern limb and the

Amundsen Sea, which results in more unmodi

fi

ed CDW from the

ACC reaching the continental slope (Fig.

d). A stronger gyre also

more directly links the open ocean and the Ross continental shelf,

with less meandering and mixing of waters in the near-slope

fl

after the bifurcation at the Marie Byrd seamounts. This linkage is

evidenced by the shift toward zero in the probability density

function of cumulative temperature change along a trajectory

(Fig.

d).

In order to quantify the partitioning of particles between the

two routes, we conducted a forward particle release experiment

along a transect that extends from 130 to 135

∘

Wat67

∘

—

capturing the core of the gyre

’

s eastern limb. The meridional heat

transport through this transect exhibits interannual variability,

with a greater southward

fl

ux throughout the CDW density layer

in 2018, when the gyre was stronger (Fig.

b). The increased heat

transport is primarily due to the higher current speeds associated

with a stronger gyre, rather than changes in the temperature

pro

fi

le itself (Supplementary Fig. 6). Differences in gyre state also

lead to changes in the pathways of poleward

fl

ow. This can be

seen by comparing trajectories from particles that were advected

by the 2014 (Fig.

a) and 2018 (Fig.

c) velocity

fi

elds, using the

same looping technique described previously. Gyre expansion

favors the movement of particles toward the Amundsen Sea

(Fig.

c), as compared to the mean state (Supplementary Fig. 7).

To demonstrate the redistribution of particles between the two

streams around the Marie Byrd seamounts, we can examine the

fraction of particles that travel eastward from their initial release

location along the transect, which varies greatly in depth and

between the end-member years (Fig.

d). For particles seeded

between 400 and 800 m, corresponding to Upper CDW, an

expanded gyre roughly doubles the percentage of trajectories that

move toward the Amundsen and Bellingshausen continental

shelves. This reorganization is linked to changes in the position of

the gyre

’

s eastern limb, especially the location where it meets the

Marie Byrd seamounts, which controls the bifurcation of the

fl

ow.

Furthermore, the substantial depth dependence of the partition-

ing implies that the

fl

ow is not barotropic. In particular, the

relatively abrupt change within the thermocline ( ~ 700 m in

Fig.

d) suggests that this is a

fi

rst baroclinic mode response. The

gyre

’

s baroclinic structure is largely unknown since our under-

standing of the gyre dynamics stems primarily from satellite

altimetry

. While the variability in gyre strength is well-

explained by the rapid barotropic response to changes in ocean

surface stress curl (Fig.

a), the Lagrangian trajectories suggest

that baroclinic adjustments can be important at longer (multi-

year) timescales.

Conclusions

Predicting future WAIS melt is crucial to constrain sea-level rise

projections

, which, in turn, inform policy to address climate

impacts. Improved prediction requires better knowledge of the

processes that transfer heat from the open ocean to the ice shelf

cavities

. While many studies have examined cross-shelf heat

fl

uxes

, comparatively few have considered the pathways

that bring CDW from the ACC to the continental slope

. Here,

we have shown, using a data-assimilating state estimate, that Ross

Gyre variability impacts the magnitude and distribution of

oceanic heat supply toward the continental shelf. This connection

arises from variations in the position of the gyre

’

s eastern limb,

which is responsible for transporting CDW to the shelf break. An

expanded gyre preferentially steers heat toward the Amundsen

Fig. 3 Ocean temperature anomalies associated with contracted and expanded gyre states.

Map of the annual mean temperature anomaly (relative to

the 2013-2018 mean) on the

γ

=

27.9 kg m

−

surface for

2014 and

2018, re

ecting the contracted and expanded gyre states, respectively. In both

panels, the green contour represents the mean gyre boundary for that year, while black contours mark bathymetry in 1000 m intervals with the 1000 m

isobath plotted thicker to indicate the approximate location of the Antarctic Slope Front. The dashed black line indicates a meridional transect mov

ing

offshore from the Amundsen Sea continental shelf (along 105

∘

W). The annual mean temperature anomaly in the top 1400 m on this transect is plotted for

2014 and

2018. Black contours denote isopycnals for each year, with the 27.9

γ

surface highlighted in bold.

