This paper reviews the progress in our understanding of the atmospheric response to midlatitude oceanic fronts and eddies, emphasizing the Kuroshio-Oyashio Extension (KOE) region. Oceanic ...perturbations of interest consist of sharp oceanic fronts, temperature anomalies associated with mesoscale eddies, and to some extent even higher-frequency submesoscale variability. The focus is on the free atmosphere above the boundary layer. As the midlatitude atmosphere is dominated by vigorous transient eddy activity in the storm track, the response of both the time-mean flow and the storm track is assessed. The storm track response arguably overwhelms the mean-flow response and makes the latter hard to detect from observations. Oceanic frontal impacts on the mesoscale structures of individual synoptic storms are discussed, followed by the role of oceanic fronts in maintaining the storm track as a whole. KOE fronts exhibit significant decadal variability and can therefore presumably modulate the storm track. Relevant studies are summarized and intercompared. Current understanding has advanced greatly but is still subject to large uncertainties arising from inadequate data resolution and other factors. Recent modeling studies highlighted the importance of mesoscale eddies and probably even submesoscale processes in maintaining the storm track but confirmation and validation are still needed. Moreover, the atmospheric response can potentially provide a feedback mechanism for the North Pacific climate. By reviewing the above aspects, we envision that future research shall focus more upon the interaction between smaller-scale oceanic processes (fronts, eddies, submesoscale features) and atmospheric processes (fronts, extratropical cyclones etc.), in an integrated way, within the context of different climate background states.
Submesoscale processes (SMPs) play profound roles in energy cascade, air-sea heat flux, and marine ecosystem, the variability of which significantly regulates regional and global climate. The ...southeastern tropical Indian Ocean (SETIO) has abundant and complex dynamic processes, yet the dynamics of SMPs in this region remain unclear. Based on the outputs of two high-resolution models, the seasonality and potential mechanisms of SMPs in the SETIO, as well as accompanying submesoscale vertical heat transport (SVHT) are investigated in this study. The SMPs and SVHT are much stronger during the southeast monsoon period (June-October). Mixed layer instability (MLI) dominates the generation of SMPs, while frontogenesis only plays a minor role. The enhanced horizontal density gradients partly resulting from strengthened large-mesoscale flow strain, coupled with a deeper mixed layer induced by surface cooling and strong southeast monsoon, account for the stronger MLI in the southeast monsoon period. Besides, symmetric instability (SI) also contributes to the generation of SMPs by extracting kinetic energy from the geostrophic flows. Upward SVHT in medium- and high resolution ROMS simulations surpasses that in low-resolution ROMS simulation by a factor of three during the austral winter and is significantly stronger than mesoscale vertical heat transport. These results confirm the importance of SMPs in upper-layer vertical heat transport, and SMPs resolving models can represent the vertical heat transport much better. Our findings could deepen our understanding on multiscale dynamic processes and vertical heat transport in the SETIO.
Intraseasonal variability (ISV) of sea surface height (SSH) over the Bay of Bengal (BoB) is studied using altimetry data and an eddy‐resolving ocean model. In both the model hindcast and satellite ...observations, large SSH ISV is found along the eastern and northern coasts of the BoB, in the western BoB, and in a zonal band across the Bay centered near 5°N. The ISV displays a clear seasonality. In the western BoB, it reaches its annual maximum in spring, whereas it does so in summer and autumn southeast of Sri Lanka. Driven by equatorial intraseasonal winds, equatorial Kelvin waves propagate eastward, reach the western coast of Sumatra, and reflect there to propagate around the perimeter of the BoB as coastally trapped waves. Two distinct bands of high eddy activity are detected in the western and central BoB, respectively. In both bands, isolated eddies propagate southwestward. Eddy formation in the eddy train in the central Bay is linked to the coastal wave as it bends around the corner of the Irrawaddy Delta off Myanmar. Eddy activity contributes to the high ISV in the central Bay to some extent. An energetics analysis indicates that high total eddy energy in the western BoB is due to barotropic/baroclinic instability of the mean current.
