Semyon A. Grodsky and James A. Carton, University of Maryland, College Park, MD

The PIRATA (PIlot Research Array moored in the Tropical Atlantic) project is an international program which maintains a network of surface or near-surface measurements including temperature, salinity, and air-sea fluxes [Servain et al., 1998]. This data (now up to 8 years long) should capture the interannual variability of the tropical Atlantic. Although at a weaker magnitude than the Pacific Ocean, the Tropical Atlantic Ocean also seats a strong climatic Variability (TAV). This variability is decomposable in two modes: the zonal primarily equatorial mode and a trans-equatorial, or meridional mode, both associated with the variations of SST, atmospheric pressure and position in latitude of the InterTropical Convergence Zone (ITCZ). The TAV time scales range from interannual to decadal. During the PIRATA observations (1997-present, Fig.1) two major interannual equatorial warm events were observed in 1998-1999 and in 2003 (Fig.2b). With the very first PIRATA data Servain et al. [2003] have demonstrated how the thermal structure of the two modes of TAV is resolved by the PIRATA at interannual periods. Less is known about the salinity contribution to the TAV. The two modes are strongly distinguishable by their seasonality and impact on the meridional migration of rainfall. Warm phase of the zonal mode (Atlantic Nino) peaks in JJA and shifts rainfall in the east southward from the continent into the northern Gulf of Guinea [Hisard, 1980]. The meridional SST gradient has significant impact on the southern extension of the ITCZ rainfall band during MAM [Moura and Shukla, 1981]. The ITCZ crosses the equator in the west and brings an abundant precipitation to the NE Brazil when the interhemispheric SST gradient is southward. It is expected that both modes of the TAV impacts the equatorial salinity by modifying the net surface freshwater flux.

Figure 1. (left) PIRATA buoy location and (right) availability of the sea surface salinity data

This work presents the interannual variation of salinity in the equatorial Atlantic observed by the PIRATA array. Along the equator we find the surface intensified interannual salinity variations (e.g. Fig.2a) that are decomposed into two EOF modes weighted towards the east and the west (Fig.3). These two modes of salinity variability are related with the zonal and meridional modes of SST variability, respectively (Figs.4, 5, 6)

Figure 2. (a) Monthly salinity anomaly in the Gulf of Guinea (missing data are shaded gray). (b) JJA Atlantic Nino index (SST anomaly averaged 20W-10E, 4S-2N). Warm events correspond to freshening. Reynolds and Smith SST is used. Monthly anomaly is defined as deviation from the seasonal cycle.

Interannual variation of salinity in the equatorial Atlantic observed by the PIRATA moorings

References•Hisard, P., El-Nino Response of the Eastern Tropical Atlantic, Oceanologica Acta, 3 (1), 69-78, 1980.•Moura A.D., and J. Shukla, On The Dynamics Of Droughts In Northeast Brazil - Observations, Theory and Numerical Experiments with a General-Circulation Model, J. Atmos. Sci., 38 (12): 2653-2675, 1981.•Servain, J., A. J. Busalacchi, M. J. McPhaden, A. D. Moura, G. Reverdin, M. Vianna, and S. E. Zebiak, A Pilot Research Moored Array in the Tropical Atlantic (PIRATA), Bull. Amer. Meteorol. Soc., 79, 2019-2031, 1998.•Servain, J., G. Clauzet, and I.C. Wainer, Modes of tropical Atlantic climate variability observed by PIRATA, Geoph. Res. Lett. 30 (5), Art. No. 8003, 2003.

PIRATA salinity data coverage

Salinity and SST anomalies in the Gulf of Guinea

Leading EOFs salinity variability along the equator based on the PIRATA moorings data

Figure 3. The first and second EOFs of salinity anomaly along the equator: (a) longitude-depth patterns, (b) time series. +/o markers label each July/April. Positive phase of the leading EOF (negative salinity anomaly in the Gulf of Guinea) takes place in 1998 and 2003 in phase with the Atlantic Nino warming (compare to Fig.2b).

Projection of the leading EOFs time series

Figure 4. Time regression of (a) the first EOF time series (PC#1) on the JJA SST anomaly, (b) the second EOF time series (PC#2) on the MAM SST anomaly. Areas with time correlation exceeding 0.5 in magnitude are contoured.

Figure 5. Time regression of (a) the first EOF time series (PC#1) on the JJA rainfall anomaly, (b) the second EOF time series (PC#2) on the MAM rainfall anomaly. Precipitation is the Tropical Rainfall Measuring Mission (TRMM) satellite data. Areas with time correlation exceeding 0.5 in magnitude are contoured.

Projection of the leading EOFs time series

Figure 6. Time regression of (a) the first EOF time series (PC#1) on the JJA wind anomaly (arrows), (b) the second EOF time series (PC#2) on the MAM wind anomaly. Wind velocity is combination of the monthly mean SSM/I and QuikSCAT satellite data. Areas with time correlation exceeding 0.5 in magnitude are shaded gray.

JJA regression pattern displays anomaly convergence of the equatorial winds and some weakening of the easterly wind in the west in response to the Gulf of Guinea warming/ freshening.

In MAM the northerly cross-equatorial wind anomaly is indicative of the southward shift of the ITCZ in response to colder North Tropical Atlantic. The southward shift of the ITCZ increases equatorial rainfall in the west that decreases salinity.

ResumeSalinity measurements from the equatorial PIRATA moorings span 8 years (1998-2005) that provides data for assessing the interannual salinity variation in the equatorial Atlantic. In this still too limited records we find an impact of the two modes of the Tropical Atlantic Variability. •During Atlantic Nino years near surface salinity in the Gulf Guinea decreases by around 0.1 psu in response to 0.5oC SST warming and 1-3 mm/day stronger rainfall along the northern Gulf of Guinea.•North Tropical Atlantic Cold SST anomaly of around -0.5oC shifts ITCZ southward and produces an excess rainfall of around 1-3 mm/day in the western equatorial Atlantic in MAM that results in lowering of salinity by 0.1 psu there.