1. Western boundary circulation in the tropical South Atlantic and its relation to Tropical Atlantic Variability
Rebecca Hummels1, Peter Brandt1, Marcus Dengler1, Jürgen Fischer1
1GEOMAR Helmholtz Zentrum für Ozeanforschung, Kiel, Germany
Workshop Brazil-Germany, Fortaleza, Brazil,

2. Tropical Atlantic Variability
Two modes of climate variability
Meridional gradient mode
Zonal mode
interannual SST anomalies associated with these patterns are related to rainfall anomalies over the adjacent continents
Lets first start with clarifying what we mean with tropiacl Atlantic variability and why are we so interested in it? In the context of climate variability two patterns dominate the tropical regions namely the meridional as well as the zonal gradient mode. The inter annual variability of these SST patterns are related to rainfall anomalies over the adjacent continents on interannual timescales, which crucially affect the economy of these regions. The hope is that improved forecast skill of SST would improve predictions of rainfall anomalies. However, prediction of SST anomalies is still rather difficult, as various processes interact. one crucial factor is to understand the circulation system of this region and especially the variability of this system.
Kushnir et al. 2006

3. Tropical Ocean Dynamics - Lecture 12
Circulation in the tropical Atlantic is a superposition of 1) Meridional Overturning Circulation (MOC)
February 1, 2013
The circulation system in the tropical Atlantic is a superposition of three consituents. The first is the Meridional Overturning Circulation. Probably everybody is familiar with this picture of the conveyer belt connecting deep water formation sites at high latitudes with areas of upwelling mainly at lower latitudes. Within the tropical Atlantic the MOC is comprised of a warm water route bringing surface and intermediate waters to the north and a cold water route bringing cold deep water to the south.
Kuhlbrodt et al. 2007
Peter Brandt - GEOMAR

4. Tropical Ocean Dynamics - Lecture 12
Circulation in the tropical Atlantic is a superposition of 1) Meridional Overturning Circulation (MOC)
2) Shallow Subtropical-tropical Overturning Circulation
3) Wind driven circulation
Circulation in the tropical Atlantic is a superposition of 1) Meridional Overturning Circulation (MOC)
2) Shallow Subtropical-tropical Overturning Circulation
February 1, 2013
Subduction (blue)
Upwelling (green)
Equatorward transport (circles and numbers in [Sv])
Poleward Ekman transport (red arrows)
The second constituent of the circulation system in the tropical Atlantic are the shallow Subtropical Cells of both hemispheres. These connect subduction areas in the subtropics shaded here in blue with upwelling areas in the eastern equatorial region shaded in green. These cells are wind driven by the poleward Ekman transports draining water out of the equatorial region and forcing a compensating return flow. This compensation occurs mainly over the western boundary, although also interior pathways exist.
The third component is the wind driven circulation, which constitues of the subtropical gyres and is responsible for the complicated zonal current system at the equator.
Schott et al. 2004
Peter Brandt - GEOMAR

5. Circulation in the tropical Atlantic is a superposition of 1) Meridional Overturning Circulation (MOC)
2) Shallow Subtropical-tropical Overturning Circulation
3) Wind driven circulation
If we consider this complex circulation system for the tropical Atlantic it gets obvious that the interaction between both hemispheres is focused at the western boundary, which appears as a chokepoint. It seems reasonale that the best place to monitor the variability of the circulation system of the tropical Atlantic as part of the MOC and STCs is here, where everything seems to converge. This is the reason, why this region already attracted observational interest
 Interaction between the hemispheres is focused on the western boundary

6. Observations at 5°S and 11°S between 1990-2004:
9 research cruises: repeatedly occupied the 5°S and 11°S section
Mooring array at 11°S
Between 1990 and research cruises to this area have been undertaken to investigate the mean transports of the warm and cold water route as well as their variability. The sections at 5°S and 11°S which are shown here are slanted to observe the alongshore velocities and transport. So in the following I will mainly talk about alongshore velocities. To better resolve the variability of the warm and cold water route additionally a mooring array was deployed at 11°S.
Schott et al. 2005

