# Presentation: 893678

## South Atlantic oceanographic and meteorological characteristics

*Claudia Schmid (NOAA/AOML, USA)*

**Notes:**
IRD = Institut de Recherche pour le Développement

## Outline

**Hydrography & Water masses**
**Circulation:**
**surface**
**intermediate depth**
**THC & inter-ocean exchange**
**Atmosphere ocean interaction & Meteorology**
**upwelling off Africa**
**forcing the circulation**
**correlations**
**South Atlantic tropical storms**

## Water masses

- Peterson & Withworth (1989)

## Meridional sections

- 30W

- 1E

**27.35**

**27.18**

**27.00**

**Notes:**
Figure 8. Quasi-meridional section along approximately 30°W, obtained during SAVE 5&6. PF, STF and SAF indicate the locations of the Polar, Subtropical, and Subantarctic fronts, respectively. Upper panel: potential temperature. Lower panel: Salinity, with the thick lines representing the isopycnals. sq = 27.00, 27.18 and 27.35 kg/m3. Figure reproduced from Schmid et al. (2000).

Figure 9. Quasi-meridional section along approximately 1°E, obtained during the AJAX expedition. PF, STF and SAF indicate the locations of the Polar, Subtropical, and Subantarctic fronts, respectively. Upper panel: potential
temperature. Lower panel: Salinity, with the thick lines representing the isopycnals. sq = 27.00, 27.18 and 27.35 kg/m3. Figure reproduced from Schmid et al. (2000).

## Zonal section at about 30oS

**27.35**

**27.18**

**27.00**

**Notes:**
Figure 10. Quasi-zonal section at about 30°S (WOCE line A10) in the South Atlantic obtained from the Meteor 22/5 cruise in austral autumn of 1994. The upper and lower panels show potential temperature and salinity, as in Figs 8 and 9. Reproduced from Schmid et al. (2000).

## Mean zonal currents at 35oW

- Schott et al (2003)

- Velocities in 700-900m

- EIC was defined as the westward flow under the EUC.
- EDJ make it hard to derive mean currents below the EUC due to their poorly understood variability on seasonal to interannual time scales.

**Notes:**
Mean zonal current distribution across 35W from the 13 sections, with transports (in Sv = 106 m3s1)  of the different current branches marked

Average EIC velocities at 700–900 m depth and in latitude range 1S–1N from 12 ship sections vs. time of year and fit of annual and semiannual harmonics and their superposition; also marked are shipname and year of survey.

## Intermediate depth circulation

**Intermediate depth circulation**

- Nunez-Riboni et al. (2005)

**Notes:**
I. Nu ́n ̃ez-Riboni et al. / Deep-Sea Research II 52 (2005) 545–564
Fig. 6. Average velocities for grid shown in Fig. 4. The corresponding 63% probability error ellipses are centered on the tip of each velocity arrow.

Fig. 7. Objectively mapped velociti es from float data within isoneutral layer (27.40). Grey arrows indicate eastward and black arrows Westward currents. A reference arrow of 5 cms1 is added.

Fig. 8. Stream function calculatedfrom float data within isoneutral layer. Contour values are in units of transport per depth (Svkm1).

## Intermediate depth Circulation

**Notes:**
Fig. 2. Schematic of AAIW circulation in the South Atlantic (adapted from You (1999)).

## Mean currents (20d integrated trajectories)

- Mean currents *(20d integrated trajectories)*

## Surface circulation

**Notes:**
Figure 2. Schematic representation of the circulation in the upper levels of the South Atlantic, based on Peterson and Stramma (1991) and Stramma and England (1999).

## Conveyor belt

- After Broecker (1991)

**Notes:**
Figure 3. Schematic diagram of the global ocean circulation pathways, the ‘conveyor belt’ (after W. Broecker).

