Western Boundary Pathways Research Articles

Subtropical-tropical pathways of spiciness anomalies and their impact on equatorial Pacific temperature

Understanding mechanisms of tropical Pacific decadal variability (TPDV) is of high importance for differentiating between natural climate variability and human induced climate change as this region sustains strong global teleconnections. Here, we use an ocean general circulation model along with a Lagrangian tracer simulator to investigate the advection of density compensated temperature anomalies (“spiciness mechanism”) as a potential contributor to TPDV during the 1980–2016 period. Consistent with observations, we find the primary regions of spiciness generation in the eastern subtropics of each hemisphere. Our results indicate that 75 $$\%$$ of the equatorial subsurface water originates in the subtropics, of which two thirds come from the Southern hemisphere. We further show two prominent cases where remotely generated spiciness anomalies are advected to the equatorial Pacific, impacting subsurface temperature. The relative contribution of Northern versus Southern Hemisphere prominence and/or interior versus western boundary pathways depends on the specific event. The anomalously warm case largely results from advection via the Southern hemisphere interior (65 $$\%$$ ), while the anomalously cold case largely results from advection via the Northern hemisphere western boundary (48 $$\%$$ ). The relatively slow travel times from the subtropics to the equator (> 4 years) suggests that these spiciness anomalies underpin a potentially predictable contribution to TPDV. However, not all decadal peaks in equatorial spiciness can be explained by remotely generated spiciness anomalies. In those cases, we propose that spiciness anomalies are generated in the equatorial zone through changes in the proportion of Northern/Southern hemisphere source waters due to their different mean spiciness distribution.

Origins and Pathways of the Pacific Equatorial Undercurrent Identified by a Simulated Adjoint Tracer

AbstractThe origins and pathways of the Pacific Equatorial Undercurrent (EUC) are investigated using a simulated adjoint tracer of the consortium Estimating the Circulation and Climate of the Ocean. The main source waters of the EUC, as well as their transit time and contributions, are identified. The zonal variability of the EUC water composition and the contributions from different pathways are also estimated. Results show that the ratio of the EUC water coming from the tropics to that from extratropics is relatively stable (1 vs. 4) along the equator, except in the western Pacific where extratropical water is more dominant. The main body of extratropical water within the EUC is transported through the western boundary pathways, while the percent transported via the interior pathways gradually increases toward the east. Tropical water merges into the EUC mainly through the tropical cells with a larger portion through the northern side of the equator.

The fate of the Deep Western Boundary Current in the South Atlantic

The pathways of recently ventilated North Atlantic Deep Water (NADW) are part of the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). In the South Atlantic these pathways have been the subject of discussion for years, mostly due to the lack of observations. Knowledge of the pathways of the AMOC in the South Atlantic is a first order prerequisite for understanding the fluxes of climatically important properties. In this paper, historical and new observations, including hydrographic and oxygen sections, Argo data, and chlorofluorocarbons (CFCs), are examined together with two different analyzes of the Ocean general circulation model For the Earth Simulator (OFES) to trace the pathway of the recently ventilated NADW through the South Atlantic. CLIVAR-era CFCs, oxygen and salinity clearly show that the strongest NADW pathway in the South Atlantic is along the western boundary (similar to the North Atlantic). In addition to the western boundary pathway, tracers show an eastward spreading of NADW between ~17 and 25°S. Analyzed together with the results of earlier studies, the observations and model output presented here indicate that after crossing the equator, the Deep Western Boundary Current (DWBC) transports water with the characteristics of NADW and a total volume transport of approximately 14Sv (1Sv=106m3s−1). It crosses 5°S as a narrow western boundary current and becomes dominated by eddies further south. When this very energetic eddying flow reaches the Vitória-Trindade Ridge (~20°S), the flow follows two different pathways. The main portion of the NADW flow continues along the continental shelf of South America in the form of a strong reformed DWBC, while a smaller portion, about 22% of the initial transport, flows towards the interior of the basin.

