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
Abstract
The direct response of the tropical mixed layer to near-inertial waves (NIWs) has only rarely been observed. Here, we present upper-ocean turbulence data that provide evidence for a strongly ...elevated vertical diffusive heat flux across the base of the mixed layer in the presence of a NIW, thereby cooling the mixed layer at a rate of 244 W m
−2
over the 20 h of continuous measurements. We investigate the seasonal cycle of strong NIW events and find that despite their local intermittent nature, they occur preferentially during boreal summer, presumably associated with the passage of atmospheric African Easterly Waves. We illustrate the impact of these rare but intense NIW induced mixing events on the mixed layer heat balance, highlight their contribution to the seasonal evolution of sea surface temperature, and discuss their potential impact on biological productivity in the tropical North Atlantic.
Inclusive science in the South Atlantic Perez, Renellys; Garzoli, Silvia; Hummels, Rebecca ...
Communications earth & environment,
12/2023, Letnik:
4, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The South Atlantic Meridional Overturning Circulation initiative began as a grassroots effort to study the South Atlantic Ocean and its impact on climate. In striving towards this goal, it has also ...become a platform for the empowerment of women and international scientists.The South Atlantic Meridional Overturning Circulation initiative began as a grassroots effort to study the South Atlantic Ocean and its impact on climate. This Comment discusses how, in striving towards this goal, it has also become a platform for the empowerment of women and international scientists.
Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) is a multinational program initiated in 1997 in the tropical Atlantic to improve our understanding and ability to predict ...ocean‐atmosphere variability. PIRATA consists of a network of moored buoys providing meteorological and oceanographic data transmitted in real time to address fundamental scientific questions as well as societal needs. The network is maintained through dedicated yearly cruises, which allow for extensive complementary shipboard measurements and provide platforms for deployment of other components of the Tropical Atlantic Observing System. This paper describes network enhancements, scientific accomplishments and successes obtained from the last 10 years of observations, and additional results enabled by cooperation with other national and international programs. Capacity building activities and the role of PIRATA in a future Tropical Atlantic Observing System that is presently being optimized are also described.
Plain Language Summary
Long data records are essential for improving our understanding of the weather and climate, their variability and predictability, and how the climate may change in the future in response to anthropogenic greenhouse gas emissions. Climate variability in the tropical Atlantic Ocean has strong impacts on the coastal climate in particular and, consequently, the economies of the surrounding regions. Since 1997, the Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) program has maintained a network of moored buoys in the tropical Atlantic in order to provide instantaneous high‐quality data to research scientists and weather forecasters around the world. This paper describes PIRATA successes in terms of scientific discoveries and observing technology enhancements. Perspectives are also provided on PIRATA's role in the future Tropical Atlantic Observing System, currently under design, that will consist of a variety of coordinated measurements from satellites, ships, buoys, and other ocean technologies.
Key Points
We describe the sustained tropical Atlantic observing system PIRATA maintained through a multinational cooperative program in ocean science
Major scientific results and accomplishments obtained from the last 10 years are presented
The potential role of PIRATA in a future Tropical Atlantic Observing System is described
Abstract
Since the inception of the international South Atlantic Meridional Overturning Circulation initiative in the 21st century, substantial advances have been made in observing and understanding ...the Southern Hemisphere component of the Atlantic Meridional Overturning Circulation (AMOC). Here we synthesize insights gained into overturning flows, interocean exchanges, and water mass distributions and pathways in the South Atlantic. The overturning circulation in the South Atlantic uniquely carries heat equatorward and exports freshwater poleward and consists of two strong overturning cells. Density and pressure gradients, winds, eddies, boundary currents, and interocean exchanges create an energetic circulation in the subtropical and tropical South Atlantic Ocean. The relative importance of these drivers varies with the observed latitude and time scale. AMOC, interocean exchanges, and climate changes drive ocean warming at all depths, upper ocean salinification, and freshening in the deep and abyssal ocean in the South Atlantic. Long-term sustained observations are critical to detect and understand these changes and their impacts.
The Atlantic Subtropical Cells Inferred from Observations Tuchen, Franz Philip; Lübbecke, Joke F.; Schmidtko, Sunke ...
Journal of geophysical research. Oceans,
November 2019, 2019-11-00, 20191101, Letnik:
124, Številka:
11
Journal Article
Recenzirano
Odprti dostop
The Atlantic Subtropical Cells (STCs) are shallow wind‐driven overturning circulations connecting the tropical upwelling areas to the subtropical subduction regions. In both hemispheres, they are ...characterized by equatorward transport at thermocline level, upwelling at the equator, and poleward Ekman transport in the surface layer. This study uses recent data from Argo floats complemented by ship sections at the western boundary as well as reanalysis products to estimate the meridional water mass transports and to investigate the vertical and horizontal structure of the STCs from an observational perspective. The seasonally varying depth of meridional velocity reversal is used as the interface between the surface poleward flow and the thermocline equatorward flow. The latter is bounded by the 26.0 kg m−3 isopycnal at depth. We find that the thermocline layer convergence is dominated by the southern hemisphere water mass transport (9.0 ± 1.1 Sv from the southern hemisphere compared to 2.9 ± 1.3 Sv from the northern hemisphere) and that this transport is mostly confined to the western boundary. Compared to the asymmetric convergence at thermocline level, the wind‐driven Ekman divergence in the surface layer is more symmetric, being 20.4 ± 3.1 Sv between 10°N and 10°S. The net poleward transports (Ekman minus geostrophy) in the surface layer concur with values derived from reanalysis data (5.5 ± 0.8 Sv at 10°S and 6.4 ± 1.4 Sv at 10°N). A diapycnal transport of about 3 Sv across the 26.0 kg m−3 isopycnal is required in order to maintain the mass balance of the STC circulation.
