Impact of intraseasonal wind bursts on SST variability in the 1 far eastern Tropical Atlantic Ocean during boreal spring 2 2005 and 2006 . Focus on the mid-May 2005 event

The impact of spring intraseasonal wind bursts on s ea surface temperature variability in the eastern 9 Tropical Atlantic Ocean in 2005 and 2006 is investi gated using numerical simulation and observations. We 10 specially focus on the few documented coastal regio n east of 5° E and between the equator and 7° S. Fo r both 11 years, the southerly winds strengthening induced co oling events through i) upwelling processes; ii) v ertical 12 mixing due to vertical shear of zonal current; and for some particular events iii) a decrease of incom ing surface 13 shortwave radiation. The strength of the cooling ev ents was modulated by subsurface conditions affecte d by the 14 arrival of Kelvin waves from the west influencing t he depth of the thermocline. Once impinging the eas tern 15 boundary, the Kelvin waves excited westward-propaga tin Rossby waves which, combined with the effect o f 16 enhanced westward surface currents, contributed to the westward extension of the cold water. A particu larly 17 strong wind event occurred in mid-May 2005 and caus ed an anomalous strong cooling off Cape-Lopez and i n 18 the whole eastern Tropical Atlantic Ocean. From the analysis of oceanic and atmospheric conditions dur ing this 19 particular event, it appears that anomalous strong spring wind strengthening associated to anomalous s trong 20 Hadley cell activity made the event as a decisive e vent which prematurely triggered the rainfall coast al onset in 21 the northern Gulf of Guinea. Results show that no s imilar atmospheric conditions were observed over th 199822 2008 period. It is also found that the anomalous oc eanic and atmospheric conditions associated to the event 23 exerted strong influence on rainfall off Northeast Brazil. This study highlights the different process through 24 which the wind power from South Atlantic is brought to the ocean in the Gulf of Guinea and emphasizes th 25 need to further document and monitor the South Atla ntic region. 26

influence on the WAM.In a second part of the study, we thus focus on this particular wind event that preceded a strong cold event in the far eastern Tropical Atlantic along with an early ACT development.We aim to describe i) the atmospheric and oceanic conditions during this particular event; ii) to what extent it is involved in the WAM system; and iii) which processes make it an exceptional event.
The remainder of the paper is organized as follows.In Sect.2, the model and observational data used in this study are described.The seasonal and interannual variability of SST, winds, currents, 20° C-isotherm depth and sea surface heat flux in the CLR are analyzed in Sect.3. The cooling events generated in response to southerly wind bursts and the other forcing mechanisms implied in the CLR are investigated in details for the years 2005 and 2006 in Sect. 4. In Sect.5, we focus our analysis on the unusual wind burst occurring in mid-May 2005.
Finally, the main results are summarized and discussed in Sect.6.

Model and data
The numerical model used in this paper is the Regional Oceanic Modeling System (ROMS) (Shchepetkin and McWilliams, 2005).The model configuration is the same as employed in Herbert et al. (2016), and the following text is derived from there with minor modifications.
ROMS is a three-dimensional free surface, split-explicit ocean model which solves the Navier-Stokes primitive equations following the Boussinesq and hydrostatic approximations.We used the ROMS version developed at the Institut de Recherche pour le Développement (IRD) featuring a two-way nesting capability based on AGRIF (Adaptative Grid Refinement In Fortran) (Debreu et al., 2012).The two-way capability allows interactions between a large-scale (parent) configuration at lower resolution and a regional (child) configuration at high resolution.The ROMSTOOLS package (Penven et al., 2008) is used for the design of the configuration.The model configuration is built following the one performed by Djakouré et al. (2014) over the Tropical Atlantic.
The large scale domain extends from 60° W to 15.3° E and from 17° S to 8° N and the nested high resolution zoom focuses between 17° S and 6.6° N and between 10° W and 14.1° E domain.This configuration allows for equatorial Kelvin waves induced by trade wind variations in the western part of the basin to propagate into the Gulf of Guinea and influence the coastal upwelling (Servain et al., 1982;Picaut, 1983).The horizontal grid resolution is 1/5° (i.e.22 km) for the parent grid and 1/15° (i.e. 7 km) for the child grid (see Herbert et al. (2016), their Fig. 1).This allows an accurate resolution of the mesoscale dynamics since the first baroclinic Rossby radius of deformation ranges from 150 to 230 km in the region (Chelton et al., 1998).The vertical coordinate is discretized into 45 sigma levels with vertical S-coordinate surface and bottom stretching parameters set respectively to theta_s = 6 and theta_b = 0, to keep a sufficient resolution near the surface (Haidvogeland Beckmann, 1999).The vertical S-coordinate Hc parameter, which gives approximately the transition depth between the horizontal surface levels and the bottom terrain following levels, is set to Hc = 10 Ocean Sci.Discuss., https://doi.org/10.5194/os-2017-74Manuscript under review for journal Ocean Sci. Discussion started: 4 October 2017 c Author(s) 2017.CC BY 4.0 License.m.The GEBCO1 (Global Earth Bathymetric Chart of the Oceans) is used for the topography (www.gebco.net).
The runoff forcing is provided from Dai and Trenberth's global monthly climatological run-off data set (Dai and Trenberth, 2002).The rivers properties of salinity and temperature are prescribed as annual mean values.One river (Amazon) is prescribed in the parent model while five rivers, that correspond to the major rivers present around the Gulf of Guinea, are prescribed in the child model (Congo, Niger, Ogoou, Sanaga, Volta).At the surface, the model is forced with the surface heat and freshwater fluxes as well as 6 hourly wind stress derived from the Climate Forecast System Reanalysis (CFSR) (horizontal resolution of ¼°x ¼°) (Saha et al., 2010).Our model has three open boundaries (North, South, and West) forced by temperature and salinity fields from the Simple Ocean Data Analyses (SODA) (horizontal resolution of ½°x½°) (Carton et al., 2000a(Carton et al., , 2000b;;Carton and Giese, 2008).The simulation has been performed on IFREMER Caparmor super-computer and integrated for 30 years from 1979 to 2008 with the outputs averaged every 2 days.A statistical equilibrium is reached after ~10 years of spin-up.Model analyses are based on the 2-days averaged model outputs from year 1998 to year 2008.The model has already been validated successfully with a large set of measurements and climatological data, and more detailed information about the model validations can be found in Herbert et al. (2016).
For SST observations, we use data obtained from measurements made by the Tropical Rainfall Measuring Mission microwave imager (TMI).The dataset is a merged product available at www.remss.com.The SST data have a spatial resolution of ¼° and for the present study the 10 years' time series, from 1 January 1998 to 31 December 2008, obtained as 3-daily field.The important feature of the microwave retrievals is that it can give accurate SST measurements under clouds (Wentz et al., 2000).However, the major limitation to the microwave TMI observations is land contamination which results in biases of the order of 0.6°K within about 100 km from the coast (Gentemann et al., 2010).Thus, in the Optimal Interpolation TMI product the offshore zone with no data extends at approximately 100 km from the coast.This limits to some degree the analysis of near-coastal regions, in particular those dominated by coastal upwelling dynamics.
We also use for this study daily sea surface height (SSH) data, which are available for the period 1993-2012 and maintained by the organization for Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO; www.aviso.altimetry.fr).The sea surface height dataset is a merged product of observations from several satellite missions Ssalto/Duacs (Segment Sol multimissions d'ALTimétrie, d'Orbitographie et de localisation précise/Developing Use of Altimetry for Climate Studies) mapped onto a 0.25° Mercator projection grid.All standard corrections have been made to account for atmospheric (wet troposphere, dry troposphere and ionosphere delays) and oceanographic (electromagnetic bias, ocean, load, solid Earth and pole tides) effects.
The mean sea surface topography for the period 1993-2012 was removed from the SSH to produce sea surface height anomalies.
In addition, surface pressure data were studied using ECMWF Atmospheric Reanalysis (ERA) for the 20th Century product.The four-hourly data are daily averaged and is available on https://rda.ucar.eduwebsite.The product assimilates surface pressure and marine wind observations.Ocean Sci.Discuss., https://doi.org/10.5194/os-2017-74Manuscript under review for journal Ocean Sci. Discussion started: 4 October 2017 c Author(s) 2017.CC BY 4.0 License.

