Sverdrup Transport Research Articles

Equatorial Dynamics Significantly Weakened the Southward Somali Current Circulation during 1997–98

Abstract Based on reanalysis data and numerical model, this study documented an unusually weak southward Somali Current (SSC) circulation, an anticlockwise circulation in the western Arabian Sea, during the winter of 1997 to spring 1998, throughout the two-decade period from 1994 to 2017. Theoretical analysis and model simulations suggest that the exceptionally weak SSC circulation was primarily caused by abnormal wind forcing around the equator, associated with extreme climate events. The strong equatorial easterly anomalies are mainly responsible for the weakening of the SSC circulation north of ∼6°N and contribute to about half of the weakening south of ∼6°N, by regulating the layer thickness along the eastern boundary, following the time-dependent Sverdrup relation. The large negative wind stress curl anomalies near the equator contribute to another half of the weakening south of ∼6°N. This research underscores the dynamic connection between the circulation in the western basin and equatorial dynamics during extreme climate modes. Significance Statement The dynamic linkage between the western basin circulation and the equatorial forcing remains elusive. Based on reanalysis data, theoretical analysis, and numerical simulations, this study has confirmed that the exceptionally weak southward Somali Current circulation during the winter of 1997 to spring 1998 was significantly influenced by abnormal wind patterns around the equator, triggered by extreme climate modes.

Influence of rossby wave in southern Indian Ocean on the low frequency variability of eddy kinetic energy within agulhas current system

The basin-scale wave propagation in the Southern Indian Ocean (SIO) is inferred to affect the low-frequency variability of eddy kinetic energy (EKE) within the Agulhas current system (ACS). Observations and numerical simulations indicate that westward-propagating waves, in the form of sea level variability from the east boundary, may cause “non-linear” oceanic responses (e.g., sea level and EKE) in the ACS. This is likely due to two factors: (i) the movement of wave crests and troughs continuously alters the stratification of the upper ocean, generating a positive baroclinic instability and prompting conversions of EKE from mean flow in the source regions, and (ii) wave-current interactions induce a rapid intension of an anticyclone gyre in the interior SIO and the corresponding EKE in the downstream regions. These basin-scale wave modulations may provide an additional perspective to the traditional wind driving view based on the barotropic Sverdrup relation. The local winds over the SIO play an important role in maintaining basin-scale Rossby waves in the southwest Indian Ocean and further modulating oceanic low-frequency variability in a “quasi-linear” way.

Contribution of Deep Vertical Velocity to Deficiency of Sverdrup Transport in the Low-Latitude North Pacific

Abstract Deep vertical velocity is a critical factor causing deficiencies in Sverdrup theory. However, few studies have focused on its influence in the low-latitude western Pacific. Through multiple analyses of observational, reanalysis, and simulation data, this study explored the contribution of deep nonzero vertical velocity to the Sverdrup transport inaccuracy in the low-latitude North Pacific. The vertical velocities inducing relatively small non-Sverdrup transport exist within 1500–2500 m, which exhibit similar patterns with opposite values to the south and north of 13°N. The zonally integrated meridional volume transport associated with these vertical velocities displays nonnegligible dipolar zonal bands west of approximately 150°W. The positive and negative transport bands, centered at 11° and 17°N, can reach an amplitude of approximately 8.0 Sv (1 Sv ≡ 106 m3 s−1) when integrated from the eastern boundary to 140°E. On average, such integrated meridional transport makes up roughly half of the prominent Sverdrup transport discrepancies in the central-western Pacific. Further investigation indicated that the spatial pattern of these vertical velocities is modulated by ocean topography and deep meridional currents. Moreover, a near-global test suggested that the meridional non-Sverdrup transport related to deep vertical velocity is widespread and undergoes remarkable multidecadal variation. This study reveals the disruptive role of deep vertical velocity in disturbing the Sverdrup balance and emphasizes the consideration of its long-term variation when diagnosing wind-driven circulation changes using Sverdrup theory.

