The semienclosed Solomon Sea is the final passage in the equatorward transit of the South Pacific western boundary currents (WBCs) that play a key role in heat and mass budgets of the equatorial Pacific. The Solomon WBCs and their associated water properties are examined using data from two oceanographic cruises undertaken during the contrasting trade wind seasons: July 2012 and March 2014. The mean circulation and associated transports with uncertainties is determined from the cruise data using a unique configuration of an inverse box model formulated based on measured shipboard acoustic Doppler current profiler velocities. An intense inflow of 36 Sv is found entering the Solomon Sea in July–August 2012 that falls by 70% to 11 Sv in March 2014. Large differences are also found in the total transport partitioning through each of the major exit passages during each season. Different water masses are found in the WBC stream northeast of the Solomon Islands that are likely related to a northern stream of the South Equatorial Current. Within the Solomon Sea, isopycnal salinity gradients are gradually stronger than within the subtropical Pacific, likely induced by stronger diapycnal mixing processes. WBC pathways exhibit distinct water mass signatures in salinity, oxygen, and nutrients that can be traced across the Solomon Sea, associated with significant water mass modifications at the northern exit straits and south of the Woodlark Island.

The semi-enclosed Solomon Sea in the southwestern tropical Pacific is on the pathway of a major oceanic circuit connecting the subtropics to the equator via energetic western boundary currents. Waters transiting through this area replenish the Pacific Warm Pool and ultimately feed the equatorial current system, in particular the equatorial undercurrent. In addition to dynamical transformations, water masses undergo nutrient and micronutrient enrichment when coming in contact with the coasts, impacting the productivity of the downstream equatorial region. Broadscale observing systems are not well suited for describing the fine-scale currents and water masses properties in the Solomon Sea, leaving it relatively unexplored. Two multidisciplinary oceanographic cruises were conducted in the Solomon Sea region, the first in July–August 2012 and the second in March 2014, by investigators from France and the United States. The experimental approach combined physical, chemical, geochemical and biogeochemical analyses, providing access to a wide range of space and time scales of the circulation. This collection of data allows describing the fine-scale structure of the currents and the water properties, transformations and mixing from the surface to the sill depth in the Solomon Sea and in the straits connecting it to the equator. Ocean-margin exchanges were documented through a comprehensive sampling of trace elements and isotopes as efficient tracers of natural fertilization processes. As air chemistry is largely impacted by the regional volcanic plumes, rainwater pH was also sampled. Dinitrogen fixation rates were measured and found to be among the highest in the global ocean, highlighting this region as a hot spot of nitrogen fixation. This study provides an overview of the climatic context during both cruises and the physical circulation and water masses properties. It provides a comprehensive description of all measurements made onboard, and presents preliminary results, aiming to serve as a reference for further physical, geochemical and biogeochemical studies.

The gyre-scale, dynamic sea surface height (SSH) variability signifies the spatial redistribution of heat and freshwater in the ocean, influencing the ocean circulation, weather, climate, sea level, and ecosystems. It is known that the first empirical orthogonal function (EOF) mode of the interannual SSH variability in the North Atlantic exhibits a tripole gyre pattern, with the subtropical gyre varying out of phase with both the subpolar gyre and the tropics, influenced by the low-frequency North Atlantic Oscillation. Here, we show that the first EOF mode explains the majority (60 %–90 %) of the interannual SSH variance in the Labrador and Irminger Sea, whereas the second EOF mode is more influential in the northeastern part of the subpolar North Atlantic (SPNA), explaining up to 60 %–80 % of the regional interannual SSH variability. We find that the two leading modes do not represent physically independent phenomena. On the contrary, they evolve as a quadrature pair associated with a propagation of SSH anomalies from the eastern to the western SPNA. This is confirmed by the complex EOF analysis, which can detect propagating (as opposed to stationary) signals. The analysis shows that it takes about 2 years for sea level signals to propagate from the Iceland Basin to the Labrador Sea, and it takes 7–10 years for the entire cycle of the North Atlantic SSH tripole to complete. The observed westward propagation of SSH anomalies is linked to shifting wind forcing patterns and to the cyclonic pattern of the mean ocean circulation in the SPNA. The analysis of regional surface buoyancy fluxes in combination with the upper-ocean temperature and salinity changes suggests a time-dependent dominance of either air–sea heat fluxes or advection in driving the observed SSH tendencies, while the contribution of surface freshwater fluxes (precipitation and evaporation) is negligible. We demonstrate that the most recent cooling and freshening observed in the SPNA since about 2010 were mostly driven by advection associated with the North Atlantic Current. The results of this study indicate that signal propagation is an important component of the North Atlantic SSH tripole, as it applies to the SPNA.

The Solomon Sea is a marginal sea in the western Pacific warm pool that contains the South Pacific low latitude western boundary currents. These low latitude western boundary currents chiefly exit the Solomon Sea through three channels (Vitiaz Strait, St. George's Channel, and Solomon Strait) and serve as the primary source water for the Equatorial Undercurrent. Simulations have shown that transport partitioning between the straits determines the water mass structure of the Equatorial Undercurrent, but the relative contributions of transport through each strait have not been observed before. As part of the Southwest Pacific Ocean Circulation and Climate Experiment, an array of moorings was deployed simultaneously in the three outflow channels of the Solomon Sea from July 2012 until March 2014 to resolve transport and water properties in each strait. Above deep isopycnals (σ₀ ≤ 27.5), Vitiaz and Solomon Straits account for 54.2% and 36.2% of the mean transport, respectively, with the remaining 9.6% exiting through St. George's Channel. The strongest subinertial transport variability is observed in Solomon Strait and dominates total Solomon Sea transport variability, and a significant fraction of this variability is at intraseasonal time scales. Finally, a previously unobserved deep current at 1,500‐m depth is found to enter the Solomon Sea through Solomon Strait, with a deployment‐mean transport of 4.6 Sv (Sv ≡10⁶ m³/s).

Over the past few decades, the global mean sea level rise and superimposed regional fluctuations of sea level have exerted considerable stress on coastal communities, especially in low-elevation regions such as the Pacific Islands in the western South Pacific Ocean. This made it necessary to have the most comprehensive understanding of the forcing mechanisms that are responsible for the increasing rates of extreme sea level events. In this study, we explore the causes of the observed sea level variability in the midlatitude South Pacific on interannual time scales using observations and atmospheric reanalyses combined with a 1.5 layer reduced-gravity model. We focus on the 2004–2020 period, during which the Argo’s global array allowed us to assess year-to-year changes in steric sea level caused by thermohaline changes in different depth ranges (from the surface down to 2000 m). We find that during the 2015–2016 El Niño and the following 2017–2018 La Niña, large variations in thermosteric sea level occurred due to temperature changes within the 100–500 dbar layer in the midlatitude southwest Pacific. In the western boundary region (from 30°S to 40°S), the variations in halosteric sea level between 100 and 500 dbar were significant and could have partially balanced the corresponding changes in thermosteric sea level. We show that around 35°S, baroclinic Rossby waves forced by the open-ocean wind-stress forcing account for 40 to 75% of the interannual sea level variance between 100°W and 180°, while the influence of remote sea level signals generated near the Chilean coast is limited to the region east of 100°W. The contribution of surface heat fluxes on interannual time scales is also considered and shown to be negligible.