Introduction
1. Introduction¶
In 2003, Robert Weller (Woods Hole Oceanographic Institution [WHOI]) , Albert Plueddemann (WHOI), and Roger Lukas (The University of Hawaii [UH]) proposed to establish a long-term surface mooring at the Hawaii Ocean Time-series (HOT) Station ALOHA (22°45’N, 158°W) to provide sustained, high-quality air-sea fluxes and the associated upper ocean response as a coordinated part of the HOT program, and as an element of the global array of ocean reference stations supported by the National Oceanic and Atmospheric Administration’s (NOAA) Office of Climate Observation.
With support from the NOAA and the National Science Foundation (NSF), the WHOI HOT Site (WHOTS) surface mooring has been maintained at Station ALOHA since August 2004. This project aims to record long-term, high-quality air-sea fluxes as a coordinated part of the HOT program and contribute to the goals of observing heat, freshwater, and chemical fluxes at a site representative of the oligotrophic North Pacific Ocean. The approach is to maintain a surface mooring outfitted for meteorological and oceanographic measurements at a site near Station ALOHA by successive mooring turnarounds. These observations will be used to investigate air-sea interaction processes related to climate variability.
The original mooring system is described in the mooring deployment and recovery cruise reports [Plueddemann et al., 2006, Whelan et al., 2007, Whelan et al., 2008, Whelan et al., 2010, Santiago-Mandujano et al., 2024, Santiago-Mandujano et al., 2024]. Briefly, a Surlyn foam surface buoy is equipped with meteorological instrumentation, including two complete Air-Sea Interaction Meteorological (ASIMET) systems, measuring air and sea surface temperatures, relative humidity, barometric pressure, wind speed and direction, incoming shortwave and longwave radiation, and precipitation [Hosom et al., 1995, Colbo and Weller, 2009]. Complete surface meteorological measurements are recorded every minute, as required to compute air-sea fluxes of heat, freshwater, and momentum. Each ASIMET system also transmits hourly averages of the surface meteorological variables via the Argos satellite system. The mooring line is instrumented to collect time series of upper ocean temperatures, velocities, and salinities coincident with the surface forcing record. This mooring includes conductivity, salinity, and temperature recorders, two Vector Measuring Current Meters (VMCMs), and two Acoustic Doppler current profilers (ADCPs). See the WHOTS-19 mooring diagram in the Fig. 1.1.
The subsurface instrumentation is located to resolve the temporal variations of shear and stratification in the upper pycnocline to support the study of mixed layer entrainment. Experience with moored profiler measurements near Hawaii suggests that Richardson number estimates over 10 m scales are adequate. Salinity is essential to the stratification, as salt-stratified barrier layers are observed at HOT and in the region [Kara et al., 2000]. Hence, we use Sea-Bird SeaCATs and MicroCATs with vertical separation ranging from 5 to 20 m to measure temperature and salinity. We use two ADCPs made by Teledyne RD Instruments to obtain current profiles across the entrainment zone and in the mixed layer zone. Both ADCPs are in an upward-looking configuration, one is at 125 m, using 4 m bins, and the other is at 47.5 m using 2 m bins. To provide near-surface velocity (where ADCP estimates are less reliable), we deploy two Vector Measuring Current Meters (VMCMs). The nominal mooring design is a balance between resolving extremes versus the typical annual cycling of the mixed layer [Plueddemann et al., 2006, Santiago-Mandujano et al., 2007]. A pair of Sea-Bird SeaCATs (SBE-16) or MicroCATs (SBE-37) have been included since the WHOTS-9 deployment (June 2012) to measure near-bottom temperature and salinity.
Fig. 1.1 WHOTS-19 mooring design¶
The WHOTS-19 mooring was deployed on June 17, 2023 (WHOTS-19 cruise) and was recovered on June 06, 2024 (WHOTS-20 cruise). Both cruises were carried out aboard the R/V Oscar Elton Sette. The WHOTS-20 mooring was deployed on June 02, 2024, during the WHOTS-20 cruise and was recovered on September 28, 2025.
This report documents and describes the oceanographic observations made on the WHOTS-19 mooring for nearly one year and from shipboard measurements during the two cruises when the mooring was deployed and recovered. Sections II and III include a detailed description of the cruises and the mooring, respectively. Sampling and processing procedures of the hydrographic casts, thermosalinograph, and shipboard ADCP data collected during these cruises are described in Section IV. Section V includes the processing procedures for the data collected by the moored instruments: SeaCATs, MicroCATs, Moored ADCPs and VMCM. Plots of the resulting data and preliminary analysis are presented in Section VI.