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and Bellingshausen Seas, leading to a zonal heat transport con-

vergence that warms the subsurface waters offshore of the

Amundsen continental shelf (Fig.

c). These results suggest a

possible link between Ross Gyre size and melt rates for West

Antarctic ice shelves.

It is important to recognize, however, that other processes

affect the eventual heat delivery to the ice shelf cavities. These

include the on-shelf CDW

fl

and the shelf

circulation

, both of which exhibit variability

—

in some

cases due to the same atmospheric forcing that drives the

gyre

. Variability in the thickness of the CDW layer at the

Amundsen Sea Embayment is typically attributed to the zonal

winds at the shelf break (yellow box in Fig.

a). Eastward wind

anomalies in this location have been shown to enhance onshore

CDW transport at interannual timescales due to barotropic

acceleration of the Amundsen undercurrent

. However, the

opposite relationship between winds and cross-shelf heat

fl

ux has

been observed at decadal timescales, which was linked to bar-

oclinic adjustment of the undercurrent

. The interaction of these

mechanisms with Ross Gyre variability has not been explored,

although large-scale sea level pressure differences between the

end-member years (Fig.

a) would presumably in

fl

uence both the

gyre forcing and the wind-driven onshore transport.

To begin assessing this relationship, we can examine, in con-

cert, the ocean surface stress curl over the central gyre and the

zonal wind anomalies at the Amundsen shelf break. The two are

weakly correlated (

=

0.19) at monthly timescales (Fig.

b),

suggesting that the gyre conditions that preferentially steer CDW

toward the Amundsen may also be associated with enhanced on-

shelf CDW transport. Note that this conclusion is independent of

the model

’

s explicit representation of cross-shelf exchange. Ulti-

mately, quantifying the full pathways of CDW from its origin in

the ACC to the ice shelf itself is necessary to understand the

regional response of AIS mass loss to changes in forcing. How-

ever, our focus here is on the pathways from the open ocean to

the continental slope given that the horizontal grid spacing of

B-SOSE is too coarse to resolve some of the dynamics relevant to

cross-shelf heat transport, which has been shown to be sensitive

to model resolution

. Still, we have demonstrated that Ross

Gyre variability can modulate the heat reservoir offshore of the

Amundsen Sea at interannual timescales and thus deserves fur-

ther investigation in the context of continental shelf heat content

and WAIS melt. Future work should consider how changes in

gyre circulation interact with the processes that enable cross-shelf

exchange across a range of timescales.

Given the 6-year length of the B-SOSE simulation, it is dif

fi

cult

to reach conclusions about long-term changes induced by

anthropogenic forcing. Recent work projected an expansion of

the Ross Gyre over the 21st century due to increased vorticity

input by the winds, which led to changes in continental shelf

properties in West Antarctica

. Another recent study projects

unavoidable warming of the Amundsen continental shelf by the

end of the century regardless of the forcing scenario

. B-SOSE

has an evident trend in gyre strength and associated CDW

temperature anomalies, but this could be due to subsampling

decadal

fl

uctuations. In that case, the end-member years can be

thought of as representing earlier and later periods within a

longer-term signal. Furthermore, due to the distance between the

Fig. 4 Reverse particle trajectories showing differences in the origin of continental slope waters during contracted and expanded gyre states.

100

randomly selected trajectories for particles released at the contentinal slope in the Ross (blue) and Amundsen (orange) and run backwards with the

2014

(weak gyre) and

2018 (strong gyre) velocity

elds, looped to achieve a 6-year integration time. Underlying shading denotes bathymetry and black lines mark

the 1000 m isobath, Polar Front (PF) and Southern Boundary of the Antarctic Circumpolar Current (SBdy)

Vertical temperature pro

les at the release

locations (circles in

and

) in the Ross (blue) and Amundsen (orange) sectors for 2014 (solid) and 2018 (dotted).