Plain Language Summary
The Atlantic Subtropical Cells (STCs) are shallow wind‐driven overturning circulations connecting the tropics to the subtropical regions within the upper 300 m. In both hemispheres, they are characterized by equatorward transport at subsurface level and poleward transport in the surface layers. They are closed by upwelling at the equator and subduction in the subtropics. STCs are suggested to impact sea‐surface temperature variability in tropical upwelling regions thereby influencing, for example, precipitation patterns. The boundary between the two branches is approximated by the depth at which the meridional velocity reverses. The lower boundary of the deep equatorward branch is defined by an isoline of potential density. We find that at subsurface level, the equatorward branches converge in the tropics with more transport coming from the southern hemisphere. At the surface, a more symmetric divergence of water mass is observed in the tropics. The surface layers are also influenced by geostrophic transport generally counteracting the wind‐driven divergence. In total, the net surface divergence and the subsurface convergence yield a residual. It is suggested that this water mass volume deficit originates from below the STCs and enters the subsurface layers in the tropics where it is lifted to the surface.
Key Points
Equatorward and poleward transports associated with the Atlantic Subtropical Cells are estimated from observations and reanalysis data
Estimates show asymmetry in thermocline transports (three times more transport from the south) and symmetric flow divergence at the surface
Transport budget reveals a residual of 3 Sv likely linked to the upper‐layer flow of the Atlantic meridional overturning circulation
Upwelling velocities w in the equatorial band are too small to be directly observed. Here, we apply a recently proposed indirect method, using the observed helium isotope (3He or 4He) disequilibria ...in the mixed layer. The helium data were sampled from three cruises in the eastern tropical Atlantic in September 2005 and June/July 2006. A one‐dimensional two‐box model was applied, where the helium air‐sea gas exchange is balanced by upwelling from 3He‐rich water below the mixed layer and by vertical mixing. The mixing coefficients Kv were estimated from microstructure measurements, and on two of the cruises, Kv exceeded 1 × 10−4 m2/s, making the vertical mixing term of the same order of magnitude as the gas exchange and the upwelling term. In total, helium disequilibrium was observed on 54 stations. Of the calculated upwelling velocities, 48% were smaller than 1.0 × 10−5 m/s, 19% were between 1.0 and 2.0 × 10−5 m/s, 22% were between 2.0 and 4.0 × 10−5 m/s, and on 11% of upwelling velocities exceeded this limit. The highest upwelling velocities were found in late June 2006. Meridional upwelling distribution indicated an equatorial asymmetry with higher vertical velocities between the equator and 1° to 2° south compared to north of the equator, particularly at 10°W. Associated heat flux into the mixed layer could be as high as 138 W/m2, but this depends strongly on the chosen depths where the upwelled water comes from. By combining upwelling velocities with sea surface temperature and productivity distributions, a mean monthly equatorial upwelling rate of 19 Sv was estimated for June 2006 and a biweekly mean of 24 Sv was estimated for September 2005.
The mixed layer (ML) temperature and salinity changes in the central tropical Atlantic have been studied by a dedicated experiment (Cold Tongue Experiment (CTE)) carried out from May to July 2011. ...The CTE was based on two successive research cruises, a glider swarm, and moored observations. The acquired in situ data sets together with satellite, reanalysis, and assimilation model data were used to evaluate box‐averaged ML heat and salinity budgets for two subregions: (1) the western equatorial Atlantic cold tongue (ACT) (23°–10°W) and (2) the region north of the ACT. The strong ML heat loss in the ACT region during the CTE was found to be the result of the balance of warming due to net surface heat flux and cooling due to zonal advection and diapycnal mixing. The northern region was characterized by weak cooling and the dominant balance of net surface heat flux and zonal advection. A strong salinity increase occurred at the equator, 10°W, just before the CTE. During the CTE, ML salinity in the ACT region slightly increased. Largest contributions to the ML salinity budget were zonal advection and the net surface freshwater flux. While essential for the ML heat budget in the ACT region, diapycnal mixing played only a minor role for the ML salinity budget. In the region north of the ACT, the ML freshened at the beginning of the CTE due to precipitation, followed by a weak salinity increase. Zonal advection changed sign contributing to ML freshening at the beginning of the CTE and salinity increase afterward.
Key Points
Atlantic cold tongue development from May to July 2011 was examined
Diapycnal mixing is key process for cooling in western cold tongue region
Zonal advection is main contributor to mixed layer salinity changes
Sea surface temperatures (SSTs) in the eastern tropical Atlantic are crucial for climate variability within the tropical belt. Despite this importance, state-of-the-art climate models show a large ...SST warm bias in this region. Knowledge about the seasonal mixed layer (ML) heat budget is a prerequisite for understanding SST mean state and its variability. Within this study all contributions to the seasonal ML heat budget are estimated at four locations within the Atlantic cold tongue (ACT) that are representative for the western (0°N, 23°W), central (0°N, 10°W) and eastern (0°N, 0°E) equatorial as well as the southern (10°S, 10°W) ACT. To estimate the contribution of the diapycnal heat flux due to turbulence an extensive data set of microstructure observations collected during ten research cruises between 2005 and 2012 is analyzed. The results for the equatorial ACT indicate that with the inclusion of the diapycnal heat flux the seasonal ML heat budget is balanced. Within the equatorial region, the diapycnal heat flux is essential for the development of the ACT. It dominates over all other cooling terms in the central and eastern equatorial ACT, while it is of similar size as the zonal advection in the western equatorial ACT. In contrast, the SST evolution in the southern ACT region can be explained entirely by air-sea heat fluxes.
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