Mooring and Seafloor Pressure End Point Measurements at the Southern Entrance of the Solomon Sea: Subseasonal to Interannual Flow Variability. Issue 7 (22nd July 2019) - Anutaliya, Arachaporn
Abstract: Variability of the flow across the Solomon Sea's southern entrance was examined using end point subsurface moorings and seafloor pressure sensors, reconstructed velocity profiles based on satellite‐derived surface velocity and bottom pressure‐derived subsurface velocity, and 1993–2017 proxy volume transport based on satellite altimetry. The reconstructed velocity correctly represents the fluctuating surface flow and subsurface core providing a high‐frequency continuous observing system for this sea. The mean equatorward volume transport over 0‐ to 500‐m depth layer is 15.2 Sv (1 Sv ≡ 10 6 m 3 /s) during July 2012 to May 2017. The measurements resolve the full spectrum of the volume transport including energetic subseasonal variability that fluctuates by as much as 25 Sv over one week. At low‐frequency timescales, the study finds that linear Rossby waves forced by Ekman pumping in the interior of the Pacific influence not only seasonal fluctuations as found by previous studies but also interannual variability. As found previously, the El Niño–Southern Oscillation highly influences interannual volume transport. During the 2015/2016 El Niño, observations show the seasonal cycle to be suppressed from the second half of 2014, prior to the mature phase of the El Niño, to September 2016 along with an increase in across‐transect transport. At subseasonal timescales, local Ekman pumping and remote wind stress curl are responsible for a third of the subseasonal variance. The study highlights the importance of high‐frequency observations at the southern entrance of the Solomon Sea and the ability of a linear Rossby model to represent the low‐frequency variability of the transport. Plain Language Summary: The Solomon Sea, located in the tropical southwest Pacific Ocean, is characterized by energetic currents that transport water from the subtropical South Pacific to the equatorial Pacific. In this study, we quantify the amount of water being transported through the Solomon Sea to understand how and why it changes over time. Over the observing period (July 2012 to May 2017), a mean water volume of 15.2 Sv (1 Sv ≡ 10 6 m 3 /s) flows into the Solomon Sea at its southern entrance. The flow can change greatly over short time period (±25 Sv over 1 week) as being modified by winds over the Solomon Sea and further upstream over the Coral Sea. The observed inflow also has distinct interannual variability and a seasonal cycle with a minimum in March/April and a maximum in July/August. On these timescales, the flow fluctuates up to ±10 Sv as being influenced by the winds over the equatorial and tropical Pacific Ocean. During the 2015/2016 El Niño, our observations show that the equatorward flow via the Solomon Sea strengthens but the seasonal cycle is suppressed. We also showed that the flow can be efficiently monitored using only a pair bottom‐pressure sensors and satellite altimetry. Key Points: The observations reveal energetic subseasonal flow (37% of total variance) into the Solomon Sea associated with local and remote winds Rossby waves propagating from the central Pacific Ocean are well correlated with the inflow variability at seasonal and longer timescales The inflow strengthens and the seasonal cycle is suppressed during the 2015/2016 El Niño