7. Observations at 5°S and 11°S between 2000-2004: Mean state
cm/s
cm/s
The mean longitude depth sections consisting of 9 and 5 individual sections respectively are shown here. This is a remake of the pictures in Schott et al In the upper 1000m we see the NBUC flowing „northward“ in the central and intermediate water range and the DWBC flowing southward within the layers of the North Atlantic deep water. At 11°S the NBUC is already strongly developed and of similar strength then at 5°S. The structure in the NADW layer is somewhat different at this more southern section, but I will come back to this in a moment. Average transports of the sections were estimated to be ... But in general it seems that northward and southward flow are more or less compensating.
AS mentioned previously to better resolve the variability of this average picture the mooring array at 11°S was deployed.
Für Fragen es wurde gaussian weighted interpolation between mooring secments gemacht. 4 EOFs genutzt um Lücken in der Zeitserie zu füllen
Average transports at 5°S:
NBUC /- 5.3 Sv [=1 x106 m3s-1]
NADW /-10.1 Sv
Average transports at 11°S:
NBUC /-5.3 Sv
NADW /-5 Sv

8. Observations at 5°S and 11°S between 2000-2004: Variability
NBUC:
Average transport is similar to ship sections (23.3 Sv)
Seasonal variability : amplitude of annual and semi-annual harmonics < 2 Sv
Interannual variability: for the 4 years is estimated to +/- 1.2 Sv
Looking at the transport time series for the warm and cold water route shows quite some variability. The NBUC transport ranges between 10 and 40 Sv. However, the average transport is similar to the one estimated from the ship sections, seasonal variations small, inter annual variability small, but only a limited timeseries could be used to estimate the interannual variations
In the deeper layer, the transport variability is even larger ranging from 60 southward to 20 northward. Further analysis of this NADW timeseries showed a spectral peak at days which could be associated with deep eddies and led to the discovery that the transport at NADW levels is accomplished not by a constant current feature, but by
NADW:
Average transport similar to ship sections
Extremely large variability
Schott et al. 2005

9. Observations at 5°S and 11°S between 2000-2004:
Break up of DWBC in to eddies at around 8°S
Deep eddies. The constant DWBC seems to break up at around 8°S and from then on eddies accomplish the transport, leading to this large transport variability depending on the actual eddy state. After this intense observational period, other studies have further invesigated the variability in this region and its links to the large scale circulation
After this period of intense observations within this area, other studies have addressed the variability within this region.
Dengler et al. 2004

10. Other studies after observational period:
Biastoch et al. 2008
extremely close correspondence between AMOC strength at 6°S (red) and NBUC transport (black) in a model study
Biastoch find in a model study an extremely good correspondence of the variability in AMOC strength at 6°S and the NBUC transport varaibility over decades

11. Other studies after observational period:
In another study Biastoch find that the Agulhas leakage has increased within the past decades, bringing more saline and warmer waters into the South Atlantic. They find that these anomalies can already be traced in the thermocline waters within the NBUC in observations. Furthermore they point out that these waters have already begun to invade the North Atlantic an have potentially an implication for the future evolution of the MOC.
Biastoch et al. 2009
Salinity anomalies within the NBUC are related to the variability of the Agulhas leakage and might have implications for further evolution of MOC

12. Other studies after observational period:
Zhang et al. 2011
In another study by Zhang et al- they contructed a geostrophic transport timeseries of the NBUC from available historial hydrographic observations within the NBUC region and find strong interannual transport variability contradicting the findings of Schott. Surely the timeseries of Schott is rather time limited, nevertheless, it would have been nice if some overlap would exist between the two to confirm that the exact procedure of estimating the geostrophic transport in Zhang is adequate and to strengthen this finding of strong interannual variations
Strong interannual NBUC transport variability when constructing the geostrophic transport timeseries based on historical hydrographic observations in the NBUC region

