Future changes to the upper ocean Western Boundary Currents across two generations of climate models

In the following, model transports and projected changes are expressed as ensemble interquartile ranges with individual model details provided in Tables S1-S10, reanalysis transport estimates are provided as the range across the three products examined (Table S11), and observational transports and associated references are provided in Table S12. Projections represent differences between the 1900–2000 historical mean and 2050–2100 means from the business-as-usual SSP5-8.5 (RCP8.5) scenarios for CMIP6 (CMIP5).

Indian Ocean

In the Indian Ocean, the South Equatorial Current (SEC) forms the northern limb of the subtropical gyre, carrying fresh ITF water to the western basin. The SEC bifurcates east of Madagascar, forming the Northeastern and Southeastern Madagascar Currents (NMC and SMC, Fig. 1). Along the African shelf, the NMC further splits southward through the Mozambique Channel (MZC) and northwards as the East African Coastal Current (EACC). Further south, the SMC and MZC transport combine into the Agulhas Current (AC). The AC extension continues westwards beyond the African cape where it retroflects, returning most water eastwards to the Indian basin³³, while a part of this water (~ 21Sv³⁴) escapes into the South Atlantic as Agulhas Leakage.

The EACC transport across the CMIP6 models (interquartile range: 16.5–19.9 Sv Fig. 2, Fig. 3a) is consistent with the observed 19 Sv peak near 5°S and lies within the broad range of reanalysis estimates (7.4–23 Sv). The simulated NMC (19.4–22.7 Sv) and SMC (−10.2 to −15.7 Sv) are generally weaker than the range of observations (27–48 Sv and 20– 30 Sv, respectively), based on multiple short-term estimates (Table S12), but span similar ranges to the reanalysis (Fig. 3). Conversely, the simulated transports through the MZC (17–24.6 Sv) are slightly stronger than observations and reanalysis (15-19 Sv and 11.8-21 Sv, respectively). The simulated MZC transport seasonality, which is maximum around austral autumn, agrees well with observations and reanalysis (Figure S1). Further south, the CMIP6 AC transport increases to 50.8–61.6 Sv near Africa’s southern tip, somewhat weaker than the observational (70–77 Sv) but overlapping the weaker reanalysis estimates (47.2–53.7 Sv). A recent 3-year campaign³⁴ found AC transport at ~ 27°E to be strongest in austral summer and weakest in winter, although large interannual variability was evident. This seasonality is qualitatively consistent with the models and reanalysis, although the observed seasonal range ~ 15 Sv is considerably larger than in the models ~ 3 Sv (Figure S1).

Previous work²⁸ showed a broad-scale projected slowdown of the south Indian Ocean circulation by the end of the twenty-first century in CMIP5 models. Their reported weakening of both the western boundary Agulhas system and eastern boundary Leeuwin Current system is consistent with our CMIP6 and CMIP5 results. There is near-unanimous agreement across CMIP6 for reduced transport for the MZC (3.3 to 5.3 Sv), SMC (0.9 to 1.8 Sv), NMC (−2.1 to −3.9 Sv) and AC (3.4–7.6 Sv) (Fig. 2, Fig. 3b). However, neither the CMIP5 nor CMIP6 models show a consistent change in the EACC. In contrast to the reduction in transport along much of the southern African coast, the westward flowing AC extension south of Africa intensifies in all models (−3.6 to −7.7 Sv at 25°E) – a ~ 15% strengthening.

Atlantic Basin

At the northern extent of the South Atlantic subtropical gyre, the westward SEC bifurcates with most of its water entering the equatorward North Brazil Current (NBC)—responsible for large upper-ocean cross-equatorial heat transport³⁵. The remainder flows southward from ~ 10°S forming the relatively weak Brazil Current (BC). In the North Atlantic, the poleward flow, partly fed by the NBC, follows the western boundary of Central America as the Caribbean and Yucatan Currents ultimately emerging via the Florida Straits to form the GS. The GS breaks away from the coast at ~ 40°N, feeding the north-eastward North Atlantic Current.

BC transport estimates from observations range from −19 and −23 Sv between 36 and 38°S. CMIP6 models generally simulate the maximum BC transport between about 35-40°S with values ranging from −13.9 to −25.8 Sv, which lies in the very broad range of reanalysis transports (−8.7 to −32.5 Sv). The observed NBC transport (23–26 Sv) is slightly underestimated by the ensemble (19–22 Sv, 5-10°S) with even weaker estimates from reanalysis (11.5–18 Sv). The models simulate maximum (minimum) transport in July-Aug (April–May, Fig. 4) (observed NBC seasonality estimates are not available at the latitudes examined). The BC and NBC forms at ~ 10°S³⁷ just north of the basin-averaged zero wind-stress curl latitude. This bifurcation typically sits about 10° too far south in the CMIP models, in part related to a systematic southward bias in the model Atlantic wind field (Fig. 2, Figure S2).