## THC in the South Atlantic

- The compensating meridional flows

- After Stramma & England (1999)

**NADW (>2000m)**

**AAIW (500-1200m)**

**SACW (100-500m)**

**THC in the South Atlantic**

**Notes:**
NADW: the DWBC transports NADW from the northern Hemisphere into the South Atlantic.
AAIW: Originates in the South Atlantic from a surface region of the circumpolar layer, especially in the Northern Drake Passage and the Malvinas Current Loop. AAIW from the Indian Ocean is contributed to the South Atlantic AAIW via the Agulhas leakage.
SACW: forms in the Confluence region, Part of the SACW is not subducted in the Atlantic portion of the subtropical convergence but is in fact Indian central Water brought into the Atlantic Ocean by the Agulhas Current intrusions.

## Transports (from Gordon et al., 1992)

- Donners and Drijfhout (2004)

**T>9****o****C:**
**~63% from IO**
**T<9****o****C, p>1500dbar: ~56% from IO**

**Notes:**
Schematic circulation (a) from Gordon et al. (1992) in comparison with (b) the analog scheme for OCCAM. Black arrows indicate the thermocline layer (T . 98C); gray indicates the intermediate layer (T , 98C, shallower than 1500 dbar). The dashed line in (a) is the hydrographic section SAVE leg 4 used for the study of Gordon et al. The fluxes across the dashed line between the solid dots in (b) are calculated from two trajectory analyses (all mass fluxes are in Sverdrups). Underlined transports are calculated explicitly from hydrographic data; all other transports in (a) are estimated from chemical tracers and salinity.

## Transports from OCCAM

- Donners and Drijfhout (2004)

**T>9****o****C:**
**~73% from IO**
**T<9****o****C, p>1500dbar: ~69% from IO**

**Notes:**
Schematic circulation (a) from Gordon et al. (1992) in comparison with (b) the analog scheme for OCCAM. Black arrows indicate the thermocline layer (T . 98C); gray indicates the intermediate layer (T , 98C, shallower than 1500 dbar). The dashed line in (a) is the hydrographic section SAVE leg 4 used for the study of Gordon et al. The fluxes across the dashed line between the solid dots in (b) are calculated from two trajectory analyses (all mass fluxes are in Sverdrups). Underlined transports are calculated explicitly from hydrographic data; all other transports in (a) are estimated from chemical tracers and salinity.
Gordon et al. concluded that 60%–65% of the thermocline water in the Benguela Current originated from the Indian Ocean (Fig. 9), slightly less than in OCCAM (12.3 Sv/16.9 Sv = 73%), the other 35%–40% recirculates in the South Atlantic subtropical gyre.

## Intermediate depth transports (Schmid et al., 2000)

**AAIW: <=45% from IO?**

**Notes:**
Figure 11. Schematic representation of the AIW circulation in the South Atlantic, with transport estimates, based on Schmid et al. (2000). Three important regions for understanding the origins and pathways of the AIW in the South Atlantic are indicated (BISEC, BMC and Agulhas).

## Brazil-Malvinas Confluence

**Brazil-Malvinas Confluence**

**Notes:**
If the oceanic circulation is explained by the closure of the wind stress, all WBC should have similar intensity. BC is lees that 1/3 of the Gulf Stream. Stommel(1965) suggested that this was due to the THC.
BC weak and therefore the wind stress play an important role in the intensity of the BC transport.  The wind stress has an annual cycle and this cycle apparently follows the pattern of southward penetration of the BC.

Figure 7. The thermohaline front of the Brazil and Malvinas currents in the different seasons and the SST difference southernmost position–northernmost position. The images were provided by O. Brown and G. Podesta (RSMAS/Univ. of Miami).

**Eddies in the Agulhas Region**

- Boebel et al. (2003)

**Notes:**
RAFOS floats

## SSH movie

## Ring A

- Schmid et al. (2003)

## Eddy kinetic energy (cm2/s2)

**Eddy kinetic energy (cm****2****/s****2****)**

## Cape Cauldron

**Cape Cauldron**

## Wind and SLP anomaly off Africa

- Risien et al.(2003)

- January–March 2000 falls within the protracted 1998–2001 La Nina -> high pressure anomaly in mid-latitude South Atlantic and South Indian Oceans and low pressure anomaly over southern Africa, stronger easterlies over Benguela region.

**Notes:**
Figure 1. The composite anomaly (January–March 2000) of both wind, off the coast of southern
Africa, and sea level pressure (SLP) over the South Indian/South Atlantic sectors of the Southern
Hemisphere, as derived from the 2.5 resolution NCEP/NCAR reanalysis data set [Kalnay et al., 1996].
The contour intervals are 0.3 m s1 and 1  102 hPa, respectively.