Surface Heating Steers Planetary-Scale Ocean Circulation

Abstract Gyres are central features of large-scale ocean circulation and are involved in transporting tracers such as heat, nutrients, and carbon dioxide within and across ocean basins. Traditionally, the gyre circulation is thought to be driven by surface winds and quantified via Sverdrup balance, but it has been proposed that surface buoyancy fluxes may also contribute to gyre forcing. Through a series of eddy-permitting global ocean model simulations with perturbed surface forcing, the relative contribution of wind stress and surface heat flux forcing to the large-scale ocean circulation is investigated, focusing on the subtropical gyres. In addition to gyre strength being linearly proportional to wind stress, it is shown that the gyre circulation is strongly impacted by variations in the surface heat flux (specifically, its meridional gradient) through a rearrangement of the ocean’s buoyancy structure. On shorter time scales (∼10 years), the gyre circulation anomalies are proportional to the magnitude of the surface heat flux gradient perturbation, with up to ∼0.15 Sv (1 Sv ≡ 106 m3 s−1) anomaly induced per watt per square meter change in the surface heat flux. On time scales longer than a decade, the gyre response to surface buoyancy flux gradient perturbations becomes nonlinear as ocean circulation anomalies feed back onto the buoyancy structure induced by the surface buoyancy fluxes. These interactions complicate the development of a buoyancy-driven theory for the gyres to complement the Sverdrup relation. The flux-forced simulations underscore the importance of surface buoyancy forcing in steering the large-scale ocean circulation. Significance Statement Ocean gyres are large swirling circulation features that redistribute heat across ocean basins. It is commonly believed that surface winds are the sole driver of ocean gyres, but recent literature suggests that other mechanisms could also be influential. We perform a series of numerical simulations in which we artificially change either the winds or the heating at the ocean’s surface and investigate how each factor independently affects the ocean gyres. We find that gyres are steered by both winds and surface heating, and that the ocean circulation responds differently to heating on short and long time scales. In addition, the circulation depends on where the heating is applied at the ocean’s surface. Through these simulations, we argue that a complete theory about ocean gyres must consider heating at the ocean’s surface as a possible driver, in addition to the winds.

Local Topographic Rossby Modes Observed in the Abyssal Sea of Japan

Abstract The short-period current fluctuations (topographic wave fluctuations, TWFs) on the southern rim slope of the abyssal Sea of Japan were investigated using current meter datasets from closely spaced mooring arrays. The TWFs occurred almost continuously throughout the year with short periods in a narrow band (1.5–5 days), showing a seasonal modulation in their amplitude. The TWFs were attributable to alternate passage of cyclonic and anticyclonic eddies on the rim slope, which propagated eastward at a speed of 0.15–0.23 m s−1. In addition, the TWFs showed a bottom-intensified characteristic, along with the two-layer structure consisting of an almost barotropic lower layer and a marginally baroclinic upper layer. The lowest topographic Rossby mode, which is a normal mode of the topographic Rossby waves prescribed by the two ridges on the rim slope, was considered as a cause of the TWFs because of its eastward-propagating eddy train structure along the rim slope and the eigenperiod (3–5 days) near the TWF band. In addition, the local time-dependent Sverdrup balance was considered as a mechanism of the TWF generation, since the TWFs significantly correlated with the wind stress curl variations over the observation area with time lags. That is, the current fluctuations near the eigenperiod were selectively amplified via the resonance between the lowest topographic Rossby mode and the Ekman pumping variations induced by the TWF-band wind stress curl. We concluded that the observed TWFs were a manifestation of the wind-induced lowest topographic Rossby mode prescribed by the bottom topography. Significance Statement The dispersion relation teaches us that short-period (<10 days) Rossby waves have a very long wavelength (>103 km). However, as atmospheric forcing with both such period and wavelength is absent, the short-period Rossby waves excited by a local forcing generally dissipate quickly in a limited area. Nevertheless, we observed short-period (1.5–5 days) current fluctuations occurring continuously throughout the year in the abyssal (>1000 m) Sea of Japan. The deep current fluctuations were attributable to the propagation of cyclonic and anticyclonic eddy trains on the zonally extended slope. This is the wind-induced lowest topographic Rossby normal mode prescribed by the bottom topography. This study suggests that short-period current fluctuations can occur everywhere if appropriate topographic and atmospheric conditions were established.

