Tropical Instability Waves (TIWs) are the dominant source of intraseasonal variability in the central equatorial Atlantic and play an important role in the redistribution of heat in the upper ocean. ...Here we use multidecadal records of sea surface temperature, sea level anomaly, sea surface salinity, and near‐surface currents constructed from in situ and satellite observations to reveal a long‐term intensification of the intraseasonal variability of these variables due to an increase of TIW activity. Enhanced barotropic energy conversion from increased covariance of horizontal current fluctuations, rather than low‐frequency changes of the mean zonal currents, drives the TIW intensification. As a consequence, boreal summer cooling of tropical North Atlantic surface waters through horizontal eddy temperature advection increased by 0.03°C month−1 decade−1 during 1993–2021, a change of 74% ± 53% relative to the long‐term mean. The presented multidecadal TIW trends are strongly modulated by interannual variations like the 2021 Atlantic Niño.
Plain Language Summary
In the equatorial Atlantic, temperature, salinity, sea level anomaly, and ocean velocity variations on time scales of tens of days are dominated by the presence and westward passage of large‐scale Tropical Instability Waves (TIWs). Several decades of satellite and surface drifter data as well as moored velocity observations show a long‐term intensification of TIW activity in all of these variables in the tropical North Atlantic where TIWs are most pronounced. We find that increased high‐frequency flow variability, and not long‐term changes of the mean zonal current system, drives the TIW intensification. One consequence of increased Atlantic Ocean TIW activity is the corresponding intensification of the horizontal eddy temperature advection pattern in boreal summer leading to stronger cooling of surface waters north of the equator. This equates to an increase in TIW‐driven sea surface temperature cooling of 74% ± 53% in the tropical North Atlantic during the last 3 decades. The presented multidecadal TIW trends are strongly modulated by interannual variations such as the 2021 Atlantic Niño. We further explore potential large‐scale drivers of the TIW intensification, including changes in high‐frequency wind variability.
Key Points
In situ and satellite observations show a long‐term intensification of Tropical Instability Waves (TIWs) in the tropical North Atlantic
Enhanced TIW activity is mainly due to increased barotropic instability associated with increased covariance of velocity fluctuations
As a result, TIW‐driven sea surface cooling north of the equator due to eddy temperature advection has increased by 74% from 1993 to 2021
Based on velocity data from a long‐term moored observatory located at 0°N, 23°W we present evidence of a vertical asymmetry during the intraseasonal maxima of northward and southward upper‐ocean flow ...in the equatorial Atlantic Ocean. Periods of northward flow are characterized by a meridional velocity maximum close to the surface, while southward phases show a subsurface velocity maximum at about 40 m. We show that the observed asymmetry is caused by the local winds. Southerly wind stress at the equator drives northward flow near the surface and southward flow below that is superimposed on the Tropical Instability Wave (TIW) velocity field. This wind‐driven overturning cell, known as the Equatorial Roll, shows a distinct seasonal cycle linked to the seasonality of the meridional component of the south‐easterly trade winds. The superposition of vertical shear of the Equatorial Roll and TIWs causes asymmetric mixing during northward and southward TIW phases.
Plain Language Summary
Tropical Instability Waves (TIWs) are clear in satellite measurements of sea surface temperature as horizontal undulations with wavelength of the order of 1,000 km in equatorial regions of both Atlantic and Pacific Oceans. TIWs are characterized by their distinctive upper‐ocean meridional velocity structure. TIWs amplify vertical shear and thus contribute to the generation of turbulence which in turn leads to the mixing of heat and freshwater downward into the deeper ocean. In this study we show that the prevailing southerly winds in the central equatorial Atlantic drive near‐surface northward and subsurface southward flows, which are superposed on the meridional TIW velocity field. The strength of this wind driven cell is linked to the seasonal cycle of the northward component of the trade winds, peaking in boreal fall when TIWs reach their maximum amplitude. The overturning cell affects the vertical structure of the meridional velocity field and thus has impact on the generation of current shear and turbulence. We show that the overturning reduces/enhances shear during northward/southward TIW flow, an asymmetry that is consistent with independent measurements showing asymmetric mixing.