It should be noted that the period of analysis falls within the protracted 1998–2001 La Nin ̃a episode. Typically, high-pressure anomalies develop over the midlatitude South Atlantic and South Indian Oceans during La Nin ̃as with low-pressure anomalies over southern Africa [Reason et al., 2000], and hence stronger south easterlies tend to occur over the Benguela region (Figure 1).

2003JGR_01.pdf

## Upwelling regions off Africa & wind variability

- Risien et al.(2003)

- Lutjeharms and Meeuwis (1987)

- pulses of stronger winds during the April–October 2000 period

- Benguela

- 34oS

- 26oS

- 22oS

- 12oS

- 31oS

- 17oS

**Variability at periods:**
** ****4 – 16 days in 10-23.5****o****S**
**4-12 and 25-50 days in 24-35****o****S**

**Notes:**
Variability at periods:
4 – 16 days in 10-23.5 S
4-12 and 25-50 days in 24-35S

Figure 2. Distribution of the well-defined upwelling regions in the southeast Atlantic Ocean. Each dot represents the center of an upwelling event observed over a period of 156 weeks [after Lutjeharms and Meeuwis, 1987].

Figure 3. Feather plots of each of the six distinct wind regions (10–15, 15.5–18.5, 19–23.5, 24–28.5, 29–32.5, and 33–35S) as identified via the SOM analysis (01 August 1999 to 29 November 2000).
2003JGR_01.pdf

## Angola-Benguela Front

- John et al. (2004)

- quiescent situation

- after strong upwelling

**Notes:**
Fig. 3. Surface chlorophyll-a concentrations at the end of the quiescent situation (left panel), and at the time the biological samples were taken (right panel) 1 week after the onset of strong winds and upwelling.

## Angola-Benguela Front

- John et al. (2004)

- SST and faunistic advection due to the anticyclonic circulation off Angola.

**Notes:**
Fig. 15. The sea surface temperature distribution at the time of surveying the southern transect, overlaid by a conceptual model suggesting advective paths of faunistic elements relative to the anticyclonic circulation at the tip of the Angola Current intrusion into northern Namibian waters. Abbreviations are the same as for Figs. 8 and 9, respectively, and Table 3.
N= neritic, O= oceanic, T = tropical, S = southern, U= ubiquitous or uncertain

## Correlation of SSTA and Wind

**Correlation of SSTA and Wind**

- Enfield and Mestas-Nunez (2000)

**Notes:**
Enfield, D.B., and A.M. Mestas-Nunez (2000)
Fig. 7: Second mode of a singular vector decomposition (SVD) analysis of sea surface temperature anomalies (SSTA) and Xie and Arkin (1996) rainfall anomaly over the spatial domains shown in the upper and lower panels, respectively. The upper panel shows the homogeneous correlation map for gridded SSTA vs. the temporal expansion coefficients for SSTA (colored shading).

## SSTA and SLPA

**SSTA and SLPA**

-  Sterl and Hazeleger (2003)

- Based on 52 years of data (1949–2000) from the NCEP/NCAR
- Reanalysis (Kalnay et al. 1996).

- maxima of SST coincide with regions in which the pressure has a large gradient

**Notes:**
Fig. 1 Patterns of the first two EOFs of SST anomalies (colors, in K) with the climatological SST superimposed (contours, in C). Explained variance is displayed on top of each panel

Fig. 2 Patterns of the first two EOFs of SLP anomalies (colors, in hPa) with the climatological pressure superimposed (contours, in hPa). Explained variance is displayed on top of each panel

Fig. 3 First two leading modes of a combined SVD analysis of anomalies of SST (colors, in K) and SLP (contours, negative contours dashed, in hPa)