Rossby Waves and Zonal Flux Anomalies in Hadley and Ferrell Cell Analogs of the General Atmospheric Circulation: Model and Experiments

The circulation of the form of axysimmetrically flow with induced anomalies (disturbances) in closed rotating annular channel with conical bottom is generated by sources-sinks and MHD-method. The reducing of external forcing affects the changes of the anticyclones translation speed, but not the cyclones. The great part of moving anticyclones can disappear or stoped or new quasistationary cyclones may realized, whereas there are no change in the flow pattern in the sector where external anomalies take a place. But the variations in averaged parameters of the flow vortex are distinguished for the entire channel area and for their part. These anomalies physically are the weakness of subtropical Hadley cell with diminishing of passats in some sector of subequatorial atmospheric circulation and also the weakness the westerlies in the middle latitudes. The superposition of blocked and transported vortices are investigated with the help of simple analytical resonance interaction model for transient (with the maximum of velocity) modes in the shear flow. The amplitude of stationary background and the famous Sverdrup relation of the free surface streamfunction in the west oceanic shore intensification are affected in the same way from the beta-effect.

Topography Effects on the Seasonal Variability of Ocean Bottom Pressure in the North Pacific Ocean

Abstract Ocean bottom pressure pB is an important oceanic variable that is dynamically related to the abyssal ocean circulation through geostrophy. In this study we examine the seasonal pB variability in the North Pacific Ocean by analyzing satellite gravimetric observations from the GRACE program and a data-assimilated ocean-state estimate from ECCOv4. The seasonal pB variability is characterized by alternations of low and high anomalies among three regions, the subpolar and subtropical basins as well as the equatorial region. A linear two-layer wind-driven model is used to examine forcing mechanisms and topographic effects on seasonal pB variations. The model control run, which uses a realistic topography, is able to simulate a basinwide seasonal pB variability that is remarkably similar to that from GRACE and ECCOv4. Since the model is driven by wind stress alone, the good model–data agreement indicates that wind stress is the leading forcing for seasonal changes in pB. An additional model simulation was conducted by setting the water depth uniformly at 5000 m. The magnitude of seasonal pB anomaly is amplified significantly in the flat-bottom simulation as compared with that in the control run. The difference can be explained in terms of the topographic Sverdrup balance. In addition, the spatial pattern of the seasonal pB variability is also profoundly affected by topography especially on continental margins, ridges, and trenches. Such differences are due to topographic effects on the propagation pathways of Rossby waves.

Topographical and Thermal Forcing in Favorable Circulation Pattern to Early Spring Precipitation over the Southeastern Tibetan Plateau

During the boreal spring (March–May), the precipitation that occurs from March over the southeastern Tibetan Plateau (TP) can account for 20–40% of the total annual amount. The origin of this phenomenon has not been clearly understood from a climatological perspective. In this study, the role of topographical and thermal forcing on the precipitation over the southeastern TP in early spring (March) was investigated through sensitivity numerical simulations based on general circulation model. The simulated results show the favorable circulation and static stability to early spring precipitation over the southeastern TP when the model is simultaneously forced by realistic topography, zonal symmetric radiative equilibrium temperature, and diabatic heating over the TP and its surrounding areas. The quasi-stationary wave pattern over the Eurasian continent forced by realistic and TP topographical forcing leads to prolonged low pressure and intensified zonal winds over the southeastern TP due to quasi-steady wave activities. Thermal forcing experiments reveals that sensible heating over the southeastern TP not only strengthens the cyclonic circulation, ascending motion and statically unstable over the southeastern TP through thermal adaptation and the Sverdrup balance, but also triggers an anticyclone at upper tropospheric level extending from north of the Bay of Bengal to the eastern TP, which further favors precipitation over the southeastern TP. This work will provide useful background information for spring climate prediction over the TP.

A Time-Dependent Sverdrup Relation and Its Application to the Indian Ocean

Abstract The Sverdrup relation is the backbone of wind-driven circulation theory; it is a simple relation between the meridional transport of the wind-driven circulation in the upper ocean and the wind stress curl. However, the relation is valid for steady circulation only. In this study, a time-dependent Sverdrup relation is postulated, in which the meridional transport in a time-dependent circulation is the sum of the local wind stress curl term and a time-delayed term representing the effect of the eastern boundary condition. As an example, this time-dependent Sverdrup relation is evaluated through its application to the equatorial circulation in the Indian Ocean, using reanalysis data and a reduced gravity model. Close examination reveals that the southward Somali Current occurring during boreal winter is due to the combination of the local wind stress curl in the Arabian Sea and delayed signals representing the time change of layer thickness at the eastern boundary. Significance Statement Sverdrup balance dictates the law of meridional transport of steady circulation in the upper ocean, and has been one of the foundations upon which our understanding of ocean circulation is built. However, for circulation forced by time-varying wind stress, with annual, interannual and decadal frequency, the governing law remains elusive. In this study, we introduce a time-dependent Sverdrup relation applicable to time-dependent wind-driven circulation. As an example, this relation is used to diagnose the monsoon-driven circulation in the Indian Ocean.

