Search

Oceanic eddy-induced meridional heat transport (EHT) is an important process in the Southern Ocean heat budget, the variability of which significantly modulates global meridional overturning circulation (MOC) and Antarctic sea-ice extent. Although it is recognized that mesoscale eddies with scales of ~40–300 km greatly contribute to the EHT, the role of submesoscale eddies with scales of ~1–40 km remains unclear. Here, using two state-of-the-art high-resolution simulations (resolutions of 1/48° and 1/24°), we find that submesoscale eddies significantly enhance the total poleward EHT in the Southern Ocean with an enhancement percentage reaching 19–48% in the Antarctic Circumpolar Current band. By comparing the eddy energy budgets between the two simulations, we detect that the primary role of submesoscale eddies is to strengthen mesoscale eddies (and thus their heat transport capability) through inverse energy cascade rather than directly through submesoscale heat fluxes. Due to the submesoscale-mediated enhancement of mesoscale eddies in the 1/48° simulation, the clockwise upper cell and anti-clockwise lower cell of the residual-mean MOC in the Southern Ocean are weakened and strengthened, respectively. This finding identifies a potential route to improve the mesoscale parameterization in climate models for more accurate simulations of the MOC and sea ice variability in the Southern Ocean.

This study investigates the responses of the meridional ocean heat transport (OHT) to heat fluxes of equal amplitude but opposite sign into the global ocean surface using the Community Earth System Model (CESM). Results show that the poleward OHT in both hemispheres are weakened in response to the positive forcing (i.e., warming) and strengthened in response to the negative forcing (i.e., cooling), with the latter change exceeding the former, manifesting an overall asymmetric response. The OHT responses in the Indo-Pacific to both the warming and the cooling and thus their asymmetry are dominated by its Eulerian-mean component, due primarily to changes in the Indo-Pacific Subtropical Cells. Similarly, the OHT responses in the Atlantic are determined by changes in the Atlantic Meridional Overturning Circulation, yet their asymmetry is small due to the mediation from the nonlinear effect of temperature and velocity changes. For the Southern Ocean, the Eulerian-mean component still controls the total OHT responses, yet the largest contribution is from changes in vertical temperature structure. In addition, the asymmetric response in the Southern Ocean is a joint effort among the overturning circulation cells, horizontal gyres, eddies, as well as temperature changes.

Under external heating forcing in the Southern Ocean, climate models project anomalous northward atmosphere heat transport (AHT) across the equator, accompanied by a southward shift of the intertropical convergence zone (ITCZ). Comparison between a fully coupled and a slab ocean model shows that the inclusion of active ocean dynamics tends to partition the cross-equatorial energy transport and significantly reduce the ITCZ shift response by a factor of 10, a finding which supports previous studies. To understand how ocean dynamics damps the ITCZ’s response to an imposed thermal heating in the Southern Ocean, we examine the ocean heat transport (OHT) and ocean circulation responses in a set of fully coupled experiments. Results show that both the Indo-Pacific and the Atlantic contribute to transport energy across the equator mainly through its Eulerian-mean component. However, different from previous studies that linked the changes in OHT to the changes in the wind-driven subtropical cells or the Atlantic meridional overturning circulation (AMOC), our results show that the cross-equatorial OHT anomaly is due to a broad clockwise overturning circulation anomaly below the subtropical cells (approximately bounded by the 5°C to 20°C isotherms and 50°S to 10°N). Further elimination of the wind-driven component, conducted by prescribing the climatological wind stress in the Southern Ocean heat perturbation experiments, leads to little change in OHT, suggesting that the OHT response is predominantly thermohaline-driven by air-sea thermal interactions.