Thermohaline intrusions in the thermocline of the western tropical Pacific Ocean

The existing high-resolution hydrographic data in the western tropical Pacific Ocean are used to explore the spatial distribution and primary characteristics of thermohaline intrusions in the thermocline. Statistics show that the vertical scales of intrusions are 20-40 m in the upper thermocline (22.0-26.0 sigma(0)) and 40-80 m in the lower thermocline (26.0-27.2 sigma(0)). In the upper thermocline, the most intensive intrusions exist at the equatorial front (EF) where north/south Pacific water masses converge, and the westward spreading of the north Pacific tropical water (NPTW) in the Philippines Sea also produces patches of intrusions surrounding its high-salinity tongue. In the lower thermocline, intrusions are also strong at the tropical front (TF) which is the boundary between the north Pacific subtropical/tropical waters. At the bottom of the thermocline (at about 27.0 sigma(0)), intrusions mainly exist near the western boundary, which are produced by intermediate water convergence through the advection of subthermocline western boundary flows. Most strikingly a "C"-shape distribution of intrusions at around 26.4 sigma(0) is revealed, covering the vicinity of the EF, the TF, and the Mindanao Current (MC), i.e., the western boundary pathway of the north Pacific subtropical cell (STC). Synoptic section analysis reveals that intrusions are more prominent on the warm/salty flank of the fronts, implying more cross-front tongues of cold/fresh water. Among the intrusions, those at the EF are of best lateral coherence which implies a unique driving mechanism involving near-inertial velocity perturbations near the equator.

Pacific Shallow Meridional Overturning Circulation in a Warming Climate

Abstract By analyzing a set of the Coupled Model Intercomparison Project phase 3 (CMIP3) climate model projections of the twenty-first century, it is found that the shallow meridional overturning of the Pacific subtropical cells (STCs) show contrasting trends between two hemispheres in a warming climate. The strength of STCs and equivalently the STC surface-layer transport tend to be weakening (strengthening) in the Northern (Southern) Hemisphere as a response to large-scale surface wind changes over the tropical Pacific. The STC pycnocline transport convergence into the equatorial Pacific Ocean from higher latitudes shows a robust weakening in the twenty-first century. This weakening is mainly through interior pathways consistent with the relaxation of the zonal pycnocline tilt, whereas the transport change through western boundary pathways is small and not consistent across models. It is found that the change of the western boundary pycnocline transport is strongly affected by the shoaling of the pycnocline base. In addition, there is a robust weakening of the Indonesian Throughflow (ITF) transport in a warming climate. In the multimodel ensemble mean, the response to greenhouse warming of the upper-ocean mass balance associated with the STCs is such that the weakening of the equatorward pycnocline transport convergence is balanced by a weakening of the poleward surface-layer transport divergence and the ITF transport of similar amounts.

Anti‐correlated variability in subduction rate of the western and eastern North Pacific Oceans identified by an eddy‐resolving ocean GCM

Analysis of results from an eddy‐resolving ocean general circulation model has revealed the existence of an anti‐correlated variability in subduction rate of the western and eastern North Pacific Oceans, consistent with the leading mode of Pacific Decadal Oscillation (PDO). A composite analysis shows that during the positive phase of PDO, the winter mixed layer depth (MLD) generally gets deeper in the west and shallower in the east, being closely related to the variability in surface heat flux stemming from an enhanced Aleutian Low, and the situation during the negative phase of PDO is reversed. Changes in MLD are primarily responsible for the anti‐correlated variability in subduction rate, though changes in subtropical gyre circulation also play a role. The result also provides useful evidence for the interior and western boundary pathways, through which variability in eastern subtropical mode water can be conveyed into the equatorial region.

Response of Pacific subtropical‐tropical thermocline water pathways and transports to global warming

Global warming may change the thermocline water pathways and transports from the subtropics to the tropics in the Pacific Ocean, which are known to have profound implications for the El Niño‐Southern Oscillation (ENSO) and thereby global climate. This study investigates the changes by comparing solutions between a present‐day climate and a future, warmer climate from a set of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models. As the climate warms, although the total transport from the subtropics to the tropics exhibits no significant change, transport via western boundary pathways increases and via interior pathways decreases. This shift is due to high potential vorticity (PV) zones that extend further westward, thus dynamically guiding thermocline water away from interior pathways to prefer western boundary pathways from the subtropics to the tropics. Additionally, a warmer climate induces a large temperature increase near the sea surface in the eastern tropics and a significantly enhanced Equatorial Undercurrent (EUC) in the western and central Pacific; the former is related to the decreased transport through interior pathways and the latter is linked to the increased transport through western boundary pathways. Implications of the results of this study are also discussed.