On the Role of Eddy Mixing in the Subtropical Ocean Circulation

Most of the classic wind-driven circulation theories based on Sverdrup balance have neglected the profound influence of eddy mixing on the large-scale potential vorticity (PV) distribution, thus failing to explain some prominent features of the observed circulation. In this study, using a series of numerical experiments based on the MITgcm, we diagnose the PV balance to quantify the effect of eddy mixing on the subtropical gyre. Four horizontal grid-spacings of 1°, 1/3.2°, 1/10°, and 1/32° are selected to compare the structure of the upper-ocean circulation to examine the effect of different eddy influences. In the 1° grid case, the thermocline structure is as predicted by the Sverdrup dynamics, with its maximum depth located in the subtropical interior where the wind stress curl is strongest. With increasing resolution, however, this maximum depth is displaced towards the subtropical front, which more closely resembles the observed thermocline. From 1° to 1/32°, the enhanced eddy mixing tends to homogenize the macroscopic PV in the subtropical gyre and reduces the meridional PV range by about 75% of the non-eddy (1°) solution; and the region where Sverdrup balance holds is relegated to isolated patches, with its area reduced by about 60%. Furthermore, sensitivity experiments show that the observed thermocline structure is well reproduced in eddy-resolving runs, indicating that the PV mixing provides a reasonable explanation of the subtropical circulation. Our results suggest that the Sverdrup relationship should be treated carefully in the eddy-rich region, even in the subtropical interior.

Seasonal variation of deep vertical velocity in the South China Sea

On the basis of the Simple Ocean Data Assimilation (SODA) and the Ocean General Circulation Model for the Earth Simulator (OFES), it is surprised that a distinct seasonal variation is found in the deep vertical velocity fields of the South China Sea (SCS). In summer, an upwelling band extending from the northeast to the southwest across the SCS deep basin is mainly located in the interior region of the basin, surrounded by the downwelling areas situated in the marginal region of the basin. In winter, the vertical motion is almost completely reversed, which is also enhanced in its strength and transport. A more extended downwelling band in winter replaces the upwelling band in summer, and the upwelling occurs in the marginal region. The upwelling or downwelling in the interior region of the basin is compensated by the reverse flow near the marginal basin in summer or winter, maintaining the balance of the whole deep basin without water exchange. The seasonal structures of potential density and potential temperature are consistent with the seasonal variation of the SCS deep vertical velocity. In the interior region, their isolines uplift in summer and drop in winter. The seasonal variation of the SCS deep vertical velocity in the interior region also has a good correspondence with that of basin-scale horizontal circulation, the seasonal pattern of which is cyclonic in summer and anticyclonic in winter. Just as the vertical transports always maintain a balanced, the vorticity averaged over the entire basin always remains positive, and the values in winter are one order of magnitude smaller than that in summer. This correspondence can be explained by the Sverdrup relation. The mechanism of the seasonal variation of the SCS deep vertical velocity may be related to the seasonal deepwater overflow through the Luzon Strait.

Baroclinic instability and large‐scale wave propagation in a planetary‐scale atmosphere

AbstractMidlatitude atmospheric variability is dominated by the dynamics of the baroclinically unstable jet stream, which meanders and sheds eddies at the scale of the Rossby deformation radius. The eddies interact with each other and with the jet, affecting the variability on a wide range of scales, but the mechanisms of planetary‐scale fluctuations of the jet are not well understood. Here, we develop a theoretical framework to explore the stability of planetary‐scale motions in an idealized two‐layer model of the atmosphere. The model is based on a combination of vertical shear and the Sverdrup relation, providing the dynamic link between the two layers, with meridional eddy heat fluxes parameterized as a diffusive process with the memory of past baroclinicity of the jet. We find that a planetary‐scale instability exists if the vertical shear of the jet does not exceed a particular threshold. The inclusion of the eddy‐memory effect enables westward or eastward propagation of planetary waves relative to the barotropic mean flow. Importantly, we find growing planetary waves that propagate slowly westward or are stationary, which could have important implications for the formation of atmospheric blocking events. Our theoretical results suggest that, with ongoing polar amplification due to global warming and the corresponding reduction of the vertical shear of the mean wind, the background conditions for the growth of planetary‐scale waves via planetary‐scale baroclinic instability are becoming more favorable.

