Discoveries of persistent coastal species in the open ocean shift our understanding of biogeographic barriers. Floating plastic debris from pollution now supports a novel sea surface community ...composed of coastal and oceanic species at sea that might portend significant ecological shifts in the marine environment.
The Somali Current system in the western Arabian Sea reverses seasonally with the South Asian Monsoon and is associated with localized upwelling cells or cold wedges during the summer southwest ...monsoon. Drifter trajectories in boreal summer and fall 2014 provide rare observational evidence that the northward Somali Current and associated cold wedges can persist into the boreal fall intermonsoon period. The near‐surface circulation and sea surface temperatures further suggest that the wedge‐like surface signatures may intermittently be capped and then reappear at a later time. Our observations show that the northward Somali Current system rapidly decayed within 1 week after the onset of the winter northeast monsoon and the arrival of a cyclonic eddy at the coast in early November 2014. This eddy may not only have affected regional ocean‐atmosphere interactions but also biogeochemical processes and the marine ecosystem through the transport of water properties and locally induced upwelling.
Plain Language Summary
The Indian Ocean rim countries are home to about one third of the global population and depend on rain‐fed agriculture as well as fisheries supported by coastal upwelling systems, which are linked to monsoon variability. A better understanding of regional air‐sea phenomena is therefore needed, and we report here on recent drifter observations in the western Arabian Sea that provide new insights into the persistence of upwelling cells or cold wedges and associated circulation patterns in the southern part of the Somali Current system, which reverses seasonally with the monsoon. Our observations during the 2014 drought year reveal that the northward Somali Current and associated cold wedges can persist into the intermonsoon period following the summer southwest monsoon. The wedge‐like surface signatures in the surface circulation and sea surface temperature may intermittently be capped and then reappear at a later time. During our observational period, the northward Somali Current system of the summer southwest monsoon rapidly decayed within 1 week after the onset of the winter northeast monsoon and the arrival of a mesoscale feature at the coast in early November. The latter may not only have affected regional ocean‐atmosphere interactions but also biogeochemical processes and the marine ecosystem.
Key Points
First observational evidence that the northward Somali Current and associated cold wedges may persist into the boreal fall intermonsoon
Rapid breakdown of the Somali Current system after the onset of the winter northeast monsoon and the arrival of a cyclonic eddy
Persistence of cold wedges and associated circulations may affect air‐sea interactions and biogeochemical processes within the ecosystem
Monsoon driven water mass exchange between the Bay of Bengal (BoB) and Arabian Sea (AS) is the common experience. However, it is not yet firmly confirmed that the exchange pathway is either passing ...through southern tip of Sri Lanka or Palk Strait. Local circulation patterns impact the pathways followed by the East Indian Coastal Currents (EICC) that drive exchange, thereby modulating mixing and water mass transformation in the Bay of Bengal around Sri Lanka. In this study, observations from surface drifters were incorporated with the satellite derived data to understand the monsoonal impact on circulation patterns in the Indian Ocean. This was the first multi-national scientific effort which was conducted in the BoB and AS during 2013 to 2015 to understand the monsoonal impact on circulation patterns in the complex region. The results indicated that seasonally reversing monsoonal currents of southern Sri Lanka, traced by the wintertime freshwater export pathways of the EICC. The deflection of monsoon currents running along the east coast of Sri Lanka by forming cyclonic and anti-cyclonic eddies, which influence the mixing and stirring associated with these flows. Results further indicate the low salinity cold water flows from the BoB to AS along the western boundary of the BoB during northeast monsoon. In the same way, reverses the phenomena during southwest monsoon, transporting high salinity warm water from AS to the BoB. This maintain the bay status which occurred due to freshwater influx from large rivers and high saline water from AS. However, no evidences were observed for the exchange through Palk Strait during the study. Also, there are some mis-matches in
in-situ
and remotely sensed measurements which imply the necessity of systematic observation system for the complex region as an alternative approach.