In recent decades, the two polar regions have exhibited strikingly different changes, with much greater warming in the Arctic than the Antarctic. However, the warming asymmetry between the two polar regions is quite small during the second half of the twentieth century. By using a multi-member ensemble of simulations with the Community Earth System Model, this study investigates the relative contributions of greenhouse gases, aerosol, and ozone forcings to the responses of Arctic and Antarctic surface temperature during 1955–2000. Results show that both the greenhouse gases- and aerosols-induced changes are greater in the Arctic than in the Antarctic, yet they are opposite and act to balance each other, leaving a limited warming in the Arctic and hence a small bipolar asymmetry. Using a radiative kernel, feedback analysis reveals that both greenhouse gases and aerosol forcings influence the polar surface temperature through albedo feedback related to sea ice changes and lapse rate feedback related to strong surface temperature inversion. The ozone forcing can hardly excite any surface temperature changes over the polar regions even in the Antarctic with the strongest ozone depletion, which is due to a cancellation between the cooling effect from radiative forcing and cloud radiative feedback, and the warming effect from lapse rate feedback and enhanced atmospheric heat transport from lower latitudes.

Seasonal-to-decadal variability of spice injection in the upper ocean of the subtropical southeastern Pacific (SEP) is investigated using 3-day and 0.25° simulations spanning from 1992 to 2016 in the Consortium for Estimating the Circulation and Climate of the Ocean. The spice injection refers to convective mixing through which saline water in the mixed layer is injected into the interior to generate positive temperature and salinity anomalies or spiciness anomaly (SPA). Results show that the spice injection in the SEP occurs during austral winter when the mixed layer is deep, and it leads to a positive SPA in the interior. The interior SPA is found to experience significant interannual variability, which is well correlated with winter mixed layer depth (MLD), with more interior SPA corresponding to deeper MLD. During a strong winter of injection, the interior spiciness change rate can reach up to ~ 0.005 kg m−3 day−1 and the gain of spiciness in the interior ~ 0.2 kg m−3 over the SEP region. The interannual-decadal variability of interior SPA in the SEP has a significantly negative correlation to the low-frequency El Nino–Southern Oscillation index, with larger (smaller) SPA during La Nina (El Nino) conditions.

This study investigates the contribution of mesoscale eddies to the subduction and transport of North Pacific Eastern Subtropical Mode Water (ESTMW) using the high-frequency output of an eddy-resolved ocean model spanning the period 1994–2010. Results show that the subduction induced by mesoscale eddies accounts for about 31% of the total subduction of ESTMW formation. The volume of ESTMW trapped by anticyclonic eddies is slightly larger than that trapped by cyclonic eddies. The ESTMW trapped by all eddies in May reaches up to about 2.8 × 1013 m3, which is approximately 16% of the total ESTMW volume. The eddy-trapped ESTMW moves primarily westward, with its meridional integration at 18°–30°N reaching about 0.17 Sv, which is approximately 18% of the total zonal ESTMW transport in this direction, at 140°W. This study highlights the important role of eddies in carrying ESTMW westward over the northeastern Pacific Ocean.

The response of the eastern tropical Indian Ocean (ETIO) to heat fluxes of equal amplitude but opposite sign is investigated using the Community Earth System Model (CESM). A significant positive asymmetry in sea surface temperature (SST) is found over the ETIO—the warming responses to the positive forcing exceeds the cooling to the negative forcing. A mixed layer heat budget analysis is carried out to identify the mechanisms responsible for the SST asymmetry. Results show that it is mainly ascribed to the ocean dynamical processes, including vertical advections and diffusion. The net surface heat flux, on the contrary, works to reduce the asymmetry through its shortwave radiation and latent heat flux components. The former is due to the nonlinear relationship between SST and cloud, while the latter is resulted mainly from Newtonian damping and air-sea stability effects. Changes in the SST skewness are also evaluated, with more enhanced negative SST skewness over the ETIO found for the cooling than heating scenarios due to the asymmetric thermocline-SST feedback.

