In the past decade, two large marine heatwaves (MHWs) formed in the northeast Pacific near Ocean Station Papa (OSP), one of the oldest oceanic time series stations. Physical, biogeochemical and biological parameters observed at OSP from 2013 to 2020 are used to assess ocean response and potential impacts on marine life from the 2019 northeast Pacific MHW. The 2019 MHW was preceded by calm and stratified surface conditions, lower dissolved inorganic carbon, and higher pH of surface waters relative to the 2013-2020 period. A spike in the summertime chlorophyll followed by a decrease in surface macronutrients suggests increased productivity in the well-lit stratified upper ocean during summer 2019. More blue whale calls were recorded at OSP in 2019 compared to the prior year. Large subsurface temperature anomalies were also found, suggesting that the earlier northeast Pacific MHW (2013-2015, previously referred to as “Blob”) as well as the long-term increase in sea surface temperatures in the region contributed to the intensity of the 2019 MHW. This study shows how the utility of long-term, continuous oceanographic datasets and analysis with an interdisciplinary lens is necessary to understand the potential impact of MHWs on marine ecosystems.

The equatorial region of the slow-spreading Mid-Atlantic Ridge is characterized by several major transform faults, which are some of the longest on Earth. Among them, the St. Paul Transform system (SPTS) is a complex group of four transform faults, bounding three short intra-transform segments with total offset of 630 km. The northernmost transform is the 200 km-long, 30 km-wide Atoba Ridge, which represents a major topographic feature that rises above sea level at the St. Peter and St. Paul islands (SPSPA). This push-up ridge formed from transpressive stresses along several transform fault step-overs and restraining bends, uplifting mantle rocks at a rate of ~1.5 mm/yr. Moderate-sized earthquakes (>4.0 Mw) have been located by global teleseismic networks along the SPTS and near region. These earthquakes are recorded at large epicentral distances, and include raypaths that travel within the upper mantle (Pn and Sn phases). Pn velocity estimates can help to understand the dynamics of upper mantle structure around of the transform faults. Here, waveforms recorded over ~6 months of 2012 by two autonomous hydrophones moored north and south of SPTS (EA-2 and EA-8), and a seismographic station installed on SPSPA island (ASPSP station) are examined. These data allow us to make Pn velocity estimates from 32 earthquakes that occurred in the SPTS region from 1.5º S to 4.5º N. Pn wave velocities are typically thought to be 8.0–8.2 km/s in upper mantle, however we identify Pn velocities ranging from 7.5 to 9.0 km/s. The slower velocities (7.5-8.0 km/s) are from ray paths oriented parallel to the ridge axis and could be explained by elevated mantle potential temperature and the presence of melt. Ray paths passing through the transform fault system have Pn velocities from 8.1 to 9.0 km/s, indicating that upper mantle conditions are strongly affected by the presence of the crustal fault system. We will also compare our velocity estimates to global shear-wave tomographic models of the upper mantle. Hence it is our goal to show that the availability of autonomous hydrophones and a single island seismic station can be used to make rare estimates of Pn velocities, as well as provide insights into upper mantle structure, in this remote part of the Atlantic Ocean.