In the Northern Hemisphere, the complex circulation of the Caribbean Sea and Gulf of Mexico is represented very differently across the coarse resolution models. Compared to observations, most models (and reanalysis) underestimate the LLWBC transport of the Yucatan Current (30 Sv) with a model range of 13.5–23.3 Sv (reanalysis: 8.5–25.6 Sv). The GS transport intensifies moving northwards (> 90 Sv) where it diverges from the coast, with the strongest transport occurring in boreal fall³⁸. This northward intensification is absent in the models and reanalysis: northward transports peaks at 38 to 42.6 Sv between about 28-33°N (reanalysis: 37.1–46.7 Sv). The simulated winter intensification of the GS is consistent from the western margin to the extension region (Figure S1). However, the models are generally out of phase with the observations that indicate maximum transports during summer at 26.5°N³⁹ and in the extension region³⁸. While the reanalysis seasonality matches the models along the coast, there is poor agreement in the extension region.

In the Southern Hemisphere, the BC is projected to intensify (4.2 to 6.0 Sv), especially south of 30ºS, associated with an increased northward basin interior transport²⁷. This intensification is consistent with intensified westerlies across the Indian Ocean basin (Figure S3, Figure S4), which can increase northward Ekman transport and intensify the Indian Ocean input to the Atlantic via Agulhas Leakage³¹.

Conversely, WBC transports weaken northwards of ~ 15°S. The cross-equatorial NBC flow is projected to weaken (−1.7 to −4.7 Sv). Similarly, the GS reduces at all latitudes with a −4.9 to −10.8 Sv (~ 15%) decrease around the GS maximum. These changes are poorly explained by surface wind changes and are likely associated with a weakened Atlantic Meridional Overturning Circulation (see below).

Pacific Basin

In the South Pacific, the broad westward SEC bifurcates at the Australian margin forming the poleward EAC and equatorward Gulf of Papua Current (GPC). The EAC partially separates from the coast near 30ºS forming the Tasman Front, which continues southward to the east of New Zealand as the East Auckland Current and East Cape Currents (ECC). The remaining EAC water feeds a series of eddies that move southwards, forming the EAC extension and Tasman Leakage that provides a high-latitude pathway of water to the Indian Basin. The northward flowing GPC feeds the NGCU that exits the Northern Solomon Sea via multiple straits providing water to the subsurface Equatorial Undercurrent (EUC)¹⁷. In the Northern Hemisphere, the MC also feeds the EUC and forms the primary source of the ITF that transports warm water into the Indian Ocean¹⁷. Further north, the Kuroshio Current (KC) extends northwards from ~ 15ºN along eastern Japan, where it eventually separates and continues eastward.

The observed EAC transport reaches about −22 Sv at 27°S⁴⁰, with maximum/minimum transport in austral winter/summer⁴¹. CMIP6 transports are generally similar in strength (−20.3 to −23.4 Sv) and seasonality (Figure S1) to observations. While the seasonality is similar for the reanalysis products, they tend to underestimate the transport (−7 to −17 Sv). The EAC extension transport (~ 7 Sv) and Tasman Leakage (~ 8 Sv) are, however, systematically underestimated in the models (−1.4 to −7 Sv and −0.9 to −4 Sv, respectively), with some models simulating an EAC extension with northward mean flow, related to a poor representation of regional winds²⁵.

In the Northern Hemisphere, the KC intensifies from about 15 Sv at 18°N to over 20 Sv between 25 and 30°N. The CMIP6 models systematically overestimate the transport with an interquartile range of 30.1 to 44.2 Sv, which encompasses the reanalysis estimates of 38.6 to 39 Sv. Observations suggest that KC strength is weakest during winter, to the east of Taiwan⁴² while models and reanalysis display minimum transports earlier in autumn (Figure S1). Further north (28°N) observed transport is minimum in autumn⁴³, while the models show no distinct seasonality. In the extension region, surface transport is weakest in winter/spring and strongest in summer/autumn⁴⁴; while the model transports tend to peak in spring.