Seasonal variability of surface conditions in CLR
The purpose of this section is to describe the seasonal atmospheric and ocean surface conditions in the CLR.
The seasonal variability of SST, surface winds stress, horizontal current intensity, depth of 20° C-isotherm (hereafter referred to as z20), and the surface net heat flux from monthly averaged model outputs in the CLR for each year from 1998 to 2008 and averaged over the period are shown on Fig. 1.The reliability of the model is also provided by comparing the simulated and the corresponding TMI SST climatological seasonal cycle in the CLR (Fig. 1a).The SST variations display an annual cycle with highest temperature in boreal winter (warm season), when the ITCZ reaches its southernmost position and the trade winds are weakest, and minimum values in boreal summer (cold season), when the trades intensify.The most salient features of the atmospheric and hydrographic fields during May-June are also illustrated on Fig. 1 by May-June averaged maps.Despite a warm bias (~1° C) compared to satellite observations, well known in the eastern tropical Atlantic region (e.g.Zeng et al., 1996;Davey et al., 2002;Deser et al., 2006;Chang et al., 2007;Richter and Xie, 2008), the model pretty well reproduces the satellite SST pattern.The SST May-June average map indicates that the boreal summer SST minimum is related with intensified cool SST around 6°S, in the Congo mouth region.In this region, the coast is oriented parallel to the trade flow which reinforces in boreal summer, thus favorable to coastal upwelling processes.The mean alongshore wind stress during May-June reveals in fact that upwelling conditions are observed over most of the CLR.Wind stress magnitude exhibits a semi-annual variability with a second maximum in October-December and a weakening during July-September season (Fig. 1b).The strengthening of winds in spring is associated with a strengthening of mean current speed, particularly off Cape-Lopez between 2° S to 4° S and west of 8° E in May-June (Fig. 1c).The orientation of surface current is mostly westward for the May-June season, while it is northward from October to January (not shown).This general picture of surface circulation is consistent with observations (Merle, 1972;Piton, 1988;Rouault et al., 2009).
The region is also characterized by a shallow thermocline which depicts a strong semi-annual cycle (Fig. 1d).
The evolution of z20 reveals a thinning of the thermocline during May-July and a thickening up to October-November when it exhibits a minimum.
The surface net heat flux exhibits a maximum in winter and a minimum in July (Fig. 1e), following the seasonal cycle of solar shortwave radiations.As visible on the May-June average map, greater heating is found over cool waters, due to weaker heat loss via latent heat flux in these areas.

Analyze of cooling events in CLR in 2005 and 2006
In this section, we examine the impact of intraseasonal wind bursts on SST in CLR during the particular years 2005and 2006(Marin et al., 2009;;Caniaux et al., 2011).We propose here to analyze in details the SST conditions in CLR, east of 5° E, for both years.

SST variations
In order to delineate the sequence of cooling events, we analyze the SST variations from 2-days averaged model outputs in 2005 and 2006 over the CLR, i.e. between 5° E and 12° E (Fig. 3a&4a).In 2005, the intraseasonal cooling events took place on [22][23][24][8][9][10][11][12][16][17][18][19][20][26][27][28][29][30][12][13][14][15][16] June and 30 June-2 July, with a temperature drop ranging between -0.2°C to -1.7°C.The cooling events occurred east of 5° E from May to September.They concerned especially the southern equatorial region (around ~3-4° S), except for the strongest events where they reached more northern equatorial regions, especially for the mid-May and late-May 2005 events.These latter were associated with an intense meridional SST front between the cold water south of the equator and the warmer water north of the equator, as visible on SST map for 12 May 2005 presented on Fig. 2. We can see cold waters extending along the eastern coast and in ACT region west of 5° W. In the model, cold waters are deflected offshore off Cape-Lopez, due to recursive bias in warm water intrusion toward the south.
Besides, model SST fields (Fig. 3a) indicate that the SST minimum (~24° C) in 2005 was reached in July, i.e. one month earlier than in 2006, as also noticed in seasonal variations of SST averaged in the region (Fig. 1a).
These results illustrate the important role in the CLR of the succession of quick and intense cooling events in the establishment of persistent cold anomalies, as highlighted by Marin et al. (2009) in the equatorial region.