Key Points
Composites of Tropical Instability Waves at 0°N, 23°W show a surface (subsurface) velocity maximum during northward (southward) phases
Meridional wind stress forces a seasonally‐varying, shallow cross‐equatorial overturning cell‐the Equatorial Roll
The superposition of Tropical Instability Waves and Equatorial Roll causes asymmetric mixing during north‐ and southward phases
Abstract
Besides the zonal flow that dominates the seasonal and long-term variability in the equatorial Atlantic, energetic intraseasonal meridional velocity fluctuations are observed in large parts ...of the water column. We use 15 years of partly full-depth velocity data from an equatorial mooring at 23°W to investigate intraseasonal variability and specifically the downward propagation of intraseasonal energy from the near-surface into the deep ocean. Between 20 and 50 m, intraseasonal variability at 23°W peaks at periods between 30 and 40 days. It is associated with westward-propagating tropical instability waves, which undergo an annual intensification in August. At deeper levels down to about 2000 m considerable intraseasonal energy is still observed. A frequency–vertical mode decomposition reveals that meridional velocity fluctuations are more energetic than the zonal ones for periods < 50 days. The energy peak at 30–40 days and at vertical modes 2–5 excludes equatorial Rossby waves and suggests Yanai waves to be associated with the observed intraseasonal energy. Yanai waves that are considered to be generated by tropical instability waves propagate their energy from the near-surface west of 23°W downward and eastward to eventually reach the mooring location. The distribution of intraseasonal energy at the mooring position depends largely on the dominant frequency and the time, depth, and longitude of excitation, while the dominant vertical mode of the Yanai waves plays only a minor role. Observations also show the presence of weaker intraseasonal variability at 23°W below 2000 m that cannot be associated with tropical instability waves.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
On the Genesis of the 2021 Atlantic Niño Lee, Sang‐Ki; Lopez, Hosmay; Tuchen, Franz Philip ...
Geophysical research letters,
28 August 2023, Letnik:
50, Številka:
16
Journal Article
Recenzirano
Odprti dostop
An extreme Atlantic Niño developed in the boreal summer of 2021 with peak‐season sea surface temperature anomalies exceeding 1°C in the eastern equatorial region for the first time since global ...satellite measurements began in the early 1970s. Here, we show that the development of this outlier event was preconditioned by a series of oceanic Rossby waves that reflected at the South American coast into downwelling equatorial Kelvin waves. In early May, an intense week‐long westerly wind burst (WWB) event, driven by the Madden‐Julian Oscillation (MJO), developed in the western and central equatorial Atlantic and greatly amplified one of the reflected Kelvin waves, directly initiating the 2021 Atlantic Niño. MJO‐driven WWBs are fundamental to the development of El Niño in the Pacific but are a previously unidentified driver for Atlantic Niño. Their importance for the 2021 event suggests that they may serve as a useful predictor/precursor for future Atlantic Niño events.
Plain Language Summary
Atlantic Niño is the Atlantic counterpart of El Niño in the Pacific, often referred to as El Niño's little brother. It was previously thought to have only regional influence on rainfall variability in West Africa, but a growing number of studies have shown that Atlantic Niño also plays an important role in the development of El Niño–Southern Oscillation, as well as in the formation of powerful hurricanes near the coast of West Africa. This study investigates the development of an extreme Atlantic Niño in the summer of 2021. Here, we show that the 2021 event was preconditioned by warm waters piled up near the South American coast, and then directly triggered by a westerly wind burst event that drove the warm waters eastward. The westerly wind burst event was driven by a patch of tropical thunderstorms that formed across the Indian Ocean and moved slowly eastward across the Pacific, South America, and the Atlantic, also known as the Madden‐Julian Oscillation. Westerly wind bursts driven by the Madden‐Julian Oscillation are fundamental for the development of El Niño in the Pacific, but a previously unidentified driver for Atlantic Niño, and thus may improve our ability to predict future Atlantic Niño events.
