Sampling surface salinity in a high-precipitation region of the eastern tropical Pacific
The 2016-2017 Salinity Processes in the Upper-ocean Regional Studies (SPURS-2) field experiment sampled the westward extension of the eastern tropical Pacific fresh pool (EPFP; Alory et al., 2012). This is a low surface salinity feature which extends westward from the coast of Panama and Colombia. It contains some of the freshest surface water in the global ocean due to runoff from rivers in Central America and atmospheric transport of freshwater from the Atlantic across the Central American isthmus (Fig. 1). The EPFP is also associated with the intertropical convergence zone (ITCZ). This is a band of converging air, thick clouds and heavy rain (Fig. 2) which makes seasonal excursions between the equator and about 10°N.
The EPFP extends westward seasonally from the eastern boundary of the tropical Pacific reaching its maximum extent in the October/November period (Guimbard et al., 2017; Fiedler and Talley, 2006; Melnichenko et al., 2019). At the southern edge of this fresh pool extension is a surface front separating the relatively fresh EPFP water from higher salinity surface water found at the equator (Yu, 2015). This front is the location of the North Equatorial Countercurrent (NECC; Kessler, 2006), which forms in January near 3°N, migrates to the north over the course of the year, and dissipates in the months of January-February after reaching a maximum latitude of about 12°N (Yu, 2015).
As is clear from Figs. 1 and 2, the region where SPURS-2 took place is a highly dynamic one. Two powerful currents, the westward NEC and eastward NECC exist in this area, contributing to the back and forth motion of the EPFP. These currents are generated by wind patterns in the ITCZ region, including a surface convergence. Rising air associated with this convergence, leads to the rainfall we see in Fig. 2. The SPURS-2 site is one of the rainiest places on Earth on average.
The purpose of SPURS-2 was to elucidate the relationship between rainfall, ocean dynamics and surface salinity. Rainfall in the region is heavy on average, but also very patchy (Thompson et al., 2019; Rutledge et al., 2019a,b) and seasonal. When it does rain, the freshwater input into the ocean is buoyant and tends to sit at the surface in very thin (<5m) fresh puddles (Drushka et al., 2019; Clayson et al., 2019) for some period of time before it mixes into the ocean below. How do these small-scale puddles combine with the larger-scale current systems to generate the low salinity feature at the surface that stretches across the entire tropical Pacific that we see in maps like that of Fig. 3? How large is the near-surface salinity stratification induced by rainfall? How does this stratification depend on rain rate and wind speed (that causes vertical mixing)? Moreover, the patchiness of rainfall and the small-scale ocean dynamics also result in small-scale variation of SSS within satellite footprints, namely, sub-footprint variability (SFV). SFV can lead to differences between the “match-up” of satellite SSS (averaged over satellite footprints) and pointwise in-situ measurements because of the differences in spatial scales captured by satellites and in-situ platforms as well as their temporal sampling mis-match.
Key results from SPURS-2 were discussed in a collection of articles published in the SPURS-2 Special Issue in the TOS Oceanography Magazine: https://tos.org/oceanography/issue/volume-32-issue-02.
The editorial from the guest editors (Lindstrom et al., 2019) of the special issue provided highlights of main findings from the collection of papers.
SPURS2 Special Issue in Oceanography Magazine: https://tos.org/oceanography/issue/volume-32-issue-02.