Local forcing
To examine the local forcing mechanisms responsible for the observed cooling events in CLR, the intraseasonal variations of wind stress magnitude anomalies are examined and compared in 2005 and2006 (Fig. 3b &4b).
In 2005, successive periods of 6-16 days wind intensification occurred from late-March to late-May.The main cooling events described above are associated with positive wind stress speed anomalies occurring on 12-24 April, 2-6, 12-16 & 24-28 May, 8-12 & 28 June, with a maximum for the 12-16 May event peaking on 14 May (at ~0.03 N.m -2 ).Another period of wind intensification is evidenced in late March -early April but it did not generate significant cooling despite comparable or even higher wind intensity than following wind events.In

Remote forcing a. Highlighting of Kelvin wave propagation
As previously shown, the time of occurrence of the cold events in the CLR coincides with steeper thermocline slope which allows a mixed layer temperature to be more reactive to surface forcing.Indeed, because of its proximity to the equator, the thermocline in CLR is affected by the arrival of equatorial waves, initiated in the west part of the basin.Pairs of alternate downwelling and upwelling Kelvin waves occur usually in February-March, July-September and October-November.Upon impingement with the eastern boundary, the incoming equatorial Kelvin wave excites westward-propagating Rossby waves and poleward-propagating coastal Kelvin waves (Moore, 1968;Moore and Philander, 1977;Illig et al., 2004;Schouten et al., 2005;Polo et al., 2008).The   identified as the latitude where the meridional wind stress goes to zero (Fig. 7a).The anomaly of zonal and meridional components of the wind stress (Fig. 7b&c), the wind stress curl anomaly (Fig. 7d), as well as the z20 and SSH anomalies (Fig. 7e&f), averaged in the equatorial band (over 1° S and 1° N), are also presented.For the zonal wind stress, the frequencies > 1 month are removed by a low-pass filtering and superimposed with the anomalies (red curve on Fig. 7c).Many authors suggest that the source of the equatorial Kelvin wave is mainly related to a sudden change of the western equatorial zonal wind (e.g.Picaut, 1983;Philander, 1990): a symmetric westerly (easterly) wind burst along the equator will generate Ekman convergence (divergence) and thus force downwelling (upwelling) anomalies which then propagate eastward as a Kelvin wave (Battisti, 1988;Giese and Harrison, 1990).In 2005, shallower-than-average thermocline, evidenced by positive (negative) z20 (SSH) anomalies, occurred in the end of March-beginning of April in the west part of the basin (Fig. 7e&f).The meridional and zonal wind stress anomalies indicate that the maximum of thermocline slope anomaly was associated with a strengthening of northeast trades followed by a strengthening of southeast trades from either side on the equator.At the equator, we notice indeed a sudden reversing of meridional winds which turned southward on 27-28 March 2005 related to an abrupt southward displacement of the ITCZ which was then found south of the equator in the west part of the basin (Fig. 7a&b).The ITCZ returned its initial position four days later followed by a strengthening of easterlies which persisted for ~20 days (Fig. 7c).Climatologically, the These changes contributed to a rise in the oceanic thermocline with a time lag of some days (Fig. 7e&f).The upwelling signal might then be reinforced by the symmetric easterly wind which concerned a large part of the western basin.Besides, we identify on Fig. 6d another peak of negative wind stress curl anomaly on 6-8 May 2005, more sudden than the previous winter one.It was associated with positive (negative) z20 (SSH) anomaly indicator of a thermocline rise initiated on 6 May 2005 in the west of the basin and which propagated eastward along the equator.The zonal wind stress anomalies (Fig. 6b) also indicate an easterly wind strengthening initiated in the beginning of May, which a maximum on 8-10 May, just after the minimum of wind stress curl.
In 2006, the upwelling Kelvin wave is identified in the first half of March in the west part of the basin (Fig. 7e&f).The coinciding atmospheric conditions were slightly different than the ones identified in 2005.In winter, the position of the ITCZ had a more southern position in 2006 than in 2005.It crossed the equator during a longer period (about 10 days from ~ Feb. 10 2006), reaching minimum latitude on 22-24 February.This location south of the equator induced a negative wind stress curl anomaly (Fig. 7d).As in 2005, the reversion of the meridional wind at the equator was followed by a strengthening of westward component of the wind stress few days after, which lasted for about ten days (Fig. 6c); however, it was of a lesser magnitude compared to 2005 and only concerned the westernmost part of the basin.In addition, the negative zonal wind anomaly concerned mainly the northeasterlies rather than the southesterlies, leading to an anti-symmetric meridional wind pattern as well as symmetric zonal wind pattern on either side on the equator (not shown).These wind patterns were expected to generate Ekman convergence at the Equator and thus to reinforce the observed upwelling anomalies.
Thus, for both years, Kelvin upwelling wave occurred in the west while easterly winds were strengthened from either side of the equator after the ITCZ reached its southernmost location.This latter was observed one month earlier in 2006 than in 2005, and was associated with a negative wind stress curl anomaly.In winter 2005, the ITCZ was found south of the equator after a very sudden southward shift and was followed by strong easterlies during ~20 days, while in winter 2006, the ITCZ was found closer to the equator less sharply and during a longer period, followed by weaker easterlies compared to 2005.