To investigate the combined variability of ocean and atmosphere we perform a maximum covariance analysis on SST and SLP using singular value decomposition (SVD). This technique searches for modes that explain as much as possible of the mean-squared temporal covariance between the two fields (Bretherton et al. 1992). Figure 3 shows the first two SVD modes. Clearly, there is a close correspondence between the respective first EOF patterns (see Figs. 1a, 2a) on one side and the first SVD patterns on the other.
Furthermore, the first PCs of the SVDs and those of the corresponding EOFs are highly correlated and their spectra are almost identical (not shown). For the second SVD mode the correspondence with the respective EOFs is much lower, although the corresponding PCs are still significantly correlated. The dissimilarity between the second SLP SVD and the second SLP EOF may be
taken as additional evidence for the second EOF to be an artifact of the analysis technique.
From Fig. 3 it is obvious that the maxima of SST coincide with regions in which the pressure has a large gradient. This suggests that the occurrence of SST anomalies might be related to changes in wind or wind stress. This idea is further investigated in Sect. 4, where we try to identify mechanisms that force the main variability patterns described so far into existence, as well as those that destroy them.
climdyn21_07.pdf

## Lagged regression SSTA (colors) and SLPA (10 hPa)

-  Sterl and Hazeleger (2003)

- Build-up and destruction of the anomalies takes 6 months. Relationship strong during build-up, weak during destruction phase.

**Notes:**
Fig. 5 Sequence of lagged regressions of anomalies of SST (colors, in K), SLP (contours, spacing = 10 hPa, and wind stress (arrows) on the PCs of the SST SVDs from Fig. 3. Lag is positive when the Principal Component (PC) leads

To get more insight into the temporal evolution of the anomaly patterns described in Sect. 2.3, we now present sequences of lagged regression patterns between the PCs of the SST SVDs and quantities describing the coupled evolution of oceanic and atmospheric anomalies, namely SST, SLP, and wind stress. This is done in Fig. 5. At lag zero, one can recognize the SVDs presented before. At negative lags, i.e., at times preceding the maximum of the patterns considered, we see how they gradually built up over 6 months. Likewise, the destruction of the anomalies takes 6 months, too. This time scale of about half a year is consistent with the e-folding time of the auto-correlation function of the SST PCs (not shown). Based on this e-folding time and a corresponding reduction in the degrees-of-freedom, a t-test reveals (not shown) that regression values exceeding 0.15 K (SST) or 15 hPa (SLP) are significant at the 95%-level.
Figure 5 shows a clear relation between SST anomalies and wind stress anomalies during the built-up phase, with maximum wind stress anomalies appearing over areas of maximum SST anomalies. During the destruction phase, such a relation is less apparent, if not absent. This suggests that wind (or wind stress) plays a forcing role in generating SST anomalies, but not in destroying them, meaning that SST changes do not feed back on the atmospheric circulation in a systematic way. Wind (stress) can affect SST through several mechanisms, namely by enhancing the latent heat flux, by inducing currents that advect SST anomalies, and by inducing vertical mixing that deepens the mixed layer.
climdyn21_07.pdf

## Lagged regression SSTA (colors) and

-  Sterl and Hazeleger (2003)

- Heat flux acts to dampen (positive heat flux -> cooling) or enhance (negative heat flux) the SSTA anomalies.

**total heat flux (3e+7 Km/s)**

**wind stirring (3e+7 Km/s)**

- the regions of maximum SSTA and maximum wind stirring are displaced relative to one another

**Notes:**
Fig. 6 Sequence of lagged regressions of anomalies of SST (colors, in K) and total heat flux (–Q/(qwcp)), contours, spacing= 3Æ10–7 Km/s) on the PCs of the SST SVDs from Fig. 3. Lag is positive when the PC leads

Fig. 7 Sequence of lagged regressions of anomalies of SST (colors, in K) and meridional Ekman heat transport (contours, spacing = 1Æ10–7 Km/s) on the PCs of the SST SVDs from Fig. 3. Lag is positive when the PC leads

Therefore the patterns have no direct physical meaning, but they show whether the heat flux acts to enhance or to damp the SST anomalies. As a positive heat flux cools the ocean, the heat flux damps the SST anomalies if both have the same sign, and forces them if the signs are opposite. For SVD-1 opposite signs at negative lags occur over most of the domain, most notably in the South Equatorial Current, where this SVD has its maximum signal. For SVD-2 forcing occurs in the southwestern part of the basin, also a region of maximum signal. At lag 0 and at positive lags, heat flux is damping the SST anomalies. Thus in both cases the heat flux acts to create the SST anomaly (opposite sign of SST and heat flux anomaly at negative lags), but helps to destroy it after it has reached its maximum (equal signs at positive lags).  Splitting the total heat flux up into its different components reveals that the latent heat flux is by far the most important one. The corresponding regression patterns look almost as those obtained for the total heat flux (not shown).