Probability density functions of the

cumulative temperature change along a trajectory for the Ross (blue) and Amundsen (orange) particles for 2014 (solid) and 2018 (dotted) looped veloc

ities.

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gyre and continental shelf, the anomalies on the shelf are likely

related to multi-year gyre-mediated changes to the offshore heat

reservoir, rather than variations in the gyre circulation in any one

year speci

fi

cally. Nevertheless, characterizing the expanded and

contracted gyre states within the model can provide mechanistic

insight that is relevant to both the natural variability and forced

trend.

Record low Antarctic sea ice extent in 2023, which may indi-

cate a shift in sea ice state

, is expected to strengthen the gyre by

permitting the winds to input greater vorticity. Additionally,

future changes to Southern Ocean winds and the Amundsen Sea

Low under climate forcing

also favor conditions for gyre

expansion. These results imply enhanced warming of the offshore

heat reservoir adjacent to the Amundsen in the coming decades,

and potentially a greater importance of the Ross Gyre and open

ocean sea ice anomalies to the temperature variability on the

continental shelf. Moreover, if this offshore warming co-occurs

with the same or greater magnitude of onshore volume transport,

then we would expect accelerated melting of the most vulnerable

ice shelves in West Antarctica, including those that buttress Pine

Island and Thwaites glaciers. Further work addressing these

possible feedbacks is urgently needed given the broad rami

fi

ca-

tions of constraining future WAIS melt for climate prediction.

Methods

This study investigates pathways of CDW transport using the

data-assimilating Biogeochemical Southern Ocean State Estimate

(B-SOSE)

, which constrains the Massachusetts Institute of

Technology general circulation model (MITgcm) solution

with

satellite and hydrographic measurements. The con

fi

guration used

in this analysis, Iteration 133, has 1/6

∘

horizontal grid spacing

( ~ 8 km), 52 uneven vertical levels, and runs from 1 January 2013

to 31 December 2018. The model assimilates ocean observations,

including satellite data, shipboard CTD measurements, and Argo

fl

oat pro

fi

les; bathymetry is from the ETOPO1 Global Relief

Model

. This iteration of B-SOSE is publicly available (

http://

sose.ucsd.edu

Here, we de

fi

ne the Ross Gyre using the barotropic stream-

function, or the depth-integrated volume transport, which is

calculated by integrating the model velocity

fi

eld northward from

the coast and vertically

. Speci

fi

cally, we use the

−

8 Sverdrup

(10

−

) contour of the streamfunction as the gyre boundary,

following Roach & Speer (2019), which used this de

fi

nition to

investigate Ross Gyre dynamics using an earlier iteration of the

same model. Aside from the maps illustrating the gyre boundary

(Figs.

a, c) and the calculation of gyre strength (Fig.

a), the

analysis does not rely on this

−

8 Sverdrup value. Furthermore,

the temporal gyre variability is strongly correlated (r > 0.98) for a

range of threshold values (Supplementary Fig. 8), and the results

from the Lagrangian particle release experiments are independent

of any particular gyre boundary de

fi

nition. The error bars for gyre

strength in Fig.

are

fl

ect the standard error of the monthly

means based on the model

’

s 5-day velocity

fi

elds.

To investigate the drivers of gyre variability, we consider the

ocean surface stress

—

the force applied to the ocean surface by the

winds and sea ice

—

fi

ned as

=

ατ

ice

−

ocean

+

−

)

air-water

where

is the sea ice concentration,

ice

−

ocean

is the ice-ocean

Fig. 5 Forward particle trajectories demonstrating the impact of gyre variability on poleward heat transport magnitude and spatial distribution.

500

randomly selected trajectories for particles released at 600 m along a transect that extends from 130 to 135

∘

Wat67

∘

S (black line) and advected by the

2014 and

2018 velocity

elds, looped to achieve a 6-year integration time. Underlying shading denotes bathymetry and black lines mark the 1000 m

isobath, Polar Front (PF) and Southern Boundary of the Antarctic Circumpolar Current (SBdy)

Meridional heat transport, de

ned relative to 0

∘

through the release transect for the two end-member years; a negative transport is southward. Error bars denote the standard error.

