Interannual variability in the South-East Atlantic Ocean, focusing on the Benguela Upwelling System: Remote versus local forcing (original) (raw)
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Journal Of Geophysical Research: Oceans, 2016
We investigate the respective roles of equatorial remote (Equatorial Kelvin Waves) and local atmospheric (wind, heat fluxes) forcing on coastal variability in the SouthEast Atlantic Ocean extending up to the Benguela Upwelling System (BUS) over the 2000-2008 period. We carried out a set of six numerical experiments based on a regional ocean model, that differ only by the prescribed forcing (climatological or total) at surface and lateral boundaries. Results show that at subseasonal timescales (<100 days), the coastal oceanic variability (currents, thermocline, and sea level) is mainly driven by local forcing, while at interannual timescales it is dominated by remote equatorial forcing. At interannual timescales (13-20 months), remotely forced Coastal-Trapped Waves (CTW) propagate poleward along the African southwest coast up to the northern part of the BUS at 248S, with phase speeds ranging from 0.8 to 1.1 m.s 21. We show that two triggering mechanisms limit the southward propagation of CTW: interannual variability of the equatorward Benguela Current prescribed at the model's southern boundary (308S) and variability of local atmospheric forcing that modulates the magnitude of observed coastal interannual events. When local wind stress forcing is in (out) of phase, the magnitude of the interannual event increases (decreases). Finally, dynamical processes associated with CTW propagations are further investigated using heat budget for two intense interannual events in 2001 and 2003. Results show that significant temperature anomalies (628C), that are mostly found in the subsurface, are primarily driven by alongshore and vertical advection processes.
Quarterly Journal of the Royal Meteorological Society, 2015
Tropical Atlantic sea-surface temperatures (SSTs) maximum intraseasonal variability (ISV) and their interaction with local surface winds are investigated, applying statistical analysis to observations and to a recent coupled reanalysis over the 2000-2009 decade. Five cores of strong ISV emerge, with standard deviation reaching about 1 • C in frontal areas of the three main upwelling systems: equatorial, Angola-Benguela and Senegal-Mauritania (the southern side of the Canary upwelling). West of 10 • W along the Equator, a 20-60-day peak caused by tropical instability waves is shown to generate surface wind anomalies through the adjustment of the horizontal surface pressure gradient in addition to the modification of near-surface atmospheric stratification. East of 10 • W along the Equator, an intense biweekly oscillation increases the ocean and atmosphere ISV. In the two coastal upwelling fronts, intraseasonal SST anomalies resemble each other. They are shown to be influenced by coastal Kelvin waves in addition to large-scale wind forcing. Over the Angola-Benguela upwelling, coastal wind bursts controlling the SST ISV are associated with anomalously strong pressure patterns related to the Madden-Julian Oscillation, the St Helena anticyclone and the Antarctic Oscillation. In the Senegal-Mauritania upwelling, the wind anomalies mainly linked to the Azores anticyclone in the southern front during November to May appear to be connected to the Saharan heat-low in the northern front from June to September. In all five regions and as expected for such upwelling regimes, vertical oceanic mixing represents the dominant term in the mixed-layer heat budget. In the equatorial band, as found in previous studies, horizontal advection is equally important, while it appears surprisingly weak in coastal fronts. Finally, a striking result is the general lack of surface wind signal related to the SST ISV in the coastal upwellings.
On the propagation of the Upper Ocean Heat Anomalies in the South Atlantic
Using remote sensed sea high anomalies and sea surface temperature, as well as climatological temperature and salinity data in conjunction with a regional reduced gravity model, maps of Upper Layer Heat Content (ULH) for the South Atlantic are generated. Prior to the computations of the ULH anomalies (ULHAs), the estimated seasonal cycle is calculated and then subtracted to the ULH maps. In order to study the propagation of ULHAs, Hovmöller diagrams at 25.5 o S, 30.5 o S, and along South American coast between 25.5 o S and 36.5 o S are plotted. Using visual inspection and two-dimensional spectrum technique on the Hovmöller diagrams we show that the ULHAs propagate westward with a period of 4.63 years in the Subtropical region with phase speed comparable to long first mode baroclinic Rossby waves. As they reach the South American East coast the ULHAs are advected southward by the Brazil Current (BC). We speculate the southward advection of these anomalies is probably related to the v...