For SVD-1 the patterns of heat flux and SST in the South Equatorial Current region coincide for several months. The heat flux thus systematically forces the SVD-1 SST pattern there. A comparison with Fig. 5 shows that in the same region also the anomalous wind (stress) reaches its maximum. The latter is directed northwestward, enforcing the climatological wind that can be inferred from the climatological SLP shown in Fig. 2. The anomalous wind has two impacts on the heat flux. Firstly, it increases the advection of cold air from the southeast into the region in question, and secondly, it increases the wind speed. Both changes increase the turbulent heat loss. Thus the anomalous heat flux forcing of the SVD-1 pattern is caused by an anomalous atmospheric circulation.

As has been shown in Sect. 3.3, heat flux in the extratropics is dominated by latent heat flux.

climdyn21_07.pdf

## Lagged regression SSTA (colors) and

-  Sterl and Hazeleger (2003)

**Ekman pumping (1e+7 Km/s)**

**merid. Ekman Tr. (1e+7 Km/s)**

- Impacts of Ekman heat transport and Ekman pumping are at least a factor of three smaller than heat flux.

**Notes:**
Fig. 8 Sequence of lagged regressions of anomalies of SST (colors, in K) and Ekman pumping (DT = 2 K, contours, spacing = 1Æ10–7 Km/s) on the PCs of the SST SVDs from Fig. 3. Lag is positive when the PC leads

Fig. 9 Sequence of lagged regressions of anomalies of SST (colors, in K) and wind stirring (–au3 *, contours, spacing = 3Æ10–7 Km/s) on the PCs of the SST SVDs from Fig. 3. Lag is positive when the PC leads
climdyn21_07.pdf

## South Atlantic SSTA+ dipole

- Haarsma et al (2003)

**Notes:**
Figure 1. The South Atlantic SSTA+ dipole (K).

2003GL017829.pdf
The atmosphere model is forced with the SSTA pattern shown in Figure 1. It is obtained from an SVD analysis of MSLP and SST of the output of SPEEDY coupled to the Miami Isopycnal Coordinate Model (MICOM) [Haarsma et al., manuscript in prepration, 2003]. This pattern is very similar to the one obtained by Sterl and Hazeleger [2003] from the NCEP-NCAR reanalysis. We have chosen this pattern because it is consistent with the dynamics of SPEEDY. The amplitude of this pattern, with maximum values of 1.5C, is about 4 times the standard deviation of the SST pattern in the coupled run in order to obtain a significant response. In addition, an experiment with half of the SSTA amplitude is made. 50 year integrations with a positive (SSTA+) and a negative (SSTA) SSTA dipole were made and compared with the control run of the same length. The dipole is called positive when the equatorward pole has positive SSTA’s.

The atmospheric response to the SSTA+ dipole is seasonally dependent. Most prominent is the response centred at 30W 15S during the austral summer (Jan.–March) (Figure 2). This response consists of an anomalous low pressure system and cyclonic circulation at low levels. The upper air response at 200 hPa displays anomalous anticyclonic circulation. This response is similar to the deep baroclinic response obtained by Robertson et al. [2003]. During the austral winter the response is still baroclinic but mainly confined to the lower half of the troposphere.

The location of the response suggests that it is mainly due to the equatorward SSTA pole. To check this we repeated the experiment for separately the equatorward and poleward poles of the SSTA dipole. The results confirmed that the deep baroclinic response is almost entirely due to the equatorward SSTA pole. The center of the anomalous low pressure system is not located above the maximum of the SSTA, but is translated about 10 degrees to the northwest in the direction of the climatological trade winds. The atmospheric response to the poleward SSTA pole is small and will be discussed later.