Key Points
Large SSH ISV is found along the coasts of the BoB and in the western BoB
The coastal Kelvin wave contributes to SSH ISV along the coasts of the BoB
Eddy activity and instability of current cause the SSH ISV in the western BoB
Eddy kinetic energy (EKE) in the Celebes Sea (CS) is investigated using a global, eddy-resolving, ocean general circulation model (OGCM) output. The OGCM simulation shows that a strong EKE is ...confined in the upper 300-m layer. The period of EKE variability varies with depth. In the 0–100-m layer, EKE has a distinct annual cycle that is strong in winter (December–February) and weak in summer (June–August). However, in the 100–300-m layer, semi-annual variation is dominant, which shows stronger EKE in spring and fall and weaker EKE in summer and winter. An eddy energy budget analysis reveals that the barotropic eddy energy conversion rate has a vertical structure similar to that of EKE. Compared to the barotropic eddy energy conversion rate, the baroclinic eddy energy conversion rate is much smaller and does not match the EKE vertical pattern. The budget analysis indicates that the variation in EKE in the CS is governed by barotropic instability of the background circulation. Further analysis reveals that the variation in the regional background circulation with depth is due to a combination of the forcing of the local monsoon and the Mindanao Current (MC) in the western tropical Pacific.
During the altimeter era, the sea level in the South China Sea (SCS) and western tropical Pacific (WTP) experienced significant decadal variability. The sea level rose during 1993–2009 and fell ...during 2010–2019. The decadal variability of Walker Circulation associated with the Pacific Decadal Oscillation can explain the sea level variability in the WTP to a great extent. The wind forced westward propagating Rossby waves increased (decreased) the sea level in the WTP during 1993–2009 (2010–2019). However, the interior wind forcing has a negligible contribution to the decadal variability of the sea level in the SCS. The remote forcing from WTP through the oceanic bridge was supposed to play a dominant role. The sensitive experiments of a 1½‐layer model and Regional Oceanic Modeling System suggested that the sea level signals via the Sibutu Passage and Mindoro Strait accounted for the decadal variability of sea level in the central basin of SCS.
Plain Language Summary
During the altimeter era, the global mean sea level rose continuously, while the regional sea level trends are not uniform. The atmospheric and oceanic processes associated with climate modes played an important role. The Walker Circulation anomalies, connected closely with the “atmospheric bridge” of the Pacific Decadal Oscillation (PDO), forced a reverse sea level trend in the western and eastern tropical Pacific via propagations of Kelvin/Rossby waves. The sea level signals propagated into the South China Sea (SCS) via the Sibutu Passage and Mindoro Strait, which dominated the sea level variations in the central SCS. Unlike in the tropical Pacific, the decadal variability of sea level in the SCS was associated with PDO through “oceanic bridge.” The results in this study shed light on our understanding on the dynamics of sea level and upper‐layer circulation changes in the SCS and adjacent regions.
Key Points
Sea level trends in the South China Sea (SCS) and western tropical Pacific experienced regime shift during the altimeter era
Remote forcing originated from tropical Pacific dominated the decadal variability of sea level in the SCS
Tropical Pacific conveys its impact on the sea level in the central SCS mainly via the Sibutu Passage and Mindoro Strait
In this paper, we study the existence of the families of odd symmetric periodic solutions in the generalized elliptic Sitnikov (N+1)-body problem for all values of the eccentricity e∈0,1) using the ...global continuation method. First, we obtain the properties of the period of the solution of the corresponding autonomous equation (eccentricity e=0) using elliptic functions. Then, according to these properties and the global continuation method of the zeros of a function depending on one parameter, we derive the existence of odd periodic solutions for all e∈0,1). It is shown that the temporal frequencies of period solutions depend on the total mass λ (or the number N) of the primaries in a delicate way.
Using 28-year satellite-borne Special Sensor Microwave Imager observations, features of high-wind frequency (HWF) over the southern Indian Ocean are investigated. Climatology maps show that high ...winds occur frequently during austral winter, located in the open ocean south of Polar Front in subpolar region, warm flank of the Subantarctic Front between 55°E–78°E, and south of Cape Agulhas, where westerly wind prevails. The strong instability of marine atmospheric boundary layer accompanied by increased sensible and latent heat fluxes on the warmer flank acts to enhance the vertical momentum mixing, thus accelerate the surface winds. Effects of sea surface temperature (SST) front can even reach the entire troposphere by deep convection. HWF also shows distinct interannual variability, which is associated with the Southern Annual Mode (SAM). During positive phase of the SAM, HWF has positive anomalies over the open ocean south of Polar Front, while has negative anomalies north of the SST front. A phase shift of HWF happened around 2001, which is likely related to the reduction of storm tracks and poleward shift of westerly winds in the Southern Hemisphere.