The impact of wind corrections and ocean-current influence on wind stress forcing on the modeling of Pacific North Equatorial Countercurrent

Bias correction of reanalysis-based wind stress using scatterometer derived equivalent neutral wind has been a common practice in producing the forcing datasets used in recent global ocean model intercomparisons (OMIPs). Here we systematically evaluate the effect of this wind correction procedure on the simulation of the Pacific North Equatorial Countercurrent (NECC) with multiple sets of model experiments. The weak NECC evident in earlier OMIPs employing the Coordinated Ocean-ice Reference Experiments (COREs) forcing dataset persists with the new JRA55-do (Japanese 55-year Reanalysis) forcing dataset. Two factors appear to significantly affect the Pacific NECC in forced ocean simulations: i) the bias correction procedure using QuikSCAT derived winds and ii) whether or not the ocean current is considered in the bulk formula. In the forced ocean simulations, the QuikSCAT correction weakens the averaged NECC transports by about 60%. Taking the ocean currents into account in the bulk formula may weaken the averaged NECC transports by about 26%–30%. Under the current OMIP protocol the above two procedures are used together to force the ocean model resulting in a double-counting of ocean surface current feedback on wind stress because the QuikSCAT estimates the equivalent 10-m neutral winds relative to surface current. We further systematically verify and investigate the impacts of this double-counting of the ocean surface currents on the modeled Pacific NECC using offline linear Sverdrup transport analysis, in which the observational data of vector wind and surface current are used to calculate the surface wind stress. It shows that including the ocean current in the bulk formula may reduce the zonal Sverdrup transport (ZST) by about 6.6 Sv (33%) and the further double-counting of the ocean current leads to an additional reduction of 6.4 Sv (48%). Next, using “perfect model” experiments with output from a coupled ocean–atmosphere model we further identify that the double-counting of current feedback in the bulk formula results in approximately 21% weakened volume transport. The built-in nonlinear processes in the model, such as the advection and friction terms, may partly damp the reduction due to the double-counting of the ocean current. However, the double-counting bias can only explain 26%–30% of the Pacific NECC simulation bias and the other part of the bias, around 30%, caused by the correction with QuikSCAT has not been explained. We speculate that this part may be explained by the retrieval biases in QuikSCAT wind data and the use of annual mean climatological wind adjustment factors.

On the role of domain aspect ratio in the westward intensification of wind-driven surface ocean circulation

Abstract. The two seminal studies on westward intensification, carried out by Stommel and Munk over 70 years ago, are revisited to elucidate the role of the domain aspect ratio (i.e., meridional to zonal extents of the basin) in determining the transport of the western boundary current (WBC). We examine the general mathematical properties of the two models by transforming them to differential problems that contain only two parameters – the domain aspect ratio and the non-dimensional damping (viscous) coefficient. Explicit analytical expressions are obtained from solutions of the non-dimensional vorticity equations and verified by long-term numerical simulations of the corresponding time-dependent equations. The analytical expressions as well as the simulations imply that in Stommel's model both the domain aspect ratio and the damping parameter contribute to the non-dimensional transport of the WBC. However, the transport increases as a cubic power of the aspect ratio and decreases linearly with the damping coefficient. On the other hand, in Munk's model the WBC's transport increases linearly with the domain aspect ratio, while the damping coefficient plays a minor role only. This finding is employed to explain the weak WBC in the South Pacific. The decrease in transport of the WBC for small-domain aspect ratio results from the decrease in Sverdrup transport in the basin's interior because the meridional shear of the zonal velocity cannot be neglected as an additional vorticity term.

