Seasonal and interannual variations of sea surface salinity in the tropical Pacific Ocean

Abstract

Sea surface bucket measurements, obtained through a ship‐of‐opportunity program, are used to describe the sea surface salinity (SSS) field for the tropical Pacific during the period 1969–1988. Emphasis is placed upon the mean SSS distribution and the seasonal and interannual SSS variability occurring along four well‐sampled shipping tracks. These tracks extend from New Zealand to Japan, from New Zealand to Hawaii, from Tahiti to California, and from Tahiti to Panama. They cross the equator at 155°E, 160°W, 140°W, and 100°W, respectively. Along each track, the mean SSS distribution is characterized by SSS minima which are 4°–6° further poleward than the axes of maximum precipitation associated with the Intertropical Convergence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ). It is suggested that these SSS minima owe their existence mainly to heavy rainfall and poleward Ekman salt transport associated with the trade winds. The role of zonal salt advection was found negligible for these SSS minima. Except along the eastern track, maximum seasonal SSS variations are located in the ITCZ and SPCZ regions, with minimum SSS in September‐October and March‐April, respectively. On the basis of precipitation island stations, it is demonstrated that the maximum seasonal SSS variations are closely related to the rainfall regimes of the ITCZ and SPCZ (rainfall maximum 3 months before SSS minimum; rainfall amount sufficient to account for SSS changes). Along the eastern track, a strong annual SSS cycle is found from about 4°S (110°W) to the Panama coast (minimum SSS in February–March), reflecting the combined effects of rainfall, salt advection, and vertical mixing. Notable interannual SSS variability concerns only the western half of the tropical Pacific Ocean where El Niño‐Southern Oscillation (ENSO) related SSS changes are strongly related to ENSO‐related precipitation changes. During ENSO periods, the SSS field west of about 150°W is characterized by fresher‐than‐average SSS within about 8°N to 8°S, and conversely saltier‐than‐average SSS poleward of 8° latitudes. These modifications in the SSS field are thought to result mainly from an eastward displacement in the ascending branch of the Walker and Hadley cells which induces unusually high rainfall over the western and central equatorial Pacific region bordered on all sides by rainfall deficits. Reproducing the actual SSS changes at seasonal and interannual time scales would be a very stringent test for model capability.

Interannual sea surface salinity variations observed in the tropical North Pacific Ocean

“…Comparing with seasonal SSS variability which is primarily dominated by E-P forcing according to previous studies [e.g., Delcroix and Henin, 1991;Delcroix et al, 1996;Hénin et al, 1998], our analysis demonstrate that ocean dynamics is of higher-level of importance at interannual time scale. In the central Pacific, meridional Ekman advection -V E S y and E-P forcing are both important in producing and maintaining the low-frequency, large-magnitude SSS anomalies, and the two effects are also both closely associated with ENSO events.…”

“…Figure 1a shows the standard deviation (STD) of SSS (represented by 10 dbar salinity), which is low-passed with a 13 month running mean filter to underscore interannual signals. The strongest SSS variability appears in the far western equatorial Pacific, with STD > 0.2 (psu) and extending southeastward to the SPCZ, which has been investigated by previous studies [e.g., Delcroix and Henin, 1991;Delcroix et al, 2011;Singh et al, 2011]. Another relative maximum of variability occurs in the tropical North Pacific (TNP) between 7 N-15 N with STD > 0.15 near the dateline, and extends westward.…”

“…However, quantifying sea surface salinity (SSS) variability used to be a challenging task due to the paucity of in situ observations. In the tropical Pacific Ocean, large seasonal SSS variations often occur in the intertropical convergence zone (ITCZ) and South Pacific convergence zone (SPCZ) regions and mainly controlled by rainfall changes [e.g., Delcroix and Henin, 1991;Delcroix et al, 1996;Hénin et al, 1998]. At interannual time scale, SSS variations are of larger amplitudes and more complicated patterns in response to El Niño-Southern Oscillation (ENSO)-related freshwater forcing and ocean current advection [Maes, 2000;Delcroix and McPhaden, 2002;Delcroix et al, 2011;Singh et al, 2011].…”

Measuring heat storage changes in the equatorial Pacific: A comparison between TOPEX altimetry and Tropical Atmosphere‐Ocean buoys

“…An increase in salinity will cause water to be heavier and it will tend to counteract part of the thermal expansion due to increased warming; a decrease in salinity will cause water to be lighter and it will tend to reinforce the thermal expansion due to increased warming. Observations in the warm pool indicate that there are large interannual variations in salinity connected with ENSO, with salinity increasing from the E1 Nifio to the La Nifia phase of the ENSO cycle [Delcroix and Henin, 1991]. Since the cycle from.1993 to 1996 was from E1 Nifio to La Nifia, we investigated whether this could account for the discrepancy between the TOPEX and TAO measurements.…”

“…The total heat content was computed from averaged-daily temperatures (T) at each level (h) as We have minimized this error by computing TOPEX sea level anomalies relative to a high-resolution mean sea surface (MSS). The MSS has a 1-s along-track resolution and each 1-s bin is a plane for which the along-track gradient, cross-track gradient, and mean height at the centerof the bin have been estimated from all the available data, after removing any linear trend as well as annual and semiannual variability [Chambers et al, 1998b [Delcroix and Henin, 1991], but this will not have a significant effect on the calculation of stored heat from the TAO buoys. It will, however, cause an additional signal in the TOPEX data that is not related to heat storage, which will cause a disagreement between the TOPEX and TAO measurements.…”