13. Other studies after observational period:
Biastoch et al. 2008
Biastoch et al. 2009
2000
2005
2010
2015
All these results show that it would be of great interest to have a long timeseries of the NBUC region to monitor the variability in the transport, but also the variability in water mass properties and to relate this potential variability to variability in the AMOC strength, Agulshas leakage and so on. So thats the reason why we are going back there and continue the observations. Which new observations do we have so far?
It would be great if we can get a long timeseries of NBUC transport to confirm these results - maybe improve the estimate of Zhang by trying to mimic old and new transport timeseries with help of other observations geostrophic transport plus Altimeter data ?
Zhang et al. 2011

14. New observations : a) velocities
The 5°S section as well as the 11°S section were occupied delivering another snapshot as the ones shown in Schott et al, At 5°S the section kind of fits into the row of snapshots with a rather strong NBUC of 31.9Sv. Within the NADW layer the transport is lowest in the row of snapshots, but as velocities are highly varibale on this single sections, we can not make a strong statement about it at this place. Even more extreme are the observed snapshot at 11°S, where we hardly detect any transport in the NADW layer and also the NBUC seems to have a somewhat weaker signature.
At 11°S probably in the DWBC layer difference, but that can be explained with eddy state. So maybe we do not especially observe anything different at first glance like Zhang proposes, hard to say with large variance of mean lets look at T and S characteristics

15. New observations : a) velocities
The 5°S section as well as the 11°S section were occupied delivering another snapshot in the row shown in Schott et al, At 5°S the section kind of fits into the row of snapshots with a rather strong NBUC of 31.9Sv. Within the NADW layer the transport is lowest in the row of snapshots, but as velocities are highly varibale on this single sections, we can not really say anything about the variability here. At 11°S the transport in the NADW layer is also lowest in the row, the NBUC transport in the middle of the range as observed at 5°S. However these velocity snapshots are highly variable and we can not really deduce any strong statement for decadal velocity changes here. But lets look at T and S characteristics

16. New observations : b) water mass characteristics
As Biastoch claim that there is already a salinty trend detectable at 11°S, we wanted to have a look whether we can find any trends in our salinity and oxygen sections from 2013 in comparison to the period 10 years ago. We can calculate the difference in salinity in different ways. If we plot the difference in salinity on pressure surfaces, we get the following picture. So there are different trends detectable in the different layers. The significance of these trends was tested and is robust. However differences on pressure levels do not tell us the cause of these differences. Therefore we have to decompose the trends into a part which is caused by the heave of isopycnals, which can be caused by global warming, and real changes on density surfaces, which tell us that real water mass changes are taking place. So it seems thatthe main signal on pressure surfaces is really caused by water mass changes. Similar pictures for salinity changes can be obtained for 11°S, so this is a coherent signal. We can do the same for oxygen

17. New observations : b) water mass characteristics
Here again the changes are mainly caused by water mass changes. The highest increase in oxygen is found within the oxygen minimum zone whereas in similar density classes at the western boundary oxygen decreases. To quantify the changes on density surfaces we

18. New observations : b) water mass characteristics
5°S and 11°S °C
Biastoch et al., 2009
Averaging the difference in density classes over the entire section results in the follwing trends per decade: For oxygen
For salinity we find.....
If we recall the study of Biastoch and compare the values then the decadal salinity changes are in the range they reported, although the approaches and data sets are different.
As I said it is hard to say something yet about changes in velocitites or transports, as the velocity field is highly variable. But I can show you a first glimpse on the moorings timeseries which we have just recovered
Average differences (5°S and 11°S)
ΔO1 (27.7<γn<24.5) = 3 μmol/kg/decade
ΔO2 (28.135<γn<27.7) = 1.36 μmol/kg/decade
Average differences (5°S and 11°S)
ΔS1 (27.7<γn<24.5) = / decade
ΔS2 (28.135<γn<27.7) = /decade