Impact of the Benguela coastal low-level jet on the southeast tropical Atlantic SST bias in a regional ocean model

The current generation of coupled ocean–atmosphere climate models, which are widely used for studying the dynamics of the climate system and to make projections of future climate, suffers from erroneously warm (exceeding 5 °C) sea surface temperature (SST) simulation in the eastern boundary upwelling systems (EBUS) in the Atlantic and Pacific oceans. While recent improvements in the horizontal resolution of coupled model components helped to alleviate this issue in the North and South Pacific and North Atlantic oceans, the warm bias in the Southeast Tropical Atlantic (SETA) region still remains above 5 °C. Recent studies have highlighted the importance of inaccuracies in the near-coastal winds, especially in the Benguela low-level coastal jet (BLLCJ), in the formation of this warm SST bias. In addition, unique ocean circulation features like the poleward flowing coastal current and the presence of a strong coastal SST front were argued to contribute to the complexity in simulating realistic SST in the SETA region. In this study, we investigate how ocean model resolution and the spatial structure and strength of the BLLCJ affect the SST warm bias in the SETA. We conducted a suite of ocean model experiments with varying horizontal resolutions (81–3 km) and different atmospheric forcing products. We found that the accuracy in the magnitude of BLLCJ’s alongshore component and in the spatial structure of BLLCJ are the primary factors in reducing the warm SST bias in SETA and the ocean model horizontal resolution is of only secondary importance. A weaker alongshore component of the BLLCJ leads to a weaker upwelling through Ekman transport and fails to generate a downwind equatorward surface jet in the ocean. When the core of the BLLCJ is too far offshore, the broad wind stress curl zone gives rise to anomalous poleward depth integrated flow through Sverdrup balance and weakens the wind stress curl induced Ekman pumping near the coast. Both the presence of a poleward current, which can transport warm equatorial waters farther south, and the absence of the equatorward surface jet contributes to the warm SST bias. Heat budget analysis shows that the contribution from the anomalous poleward flow is a bigger contributor to the warm bias than the weak upwelling. With more realistic, high-resolution winds, an ocean model resolution increase from 27 to 9 km slightly reduces the extent and magnitude of SST bias. However, no such improvement occurs with coarse resolution less realistic winds. Sensitivity experiments using atmospheric forcing fields from different sources confirm that the major contribution to the warm SST bias is in fact from the erroneous winds rather than other atmospheric forcing fields like net shortwave and downward longwave radiation. Further studies are needed to understand whether eddy heat advection or mean flow heat advection primarily cause the warm SST bias.

Summertime eastward jet and its relationship with western boundary current in the South China Sea on the interannual scale

The western boundary current (WBC) in the South China Sea (SCS) flows northward in summer, closely following the Vietnam coast, and splits near 11.2° N into one eastward offshore branch named summertime eastward jet (SEJ). SEJ is significant between 110–112° E, and gradually becomes wider, shallower, weaker and warmer toward eastward until it disappears after 113.5° E. Based on the monthly mean Simple Ocean Data Assimilation (SODA) data, SEJ and its relationship with WBC on the interannual scale are studied. At the cross section 111.2° E, the current axis of SEJ (YSEJ) swings in the north–south direction with climatological value of 11.7° N and standard deviation of 0.9°, and the standard deviation of surface intensity and volume transport of SEJ (SISEJ and VTSEJ) is about 50% and 88% of the climatological value 28 cm s−1 and 4.6 Sv, respectively. In El Nino developing summers, SEJ increases its volume by 37% with 0.2° southward deviation, while in the decaying summers, SEJ decreases its volume by 43% with 0.5° northward deviation. More interestingly, SEJ has prominent correlation with the southern WBC (0.54 for VT and 0.41 for SI, > 99%), and both of them strengthen (weaken) in El Nino developing (decaying) summers. The response of SEJ and the southern WBC to El Nino can be attributed to the setting up of the anticyclonic circulation in Sverdrup balance driven by the negative wind stress curl in the southern SCS. The negative wind stress curl controls the southern WBC and further determines the transport of SEJ based on Sverdrup balance.

An assessment of the CMIP5 models in simulating the Argo geostrophic meridional transport in the North Pacific Ocean

Eleven climate system models that participate in the Coupled Model Intercomparison Project phase 5 (CMIP5) were evaluated based on an assessment of their simulated meridional transports in comparison with the Sverdrup transports. The analyses show that the simulated North Pacific Ocean circulation is essentially in Sverdrup balance in most of the 11 models while the Argo geostrophic meridional transports indicate significant non-Sverdrup gyre circulation in the tropical North Pacific Ocean. The climate models overestimated the observed tropical and subtropical volume transports significantly. The non-Sverdrup gyre circulation leads to non-Sverdrup heat and salt transports, the absence of which in the CMIP5 simulations suggests deficiencies of the CMIP5 model dynamics in simulating the realistic meridional volume, heat, and salt transports of the ocean.