Fraction of particles

that move eastward from their initial release location after 1 year (i.e. toward the Amundsen Sea), plotted as a function of release depth, for both loo

ped

velocity scenarios.

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7

stress, and

air

−

water

is the wind stress. The ocean surface stress is

diagnosed directly in the model, and thus does not require

additional calculation. However, we write out the expression here

to highlight the fact that the ocean surface stress is not equivalent

to the wind stress in the presence of sea ice. In fact, the increase in

ocean surface stress curl over the central gyre that drives the gyre

expansion is due largely to negative sea ice concentration

anomalies in the region (Supplementary Fig. 3) rather than

variability in the wind stress.

To illustrate the isopycnic temperature variability associated

with the contracted and expanded gyre states, we plot the 2014

and 2018 annual mean temperature anomaly on the

=

27.9 kg m

−

surface (Fig.

a,c). This isopycnal was selected since

it corresponds to the subsurface temperature maximum at the

shelf break in the Amundsen (marked by an orange circle in

Fig.

a, c), and is thus taken to be the core of the CDW layer in

the model. However, the subsurface layer of warmer tempera-

tures at this location is broad and spans a range of densities and

depths (Fig.

b). Indeed, CDW is known to be comprised of

several density classes. While we assume throughout that CDW

is isolated from surface forcing, we note that the 27.9

surface

outcrops in winter in a small region within the central gyre

(Supplementary Fig. 9). Although, this does not correspond to

the locations with the largest isopycnic temperature anomalies,

which occur due to shifting of the strong isopycnal temperature

gradientnearthegyreedge(visibleinFig.

a). This gradient is

present on denser surfaces as well, so the isopycnic temperature

anomaly patterns look qualitatively similar for

=

28.0 kg m

−

(SupplementaryFig.10).Wepresentthe27.9

surface in the

main text since it corresponds to the subsurface temperature

maximum at the shelf break, which suggests its relevance to

the onshore heat transport. However, the conclusions of the

paper do not depend on the selection of this particular density

surface.

B-SOSE 5-day mean velocity

fi

elds were used to calculate of

fl

ine

Lagrangian particle trajectories

using the particle-tracking model

Parcels, which is available online (

http://oceanparcels.org/

). Details

of the interpolation scheme ha

ve been described previously

.For

the purposes of this study, we conducted 3 reverse and 3 forward

release experiments. Here, we are interested in subsurface pathways,

so we only consider particles that do not outcrop into the mixed

layer. The reverse releases were done

by seeding 2,500,000 particles at

different locations along the 100

0 m isobath (Supplementary Fig. 5)

and at all depths in the water column. Particles were

fi

rst released at

the end of the simulation (i.e. December 31, 2018) and integrated

backwards in time for 6 years to the beginning of the model run. Two

other experiments were done with the same initial locations and

6-year integration time. However, instead of using the full model

Fig. 6 Comparison of Ross Gyre forcing and Amundsen continental shelf break winds that drive onshore transport variability. a

2018 minus 2014 sea

level pressure, with a green contour denoting the mean gyre boundary and black lines marking bathymetry in 1000 m intervals with the 1000 m isobath plot

ted

thicker to indicate the approximate location of the Antarctic Slope Front. A purple box denotes the region of maximum negative wind stress curl that fo

rces the

gyre and a yellow box marks the offshore heat reservoir adjacent to the Amundsen.

Monthly time series of zonal wind stress anomaly (yellow) near the

Amundsen continental slope (yellow box in

) and ocean surface stress curl (purple) averaged over the gyre forcing region (purple box in

Time series of

upper ocean temperature in the offshore reservoir (yellow box in

) with isopycnals overlaid in black and the 27.9

γ

surface highlighted in bold.

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velocity

fi

eld, we loop 2014 and 2018 velocities to represent con-

tracted and expanded gyre states, respectively.