Scientific Reports, 2019
The oceanic connection between the coastal variability along the southwestern African coasts and the linear equatorial dynamics at subseasonal time-scales (<120 days) is examined using a variety of model outputs, ranging from linear to general circulation models. We focus on the equatorially-forced fast and weakly dissipative first-mode coastal trapped waves which are shown to propagate down to the southern tip of Africa. In the eastern equatorial Atlantic, the first-mode equatorial forcing is tangled with the higher-order Kelvin wave modes and is overshadowed by the dominant second baroclinic mode. The latter is slower and peaks 10 days after the concealed first-mode contribution. Within this time frame, the remotely-forced first-mode coastal trapped waves impinge on the variability of the Benguela upwelling ecosystem, almost in phase with the subseasonal sea level fluctuations in the Gulf of Guinea. Over 1993–2008, the equatorial forcing undergoes a substantial interannual modu...
Evolution of Interannual Warm and Cold Events in the Southeast Atlantic Ocean
Journal of Climate, 2004
Extreme warm episodes in the southeast Atlantic Ocean, known as Benguela Niños, have devastating environmental impacts and have been shown to be remotely forced. To place these extreme events into perspective, the investigation is here extended to minor warm events as well as to cold episodes. To this end, different sets of observations have been combined with outputs from a numerical simulation of the tropical Atlantic for the period 1982-99. It is shown that both warm and cold surface events develop regularly in the same specific region along the coast of Angola and Namibia. Some cold events compete in magnitude with major warm episodes. Local sea-air heat flux exchanges do not seem to precondition the sea surface in the Angola-Benguela region prior to the arrival of an event. Most warm and cold episodes are large-scale events despite their limited surface signature. They appear to be generated by wind anomalies in the western and central equatorial Atlantic in the same way as Benguela Niños. Seasonal fluctuations of the depth and shape of the tropical thermocline seem partly to control the way subsurface anomalies eventually impact the surface. During austral summer, surface anomalies create an identifiable pool centered near 15ЊS, whereas in winter they show an elongated pattern along the coast stretching toward the equator. Local upwelling or downwelling favorable wind regimes, as well as local net heat fluxes, may modulate the surface expression of events.
2016
We quantify the wind contribution to the development of interannual sea surface temperature (SST) anomalies along the shelf of southern Africa. We compare numerical simulations that differ only in the amount of variability kept in the ERS1/2-derived surface wind forcing. Surprisingly, most of the cold and warm episodes over the Agulhas Bank are strictly related to local fluctuations of the forcing, whereas the shelf of the west coast extending 400 km north of Cape Columbine is equally sensitive to open-sea wind fluctuations. We diagnose the respective role of mesoscale eddy activity and of low frequency and intra-monthly wind fluctuations in generating interannual SST variability. The fair degree of correlation obtained at a few locations between the model and concomitant observations confirms the interest of a regional numerical tool to study anomalous events in the Benguela system.