Mid‐latitude leading double‐dip La Niña

AbstractUnderstanding the evolution asymmetry between El Niño and La Niña events is challenging. Unlike El Niño, most La Niña events are characterised by a double‐dip cooling (a.k.a. multi‐year La Niña). Herein, we examined how single‐ and multi‐year La Niña events differ by analysing observational and climate‐model data sets. Single‐year La Niña events tend to develop narrowly within the tropics from a central Pacific‐type El Niño (Niño‐4 > Niño‐3), whereas multi‐year La Niña events tend to originate from an eastern Pacific‐type El Niño (Niño‐3 > Niño‐4) and are well‐connected to mid‐latitudes through the Pacific meridional mode, which leads to a meridionally wider response of the off‐equatorial low‐level atmospheric anti‐cyclonic circulation. As the anti‐cyclonic circulation controls the amount of equatorial upper‐ocean heat recharge through Sverdrup transport, for single‐year La Niña, efficient ocean recharging due to a narrower anti‐cyclonic circulation causes a fast transition to an El Niño or a fast termination of a La Niña. In contrast, for multi‐year La Niña, a weaker recharging causes surface cooling to persist, leading to another La Niña in the following year.

The Large-Scale Vorticity Balance of the Antarctic Continental Margin in a Fine-Resolution Global Simulation

ABSTRACTThe depth-integrated vorticity budget of a global, eddy-permitting ocean/sea ice simulation over the Antarctic continental margin (ACM) is diagnosed to understand the physical mechanisms implicated in meridional transport. The leading-order balance is between the torques due to lateral friction, nonlinear effects, and bottom vortex stretching, although details vary regionally. Maps of the time-averaged depth-integrated vorticity budget terms and time series of the spatially averaged, depth-integrated vorticity budget terms reveal that the flow in the Amundsen, Bellingshausen, and Weddell Seas and, to a lesser extent, in the western portion of East Antarctica, is closer to an approximate topographic Sverdrup balance (TSB) compared to other segments of the ACM. Correlation and coherence analyses further support these findings, and also show that inclusion of the vorticity tendency term in the response (the planetary vorticity advection and the bottom vortex stretching term) increases the correlation with the forcing (the vertical net stress curl), and also increases the coherence between forcing and response at high frequencies across the ACM, except for the West Antarctic Peninsula. These findings suggest that the surface stress curl, imparted by the wind and the sea ice, has the potential to contribute to the meridional, approximately cross-slope, transport to a greater extent in the Amundsen, Bellingshausen, Weddell, and part of the East Antarctic continental margin than elsewhere in the ACM.

Regional Versus Remote Atmosphere‐Ocean Drivers of the Rapid Projected Intensification of the East Australian Current

AbstractLike many western boundary currents, the East Australian Current (EAC) extension is projected to get stronger and warmer in the future. The CMIP5 multimodel mean (MMM) projection suggests up to 5°C of warming under an RCP85 scenario by 2100. Previous studies employed Sverdrup balance to associate a trend in basin wide zonally integrated wind stress curl (resulting from the multidecadal poleward intensification in the westerly winds over the Southern Ocean) with enhanced transport in the EAC extension. Possible regional drivers are yet to be considered. Here we introduce the NEMO‐OASIS‐WRF coupled regional climate model as a framework to improve our understanding of CMIP5 projections. We analyze a hierarchy of simulations in which the regional atmosphere and ocean circulations are allowed to freely evolve subject to boundary conditions that represent present‐day and CMIP5 RCP8.5 climate change anomalies. Evaluation of the historical simulation shows an EAC extension that is stronger than similar ocean‐only models and observations. This bias is not explained by a linear response to differences in wind stress. The climate change simulations show that regional atmospheric CMIP5 MMM anomalies drive 73% of the projected 12 Sv increase in EAC extension transport whereas the remote ocean boundary conditions and regional radiative forcing (greenhouse gases within the domain) play a smaller role. The importance of regional changes in wind stress curl in driving the enhanced EAC extension is consistent with linear theory where the NEMO‐OASIS‐WRF response is closer to linear transport estimates compared to the CMIP5 MMM.