Eddy activity in the central Bay of Bengal (BoB) is revealed using satellite observations and hydrographic data. Altimetric data show that eddies are generated near the eastern boundary, propagate ...southwestward, and have periods predominantly in the 30‐ to 120‐day band. Temperature profiles from the Research Moored Array for African‐Asian‐Australian Monsoon Analysis and Prediction (RAMA) buoy at 90°E, 15°N, have their highest amplitude in the depth range of the thermocline, indicating that the eddies are associated with a vertical motion of the thermocline (a first‐baroclinic‐mode response). To investigate the cause of the eddies, we obtained a suite of solutions to nonlinear and linear versions of a 1½‐layer (reduced‐gravity) model. They demonstrate that equatorial wind forcing, Myanmar bump and Andaman Island, and nonlinearity are all essential for eddy formation. Among the nonlinear terms, the advection terms are the primary cause.
Plain Language Summary
This study combines observations and model to understand eddy generation in the central Bay of Bengal. When equatorial wind‐driven downwelling (upwelling) Kelvin waves propagate to the tip of the Irrawaddy Delta off Myanmar, anticyclonic (cyclonic) eddies are generated there that subsequently propagate southwestward. The eddies are mainly driven by equatorial zonal winds, with both nonlinearity and coastline geometry essential for eddy generation.
Key Points
Eddies in the central BoB are associated with a vertical motion of the thermocline
The eddies are mainly driven by intraseasonal, equatorial zonal winds
Both nonlinearity and the Myanmar bump are essential for eddy generation
Abstract
The role of the tropical Pacific Ocean and its linkages to the southern hemisphere during the last deglacial warming remain highly controversial. Here we explore the evolution of Pacific ...horizontal and vertical thermal gradients over the past 30 kyr by compiling 340 sea surface and 7 subsurface temperature records, as well as one new ocean heat content record. Our records reveal that La Niña-like conditions dominated during the deglaciation as a result of the more intense warming in the western Pacific warm pool. Both the subsurface temperature and ocean heat content in the warm pool rose earlier than the sea surface temperature, and in phase with South Pacific subsurface temperature and orbital precession, implying that heat exchange between the tropical upper water column and the extratropical Southern Ocean facilitated faster warming in the western Pacific. Our study underscores the key role of the thermal coupling between the warm pool and the Southern Ocean and its relevance for future global warming.
Temporal and spatial variations in the ocean surface mixed layer are important for the climate and ecological systems. During 1980–2019, the Southern Indian Ocean (SIO) mixed layer depth (MLD) ...displays a basin‐wide shoaling trend that is absent in the other basins within 40°S–40°N. The SIO MLD shoaling is mostly prominent in austral winter with deep climatology MLD, substantially weakening the MLD seasonality. Moreover, the SIO MLD changes are primarily caused by a southward shift of the subtropical anticyclonic winds and hence ocean gyre, associated with a strengthening of the Southern Annular Mode, in recent decades for both winter and summer. However, the poleward‐shifted subtropical ocean circulation preferentially shoals the SIO MLD in winter when the meridional MLD gradient is sharp but not in summer when the gradient is flat. This highlights the distinct subtropical MLD response to meridional mitigation in winds due to different background oceanic conditions across seasons.
Plain Language Summary
The ocean surface mixed layer (ML) is a well‐mixed layer with uniform physical and chemical properties and is key for the ocean in exchanging materials and energy with the atmosphere. The present study shows that during 1980–2019, the Southern Indian Ocean (SIO) ML depth (MLD) displays a basin‐wide decreasing trend, which is absent in the other basins within 40°S–40°N. The SIO MLD shoaling primarily appears in austral winter when the climatology ML is deep but is insignificant in summer, substantially weakening the MLD seasonality. The SIO MLD changes are primarily explained by the ocean dynamical adjustment driven by the surface zonal wind changes. Specifically, the strengthened Southern Annular Mode in recent decades drives southward shifts of the subtropical anticyclonic winds and ocean gyre year‐round. However, the poleward‐shifted ocean gyre preferentially decreases the SIO winter MLD as the meridional MLD gradient is sharp and thus efficiently reduces the deep ML water converging from the Southern Ocean into the SIO. In contrast, the SIO MLD displays negligible change in summer when its meridional gradient is flat. The results highlight that despite under nearly identical southward‐shifted subtropical winds, the winter and summer MLD responses are distinct due to different background oceanic conditions.
Key Points
The seasonality of the Southern Indian Ocean surface mixed layer (ML) depth prominently weakens during 1980–2019
The weakened seasonality mainly results from a pronounced winter ML shoaling
The southward shift of the subtropical ocean gyre driven by the strengthened Southern Annular Mode dominates the ML shoaling
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