The seasonal dependence of the response is related to the changes in the meridional circulation and the vertical stability. During the austral winter the decending branch of the meridional circulation is situated over the subtropical Atlantic. Thus, the concurrent enhanced vertical stability prohibits the development of a deep baroclinic response. Because the atmospheric response is largest during Jan.–March, we will concentrate in the following on that season.

Changing the sign of the SSTA pattern did not have a large impact on the local response, indicating that it is mainly linear. This was also confirmed by analyzing the response to the SSTA dipole with half the amplitude. To facilitate the discussion we will describe below the response to the SSTA+ dipole.

The 500 hPa vertical velocity shows anomalous upward (downward) motion over positive (negative) SSTA’s. In addition, a dipole of anomalous upward and downward motion exists along the ITCZ. Associated with it the anomalous precipitation (Figure 3a) shows a strong signal, with maxima of about 1.5 mm day1, along the ITCZ, consisting of anomalous drying at the location of the anomalous descend, northward of it, and enhanced precipation at the location of anomalous ascend, southward of it. This results in a southward displacement of the ITCZ towards the warm SSTA’s.

The anomalous vertical circulation is initiated by the anomalous diabatic heating of the SSTA dipole. However, the strong signal along the ITCZ is the result of a positive feedback due to the anomalous convergence of moisture (Figure 3b). The anomalous northerly winds advect dry subtropical air into the northern regions of the ITCZ, whereas at the southern edge of the ITCZ tropical moisture is advected southward into the subtropics.

The shift in the position of the ITCZ is accompanied by changes in cloud cover with maxima in the order of 10% (Figure 4b). Over the areas of anomalous subsidence (ascend) the cloud cover is diminished (enhanced). This increases (reduces) the net downward radiation in the order of 10 Wm2, principally as a result of increased (reduced) solar radiation.

The anomalous low level northerly winds north of the equator enhance the prevailing trade winds, resulting in an increase of the latent heat flux in the order of 5 Wm2, cooling the surface waters. This effect is in the literature known as the wind-evaporation-SST (WES) feedback [Chang et al., 1997; Kushnir et al., 2002] and supposed to be responsible for the generation of an SST gradient across the equator. However in our simulation this effect is over compensated by the increased solar radiation and the net effect is a small heating of about 5 Wm2 of the ocean surface north of the equator. Although a coupled ocean-atmosphere experiment would really be required, this indicates that the South Atlantic dipole does not invoke a meridional SST dipole across the equator as proposed by the hypothesis of the WES feedback. Because of the southerly position of the SSTA, the anomalous winds northward of the equator are too weak to effectively trigger the WES feedback. On the other hand, the SSTA does affect the vertical circulation over the Atlantic and thereby the cloud cover distribution.

The net surface heatflux (Figure 4a), with a major contribution of the latent heatflux, results in a strong cooling (heating) of the warm (cold) SSTA’s. For the equatorward pole of the SSTA, this is, however, confined to the southeastern part. The anomalous warm surface air temperature and increased moisture in the deep baroclinic response downstream strongly reduce the anomalous latent heat flux to values in the order of 10 Wm2. This spatially varying anomalous latent heatflux affects the structure of the SSTA and effectively results, apart from a reduction, in a northwestward displacement of the equatorward pole of the SSTA. This suggests that the atmospheric response, next to oceanic response, is important for the time evolution of SSTA’s. This will be investigated in future research.

The response to the poleward SSTA pole south of 30S, as revealed by the experiment with only the poleward pole, is located above the SSTA, with a high (low) over the cold (warm) SSTA’s. The response is small, about 1 hPa for the MSLP, which is in the order of the natural variability of the model for a 50 year integration. The maximum response in the precipitation is about 0.5 mm day1. To better evaluate the structure of the response we doubled the amplitude of the SSTA. The results revealed that the structure is in all seasons equivalent barotropic, which is common for a midlatitude response, and strongly reduced above 500 hPa.