19. New observations : c) mooring array
As I mentioned before the variability of velocities would be hard to investigate with sections as they are too variable. Hence,the mooring array at 11°S was redeployed in July and we will get back the first timerseries in April/May As we have just recovered the data sets, data processign will be done to construct the continuation of the transport timeseries. But as a first glimpse I can just show you the continutaion of two velocity timeseries. So what you see
Mooring array deployed in July 2013
Recovered and redeployed in April/May 2014

20. New observations: mooring array
cm/s K1 K2 K3 K4
Here you see the velocity timeseries of the two moorings K3 and K4 which was deployed between 2000 and The timeseries are both from the 1900m depth level and they show the large variability in the circulation caused by the passage of the eddies. Now we can compare this to the first year of new observations at the same locations and it seems that there is still large variability within this depth range. However further processing
 further processing necessary before providing the continuation of the transport timeseries

21. Further aims
estimate the northward transport of central and intermediate water within the NBUC as part of the AMOC and STC
I hope I could convince you that the western boundary region between 5°S and 11°S is an important indicator of circulation variability in the Atlantic on different timescales. Further aims of this subproject awhe

22. Further aims
estimate the northward transport of central and intermediate water within the NBUC as part of the AMOC and STC
monitor the transport variability of the NBUC on intraseasonal to interannual timescales
I hope I could convince you that the western boundary region at 5°S and 11°S is an important region in observing variability in the tropical Atlantic which is related to variability in the entire basin. Further aims of this study are:

23. Further aims
estimate the northward transport of central and intermediate water within the NBUC as part of the AMOC and STC
monitor the transport variability of the NBUC on intraseasonal to interannual timescales
analyse the connection between transport variations in the western boundary current system of the tropical South Atlantic (warm and cold water route) and the variability of the subpolar North Atlantic with respect to the signal propagation within the AMOC
I hope I could convince you that the western boundary region at 5°S and 11°S is an important region in observing variability in the tropical Atlantic which is related to variability in the entire basin. Further aims of this study are:

24. Further aims
estimate the northward transport of central and intermediate water within the NBUC as part of the AMOC and STC
monitor the transport variability of the NBUC on intraseasonal to interannual timescales
analyse the connection between transport variations in the western boundary current system of the tropical South Atlantic (warm and cold water route) and the variability of the subpolar North Atlantic with respect to the signal propagation within the AMOC
analyse the propagation of water mass anomalies in the AMOC, which can e.g. be caused by the variability in the Agulhas leakage
I hope I could convince you that the western boundary region at 5°S and 11°S is an important region in observing variability in the tropical Atlantic which is related to variability in the entire basin. Further aims of this study are:

25. Further aims
estimate the northward transport of central and intermediate water within the NBUC as part of the AMOC and STC
monitor the transport variability of the NBUC on intraseasonal to interannual timescales
analyse the connection between transport variations in the western boundary current system of the tropical South Atlantic (warm and cold water route) and the variability of the subpolar North Atlantic with respect to the signal propagation within the AMOC
analyse the propagation of water mass anomalies in the AMOC, which can e.g. be caused by the variability in the Agulhas leakage
analyse the connection between NBUC variability at 11° S and EUC variability at 23°W on the equator and its relevance for climate variability
I hope I could convince you that the western boundary region at 5°S and 11°S is an important region in observing variability in the tropical Atlantic which is related to variability in the entire basin. Further aims of this study are:
Whereas the last point brings me back to the first slide in that sense, that if transport variability within the NBUC is indeed connected to the variability in the EUC it might have an impact on SSTs there, which then influence rainfall variability.

27. m

28. New observations: b) T/S characteristics
If we normalize this difference by double the standard deviation we are outside of the 95% confidence limits for values over 1 in this plot. So yes these changes seem to be significant.The other thing we can check is whether the signal is coherent between the 5°S and 11°S signal and ...it is.