(Station 1: 21 20.4'N, 158 16.2'W)
- Station ALOHA
A Long-term Oligotrophic Habitat Assessment
(Station 2: a 10 Km radius circle centered at 22 45'N, 158 W, approximately 100 km north of Oahu, Hawaii).
- WHOTS Mooring Station
(First deployed in August 2004, with annual turnaround, alternating between two locations on the edge of Station ALOHA: Station 50, approximately 22 45'N, 157 54'W, and Station 52, approxiamtely 22 40'N, 157 57'W).
- MOSEAN Mooring Station
(Station 51: Approximately 22 45'N, 158 6'W, location of the MOSEAN mooring, first deployed in August, 2004, semi-annual turnaround).
- North of ALOHA Station
(Station 3: 23 25.2'N, 158 W) occupied during some of the cruises starting with cruise 31 through 73.
- Kaena Point
(Station 6: 21 51'N, 158 21.6'W) occupied during cruises 50 through 67, and again starting on cruise 118 (August, 2000).
(Station 8: approx. 22 15'N, 158 6'W, location of the HALE ALOHA mooring) occupied between January 1997 and October 2002.
In 1987, the National Science Foundation established a special-focus research initiative termed 'The Global Geosciences Program'. This program is intended to support studies of the earth as the system of interrelated physical, chemical, and biological processes that act together to regulate the habitability of our planet. Two of the components of the Global Geosciences Program are the World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux Study (JGOFS) programs. The former is focused on physical oceanographic processes and the latter on biogeochemical processes.
HOT was initially funded in 1988 as a 5-year program under WOCE and JGOFS to make repeated observations of the physics, biology and chemistry of the water column at a site north of Hawaii. A second 5-years of funding were approved in 1994, 3 years more in 1998, and 3 years more in 2001. In 2003 HOT was renewed for 5 more years, and in 2008 it was extended until 2010, and again in 2009 it was extended until 2013 (this extension was funded by the American Recovery and Reinvestment Act). In 2013 the project was renewed 5 more years until 2018.
HOT is part of the ALOHA Observatory, which is being developed at HOT's station ALOHA. Measurements at the HOT site contribute to the global description of heat, fresh water and chemical fluxes at a site representative of the oligotrophic North Pacific Ocean. The PO component contributes to the objectives of the World Climate Research Programme (WCRP) Climate Variability and Predictability (CLIVAR) Programme by providing information on interannual to decadal variability of the North Pacific Ocean.
ObjectivesThe main objective of the Physical Oceanography (PO) component of HOT is to describe and understand the physical oceanography at the HOT site, located in the central subtropical gyre of the North Pacific. In particular, the documentation and understanding of the temporal variability and its role in the earth's climate is an important scientific objective.
ObservationsCruises are made on approximately monthly intervals, and repeated observations of the physics, biology and chemistry of the water column are made at the HOT site. Data collected by the PO group include CTD (conductivity, temperature and pressure) profiles, water samples, XBT (expendable bathythermographer), ADCP (acoustic Doppler current profiler), themosalinograph, and meteorological observations. When available, the University of Hawaii's R/V Moana Wave was used for HOT cruises, and the R/V Ka'Imikai O Kanaloa since July 1999, after the Moana Wave's retirement. The new University of Hawaii's AGOR 26 R/V Kilo Moana has been used since November 2002 when available. Other vessels from the University-National Oceanographic System (UNOLS) have also been used during various cruises. In addition to the cruise observations, data from IESs (inverted echo sounders) at the HOT site were obtained between 1991 and 1999.
Each HOT cruise follows the same basic pattern with the flexibility for the addition of ancillary projects. The CTD casts consist of a CTD mounted on a rosette package with typically 24 Niskin bottles, which are used to obtain water samples during the up-cast.
A CTD cast to 1000 m is conducted at Kahe Point. This cast serves as a "shakedown" for the remainder of the cruise, and the data taken represent an additional time-series of water properties at a near-shore site. After this cast the ship heads towards the station ALOHA.
Upon arrival at ALOHA, operations commence with a deep cast (maximum depth approximately 4750 m). Subsequently, a 36-hour burst of consecutive 1000 m CTD casts at the same location is carried out. The purpose of this is to average out the local near-inertial and tidal periods and therefore preventing the aliasing of longer-term fluctuations whose detection is one of the main objectives of the WOCE Hydrographic Time-series program. This ends the PO core sampling.
In addition to the core sampling, 1000 m CTD casts are made to support ancillary JGOFS work. When time permits, one additional deep cast is done at station ALOHA. Finally, a near-bottom cast is done to about 2500 m at Kaena Station (Station 6). Between January 1997 and October 2002 operations at ALOHA were followed by a visit to Station 8 (the location of the HALE ALOHA mooring), where a 1000 m CTD cast was made to calibrate instruments installed in the mooring.
Vertical temperature profiles of the upper 700 m were obtained from XBTs (Expendable Bathythermograph) launched at 10 mile intervals along
When available the shipboard ADCP (Acoustic Doppler Current Profiler) is used to measure the velocity profile in the upper 300 m during the entire cruise. The exact penetration depth depends on the ship-specific acoustic noise level and the amount of entrained bubbles.
Starting in mid 1995 a thermosalinograph installed on the R/V Moana Wave recorded continuous measurements of near-surface temperature and conductivity during the HOT cruises. A SBE-21 Seacat thermosalinograph was mounted in a sea chest located at the bow of the ship about 3 m deep. Temperature and conductivity measurements are obtained at 10 sec intervals throughout the cruise. After the Moana Wave's retirement, the thermosalinograph system was transferred to the R/V Ka'Imikai O Kanaloa to continue taking measurements since July 1999.
Every 4 hours, at 0, 400, 800, 1200, 1600, 2000 hours (GMT) meteorological observations are obtained at station ALOHA by the science personnel.
Inverted Echosounders (IES)
In early 1991, a 50-km array of 5 inverted echo sounders (IESs) was deployed as part of the WOCE deep water station and mantained until mid-1993, after which single IESs were deployed at Kaena and ALOHA stations until 1999. The purpose of these observations is to resolve the synoptic variablility which may not be resolved by the monthly sampling.
Details of the sampling program can be found in the HOT Data Reports.