Westward extension of the CLR cooling
In the east, the cooling generated by southerly wind bursts in the CLR then progressively extended westward to connect with the southern boundary of the equatorial ACT.This phenomenon was more obvious in 2005 when the cooling which first concerned coastal area extended further offshore a few days after the two strong events occurring in the second half of May.To evidence the effect of these events on SST, maps of SST anomalies and wind stress monthly anomalies averaged over May 2005 and over the weeks before the strong events (from 1 to 10 May) are presented on  To better understand the oceanic processes implied in this cooling extension, we examined the z20 anomalies, SSH anomalies and zonal velocities along 3° S (Fig. 9 b-d).They reveal that the cooling westward extension was associated with a westward propagation of a steeper thermocline and negative SSH anomalies from the African coast up to 5°-10° W combined with enhanced surface westward current fluctuations at the dates of the successive events from April-June.The fluctuations of the westward surface current occurring off Gabon with periods of ~8-10 days were related to the strengthening of southerly winds during the wind bursts at the same periods (Fig. 3b & 4b).The surface current in this area is part of the westward SEC which is known to intensify during the cold season (Okumura and Xie, 2006).Our study implies shorter time scales than seasonal scale but the intensification of the SEC during wind bursts through Ekman transport processes might contribute to the westward extension of the cooling by advection of cold eastern upwelled water.This is in agreement with DeCoëtlogon et al. ( 2010) who found from model results that at short time scale (a few days), more than half of the cold SST anomaly around the equatorial cooling could be explained by horizontal oceanic advection controlled by the wind with a lag of a few days.In addition, the z20 and SSH show respectively negative and positive anomalies propagating westward at 3° S (Fig. 9), initiated from the coast with a propagating speed of around 10 cm.s -1 , which is very close to the phase speed of Rossby waves.Indeed, the excitation of the westward waves at the coast coincided with the arrival of Kelvin waves (see Fig.
wind stress magnitude has revealed that each event is associated with strengthening of equatorward winds, especially during the 14-16 May event when the wind stress magnitude anomaly averaged over the CLR is the strongest one.This particular event has been found to be responsible for the sudden and intense SST cooling in the eastern equatorial Atlantic and identified as part of manifestation of temporal variability of the St. Helena Anticyclone (Marin et al., 2009).In this section, we focus on this mid-May event, to better understand the processes in play during this unusual event.

Wind and surface atmospheric pressure
The spatial distribution of the mid-May 2005 wind event can be inferred from Fig. 10 where anomalous CFSR wind speed fields superimposed with daily precipitation fields, surface pressure, wind speed curl, and downward shortwave radiation, are presented from 13 May to 17 May.The event was characterized by intense southeasterly wind anomalies east of 15° W and from 30°S to the equator from 13-14 May, concomitant with a strengthening of the easterlies west of 30° W between 30° and 15° S (Fig. 10a).The wind anomaly extended then westward up to 15-16 May when the maximum was located in the western part of the basin off northeastern Brazilian coast.Simultaneously, a strengthening of southerly winds occurred north of the equator in the Gulf of Guinea.The anomalous strong winds during the event were associated with anomalous high pressure core of the Saint Helena Anticyclone, especially on 13-14 May, also associated with particularly low pressure under the ITCZ.

Precipitation
The maps of precipitation rate during the event (Fig. 10a) display a band of heavy precipitation (9-17 kg.m - 2 /day) between 5° -9° N and off northeast Brazil from the coast to 15° W and from 10° S to 3° S. The maximum precipitation rate in this region occurred on 15-16 May concomitant with the easterly winds strengthening.This convective zone, located between the ITCZ north of the equator and the South Atlantic Convergence Zone (SACZ) in southern tropics, is the Southern Intertropical Convergence Zone (SICZ) (Grodsky and Carton, 2003).This zone forms usually later, by June-August, when the southern branch of the convection separates from the ITCZ which moves north of the equator.Grodsky and Carton (2003) showed that this rainfall pattern appears closely linked to the seasonal change in SST difference between the ACT region (which they defined between 15° W -5° W, 2° S -2° N) and the SITCZ region (25° W -20° W, 10° S -3° S).They argued that the seasonal appearance of the ACT along the equator sets up pressure gradients within the boundary layer that induce wind convergence in the SITCZ region.Based on Grodsky and Carton (2003)  rainfall conditions during mid-May event might thus be explained by strong SST gradient between the two regions caused by unusually early cooling in the ACT region at this time of the year.

Generation of atmospheric gravity wave
The precipitation fields during the mid-May event (Fig. 10a) also evidence rainfall pattern typical of atmospheric gravity wave train characterized by a horizontal wave length ~500 km and initiated by a front system (forming the northern boundary of a low pressure system) which developed around 17° S on 14 May and traveled northeastward until 17 May.The rainfall train was associated with oscillatory wind stress curl train alternating between positive and negative anomalies (Fig. 10c) as well as alternating downward shortwave radiation minimum (Fig. 10d) associated with the wave clouds.Gravity waves are known to play an important role in transporting the momentum and energy through long distances (Fritts, 1984).Here, they would be a way to carry momentum and energy from South Atlantic to the equator during the strong event.To estimate the effect of the passage of the wave on SST, the SST TMI fields have been analyzed.Given the time resolution of the dataset (3-days averaged) a propagating train in SST fields cannot be highlighted; instead, 3-daily TMI SST averaged (available east of 10° W) differences, as well as 3-daily averaged downward shortwave radiation (hereafter DSWR) fields differences, between 13-14-15 May averaged and 10-11-12 May averaged and between 16-17-18 May averaged and 13-14-15 May averaged.Results indicate a good correlation between the two fields, suggesting a northeastward propagation of the cooling associated to northeastward propagation of the DSWR minimum associated with the displacement of the atmospheric gravity wave.A cooling first occurred south of 24° S associated with DSWR minimum, and then reached more northern region concomitant with the northward migration of DSWR.Thus, these results suggest that the arrival of atmospheric gravity wave initiated in South Atlantic following atmospheric disturbance would contribute to the cooling observed in the Gulf of Guinea at the time of the mid-May event.