With respect to the remote extra-tropical response in the Northern Hemisphere, our calculations are still inconclusive. In our results, we detect at the equator an upper level divergence generated by the anomalous vertical circulation. This upper level divergence intiates the propagation of energy by Rossby waves into the extra-tropics, which by its turn enhances the variability over the North Atlantic (Figure 5). In our present resuts, however, the stationary response over the North Atlantic is not yet statistically significant. This might be due to the fact that the location of the anomalous upper air divergence shows variability in time. In addition climate signals do evolve in time. An analysis in the frequency domain would clarify this and will be part of future research. Our primary investigations indicate that at certain periods, the upper level divergence induces a wave train in the subtropical jet over southern part of the Asian continent, whereas in other periods a wave train over the Atlantic is generated.

## Atmospheric pressure and wind anomalies in response to the SSTA+ dipole

- Haarsma et al (2003)

- Most prominent is the response centered at 30W 15S during the austral summer (Jan.–March), mainly due to the equatorward SSTA pole (anomalous low MSLP and cyclonic circulation, anomalous anticyclonic circulation at 200 hPa).

**Notes:**
Figure 2. Jan.–Mar. anomalies of A, MSLP (hPa) and B, 200 hPa streamfunction (106 m2s1). The arrows indicate the wind anomalies (ms 1) at A, MSLP and B, 200 hPa.

2003GL017829.pdf

## Anomalies of precipitation and near-surface wind in response to the SSTA+ dipole

- Haarsma et al (2003)

- Anomalous low (high) precipitation associated with anomalous advection of moisture (dry -> N, moist -> S) causing anomalous down- (up-) welling -> ITCZ is displaced southward towards warm SSTA.

**Notes:**
Figure 3. Jan.–Mar. anomalies of A, precipitation (mm day1) B, near surface wind (ms1) and mean specific humidity (g Kg1).

The 500 hPa vertical velocity shows anomalous upward (downward) motion over positive (negative) SSTA’s. In addition, a dipole of anomalous upward and downward motion exists along the ITCZ. Associated with it the anomalous precipitation (Figure 3a) shows a strong signal, with maxima of about 1.5 mm day1, along the ITCZ, consisting of anomalous drying at the location of the anomalous descend, northward of it, and enhanced precipation at the location of anomalous ascend, southward of it. This results in a southward displacement of the ITCZ towards the warm SSTA’s.

The anomalous vertical circulation is initiated by the anomalous diabatic heating of the SSTA dipole. However, the strong signal along the ITCZ is the result of a positive feedback due to the anomalous convergence of moisture (Figure 3b). The anomalous northerly winds advect dry subtropical air into the northern regions of the ITCZ, whereas at the southern edge of the ITCZ tropical moisture is advected southward into the subtropics.

2003GL017829.pdf

## Anomalies of net upward surface heat flux and cloud cover in response to the SSTA+ dipole

- Haarsma et al (2003)

- Shift of ITCZ -> shift of cloud cover, but no significant change of NSHF since SSTA is too far south. Over the SSTA anomalies significant NSHF change mainly due to latent heat flux.

**Notes:**
Figure 4. Jan.–Mar. anomalies of A, net upward surface heat flux (Wm2) and B, total cloud cover (%).

The shift in the position of the ITCZ is accompanied by changes in cloud cover with maxima in the order of 10% (Figure 4b). Over the areas of anomalous subsidence (ascend) the cloud cover is diminished (enhanced). This increases (reduces) the net downward radiation in the order of 10 Wm2, principally as a result of increased (reduced) solar radiation.

The anomalous low level northerly winds north of the equator enhance the prevailing trade winds, resulting in an increase of the latent heat flux in the order of 5 Wm2, cooling the surface waters. This effect is in the literature known as the wind-evaporation-SST (WES) feedback [Chang et al., 1997; Kushnir et al., 2002] and supposed to be responsible for the generation of an SST gradient across the equator. However in our simulation this effect is over compensated by the increased solar radiation and the net effect is a small heating of about 5 Wm2 of the ocean surface north of the equator. Although a coupled ocean-atmosphere experiment would really be required, this indicates that the South Atlantic dipole does not invoke a meridional SST dipole across the equator as proposed by the hypothesis of the WES feedback. Because of the southerly position of the SSTA, the anomalous winds northward of the equator are too weak to effectively trigger the WES feedback. On the other hand, the SSTA does affect the vertical circulation over the Atlantic and thereby the cloud cover distribution.