The forward particle releases were done along a transect that

extends from 130 to 135

∘

Wat67

∘

S. The meridional heat trans-

port across this transect was calculated as

∫∫

Tv dA dz

using

constants for

(1027 kg m

−

) and

(3990 J kg

−

∘

−

), along

with the model temperature (

) and meridional velocity (

)

fi

elds.

The transport is de

fi

ned relative to 0

∘

, therefore, when the

temperature is negative, a southward heat transport corresponds

to a northward mass transport of water less than 0

∘

C. However,

we note that temperatures are always positive within the relevant

CDW density range, so the heat transport is in the same direction

as the mass transport. Error bars in Fig.

b and Supplementary

Fig. 4 re

fl

ect the standard error of the annual means based on the

monthly transports.

The magnitude of the heat transport across this transect does

depend on the precise segment location and width, however the

poleward heat transport is enhanced in 2018 (expanded gyre)

relative to 2014 (contracted gyre) regardless of position, although

the longitude of the maximum heat transport does shift (Sup-

plementary Fig. 11). As such, we release particles along a transect,

rather than from a point location, in order to capture the core of

the gyre

’

s eastern limb, even as it changes position in time. For

the release experiment, we seeded 100,000 particles evenly spaced

along the transect at all depths between 400 and 1400 m, which

includes both the Upper and Lower Circumpolar Deep Water

layers at this location. Particles were

fi

rst released at the beginning

of the simulation (i.e. January 1, 2013) and integrated forward in

time for 6 years to the end of the model run (Supplementary

Fig. 7). Similar to above, we also computed trajectories for the

same release locations, but looping the 2014 and 2018 velocities

(Fig.

a, c).

Data availability

Output from the Biogeochemical Southern Ocean State Estimate (B-SOSE) is publicly

available (

http://sose.ucsd.edu

); this analysis utilizes Iteration 133 of the model solution.

fl

ine Lagrangian particle trajectories were calculated using Parcels (

http://oceanparcels.

org

) and are archived on Zenodo (

http://zenodo.org/doi/10.5281/zenodo.10393325

Received: 8 August 2023; Accepted: 8 January 2024;

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Acknowledgements

C.J.P., G.A.M., M.R.M., L.D.T., and S.T.G. were supported by NSF PLR-1425989 and OPP-

1936222 (Southern Ocean Carbon and Climate Observations and Modeling project). C.J.P.

received additional support from a NOAA Climate & Global Change Postdoctoral Fel-

lowship. G.A.M. received additional support from UKRI Grant Ref. MR/W013835/1.

G.E.M. was supported by NSF OPP-2220969. R.Q.P. was supported by the High Meadows

Environmental Institute Internship Program. R.M. was supported by the General Sir John

Monash Foundation. A.F.T. was supported by NSF OPP-1644172 and NASA grant

80NSSC21K0916. M.R.M. also acknowledges funding from NSF awards OCE-1924388 and

OPP-2319829 and NASA awards 80NSSC22K0387 and 80NSSC20K1076. Thanks to Steve

Rintoul and two anonymous reviewers for their helpful comments. Thanks also to Becki

Beadling for reading an earlier draft of the manuscript, and to Maike Sonnewald and Mary-

Louise Timmermans for useful conversations.

Author contributions

C.J.P. designed the study with input and supervision from G.E.M., A.F.T., L.D.T., and

S.T.G. M.R.M. developed the model and G.A.M. con

fi

gured the Lagrangian particle

release experiments. C.J.P. and R.Q.P. conducted the rest of the analysis. R.Q.P. received

additional supervision from G.A.M. and S.M.G. R.M. provided code and insights on the

overall framing of the work. C.J.P. wrote the manuscript. All authors contributed to the

interpretation of the results and commented on the manuscript.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary information

The online version contains supplementary material

available at

https://doi.org/10.1038/s43247-024-01207-y

Correspondence

and requests for materials should be addressed to Channing J. Prend.

Peer review information

Communications Earth & Environment

thanks Steve Rintoul

and the other, anonymous, reviewer(s) for their contribution to the peer review of this

work. Primary Handling Editors: Jennifer Veitch, Joe Aslin, and Clare Davis. A peer

review

fi

le is available.

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