At low latitudes, observed MC transport estimates varies considerably (15 to 35 Sv, Table S12). Model and reanalysis transports lie within these estimates (−18 to −25.7 Sv and −14.6 to −21.8 Sv, respectively). Observational estimates of NGCU transport decrease from 29 Sv at 12°S to ~ 20 Sv at 1–2°N, with a large seasonality that is strongest (weakest) in austral winter (summer). In agreement with observations, the CMIP6 NGCU transports between 5–10°S are 17.4 to 25.5 Sv, with a large seasonality that peaks from July–October. Reanalysis transports are generally weaker (6–20.7 Sv) with seasonality matching the climate models. The inter-model spread in ITF transport is small compared to most other currents −11.9 to  −13.4 Sv (Fig. 3). This is slightly underestimated compared to the observed transport (15 Sv)⁴⁵, with sub-1000 m transport accounting for ~ 0.5 Sv of this discrepancy. Flow strengths through the multiple ITF straits each have different seasonality, largely controlled by local monsoonal wind changes and remote oceanic forcing, resulting in a bimodal seasonality in the total ITF transport, peaking in January and July⁴⁵. In the models, which do not simulate realistic flow through multiple straits, there is a single annual maximum around July, with a much larger (~ 10 Sv) seasonal range compared to observations, but consistent with reanalysis (Figure S1).

While the EAC core shows no consistent future change, the EAC extension and Tasman Leakage project large intensifications: −4.6 to −7.0 Sv (35–40°S) and −4.3 to −7.4 Sv (at 146°E), respectively. Previous studies have shown a negative low-frequency relationship between the EAC extension and the Tasman Front⁴⁶. Consistent with this, most CMIP models project a weakening of the ECC that is fed by the Tasman Front. In the Northern Hemisphere, there is a projected weakening of the KC and Kuroshio extension across most models, but the changes are small relative to the mean transport.

In the tropics, both the GPC and NGCU project unanimous model intensifications: 0.6 to 2.8 Sv and 1.9 to 4.9 Sv, respectively. In contrast, the MC and the ITF (which the MC feeds) decrease in all models (2.3–5.6 Sv and 2.4–3.2 Sv, respectively). Similar LLWBC changes in the CMIP3 models were linked to projected basin-wide negative wind stress curl anomalies flanking the equator²⁶. These curl anomalies are also evident in the CMIP5 and CMIP6 models (Figure S3). Conversely, the ITF weakening in CMIP5 models²⁵ and in an eddy-permitting ocean projection²⁹ could not be explained by regional wind changes. Instead, these studies found that the changes are related to a slowdown in deep ocean waters entering the South Pacific.

For the majority of currents examined across the basins, there is no significant difference in ensemble mean historical transports between CMIP5 and CMIP6. Only the Tasman Leakage and SMC demonstrate significantly different MMM transports (Table S13). In the case of the Tasman Leakage the MMM flow reverses direction from weakly eastwards in CMIP5 to weakly westward in CMIP6. This constitutes an important regional improvement although 20% of models still have spurious eastward flow in CMIP6 (compared to 57% for CMIP5). Only the MMM projected change in the AC extension transport is significantly different between model ensembles, with the CMIP6 suggesting a 40% smaller intensification compared to CMIP5.

Seasonal changes

As described, many currents exhibit seasonal transport changes that are consistent across models (Figure S1). While seasonal timing is realistic for many currents, some simulated currents, for example the GS and Kuroshio system, poorly simulate the observed seasonal phase or amplitude. For some currents, comparison is hampered by uncertainties in the observed seasonality due to short observational records and large internal variability^37,47,48. Both basin wide and local winds are important in setting transport seasonality, although the influence of remote winds may be lagged due to the slow propagation of ocean waves. The seasonal phase of wind stress curl in the models is broadly similar to the ERA5 reanalysis, although large discrepancies are evident, particularly near the boundaries of regions with strong seasonality differences (Figure S5a-c). We note that CMIP transports that show poor agreement with the observations or reanalysis seasonality (e.g. GS and KC extensions, NBC) are often associated with substantial biases in wind stress curl seasonality in the regions extending eastwards of the WBCs (Figure S5c).

A subset of currents also exhibits consistent projected changes in transport seasonality across both model generations (Fig. 4). In the South Indian Ocean, most models project a reduced seasonal cycle for the NMC and SMC (Figure S1). Likewise, in the South Atlantic, the models consistently simulate a substantial weakening of NBC seasonality. In the South Pacific, both the EAC extension and Tasman Leakage show an amplification in seasonality. Conversely, east of New Zealand, the seasonality of the ECC is projected to decrease. In the North Pacific, the seasonality in meridional transport where the KC separates from the coast shows consistent increases (decreases) in boreal winter (summer).