The net surface heatflux (Figure 4a), with a major contribution of the latent heatflux, results in a strong cooling (heating) of the warm (cold) SSTA’s. For the equatorward pole of the SSTA, this is, however, confined to the southeastern part. The anomalous warm surface air temperature and increased moisture in the deep baroclinic response downstream strongly reduce the anomalous latent heat flux to values in the order of 10 Wm2. This spatially varying anomalous latent heatflux affects the structure of the SSTA and effectively results, apart from a reduction, in a northwestward displacement of the equatorward pole of the SSTA. This suggests that the atmospheric response, next to oceanic response, is important for the time evolution of SSTA’s. This will be investigated in future research.

The response to the poleward SSTA pole south of 30S, as revealed by the experiment with only the poleward pole, is located above the SSTA, with a high (low) over the cold (warm) SSTA’s. The response is small, about 1 hPa for the MSLP, which is in the order of the natural variability of the model for a 50 year integration. The maximum response in the precipitation is about 0.5 mm day1. To better evaluate the structure of the response we doubled the amplitude of the SSTA. The results revealed that the structure is in all seasons equivalent barotropic, which is common for a midlatitude response, and strongly reduced above 500 hPa.

With respect to the remote extra-tropical response in the Northern Hemisphere, our calculations are still inconclusive. In our results, we detect at the equator an upper level divergence generated by the anomalous vertical circulation. This upper level divergence intiates the propagation of energy by Rossby waves into the extra-tropics, which by its turn enhances the variability over the North Atlantic (Figure 5). In our present resuts, however, the stationary response over the North Atlantic is not yet statistically significant. This might be due to the fact that the location of the anomalous upper air divergence shows variability in time. In addition climate signals do evolve in time. An analysis in the frequency domain would clarify this and will be part of future research. Our primary investigations indicate that at certain periods, the upper level divergence induces a wave train in the subtropical jet over southern part of the Asian continent, whereas in other periods a wave train over the Atlantic is generated.

2003GL017829.pdf

4. Discussion
In this study we investigated the atmospheric response to an SSTA dipole, which is generated by the dominant mode of variability of the atmosphere in the South Atlantic. Our results show that especially the equatorward pole of the dipole has a significant impact on the atmospheric circulation over the tropical Atlantic. Although our results should be confirmed by other studies with GCM’s, they are supported by earlier results reported in the literature. Simulations with GCM’s have indicated the potential importance of South Atlantic SSTA’s on the tropical circulation [e.g., Robertson et al., 2003; Okumura et al., 2001]. However, these studies have used either tropical Atlantic SSTA’s or a mixture between tropical and South Atlantic SSTA’s. Our results indicate that subtropical South Atlantic SSTA’s by themselves can influence the atmospheric tropical circulation.

Concerning the extra-tropical response the results are still inconclusive. Earlier reports in the literature, however, do suggest a connection between the South and North Atlantic ocean. For instance Robertson et al. [2000] demonstrate for the NCEP-NCAR reanalysis covariability between the North Atlantic Oscillation and South Atlantic SSTA’s at the location of the equatorward pole of our SSTA dipole. Also in Okumura et al. [2001] a pan-Atlantic pattern is detected, where the South Atlantic SSTA pattern shows a strong similarity with the equatorward pole of our dipole.

## South Atlantic tropical cyclones

- Angola tropical depression, April 10-15, 1991 (cyclone on 13.?) – for now the only one in the eastern South Atlantic
- Tropical depression, January 18-21 (cyclone on 19.?) southeast of Salvador, Brazil
- Hurricane Catarina March 22-28, 2004
- 2006: two short-lived tropical/subtropical systems were observed over 27oC water in a region of strong wind shear  (Jeff Masters, _[www.](http://www.wunderground.com/)__[wunderground](http://www.wunderground.com/)__[.com](http://www.wunderground.com/)_):
- February 23 near 29S, 36W (600 miles SE of Rio de Janeiro), survived about 4 hours
- March 15 off South Brazil

**Notes:**
2003GL017829.pdf

## Angola cyclone

## Hurricane Catarina

- Mean SST for March 21-28, 2004.
- Circle indicates Storm location on March 26.

- animation