Global Navigation Satellite System (GNSS) is now widely used for continuous ionospheric observations. Three-dimensional computerized ionospheric tomography (3DCIT) is an important tool for the reconstruction of electron density distributions in the ionosphere through effective use of the GNSS data. More specifically, the 3DCIT technique is able to resolve the three-dimensional electron density distributions over the reconstructed area based on the GNSS slant total electron content (STEC) observations. We present an Improved Constrained Simultaneous Iterative Reconstruction Technique (ICSIRT) algorithm that differs from the traditional ionospheric tomography methods in 3 ways. First, the ICSIRT computes the electron density corrections based on the product of the intercept and electron density within voxels so that the assignment of corrections at different heights becomes more reasonable. Second, an Inverse Distance Weighted (IDW) interpolation is used to restrict the electron density values in the voxels not traversed by GNSS rays, thereby ensuring the smoothness of the reconstructed region. Also, to improve the reconstruction accuracy around the HmF2 (the peak height of the F2 layer) altitude, a multiresolution grid is adopted in the vertical direction, with a 10-km resolution from 200 to 420 km and a 50-km resolution at other altitudes. The new algorithm has been applied to the GNSS data over the European and North American regions in different case studies that involve different seasonal conditions as well as a major storm. In the European region experiment, reconstruction results show that the new ICSIRT algorithm can effectively improve the reconstruction of the GNSS data. The electron density profiles retrieved from ICSIRT are much closer to the ionosonde observations than those from its predecessor, namely, the Constrained Simultaneous Iteration Reconstruction Technique (CSIRT). The reconstruction accuracy is significantly improved. In the North American region experiment, the electron density profiles in ICSIRT results show better agreement with incoherent scatter radar observations than CSIRT, even for the topside profiles.

Climate models project a significant shoaling of the thermocline over the western equatorial Pacific Ocean under global warming, which has been generally regarded as a direct response to surface wind change. This study investigates the formation processes for the equatorial Pacific thermocline response to CO2 quadrupling using the Community Earth System Model version 1 (CESM1). In particular, an overriding method is applied to isolate and quantify the wind stress effect and the direct radiative effect of CO2 emissions. Results show that both effects of the wind stress and direct radiative forcing are equally important for shoaling the equatorial thermocline, with the former responsible for its upper portion change and the latter for its lower portion change. Further passive tracer experiments with the ocean component of the CESM1 verify the role of ocean surface warming in shoaling the equatorial thermocline and identify the ocean circulation change in response to the surface warming as its dynamic cause of formation.

The response of ENSO amplitude to climate warming and cooling is investigated using the Community Earth System Model (CESM), in which the warming and cooling scenarios are designed by adding heat fluxes of equal amplitude but opposite sign onto the ocean surface, respectively. Results show that the warming induces an increase of the ENSO amplitude but the cooling gives rise to a decrease of the ENSO amplitude, and these changes are robust in statistics. A mixed layer heat budget analysis finds that the increasing (decreasing) SST tendency under climate warming (cooling) is mainly due to an enhancement (weakening) of dynamical feedback processes over the equatorial Pacific, including zonal advective (ZA) feedback, meridional advective (MA) feedback, thermocline (TH) feedback, and Ekman (EK) feedback. As the climate warms, a wind anomaly of the same magnitude across the equatorial Pacific can induce a stronger zonal current change in the east (i.e., a stronger ZA feedback), which in turn produces a greater weakening of upwelling (i.e., a stronger EK feedback) and thus a larger thermocline change (i.e., a stronger TH feedback). In response to the climate warming, in addition, the MA feedback is also strengthened due to an enhancement of the meridional SST gradient around the equator resulting from a weakening of the subtropical cells (STCs). It should be noted that the weakened STCs itself has a negative contribution to the change of the MA feedback which, however, appears to be secondary. And vice versa for the cooling case. Bjerknes linear stability (BJ) index is also evaluated for the linear stability of ENSO, with remarkably larger (smaller) BJ index found for the warming (cooling) case.

The Regional Ocean Modeling System (ROMS) is used to study the summer circulation in the southwestern Yellow Sea (SWYS). The modeled currents show good agreement with observations from both drifters and moorings. While the summer current in the SWYS flows consistently northeastward on the surface with large magnitude offshore, the current below the surface layer features a cyclonic circulation roughly along the 25 m isobath. The effect of a surface wind stress and bottom thermal fronts on the circulation is investigated through a series of process-oriented numerical experiments. It is found that the southeasterly wind dominates the surface current, whereas the bottom thermal fronts, which are formed in a transition area between the vertically well-mixed region and the stratified region, are responsible for the cyclonic circulation below the surface.