A decisive event for coastal monsoon onset
In order to analyze the air-sea pattern in the northern Gulf of Guinea during May-June 2005, we show on Fig. 12 the wind stress magnitude, precipitation rate, and SST fields averaged from 10° W to 6° W. The wind strengthening appeared first south of the equator on 12-16 May and then north of the equator from 14-18 May.It was associated with strong rainfall extending southward up to 2° N. Equatorial cooling occurred 4 days after the event and slowed down the overlying winds by feedback mechanisms.The winds north of the equator then remained stronger than in the ACT region and strengthened again north of the Equator on 22-28 May together with precipitation maximum pushed northward (around 5° N) after the event.
Thus, this mid-May event appears as the "decisive event" which triggered the abrupt transition between the two wind patterns in the northern Gulf of Guinea, when the wind north of the equator became and remained stronger

Why the mid-May 2005 event was so singular?
To better understand which makes the particularity of the mid-May 2005 event, the atmospheric and oceanic conditions (wind stress magnitude, short-wave radiation flux (hereafter RADSW), z20, SST, and meridional SST gradient) averaged over the 10° W-6° W region and between 15° S to 5° N during April-May are analyzed along the 1998-2008 period (Fig. 13).The wind stress magnitude during mid-May event appears to be one of the strongest over the whole 1998-2008 period.These strong wind conditions are usually met later in late boreal spring or summer, when the St. Helena Anticyclone strengthens and shifts northward toward the warm hemisphere.The wind intensification in mid-May 2005 was associated with particularly weak RADSW from South Atlantic to the northern equatorial region, suggesting cloud albedo effect during the event which tended to cool the mixed layer.We can notice that the April-May 2005 period was characterized by the weakest mean RADSW.
In addition, at the time of the event, the surface waters were already cooled by previous wind bursts (e.g.20 April and 8 May).The SST response to the mid-May event occurred 4-6 days later, inducing the weakest equatorial SST values for April-May season over the whole 1998-2008 period.The cooling also caused an enhanced SST front around 1° N, as shown on Fig. 13 (bottom panel), which was found to be the earliest and strongest one over the 1998-2008 period.This meridional SST gradient was responsible for the wind surface intensification north of the equator (Fig. 12 and 13a) through air-sea interaction mechanisms as described by Leduc-Leballeur et al. (2011).Another SST gradient maximum is found at the end of May 1998 but it was not extended as eastward than during the mid-May 2005 event (not shown).Anticyclone region 4 days earlier (Fig. 14b).The meridional surface pressure gradient during the event is thus found to be the strongest over 1998-2008 period (Fig. 14c).That suggests strong Hadley circulation intensity during the mid-May event and therefore strong anomalous equatorward moisture flux, allowing the deep atmospheric convection in the Gulf of Guinea to be triggered at a self-sustaining level, as previously described in Sect.5.2.Thus, the particularity of the mid-May 2005 event mainly lies in the i) anomalous atmospheric conditions related to anomalous strong St. Helena Anticyclone perturbation; ii) cooling initiated by the succession of previous wind bursts; and iii) favorable subsurface local ocean conditions preconditioned by equatorial waves which shoaled the mixed layer.Another wind burst of comparable intensity occurred at the beginning of May 2000 while the thermocline was shallow, causing SST cooling at the equator (Fig. 13&14).However, the wind strengthening was less sudden than during the mid-May 2005 event and the resulting cooling took place over a less broad region (not shown).In addition, the surface pressure drop in the ITCZ region was not as pronounced as during mid-May 2005 event.The preconditioning of subsurface conditions in the area via Kelvin wave at the dates of the wind bursts depended on the atmospheric conditions in the western part of the basin a few weeks earlier.Previous studies (e.g.Picaut, 1983;Philander, 1990) suggest that the source of the equatorial Kelvin wave is mainly related to a sudden change of the zonal wind in the west.Analysis of atmospheric and oceanic conditions at intraseasonal to daily scale in winter 2005 and 2006 showed that for both years, Kelvin upwelling wave was initiated in the west while easterly winds were strengthened from either side of the equator just after the ITCZ to be at its southernmost location.This latter was observed one month earlier in 2006 (late February -early March) than in 2005 (late March-early April), and was associated with a negative wind stress curl anomaly.In winter 2005, the ITCZ was found south of the equator after a very sudden southward shift and was followed by strong easterlies during ~20 days, while in winter 2006, the ITCZ was found closer to the equator less sharply and during a longer period, followed by weaker easterlies when compared to 2005.These results obtained for 2005 and 2006 years do not imply that same atmospheric conditions would be observed for winter upwelling Kelvin wave of other years.Especially, the year 2005 was very particular and also exhibited anomalously cold SSTs in the south Atlantic and anomalously warm SSTs in the north Atlantic initiated in fall 2004, signature of a meridional mode (Virmani and Weisberg, 2006;Foltz and Mc. Phaden, 2006;Hormann and Brandt, 2009).This event also induced a cooling at the equator but the surface pressure decrease in ITCZ region was not as pronounced than during mid-May 2005 event and the SST gradient around 1° N was weaker.In addition to coastal precipitation in the Gulf of Guinea and due to the early cooling in the ACT region, unusually rainfall conditions also occurred between the northeast coast of Brazil and 15° W within the SITCZ, which generally forms in early boreal summer.
Finally, this study highlights the impact of a strong southerly wind burst in the eastern tropical Atlantic during boreal spring season, which is a transitional period during which an anomalous strong energy input may tip the energy balance from an equilibrium state toward another one and thus impact the WAM system.The analysis of atmospheric and oceanic conditions during the mid-May 2005 wind event allows to highlight the different processes through which the wind power provided by the wind burst is brought to the ocean: i) direct effect of the wind on the SST in the eastern tropical Atlantic, ii) energy transport via atmospheric gravity waves from South Atlantic, and iii) energy supply to Rossby wave.In addition to unusual atmospheric conditions in mid-May 2005, the ocean response intensity to this event was also enhanced by the subsurface conditions, made favorable by previous wind bursts, either local (e.g. in 6-8 May) or occurring a few weeks before in the West.It is crucial to better describe the atmospheric and oceanic processes in play during such extreme event, notably in order to reduce the well known warm bias in the southeastern tropics in coupled models in both atmospheric and oceanic components (Zeng et al., 1996;Davey et al., 2002;Deser et al., 2006;Chang et al., 2007;Richter and Xie, 2008).This warm bias is well evidenced in our numerical simulation (Fig. 1&2) and our results clearly show that the cooling events were underestimated in the CLR, implying the need to investigate more in depth the oceanic and atmospheric processes in play in this particular region.As the intraseasonal wind bursts are related to the fluctuations of St. Helena Anticyclone, their impact on SST variability in the eastern tropical Atlantic and regional climate suggests the need of better understand the St. Helena Anticyclone variability.
It is also important to note that the mid-May 2005 event occurred during an unusually active year.The year 2005 exhibited a pronounced meridional mode pattern with strong SST gradient between the two hemispheres.
Several authors (Foltz et al., 2006 ;Virmani and Weisberg, 2006 ;Marengo et al., 2008aMarengo et al., , 2008b ; ;Zeng et al., 2008) studied this particular year, marked by anomalously warm SST in the tropical North Atlantic during March-July, the warmest from at least 150 years.This anomalous warming was associated with the most active and destructive hurricane season on record (Foltz et al., 2006;Virmani and Weisberg, 2006) and an extreme and rare drought in the Amazon Basin (Marengo et al., 2008a(Marengo et al., , 2008b;;Zeng et al. 2008;Erfanian et al., 2017).From these authors, primary causes of the anomalous warming in 2005 were a weakening of the northeasterly trade winds and associated decrease in wind-induced latent heat loss as well as changes in shortwave radiation and horizontal oceanic heat advection.This 2005 temperature record is made even more remarkable given that, unlike the 1998's one, it occurred in the absence of any strong El Niño anomaly (Shein, 2006).Some studies (Goldenberg et al., 2001) attribute these SST increases to the Atlantic Multidecadal Oscillation (AMO), while others suggest that climate change may instead be playing the dominant role (Emanuel, 2005;Webster et al., 2005;Mann and Emanuel, 2006;Trenberth and Shea, 2006).Comparable anomalously warm tropical Atlantic SSTs have been observed in 2010 also associated with extreme drought in the Amazon.However, from time series of monthly anomalies constructed for the two basins (North and South Atlantic) by using OISST monthly mean data, Erfanian et al. (2017) show that the warmer-than-usual SSTs in the North Atlantic in 2010 was not associated with colder-than-usual SST in South Atlantic contrarily to 2005 (their Fig. S4e).
While the warming in North Tropical Atlantic during 2005 has been investigated by several authors, the cooling in South Atlantic has received less attention.This study highlights the need to further document and monitor the South Atlantic region and the St. Helena Anticyclone, through additional high resolution analysis and observations.
Ocean Sci.Discuss., https://doi.org/10.5194/os-2017-74Manuscript under review for journal Ocean Sci. Discussion started: 4 October 2017 c Author(s) 2017.CC BY 4.0 License.onset date (around 19 May 2005 whereas they define the mean onset date on 23 June +/-8 days).According to Marin et al. (2009), the time shift in the development of the ACT between 2005 and 2006 is related to a particular wind burst event in mid-May 2005.This mid-May 2005 event therefore appears as exerting a strong The seasonal cycle is modulated by strong year-to-year variations.The mean SST in the CLR in 2005 cools as early as March from TMI data and April from the model data.SST reaches weaker values than the climatologic ones, as observed by Marin et al. (2009) and Caniaux et al. (2011) west of 4° E. This 2005 cold anomaly is associated with positive wind speed and surface current speed anomaly in April-May (Fig. 1b&c) as well as shallower-than-average thermocline depth.In 2006, SST variations are very close to the climatologic ones.