Key Points
The extreme 2021 Atlantic Niño was preconditioned by a series of oceanic Rossby waves reflected into downwelling equatorial Kelvin waves
One of the Kelvin waves was greatly amplified by an intense week‐long westerly wind burst event, initiating the 2021 Atlantic Niño
The westerly wind burst was driven by the Madden‐Julian Oscillation, which is a previously unidentified driver for Atlantic Niño
The upper‐ocean circulation of the tropical Atlantic is a complex superposition of thermohaline and wind‐driven flow components. The resulting zonally and vertically integrated upper‐ocean meridional ...flow is referred to as the upper branch of the Atlantic Meridional Overturning Circulation (AMOC)—a major component and potential tipping element of the global climate system. Here, we investigate the tropical part of the northward AMOC branch, that is, the return flow covering the upper 1,200 m, based on Argo data and repeated shipboard velocity measurements. The western boundary mean circulation at 11°S is realistically reproduced from high‐resolution Argo data showing a remarkably good representation of the volume transport of the return flow water mass layers when compared to results from direct velocity measurements along a repeated ship section. The AMOC return flow through the inner tropics (11°S–10°N) is found to be associated with a diapycnal upwelling of lower central water into the thermocline layer of ∼2 Sv. This is less than half the magnitude of previous estimates, likely due to improved horizontal resolution. The total AMOC return flow at 11°S and 10°N is derived to be similar in strength with 16–17 Sv. At 11°S, northward transport is concentrated at the western boundary, where the AMOC return flow enters the inner tropics at all vertical levels above 1,200 m. At 10°N, northward transport is observed both at the western boundary and in the interior predominantly in the surface and intermediate layer indicating recirculation and transformation of thermocline and lower central water within the inner tropics.
Plain Language Summary
The Atlantic Meridional Overturning Circulation (AMOC) is one of the major components of the global climate system. In the upper 1,200 m, the northward branch of the AMOC transports large amounts of heat, salt, and biogeochemical tracers across the equator from the South Atlantic through the tropics to the North Atlantic. In this study, we show that a realistic reconstruction of the upper‐ocean circulation at the southern hemisphere western boundary—a bottleneck for the AMOC—is possible based on high‐resolution Argo float data, further enabling transport and pathway estimates for the upper and intermediate water mass layers of the inner tropical Atlantic (11°S–10°N). At 11°S, the northward AMOC branch is largely concentrated at the western boundary, whereas, at 10°N, it preferably exits the inner tropics through the western boundary, but also through the interior basin after recirculating in the equatorial current system. When crossing the inner tropics, the water masses forming the AMOC return flow change their characteristics and the associated upwelling of water into the subsurface layer is found here to be less than half as large as previously estimated, likely due to improved horizontal resolution.
Key Points
Observed Atlantic western boundary mean transport of the upper 1,200 m at 11°S is realistically reproduced from high‐resolution Argo data
Diapycnal transport estimates from high‐resolution Argo data show upwelling of ∼2 Sv into the tropical Atlantic thermocline layer
By combining shipboard measurements with Argo data, we provide an overview of the individual water mass pathways within the Atlantic Meridional Overturning Circulation return flow
Recent evidence from mooring data in the equatorial Atlantic reveals that semiannual and longer time scale ocean current variability is close to being resonant with equatorial basin modes. Here we ...show that intraseasonal variability, with time scales of tens of days, provides the energy to maintain these resonant basin modes against dissipation. The mechanism is analogous to that by which storm systems in the atmosphere act to maintain the atmospheric jet stream. We demonstrate the mechanism using an idealized model setup that exhibits equatorial deep jets. The results are supported by direct analysis of available mooring data from the equatorial Atlantic Ocean covering a depth range of several thousand meters. The analysis of the mooring data suggests that the same mechanism also helps maintain the seasonal variability.