Climate models project an El Nino-like SST response in the tropical Pacific Ocean to global warming (GW). By employing the Community Earth System Model and applying an overriding technique to its ocean component, Parallel Ocean Program version 2, this study investigates the similarity and difference of formation mechanism for the changes in the tropical Pacific Ocean under El Nino and GW. Results show that, despite sharing some similarities between the two scenarios, there are many significant distinctions between GW and El Nino: (1) the phase locking of the seasonal cycle reduction is more notable under GW compared with El Nino, implying more extreme El Nino events in the future; (2) in contrast to the penetration of the equatorial subsurface temperature anomaly that appears to propagate in the form of an oceanic equatorial upwelling Kelvin wave during El Nino, the GW-induced subsurface temperature anomaly manifest in the form of off-equatorial upwelling Rossby waves; (3) while significant across-equator northward heat transport (NHT) is induced by the wind stress anomalies associated with El Nino, little NHT is found at the equator due to a symmetric change in the shallow meridional overturning circulation that appears to be weakened in both North and South Pacific under GW; and (4) heat budget analysis shows that the maintaining mechanisms for the eastern equatorial Pacific warming are also substantially different.

The low-frequency variability of the shallow meridional overturning circulation (MOC) in the South China Sea (SCS) is investigated using a Simple Ocean Data Assimilation (SODA) product for the period of 1900–2010. A dynamical decomposition method is used in which the MOC is decomposed into the Ekman, external mode, and vertical shear components. Results show that all the three dynamical components contribute to the formation of the seasonal and annual mean shallow MOC in the SCS. The shallow MOC in the SCS consists of two cells: a clockwise cell in the south and an anticlockwise cell in the north; the former is controlled by the Ekman flow and the latter is dominated by the external barotropic flow, with the contribution of the vertical shear being to reduce the magnitude of both cells. In addition, the strength of the MOC in the south is found to have a falling trend over the past century, due mainly to a weakening of the Luzon Strait transport (LST) that reduces the transport of the external component. Further analysis suggests that the weakening of the LST is closely related to a weakening of the westerly wind anomalies over the equatorial Pacific, which leads to a southward shift of the North Equatorial Current (NEC) bifurcation and thus a stronger transport of the Kuroshio east of Luzon.

The excessive cold tongue error in the equatorial Pacific has persisted in several generations of climate models. Based on the historical simulations and Representative Concentration Pathway (RCP) 8.5 experiments in the Coupled Model Intercomparison Project phase 5 (CMIP5) multi-model ensemble (MME), this study finds that models with an excessive westward extension of cold tongue and insufficient equatorial western Pacific precipitation tend to project a weaker east-minus-west gradient of sea surface temperature (SST) warming along the equatorial Pacific under increased greenhouse gas (GHG) forcing. This La Nina-like error of tropical Pacific SST warming is consistent with our understanding of negative SST-convective feedback over the western Pacific warm pool. Based on this relationship between the present simulations and future projections, the present study applies an “observational constraint” of equatorial western Pacific precipitation to calibrate the projections of tropical Pacific climate change. After the corrections, CMIP5 models robustly project an El Nino-like warming pattern, with a MME mean increase by a factor of 2.3 in east-minus-west gradient of equatorial Pacific SST warming and reduced inter-model uncertainty. Corrections in projected changes in tropical precipitation and atmospheric circulation are physically consistent. This study suggests that a realistic cold tongue simulation would lead to a more reliable tropical Pacific climate projection.