Figure 2 :
Figure 2: Map of the sea surface temperature (° C) on 12 May 2005 from 3-days average TMI data (a) and from the 2-days average model output (b).Note that for the model it corresponds to 11-12 May average whereas for TMI data it is 10-11-12 May average.The black square indicates the Cape-Lopez region (called 'CLR').
2006, periods of wind intensification were slightly shorter than in 2005 and extended from mid-March to mid-May, interrupted by periods of negative wind anomalies.The main wind events occurred in[16][17][18][2][3][4][4][5][6] May, with maximum wind stress magnitude anomaly in 16-24 April.Also, the wind event in late April 2006 did not generate a surface cooling as strong as the mid-May 2006 one, despite higher wind stress magnitude anomalies.To depict the subsurface conditions during cooling events in CLR for both years, anomalies of the 20° C-isotherm depth averaged from 5° E to 12° E are presented on Fig.3c& 4c.They indicate strong correlation with SST anomalies on intraseasonal time scale with maximum values (up to + 25 m) observed during the 14 May 2005 event.In early April 2005 and before the late-April 2006, the thermocline was deeper, that can explain why wind intensification did not generate a surface cooling at these times.Indeed, at the Ocean Sci.Discuss., https://doi.org/10.5194/os-2017-74Manuscript under review for journal Ocean Sci. Discussion started: 4 October 2017 c Author(s) 2017.CC BY 4.0 License.time of the strong 16-24 April 2006 wind event, the z20 anomalies was weaker south of the equator than during the 14-16 May 2005 event, making the SST less reactive to comparable wind intensification.The same feature is observed in early May 2006, when the z20 anomalies indicate deeper thermocline south of the equator around 4° S than a few days later.Besides, the thermocline appeared shallower south of the equator in 2005 than in 2006, in agreement with the difference of the cooling intensity observed between the two years.The Ekman pumping velocity w e averaged along 3-4° S is shown on Fig. 3d.It is correlated with wind intensity, with maximum around 8°E at the dates of the 2005 and 2006 events, in agreement with the cooling events identified mostly around 3-4°S (Fig. 3a & 4a).The maximum upward velocity sometimes extended west of 8° E, as during April 2005 and June 2006 events.Another process that may contribute to the cooling in the upper layer is the vertical mixing due to intense vertical shear of the zonal current.The maximum of the zonal current vertical shear fields in CLR, averaged between 5° and 12° E for 2005 and 2006 (Fig. 3e & 4e), exhibited intensification south of the equator, centered around 3-4° S. Weaker intensification also occurred occasionally at the equator (located around 80 m depth between the westward surface South Equatorial Current -SEC -and the eastward subsurface Equatorial Under-Current).Around 3-4° S, the vertical shear was driven by the SEC, reinforced by prevailing southerly winds events through Ekman transport.It thus occurred at the date of wind events previously identified for 2005 and 2006, with stronger vertical shear occurring in early May 2005.The intensity of the maximum of vertical shear during the events was quite similar between 2005 and 2006.The main difference lied in their meridional extent, related to the meridional extent of the strengthened southerly winds which reached equatorial region during the May 2005 events (not shown).We can also notice that for comparable wind intensification, the spring and summer wind events were not associated with comparable intensity of vertical shear.The meridional wind component favorable to westward Ekman transport was actually stronger during April and May events than during summer ones (not shown).The heat content within the mixed layer is also impacted by the sea surface heat fluxes.The net heat fluxes averaged between 5° E and 12° E are shown on Fig. 3f & 4f for 2005 and 2006 respectively.They indicate a net heating (~ 50-100 W.m -2 ) over the 2° S -5° S latitude band, where the SST cooling was strongest, suggesting other mechanisms involved.However, we notice some particular events during which the net heat flux was negative over most of the region.The strongest net cooling (-30 W.m -2 ) occurred during the 26-28 May 2005 event.It was mainly due to a sudden decrease of incoming surface short wave radiation (drop of about 140 W.m -2 between 22 and 28 May; not shown) suggesting increased cloud cover.Ocean Sci.Discuss., https://doi.org/10.5194/os-2017-74Manuscript under review for journal Ocean Sci. Discussion started: 4 October 2017 c Author(s) 2017.CC BY 4.0 License.
Ocean Sci.Discuss., https://doi.org/10.5194/os-2017-74Manuscript under review for journal Ocean Sci. Discussion started: 4 October 2017 c Author(s) 2017.CC BY 4.0 License.20° C-isotherm depth anomalies along the equator and along 9°E are presented on Fig. 5 and clearly evidence large positive anomalies indicating shallower-than-average thermocline, propagating eastward along the equator and then southeastward for both years.The eastward propagation of Kelvin wave along the equator and southeastward along the coast is also well visible in the basin-wide SSH anomalies (Fig. 6) with a phase velocity of about 1.1-1.3m.s -1 , which fits well in the range between the second and third baroclinic equatorial Kelvin wave modes.The upwelling wave is in fact detectable by negative SSH anomalies, associated with shallower-than-average thermocline.In 2005, negative (positive) SSH (z20) anomalies occurred in the West in early April and in mid-May, whereas they occurred around late-March and early May in 2006.The first Kelvin wave thus reached the CLR slightly earlier in 2006 than 2005, at the beginning of May.Thus, the intensity of the cold events observed in spring and summer 2005 and 2006 resulted from both the basin preconditioning by remotely forced shoaling of the thermocline, local mixing and upwelling processes in response to strong southerly local winds, as well as heat flux variations.In 2005, stronger wind intensification and favorably preconditioned oceanic subsurface conditions, made the coupling between surface and subsurface ocean processes more efficient than in 2006, resulting in stronger cooling.