On the basis of moored (35°W, 0° and 23°W, 0°) and satellite observations, we contrast the upper equatorial Atlantic variability during 2002 with those during 2005. Inspection of the interannual ...boreal summer cold tongue variability revealed a warm (cold) event in 2002 (2005), with relaxed (intensified) winds in the west and the Equatorial Undercurrent (EUC) embedded in a shallower (deeper) thermocline at 23°W. During both years, 2002 and 2005, equatorial Kelvin waves are present in moored observations as 20°C isotherm depth anomalies and dynamic height anomalies. Basin‐wide sea surface height anomalies are used to derive an equatorial Kelvin wave mode. The time evolution of this mode represents the basis for a regression analysis to investigate related oceanic variability: Compared to the exceptionally strong wave activity in 2002, equatorial Kelvin waves were generally weaker during 2005. The main effect of equatorial Kelvin waves on zonal velocity anomalies at 23°W, 0° is evident well below the EUC core. Their direct influence on cold tongue sea surface temperature is small, but they are found to affect the equatorial thermocline slope. Prior to the cold tongue onset in 2002 (2005), the presence of equatorial Kelvin waves is associated with a flattened (steeper) thermocline slope that is crucial for the shallowing (deepening) of the EUC core at 23°W and that might precondition the development of the warm (cold) event.
Climate variability in the tropical Atlantic Ocean is determined by large-scale ocean-atmosphere interactions, which particularly affect deep atmospheric convection over the ocean and surrounding ...continents. Apart from influences from the Pacific El Niño/Southern Oscillation and the North Atlantic Oscillation, the tropical Atlantic variability is thought to be dominated by two distinct ocean-atmosphere coupled modes of variability that are characterized by meridional and zonal sea-surface-temperature gradients and are mainly active on decadal and interannual timescales, respectively. Here we report evidence that the intrinsic ocean dynamics of the deep equatorial Atlantic can also affect sea surface temperature, wind and rainfall in the tropical Atlantic region and constitutes a 4.5-yr climate cycle. Specifically, vertically alternating deep zonal jets of short vertical wavelength with a period of about 4.5 yr and amplitudes of more than 10 cm s(-1) are observed, in the deep Atlantic, to propagate their energy upwards, towards the surface. They are linked, at the sea surface, to equatorial zonal current anomalies and eastern Atlantic temperature anomalies that have amplitudes of about 6 cm s(-1) and 0.4 °C, respectively, and are associated with distinct wind and rainfall patterns. Although deep jets are also observed in the Pacific and Indian oceans, only the Atlantic deep jets seem to oscillate on interannual timescales. Our knowledge of the persistence and regularity of these jets is limited by the availability of high-quality data. Despite this caveat, the oscillatory behaviour can still be used to improve predictions of sea surface temperature in the tropical Atlantic. Deep-jet generation and upward energy transmission through the Equatorial Undercurrent warrant further theoretical study.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The Atlantic Equatorial Undercurrent (EUC) is studied using a simulation for the period 1990–2002 with a high‐resolution ocean general circulation model. Simulated transports of the EUC that supplies ...the annual mean upwelling in the central and eastern equatorial Atlantic are in good agreement with new transport estimates derived from ship observations, i.e., 19.9 and 14.0 Sv at 35°W and 23°W, respectively. Although the observations are not conclusive concerning the seasonal cycle of EUC transports, the simulated seasonal cycles fit largely in the observed range. The analysis of the EUC variability associated with interannual boreal summer variability of the equatorial cold tongue showed that cold tongue indices, defined either by near‐surface temperature or steric height anomalies, are anticorrelated with thermocline EUC transport anomalies: A strong EUC corresponds to low near‐surface temperatures and steric heights. The importance of equatorial waves for the cold tongue region is shown: Surface layer transport anomalies at 23°W and 10°W are significantly correlated with both near‐surface temperature and steric height anomalies in the equatorial and coastal upwelling regions, indicating an associated eastward phase propagation along the equator toward the African coast where the signal bifurcates into two poleward branches along the coast and is reflected into a westward propagating wave.
