Quasi-decadal scale variability of upper ocean salinity in the western tropical Pacific

Low-frequency SST variations in the tropical Pacific have been investigated by numerous scientists for a past 20 years, however, low-frequency variations of salinity have not been analyzed and remained unclear at present, as compared to SST changes. In this study, the low-frequency changes of the upper ocean salinity in the tropical Pacific are investigated by 10-years (2001-2009) in-situ data, with special reference to tropical Pacific quasi-decadal (PQD) scale variability of SST. Similar to past studies, during 2000s PQD-scale SST anomalies show stronger amplitudes in the central equatorial Pacific and the PQD-scale SST anomalies in the central equatorial Pacific show positive phase (i.e., positive PQD-scale SST anomalies) for 2002-2006. In this period, PQD-scale salinity shows lower values in the western tropical Pacific than in negative PQD phase (2007-2009) in the upper layer above 80m-depth. It is also shown that the PQD-scale upper-ocean salinity is more clearly seen in the south equatorial Pacific than in the north equatorial Pacific. The average of surface zonal transport from buoy data also shows eastward during the positive PQD phase. Therefore, during positive PQD phase, warm water with low salinity anomaly in the western equatorial/south equatorial Pacific is transported eastward to the central Pacific, which is the dominant area of the PQD-scale SST variability. In contrast, during negative PQD phase, westward surface transports with higher salinity are found. The present results suggest that upper-ocean temperature, salinity and surface transport/current change in phase with positive and negative PQD phase in the western to central equatorial Pacific at PQD timescale.