Figure 6 :
Figure 6: Time evolution of the sea level anomaly (m) along the equator (between 54° W and 12° E) and along 9° E (between the equator and 3° S) for 2005 (left), and 2006 (right) from AVISO data.
Ocean Sci.Discuss., https://doi.org/10.5194/os-2017-74Manuscript under review for journal Ocean Sci. Discussion started: 4 October 2017 c Author(s) 2017.CC BY 4.0 License.latitudinal position of the ITCZ varies from a minimum close to the equator in boreal spring (March-May) in the west to a maximum extension of 10 °N -15 °N in late boreal summer (August) in the east.Positive (negative) wind stress curl is found north (south) of the ITCZ.When the ITCZ is north of the equator, it induces upward (downward) Ekman pumping to the north (south) of the ITCZ.Thus, the southward shift of the ITCZ on 27-28 March 2005 accompanied with strong northerlies led to negative anomaly of wind stress curl south of the equator resulting in upward Ekman pumping.Results show indeed a strong negative anomaly on 22-26 March 2005 associated with the southward shift of the ITCZ just before the upwelling signal, initiated on 28 March.

Figure 7 :
Figure 7: Time evolution, from 2-days averaged model outputs, of (a) the position (in latitude, between 5° S and 10° N) where the meridional wind stress value equal zero (indicator of the position of the ITCZ), over Jan-Dec 2005 (left) and Jan-Dec 2006 (right); (b) the meridional wind stress anomalies (N.m -2 ) averaged between 50° W and 35° W and between 1° S and 1° N; (c) same as (b) but for zonal wind stress anomalies (N.m -2 ) (in blue).The red curve is after the frequencies > 1 month are removed by a low-pass filtering; (d) the wind stress curl anomalies (N.m -2 ) ; (e) the 20° C isotherm depth anomalies (m); (f) the sea level anomalies (m).The black arrow in (a) indicates the southward shift of the ITCZ before the excitation of the Kevin wave (see text).

Fig. 8 .
The results illustrate an enhancement after 10 May of the cooling in the east associated with southerly wind intensification and an extension of the cooling especially south of the equator up to 20°W.Ocean Sci.Discuss., https://doi.org/10.5194/os-2017-74Manuscript under review for journal Ocean Sci. Discussion started: 4 October 2017 c Author(s) 2017.CC BY 4.0 License.

Figure 8 :
Figure 8: (a) Sea surface temperature anomalies (° C; color) superimposed with wind stress intensity anomalies (arrows) averaged over May 2005.Only the values > 0.02m.s - are plotted; (b) same but averaged between 1 May and 10 May 2005 (b) 5) suggesting the possibility of Kelvin wave's reflection processes into symmetrical westward propagating Rossby waves.A westward Ocean Sci.Discuss., https://doi.org/10.5194/os-2017-74Manuscript under review for journal Ocean Sci. Discussion started: 4 October 2017 c Author(s) 2017.CC BY 4.0 License.propagation of z20 and SSH anomalies, although less obvious, was presently also identified at 3° N (not shown).In 2006, no similar wave generation process is observed (not shown).In 2005, the locally wind-forced component of the wave might reinforce the remote part of the reflected wave signal at the coast by the sea level slope which balanced the strengthening of alongshore winds blowing during the mid-May and late-May events.The quantitative and respective contributions of local and remote wind forcing to this wave is out of the aim of this study and would require further analysis.Thus, the combined effects of westward surface currents (via advection and vertical mixing through horizontal current vertical shear), local wind influences (via vertical mixing) and wave westward propagation, resulted in the extension of cold upwelled water from the eastern coast to near 20° W.