Abstract
Changes in the ventilation of the oxygen minimum zone (OMZ) of the tropical North Atlantic are studied using oceanographic data from 18 research cruises carried out between 28.5° and 23°W ...during 1999–2008 as well as historical data referring to the period 1972–85. In the core of the OMZ at about 400-m depth, a highly significant oxygen decrease of about 15 μmol kg−1 is found between the two periods. During the same time interval, the salinity at the oxygen minimum increased by about 0.1. Above the core of the OMZ, within the central water layer, oxygen decreased too, but salinity changed only slightly or even decreased. The scatter in the local oxygen–salinity relations decreased from the earlier to the later period suggesting a reduced filamentation due to mesoscale eddies and/or zonal jets acting on the background gradients. Here it is suggested that latitudinally alternating zonal jets with observed amplitudes of a few centimeters per second in the depth range of the OMZ contribute to the ventilation of the OMZ. A conceptual model of the ventilation of the OMZ is used to corroborate the hypothesis that changes in the strength of zonal jets affect mean oxygen levels in the OMZ. According to the model, a weakening of zonal jets, which is in general agreement with observed hydrographic evidences, is associated with a reduction of the mean oxygen levels that could significantly contribute to the observed deoxygenation of the North Atlantic OMZ.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Equatorial deep jets (EDJs) are a prominent flow feature of the equatorial Atlantic below the Equatorial Undercurrent down to about 3000 m. Here we analyze long‐term moored velocity and oxygen ...observations, as well as shipboard hydrographic and current sections acquired along 23°W and covering the depth range of the oxygen minimum zones of the eastern tropical North and South Atlantic. The moored zonal velocity data show high‐baroclinic mode EDJ oscillations at a period of about 4.5 years. Equatorial oxygen observations which do not resolve or cover a full 4.5‐yr EDJ cycle nevertheless reveal large variability, with oxygen concentrations locally spanning a range of more than 60μmol kg−1. We study the effect of EDJs on the equatorial oxygen concentration by forcing an advection‐diffusion model with the velocity field of the gravest equatorial basin mode corresponding to the observed EDJ cycle. The advection‐diffusion model includes an oxygen source at the western boundary and oxygen consumption elsewhere. The model produces a 4.5‐yr cycle of the oxygen concentration and a temporal phase difference between oxygen concentration and eastward velocity that is less than quadrature, implying a net eastward oxygen flux. The comparison of available observations and basin‐mode simulations indicates that a substantial part of the observed oxygen variability at the equator can be explained by EDJ oscillations. The respective role of mean advection, EDJs, and other possible processes in shaping the mean oxygen distribution of the equatorial Atlantic at intermediate depth is discussed.
Key Points
Equatorial deep jets strongly affect oxygen distribution/variability
Mean oxygen distribution in the equatorial Atlantic at intermediate depth
Gravest equatorial basin mode forces an advection‐diffusion model
The Global Drifter Program deployed a total of 144 Lagrangian drifters drogued at 15 m depth, including 88 equipped with salinity sensors, in support of the first Salinity Processes in the ...Upper-ocean Regional Study (SPURS-1) in the subtropical North Atlantic Ocean with the goal of measuring salt fluxes associated with surface currents. The quality-controlled data set consists of 996,583 salinity observations collected between August 2012 and April 2014. A comparison of the drifter salinities with Aquarius satellite sea surface salinity (SSS) data shows that the lifespan of the salinity sensor fitted to the drifters is of the order of one year. The salinity and velocity data from the drifters were used to validate salt transport divergence computed with satellite products, with satellite salinity taken from the standard Aquarius v3.0 data set. The results indicate good agreement between the two independent methods, and also demonstrate that the effect of the eddy field combined with SSS variability at the surface dominates the signal. SSS variability within spatial bins as compared to Aquarius-beam footprints measured by drifters can be in excess of 0.1 psu. This result suggests that careful evaluation of the representation error is required when single-point in situ measurements, such as those collected by Argo floats, are used to validate spatially averaged Aquarius salinity data.
Ocean surface radiation measurement best practices have been developed as a first step to support the interoperability of radiation measurements across multiple ocean platforms and between land and ...ocean networks. This document describes the consensus by a working group of radiation measurement experts from land, ocean, and aircraft communities. The scope was limited to broadband shortwave (solar) and longwave (terrestrial infrared) surface irradiance measurements for quantification of the surface radiation budget. Best practices for spectral measurements for biological purposes like photosynthetically active radiation and ocean color are only mentioned briefly to motivate future interactions between the physical surface flux and biological radiation measurement communities. Topics discussed in these best practices include instrument selection, handling of sensors and installation, data quality monitoring, data processing, and calibration. It is recognized that platform and resource limitations may prohibit incorporating all best practices into all measurements and that spatial coverage is also an important motivator for expanding current networks. Thus, one of the key recommendations is to perform interoperability experiments that can help quantify the uncertainty of different practices and lay the groundwork for a multi-tiered global network with a mix of high-accuracy reference stations and lower-cost platforms and practices that can fill in spatial gaps.
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