Decadal Variability of Pycnocline Flows from the Subtropical to the Equatorial Pacific*

Abstract A method based on isopycnal trajectory analysis is proposed to quantify the pathways from the subtropics to the Tropics. For a continuous stratified ocean a virtual streamfunction is defined, which can be used to characterize these pathways. This method is applied to the climatological dataset produced from a data-assimilated model. Analysis indicates that in each layer contours of the virtual streamfunction are a good approximation for streamlines, even if there is a cross-isopycnal mass flux. The zonal-integrated meridional transport per unit layer thickness through each pathway varies in proportion to 1/sinθ, where θ is latitude. The vertical-integrated total transport through pathways behaves similarly. Transport through pathways has a prominent decadal variability. Results suggest that in decadal time scales the interior pathway transport (IPT) anomaly may be mainly caused by the wind stress anomaly at low latitude. The western boundary pathway transport (WBPT) anomaly often has a sign opposite to the IPT anomaly, reflecting compensation between the IPT and the WBPT. However, more often than not the wind stress anomaly within tropical latitudes can also be used to explain the WBPT anomaly.

The Origin, Pathway, and Destination of Niño-3 Water Estimated by a Simulated Passive Tracer and Its Adjoint

Abstract The nature of subtropical–tropical water mass exchange in the Pacific Ocean is investigated, focusing on the origin, pathway, and destination of water occupying the surface layer of the eastern equatorial Pacific Ocean (Niño-3 region; 5°S–5°N, 150°–90°W). Simulated passive tracers and their adjoint are employed to explicitly follow the circulation of specific water masses accounting for advective and diffusive effects and their time variabilities. The evolution of the forward passive tracer and adjoint passive tracer can be identified as describing where the tracer-tagged water mass goes and from where it comes, respectively. Over 10 years on average, water mass of the Niño-3 region can be traced back to eastern subtropical thermocline waters of the Northern (27%) and Southern Hemispheres (39%). The Niño-3 water subsequently returns to these subtropical latitudes in the upper ocean. In contrast to the hypothesized “subtropical cell,” however, this circulation is an open circuit with water returning to the western regions of the two hemispheres (subtropical gyres) and to the Indian Ocean, instead of returning to its origins. The representative transit time scale from the subtropics to the Tropics is 10–15 yr. Temporal variability causes the tropical circulation inferred from a time-mean state to differ significantly from the average circulation. In particular, stirring due to nonseasonal, intra-annual variability significantly enhances the transport magnitude of the so-called interior pathways relative to that of the circuitous low-latitude western boundary pathways. Such short-circuit in the subtropical–tropical exchange may help better to explain tracer distributions, such as the observed midbasin tritium maximum in the equatorial Pacific Ocean. Significant differences in circulation pathways are also identified that are associated with El Niño and La Niña events. The strength of the subtropical–tropical water mass exchange is estimated to have weakened during the 1990s.

Observational Evidence for Flow between the Subtropical and Tropical Atlantic: The Atlantic Subtropical Cells*

Abstract This study determines the mean pathways and volume transports in the pycnocline and surface layer for water flowing between the subtropical and tropical Atlantic Ocean, using potential vorticity, salinity, geostrophic flow maps on isopycnal surfaces, and surface drifter velocities. In both hemispheres, subducted salinity maximum waters flow into the Tropics in the pycnocline along both interior and western boundary pathways. The North Atlantic ventilating trajectories are confined to densities between about 23.2 and 26.0 σθ, and only about 2 Sv (Sv ≡ 106 m3 s–1) of water reaches the Tropics through the interior pathway, whereas the western boundary contributes about 3 Sv to the equatorward thermocline flow. Flow on shallower surfaces of this density range originates from the central Atlantic near 40°W between 12° and 16°N whereas flow on the deeper surfaces originates from near 20°W just off the coast of Africa at higher latitudes. The pathways skirt around the potential vorticity barrier located under the intertropical convergence zone and reach their westernmost location at about 10°N. In the South Atlantic, about 10 Sv of thermocline water reaches the equator through the combination of interior (4 Sv) and western boundary (6 Sv) routes in a slightly higher density range than in the North Atlantic. Similar to the North Atlantic, the shallower layers originate in the central part of the basin (along 10°–30°W at 10°–15°S) and the deeper layers originate at higher latitudes from the eastern part of the basin. However, the ventilation pathways are spread over a much wider interior window in the Southern Hemisphere than in the Northern Hemisphere that at 6°S extends from 10°W to the western boundary. The equatorward convergent flows in the thermocline upwell into the surface layer and return to the subtropics through surface poleward divergence. As much as 70% of the tropical Atlantic upwelling into the surface layer is associated with these subtropical circulation cells, with the remainder contributed by the warm return flow of the large-scale thermohaline overturning circulation.