These currents with modified seasonality generally occur at latitudes where the phase of the projected wind stress curl seasonality also show large projected changes (except for the MC where the changes are just upstream of the MC latitudes examined; Figure S5e). These projected changes in wind stress curl seasonality are zonally oriented, occurring at transition zones where the historical wind stress curl seasonality changes rapidly with latitude (Figure S5b), suggesting that the changes are associated with a poleward expansion of the wind fields, a well-established consequence of anthropogenic climate change¹⁰, and their associated seasonality (Figure S5f.).

Emergent constraints

For a subset of currents, there appears to be a significant inter-model relationship between historical and projected transports (Figure S6). These relationships may provide emergent constraints to narrow the uncertainty associated with the large spread in projections, although such constraints may be biased by common structural errors⁴⁹. For example, for the EAC extension and Tasman Leakage, models that underestimate mean transport or that have flow in the wrong direction tend to project the largest increases in southward or westward flow, respectively. Given observed EAC extension transports (~ 7 Sv, Table S12), a more moderate future change (~ 5 Sv) may therefore be more credible than the more extreme changes projected by some models. Similarly, given an observed mean transport of ~ 15 Sv, it is likely that the ITF decrease would be 3-4 Sv rather than the more extreme model estimates.

Connections to surface wind changes

Neglecting friction, non-linear processes and interactions with deep ocean circulation, the depth-integrated meridional transport away from the western boundaries can be related to gradients in the surface wind field via Sverdrup dynamics⁷. In particular, a positive (negative) wind stress curl drives northward (southward) flow in the ocean interior. WBCs provide a return flow for much of this meridional transport and a significant part of the inter-model differences in the historical mean WBC transport can be related to differences in interior transport (Table 1, Figure S7). The offset between the WBC-interior regression lines and the one-to-one line in Figure S7 for certain currents relate to inter-basin leakage or flow compensation in the deep ocean as part of the overturning circulation. For example, the ~ 10 Sv offsets for the EAC, EAC extension, GPC and NGCU result from a leakage of water via the ITF. The offset is ~ 5 Sv less than the ITF transport as there is also a net upwelling into the upper Pacific from below 1000 m. The offsets for the Atlantic basin currents, including the GS and NBC, result from the deep return flow below 1000 m.

For many currents intermodel difference in interior transports can be explained to some degree by differences in the surface wind field via Sverdrup dynamics (Table 1). As a result, up to 50% of the intermodel variance in WBC transports can be related to differences in the surface winds. Other factors, including different overturning rates, different inter-basin transports and non-linear dynamics must be invoked to explain the wide range of mean WBC transports.

Similarly, a significant fraction of intermodel projected WBC differences can be related to changes in surface wind stress curl, for most WBCs investigated (Table 1). In general, WBC whose mean differences are well explained by differences in their surface winds tend to be those whose projected transport differences are also well explained by differences in surface wind changes. The particularly poor relationship noted for the BC probably relates to the fact that the Sverdrup calculation becomes poorly defined as the eastern boundary lies at the southern tip of Africa. Other weak relationships in the Atlantic likely stem from large projected changes in the Atlantic overturning circulation⁵⁰. Indeed, projected NBC decreases in CMIP5 are largely compensated by a weakening of North Atlantic Deep Water transport²⁷.

Near-surface transport

WBCs affect the distribution of marine species via the dispersal of early-life stages and modulation of local thermal regimes^24,51. However, ecosystem impacts will be most sensitive to near-surface circulation changes within the euphotic zone where most marine life thrives. As such, we also examine WBC transport changes in the top 100 m of the water column.

For most currents examined, the change in the near-surface flow is of the same sign as the 1000 m integrated transport. An exception is the KC system, where the full-depth WBC is projected to weaken slightly along most of its length, while the near-surface flow is projected to intensify weakly north of 25°N (Figure S8, Figure S9). Previous work suggested that this intensification is associated with differences in warming rates across the KC, leading to an enhanced baroclinic flow⁵². As a tight connection between the state of the KC and the regional marine food webs has been documented⁵³, this surface intensification may have consequences for the ecosystem. In contrast, the full-depth MC, which is projected to weaken, typically intensifies near the surface south of 7°S.

In general, when the direction of a WBC is aligned with the warming signal (e.g. in the subtropics), a poleward intensified WBC will assist species dispersal at poleward range edges. In contrast, when the WBC flow opposes climate change velocities (e.g. in the tropics), strengthening would hinder dispersal at the poleward edges with greater propagule dispersal at the warming, equatorward edges²⁴. Weakened/strengthened WBCs are likely to directly modify larval transport and thermal regimes, affecting rates of poleward range shifts⁵¹. In addition, other more subtle changes such as WBC broadening or modified coastal retention or dispersal pathways may also impact marine life²⁴.

Source: Ecology - nature.com