Key Points
The meridional flux of zonal momentum associated with intraseasonal variability acts to maintain lower frequency varying zonal currents in the equatorial ocean, in particular the equatorial deep jets
The mechanism is demonstrated using both an idealized primitive equation model and using momentum fluxes computed from mooring data on, and either side of, the equator at 23°W in the Atlantic
Analysis of the mooring data shows that the same mechanism also supports the seasonal cycle in the equatorial Atlantic Ocean
The Atlantic Subtropical Cells (STCs) consist of poleward Ekman transport in the surface layer, subduction in the subtropics, and equatorward transport in the thermocline layer that largely ...compensates the surface Ekman divergence and closes the STCs via equatorial upwelling. As a result, the STCs play an important role in connecting the tropical and subtropical Atlantic Ocean, in terms of heat, freshwater, oxygen, and nutrients exchange. However, their representation in state‐of‐the‐art coupled models has not been systematically evaluated. In this study, we investigate the performance of the Coupled Model Intercomparison Project Phase 6 climate models in simulating the Atlantic STCs. Comparing model results with observations, we first present the simulated mean state with respect to ensembles of the key components participating in the STC loop, that is, the meridional Ekman and geostrophic flow across 10°N and 10°S, and the Equatorial Undercurrent (EUC) at 23°W. We find that the model ensemble reveals biases toward weak Southern Hemisphere Ekman transport and interior geostrophic transports, as well as a weak EUC. We then investigate the large inter‐model spread of these key components and find that models with strong Ekman divergence between 10°N and 10°S tend to have strong mixed layer and thermocline interior convergence and strong EUC. The inter‐model spread of the EUC strength is primarily associated with the intensity of the southeasterly trade winds in the models. Since the trade‐wind‐induced poleward Ekman transports are regarded as the drivers of the STCs, our results highlight the necessity to improve skills of coupled models to simulate the Southern Hemisphere atmospheric forcing.
Plain Language Summary
This work systematically assesses how well state‐of‐the‐art climate models simulate the Atlantic Subtropical Cells (STCs). The STCs are part of the shallow (upper 200 m) ocean circulation. They connect the upper tropical and subtropical Atlantic Ocean in each hemisphere via poleward transport in the surface layer (surface to about 100 m) and equatorward transport in the subsurface layer (100–200 m). Therefore, the STCs are important for the exchange of heat, freshwater, dissolved oxygen, and nutrients between these regions. In this work, we find that the climate models generally simulate a too weak Southern Hemisphere STC which can be related to weaker southeasterly trade winds compared to observations. The results highlight the necessity to improve the ability of coupled climate models to realistically simulate the winds in the Southern Hemisphere, in order to better simulate the tropical‐subtropical ocean circulation. The results provide an important reference for studies on the mean state and variability of the Atlantic STCs, and for studies investigating the STCs' possible change in the future under global warming using coupled climate models.
Key Points
The performance of the Coupled Model Intercomparison Project Phase 6 climate models in simulating the mean Atlantic Subtropical Cells is evaluated
The model ensemble shows large biases toward weak southern hemisphere subtropical cell and weak Equatorial Undercurrent
The large inter‐model spread of the subtropical cell and Equatorial Undercurrent strength is associated with the southeasterly trade winds
In the boreal summer of 2021, the equatorial Atlantic experienced the strongest warm event, that is, Atlantic Niño, since the beginning of satellite observations in the 1970s. Such events have ...far‐reaching impacts on large‐scale wind patterns and rainfall over the surrounding continents. Yet, developing a paradigm of how Atlantic Niño interacts with the upper‐ocean currents and intraseasonal waves remains elusive. Here we show that the equatorial Kelvin wave associated with the onset of the 2021 Atlantic Niño modulated both the background flow and the eddy flux of the equatorial upper‐ocean circulation, causing an extremely weak and delayed tropical instability wave (TIW) season. TIW‐induced variations of sea surface temperature (SST), sea surface salinity, sea surface height, and eddy temperature advection were exceptionally weak during May to July, the climatological peak of TIW activity, but rebounded in August when higher than normal variability was observed. Moored velocity data at 23°W show that during the peak of the 2021 Atlantic Niño from June to August, the Equatorial Undercurrent was deeper and stronger than usual. An anomalously weak eddy momentum flux strongly suppressed barotropic energy conversion north of the equator from May to July, likely contributing to low TIW activity. Reduced baroclinic energy conversion also might have played a role, as the meridional gradient of SST was sharply reduced during the Atlantic Niño. Despite extremely weak TIW velocities, modest intraseasonal variability of chlorophyll‐a (Chl‐a) was observed during the Atlantic Niño, due to pronounced meridional Chl‐a gradients that partly compensated for the weak TIWs.