Climate models project a positive Indian Ocean Dipole (pIOD)–like SST response in the tropical Indian Ocean to global warming. By employing the Community Earth System Model and applying an overriding technique to its ocean component (version 2 of the Parallel Ocean Program), this study investigates the similarities and differences of the formation mechanisms for the changes in the tropical Indian Ocean during the pIOD versus global warming. Results show that their formation processes and related seasonality are quite similar; in particular, wind–thermocline–SST feedback is the leading mechanism in producing the anomalous cooling over the eastern tropics in both cases. Some differences are also found, including the fact that the cooling effect of the vertical advection over the eastern tropical Indian Ocean is dominated by the anomalous vertical velocity during the pIOD but by the anomalous upper-ocean stratification under global warming. These findings are further examined through an analysis of the mixed layer heat budget.

Mode water is a distinct water mass characterized by a near vertical homogeneous layer or low potential vorticity, and is considered essential for understanding ocean climate variability. Based on the output of GFDL CM3, this study investigates the response of eastern subtropical mode water (ESTMW) in the North Pacific to two different single forcings: greenhouse gases (GHGs) and aerosol. Under GHG forcing, ESTMW is produced on lighter isopycnal surfaces and is decreased in volume. Under aerosol forcing, in sharp contrast, it is produced on denser isopycnal surfaces and is increased in volume. The main reason for the opposite response is because surface ocean-to-atmosphere latent heat flux change over the ESTMW formation region shoals the mixed layer and thus weakens the lateral induction under GHG forcing, but deepens the mixed layer and thus strengthens the lateral induction under aerosol forcing. In addition, local wind changes are also favorable to the opposite response of ESTMW production to GHG versus aerosol.

The enhanced central and eastern Pacific SST warming and the associated ocean processes under global warming are investigated using the ocean component of the Community Earth System Model (CESM), Parallel Ocean Program version 2 (POP2). The tropical SST warming pattern in the coupled CESM can be faithfully reproduced by the POP2 forced with surface fluxes computed using the aerodynamic bulk formula. By prescribing the wind stress and/or wind speed through the bulk formula, the effects of wind stress change and/or the wind-evaporation-SST (WES) feedback are isolated and their linearity is evaluated in this ocean-alone setting. Result shows that, although the weakening of the equatorial easterlies contributes positively to the El Nino-like SST warming, 80 % of which can be simulated by the POP2 without considering the effects of wind change in both mechanical and thermodynamic fluxes. This result points to the importance of the air–sea thermal interaction and the relative feebleness of the ocean dynamical process in the El Nino-like equatorial Pacific SST response to global warming. On the other hand, the wind stress change is found to play a dominant role in the oceanic response in the tropical Pacific, accounting for most of the changes in the equatorial ocean current system and thermal structures, including the weakening of the surface westward currents, the enhancement of the near-surface stratification and the shoaling of the equatorial thermocline. Interestingly, greenhouse gas warming in the absence of wind stress change and WES feedback also contributes substantially to the changes at the subsurface equatorial Pacific. Further, this warming impact can be largely replicated by an idealized ocean experiment forced by a uniform surface heat flux, whereby, arguably, a purest form of oceanic dynamical thermostat is revealed.

The Princeton Ocean Model (POM) is widely used in both the research and real-time marine forecast communities. However, there is a single heavily criticized feature and shortcoming of POM, i.e., the pressure gradient (PG) error associated with sloping topography. To overcome this problem, in this paper we present an eta-coordinate POM (ECPOM). The eta-coordinate system was originally adopted in atmospheric numerical models and has proved to be effective in removing the large errors associated with the horizontal pressure gradient force and advection and diffusion terms along a steeply sloping topography. A familiar idealized seamount problem is used to test the PG error in the ECPOM in order to isolate errors from other sources, as is done in the literature. The model is set up so that, ideally, the ocean should be at rest forever; any development of circulation is regarded as numerically artificial. Results show that in ECPOM, errors are dramatically reduced by two orders of magnitude over a wide range of parameters. As a trade-off, the bottom boundary of the ECPOM is a stepwise instead of a continuous one, as in the sigma-coordinate POM (SCPOM). Modification of the SCPOM to the ECPOM is straightforward and minimal. Compared to the z-coordinate model, the ECPOM is numerically simpler since the eta-coordinate has a surface-following character.