Journal of Geophysical Research, 1997
Between 1966 and 1995, subsurface temperature data have been collected in the western North Atlantic Ocean using expendable bathythermographs. Data coverage is sparse in both time and space, but evidence for decadal variability in the upper 400 m of the water column is found. The data were averaged by month onto a 2 ø of latitude by 4 ø of longitude grid. Thirty-one quadrangles in the region bounded by 17øN and 43øN and 78øW and 66øW have sufficient data to provide consistent results. Anomaly time series at 0, 100, 200, 300, and 400 m were estimated by subtracting a mean monthly climatology. The individual records were detrended and filtered to highlight the longer-period signals. The analysis resulted in 25-year records (1969-1993) for study. Within the therrnocline of the subtropical gyre and the Gulf Stream at 100 and 200 rn, periods of predominately positive temperature anomaly end in 1971, 1982, and 1990, while periods of negative anomaly end in 1976 and 1985. Only the events ending in 1971, 1976, and 1990 are in the majority of the records at 300 and 400 rn. Most of the events also appear in the sea surface temperature (SST) records but are somewhat masked by significant noise at the surface. Meridional-vertical temperature sections through the subtropical gyre show that transitions from negative to positive anomaly events are characterized by a deepening of the isotherms throughout the section and transitions from positive to negative events by a rising of the isotherms. Significant lateral migration of the axis of the Gulf Stream, although possibly masked by the 2 ø averaging, is not necessary to explain either type of event. The transitions in the SST and 100-m temperature time series occur at essentially the same time as the transitions in an index of the North Atlantic Oscillation (NAO) that has also been detrended (NAO events are also observed at 300 and 400 m as described earlier. Periods of positive subsurface temperature anomaly are coincidental with periods of positive NAO index, and periods of negative subsurface temperature anomaly are coincidental with periods of negative NAO index. Thus earlier results showing connections between the NAO and western Atlantic SST at decadal timescales are now extended to at least 400 m in the water column. Trends were computed from the individual 25-year records. The trends at all depths are predominately negative north of 38øN and positive south of 38øN. Inferences from the horizontal distribution of the trends and results from earlier studies suggest that the 1969-1993 period may be a phase of a 30-to 50-year signal observed in the northern Atlantic since the beginning of the century. [1994], using more extensive data sets than those available to Bjerknes [1964], provided additional evidence to support the earlier findings. Although causal relationships have not yet been firmly established, the SST variability has been shown to be correlated with atmospheric anomalies on the same timescales Deser and Blackman, 1993;. In particular, Bjerknes [1964] and Kushnir [1994] suggested that the SST variability is related This paper is not subject to U.S. copyright. Published in 1997 by the American Geophysical Union.
Sea surface temperature anomalies on the Western South Atlantic from 1982 to 1994
Continental Shelf Research, 2001
Thirteen years of weekly sea surface temperature (SST) fields derived from satellite data (January 1982-December 1994 are used to investigate spatial and temporal variabilities of SST in the western region of the South Atlantic Ocean. The analysis is focused on the large-scale variations over the continental shelf and slope region with depths less than 1000 m, between 42 and 228S.
Linking wind and interannual upwelling variability in a regional model of the southern Benguela
Geophysical Research Letters, 2002
1] We quantify the wind contribution to the development of interannual sea surface temperature (SST) anomalies along the shelf of southern Africa. We compare numerical simulations that differ only in the amount of variability kept in the ERS1/2-derived surface wind forcing. Surprisingly, most of the cold and warm episodes over the Agulhas Bank are strictly related to local fluctuations of the forcing, whereas the shelf of the west coast extending 400 km north of Cape Columbine is equally sensitive to open-sea wind fluctuations. We diagnose the respective role of mesoscale eddy activity and of low frequency and intra-monthly wind fluctuations in generating interannual SST variability. The fair degree of correlation obtained at a few locations between the model and concomitant observations confirms the interest of a regional numerical tool to study anomalous events in the Benguela system.
Journal of Physical Oceanography, 2006
Interannual sea surface height variations in the Atlantic Ocean are examined from 10 years of highprecision altimeter data in light of simple mechanisms that describe the ocean response to atmospheric forcing: 1) local steric changes due to surface buoyancy forcing and a local response to wind stress via Ekman pumping and 2) baroclinic and barotropic oceanic adjustment via propagating Rossby waves and quasi-steady Sverdrup balance, respectively. The relevance of these simple mechanisms in explaining interannual sea level variability in the whole Atlantic Ocean is investigated. It is shown that, in various regions, a large part of the interannual sea level variability is related to local response to heat flux changes (more than 50% in the eastern North Atlantic). Except in a few places, a local response to wind stress forcing is less successful in explaining sea surface height observations. In this case, it is necessary to consider large-scale oceanic adjustments: the first baroclinic mode forced by wind stress explains about 70% of interannual sea level variations in the latitude band 18°-20°N. A quasi-steady barotropic Sverdrup response is observed between 40°and 50°N.