Plain Language Summary
Every few years the eastern equatorial Atlantic Ocean is significantly warmer than usual during boreal summer. Such warm events are referred to as Atlantic Niño events, and share similarities with El Niño events in the Pacific. In 2021, the strongest Atlantic Niño in at least four decades was observed in the equatorial Atlantic. This study is the first that investigates the complex interaction between Atlantic Niño, tropical Atlantic upper‐ocean currents, and equatorial waves based on various observational data sets. We show that the developing 2021 Atlantic Niño weakened both the background flow and the variability of near‐surface currents in May, which in turn largely reduced the strength of intraseasonal (20–50 days) waves that are usually generated by instability of the upper‐ocean zonal currents. As a consequence, the cooling effect that these waves usually have north of the equator and the warming effect along the equator vanished from May to July 2021. Interestingly, variability of chlorophyll concentration was enhanced, suggesting that enhanced meridional chlorophyll gradients compensated for reduced wave activity.
Key Points
The developing 2021 Atlantic Niño led to weaker equatorial surface currents and reduced vertical shear of upper‐ocean horizontal velocity
Strong reduction of the surface flow, eddy flux, and meridional temperature gradient in May caused extremely weak and delayed tropical instability wave (TIW) season
Reduced meridional TIW advection contributed to sharpen the north equatorial Chl‐a front resulting in modest intraseasonal Chl‐a variability
The shallow meridional overturning cells of the Atlantic Ocean, the subtropical cells (STCs), consist of poleward Ekman transport at the surface, subduction in the subtropics, equatorward flow at ...thermocline level and upwelling along the equator and at the eastern boundary. In this study, we provide the first observational estimate of transport variability associated with the horizontal branches of the Atlantic STCs in both hemispheres based on Argo float data and supplemented by reanalysis products. Thermocline layer transport convergence and surface layer transport divergence between 10°N and 10°S are dominated by seasonal variability. Meridional thermocline layer transport anomalies at the western boundary and in the interior basin are anti‐correlated and partially compensate each other at all resolved time scales. It is suggested that the seesaw‐like relation is forced by the large‐scale off‐equatorial wind stress changes through low‐baroclinic‐mode Rossby wave adjustment. We further show that anomalies of the thermocline layer interior transport convergence modulate sea surface temperature (SST) variability in the upwelling regions along the equator and at the eastern boundary at time scales longer than 5 years. Phases of weaker (stronger) interior transport are associated with phases of higher (lower) equatorial SST. At these time scales, STC transport variability is forced by off‐equatorial wind stress changes, especially by those in the southern hemisphere. At shorter time scales, equatorial SST anomalies are, instead, mainly forced by local changes of zonal wind stress.
Plain Language Summary
In both hemispheres of the Atlantic Ocean, shallow meridional overturning circulations provide a connection between the subtropics and equatorial upwelling regions. The so‐called subtropical cells (STCs) consist of poleward transport at the surface driven by the easterly trade winds, subduction in the subtropics, equatorward flow at subsurface level and upwelling along the equator and at the eastern boundary. In this study, we provide the first observational time series of transport variability associated with the horizontal branches of the STCs estimated at 10°N and 10°S. It shows that both branches are dominated by variability on seasonal time scales. On longer time scales, transport anomalies at the western boundary reveal a reversed relation to transport anomalies in the interior leading to partial compensation. It is suggested that transport anomalies are affected by adjustment to wind‐forced oceanic planetary waves. We further show that the interior part of the subsurface transport anomalies is connected to equatorial sea surface temperature (SST) anomalies at time scales longer than 5 years. There, stronger (weaker) equatorward transport is associated with negative (positive) equatorial SST anomalies. At shorter time scales, equatorial SST anomalies are, instead, mainly forced by changes of local wind stress.
Key Points
Observational transport time series of the Atlantic Subtropical Cells reveals dominant seasonal variability for horizontal branches
On time scales longer than ∼5 years, interior thermocline layer transport convergence modulates equatorial sea surface temperature anomalies
Western boundary current and interior transport anomalies are partly compensating each other at thermocline level on all time scales
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