Figure 9 :
Figure 9: Time-longitude diagrams at 3° S between 10° W and 10° E, and from 2-days averaged model outputs from January to December 2005, of (from top to bottom) i) the sea surface temperature (° C); ii) the 20° C isotherm-depth anomalies (m); iii) the sea level anomalies from AVISO data (m); and iv) the zonal component of surface velocity (m.s -1 ).

Figure 11 :
Figure 11: Three daily-averaged differences, between 10° W and 15° E and between 40° S and 12° N from 15-12 May and 18-15 May 2005 (the dates indicated in the titles correspond to the last day taken into account for the calculation of the average.e.g.: 'SST 15-12' corresponds to the difference between the mean of SST over 13-14-15 May and over 10-11-12 May) of downward short-wave radiation (W.m -²) from CFSR fields (a); and SST from TMI data (b); than south of the equator.It occurred 15 days earlier than the average date (31 May) identified by Leduc-Leballeur et al. (2013) over 2000-2009 period.According to these authors, the time of occurrence of this phenomenon would be related with the strength of anomalous moisture flux.They explain that in April-May the low atmospheric local circulation is present only during an equatorial SST cooling and surface wind Ocean Sci.Discuss., https://doi.org/10.5194/os-2017-74Manuscript under review for journal Ocean Sci. Discussion started: 4 October 2017 c Author(s) 2017.CC BY 4.0 License.strengthening north of the equator, both generated by a southerly wind burst, before disappearing until the next wind burst.In June-July the low atmospheric local circulation is then always present and intensified by the wind bursts.Thus, the establishment of an abrupt seasonal transition event as observed in 2005, occurring much earlier than the reference date, supposed anomalously strong equatorial cooling caused by unusual strong southerly winds which allowed, through air-sea interactions mechanisms, to trigger the deep atmospheric convection in the Gulf of Guinea at a self sustaining level.

Figure 12 :
Figure 12: Time evolution, in May and June 2005 between 6° S and 6° N and averaged between 10° W and 6° W, of the (a) daily averaged wind stress magnitude (N.m -2 ) computed from CFSR wind speed fields ; (b) daily averaged precipitation rate (kg.m -2 /day) from CFSR fields and (c) 2-daily averaged SST (° C) fields, from the forced model.
Ocean Sci.Discuss., https://doi.org/10.5194/os-2017-74Manuscript under review for journal Ocean Sci. Discussion started: 4 October 2017 c Author(s) 2017.CC BY 4.0 License.When the wind burst occurred on 14 May 2005, the 20°C-isotherm depth in the area was shallow south of the equator and slightly deeper at the equator (Fig. 13c).The thermocline shoaling associated with the Kelvin wave appeared in fact a few days earlier providing favorable subsurface conditions which made the SST response to previous wind bursts (20 April and 8 May) more effective.At the time of the mid-May event, the wave already reached more eastern areas, as shown in previous sections.The mid-May 2005 event was also characterized by a particularly low surface pressure under the ITCZ, as shown on Fig. 14a which displays the surface pressure north of the equator averaged between 45° W and 20° W for April-May over the 1998-2008 period.The pressure fall during the mid-May 2005 event appeared as the lowest in May over the whole decade.It coincided with particularly high surface pressures in St. Helena Ocean Sci.Discuss., https://doi.org/10.5194/os-2017-74Manuscript under review for journal Ocean Sci. Discussion started: 4 October 2017 c Author(s) 2017.CC BY 4.0 License.

Figure 13 :
Figure 13: Time-latitude diagrams for April-May along the 1998-2008 period, of 2-days average, from top to bottom, i) wind stress magnitude (N.m -2 ) from CFSR fields; ii) short-wave radiation surface flux (W.m -²) from CFSR fields; iii) 20°C isotherm depth (m) computed from the forced model SST; iv) SST (°C) and v) meridional SST gradient (every 0.5° of latitude), from the forced model; averaged over 10° W-6° W. The vertical black line indicates the date of 14 May, 2005.
Ocean Sci.Discuss., https://doi.org/10.5194/os-2017-74Manuscript under review for journal Ocean Sci. Discussion started: 4 October 2017 c Author(s) 2017.CC BY 4.0 License.Upon impingement with the eastern boundary, the incoming equatorial Kelvin wave excites westwardpropagating Rossby waves and poleward propagating coastal Kelvin waves.In 2005, the Kelvin wave reached the coast around mid-May while southerly winds strengthened, allowing the reflected wave to be reinforced by the local wind.This resulted in westward propagation of positive (negative) z20 (SSH) anomalies which, combined with enhanced westward surface currents, provided favorable conditions to westward extension of cold upwelled water from the eastern coast to near 20°W through advection and vertical mixing.In the second part of the study, we specially focused on the mid-May 2005 event (13 May to 16 May) that was characterized by strong southerly wind strengthening in the eastern Tropical Atlantic Ocean.It was found to be responsible for the sudden and intense SST cooling in the Gulf of Guinea and the CLR, and involved in the early onset of the ACT development in 2005 and therefore in early onset of the WAM.The analysis of atmospheric and oceanic conditions in the Gulf of Guinea associated to this event allowed to show that the mid-May event, controlled by the St. Helena Anticyclone, can be identified as a "decisive event" which triggered the abrupt transition between two wind patterns in the northern Gulf of Guinea.Unusual strong southerly winds induced anomalously strong equatorial cooling which in turns slowed down the overlying wind feedback mechanism and generated stronger than normal southerlies north of the equator through the SST front around 1°N.This triggered the deep atmospheric convection in the Gulf of Guinea at a self-sustaining level and the beginning of coastal precipitation.The time of occurrence of this phenomenon, 15 days earlier than the averaged date (31 May from Leduc-Leballeur et al., 2013), suggests than the mid-May 2005 event was associated with anomalous strong moisture flux.The description of atmospheric conditions over the 1998-2008 period has shown that the 2005 event was characterized by the strongest surface pressure gradient between the St. Helena high pressures and the low pressures under the ITCZ, inducing anomalous strong Hadley cell activity.No similar atmospheric pattern was observed during the whole 1998-2008 period.Another wind burst of comparable wind intensity occurred at the beginning of May 2000.