# Presentation: 497211

## Impact of Atlantic Ocean Circulation on Tropical Atlantic Climate and Variability

**Impact of Atlantic Ocean Circulation on Tropical Atlantic Climate and Variability**
- E. Campos1, R. Haarsma2, W. Hazeleger2, C. Severijns2
- and R. de Almeida1
**1 ****Oceanographic Institute – Univ. São Paulo (IOUSP**
**2 ****Royal Netherlands Meteorological Institute (KNMI)**

**VAIVAE Project**** **

## Main Objective

**Investigate the impact of the Atlantic ****Circulation on****:**
-     **Atlantic climatology **
**    ****Atlantic equatorial-cold tongue mode       **
**Two Experiments are Discussed**** **
**1. NOAGU: No Agulhas leakage**
**2. NOMOC: No Meridional Overturning Circulation (MOC)**

## These experiments are conducted by changing the Northern  and Southern boundaries of MICOM-Atlantic

**These experiments are conducted by changing the Northern  and Southern boundaries of MICOM-Atlantic**
- The **NOAGU** considers no “Agulhas Leakage” into the South Atlantic, by closing the eastern boundary to the south of Africa.
- In **NOMOC**, boundaries are taken from a **globally** coupled SPEEDY_MICOM run which has no MOC in the Atlantic due to the lack of deep water formation in the North Atlantic.
- For the control run the Northern and Southern boundaries are taken from Levitus data

## “Closing of Agulhas leakage”  experiment

**“****Closing of Agulhas leakage”  experiment**
- Change only the boundary conditions in the South Eastern corner of the MICOM-Atlantic basin.

- Temperature

-  Salinity

- Difference between  Closing Agulhas leakage and Control boundaries

**decreases up to 5.5 degrees in the SE corner of the domain !**

**decreases 0.6 **

## Experiment starts from control run.

**Response averaged over 10 years**
**after 30 years spin-up**

**SST difference annual mean**

- In the NOAGU case, the results  show a decreased temperature  distribution in the  SE quadrant of the South Atlantic.
- There are also some changes in the North Atlantic, which could be due to atmospheric teleconnections (?).

## SSS difference annual mean

**SSS difference annual mean**

- Greater impact in the whole South Atlantic, with considerable increase in the southern tropics (mainly in the mixed layer).

##     Tropical Atlantic

- Temperature difference
- in layer 8 (sigma=26.18)
- Contours indicate layer thickness

-     Tropical Atlantic
-     ocean circulation

**It looks like the signal is transported to the tropics by means of the lower part of the southern Subtropical Overturning Cell (STC), represented by the yellow line (but, why a positive anomaly???)**

## Increased salinity is due to reduced precipitation

**Salinity at 10s; 10w**

* **Hypothesis: *Decreased temperature in SE Atlantic leads to lower convection and, consequently, reduced precipitation (????).

##              Equatorial plane

-              Equatorial plane

- Equator, 40W

**Temperature difference (NOAGU- CONTROL)**

- NOAGU

- CONTROL

-  Temperature

## S

- T

-           **Pirata SW-extension**

- 34w; 19s

- 32w; 24s

- 30w; 8s

- To illustrate how convenient are the PIRATA SWE buoys for monitoring the anomalies. They are just in the way!

- See below the differences between model (dashed) and pirata data (solid lines)

-    Salinity

-              Equatorial plane

- Equator, 40W

- NOAGU

- CONTROL

**Salinity difference (NOAGU- CONTROL)**

## Total collapse of Meridional Overturning Circulation

**Total collapse of Meridional Overturning Circulation**
**All boundaries are now taken from global run without MOC. **

## Overturning (Sv)

**Overturning (Sv)**

-     Control

-     NO-NTHC

- Meridional heat transport (Pw)

- No-THC leads to a sharp decrease in meridional heat transport

- These graphics show the MOC
- in latitude X density space. The colors represent isohalines of Transport as function of density and latitude.
- (One should look at them upside down!).

- Control

- No-THC

## NO-THC minus CONTROL

- SST difference
- annual mean

- SSS difference
- annual mean

**Temperature:** The greater impact is in the northen North Atlantic, with decreases of the order of 10oC
- .

**Salinity:** The difference shows alternating zonal stripes of positive and negative salinity anomalies, up to 2.4 in magnitude.

## Change in T and S in NO-MOC experiment

- Equatorial Plane

- Equator; 20w

- T

**Change in T and S in NO-MOC experiment**

- CONTROL

- NO-MOC

- NO-MOC minus CONTROL

## Preliminary Results

- Cooling and freshening of equatorial thermocline
- Cause: advection of cooler and fresher water due to shutdown of cold water path into the Atlantic
- Limited impact on atmospheric equatorial climatology and variability
**NO MOC experiment:**
- equatorial thermocline becomes warmer and more saline
- Cause: Weaker MOC advects less cold and fresh water from the midlatitudes into the equatorial region.
- Large impact on equatorial climatology
- Disappearance of cold-tongue mode
- North Atlantic response: Enhancement and southward shift of tropospheric jet.

**Preliminary Results**

## FIM