Celebrating 17 Years of Observations at Station ALOHAThomas K. Gregory, Fernando Santiago-Mandujano and The HOT Team*

The Hawaii Ocean Time-series was established in 1988 as part of the U.S.-JGOFS and WOCE programs in an effort to characterize the temporal dynamics of biology, chemistry and physics in the oligotrophic North Pacific subtropical gyre. As of February 2006, we have completed 178 cruises to Station ALOHA (A Long-term Oligotrophic Habitat Assessment). More than 17 years of monthly cruises to this open ocean location have revealed striking dynamics on time scales ranging from diurnal to inter-decadal. Important results stemming from observations at Station ALOHA include the non-steady state behavior in both organic and inorganic nutrient inventories; inter-decadal shifts in plankton community structure; the importance of ocean climate connections in regional and basin-scale physical forcing (rainfall, ENSO, PDO) on biogeochemical cycles and salinity changes; enhanced levels of turbulence near the seabed caused by intermittent water flow between abyssal basins; and the role of eddies on the mean distribution of properties and ocean biogeochemistry changes. Future integration of autonomous sampling platforms (moorings and satellite remote sensing) will strengthen existing shipboard observations and help resolve high frequency variability in physical and biogeochemical processes.

Abstract

University of Hawaii, Department of Oceanography, Honolulu, HI 96822

Acknowledgements

We would like to acknowledge the dedicated efforts of the HOT program scientific and technical staff, the captains and crew of the various research vessels used during the time-series, as well as the logistic support from the UH Marine Center. Of particular note is the vision and continuing achieve-ments of Principal Investigators Roger Lukas and David Karl. The help of Lance Fujieki and Matt Church in creating this poster is appreciated. The Hawaii Ocean Time-series is supported by NSF grants OCE-0327513 (Lukas) and OCE-0326616 (Karl).

Conclusions

Almost two decades of monthly, ship-based monitoring at Station ALOHA has provided a robust dataset which has proven useful in resolving dynamics on a wide range of time scales. Nevertheless, phenomena occurring on short timescales, including aperiodic events, are routinely undersampled by the current observational program. Autono-mous sampling platforms are needed to con-tinuously monitor and record biogeochemi-cal and physical conditions at Station ALOHA. Two instrumented moorings with surface buoys are currently in place near Station ALOHA. Additionally, a cabled observatory connecting Station ALOHA to shore-based facilities with broadband data and power ca-pabilities is currently being developed.

*The HOT Team

Current HOT (co)P.I.s

Robert BidigareJohn DoreDavid KarlMichael LandryRicardo LetelierRoger Lukas

Karin BjörkmanMatthew ChurchTara ClementeSusan CurlessSuzanne DefeliceKen DoggettAllison FongLance Fujieki*Eric GrabowskiTom Gregory

Adriana HarlanPaul LethabyLisa LumClaire MahaffeyNancy PaquinDan Sadler*Fernando S.-Mandujano*Blake WatkinsMark Valenciano

Current HOT Personnel

*Have been on at least 100 of the past 178 HOT cruises!

http://hahana.soest.hawaii.edu/hot/hot.html

Time scales and processes driving biogeochemical variability at Station ALOHA

Global-scale climate variability

Pacific Decadal Oscillation ~ 10-30 year period

ENSO ~ 3-5 year period

Millennia

Decadal

Interannual

Intraannual Seasonal (light, temperature, nutrients)

DiurnalTidal, light, predation

Initial planning committee for the Hawaii Ocean Time-series project. From left: Steve Chiswell, Chris Winn, David Karl, Roger Lukas and Eric Firing

After more than 17 years of monthly observations, we are beginning to decipher regional and basin scale processes that control ocean biogeochemis-try at Station ALOHA. These processes influence biogeochemical inventories and fluxes across time scales ranging from diurnal to decadal, and per-haps longer. Continuation of these observations will provide insight into the connections between climate variability and ocean biogeochemistry.

Poster#OS35F-11

J F M A M J J A S O N D1988

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Map showing the location of Station ALOHA approxi-mately 100 km north of Oahu, Hawaii in deep water. Also shown is the schedule of cruises since October 1988 and a graph showing the average percentage of each month during which observations were made over the 17 year period.

Long-term salinity variations associated with changes in rainfall

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The salinity maximum (Smax) at Station ALOHA is generally centered between 100-150 m (24.2 and 25.0 σθ). It is associated with the sub-duction of salty water in the north-central gyre and its advection southward to Station ALOHA. A pronounced salinity decrease is ob-served between 1991 and 1997. This corresponds to the downward penetration of decadal thermal anomalies in the Central North Pa-cific Ocean, suggested to be a manifestation of thermocline ventila-tion (Deser et al., 1996). The Smax nearly disappeared in early 1993 and during 1997. The rapid increase of salinity in 1998 coincided with the ENSO-related drought around Hawaii in the same year, and continued with the regional decadal rainfall deficiency of winter net freshwater flux over the North Pacific (Lukas, 2001).

The main thermocline (25-26 σθ) shows a trend towards low salini-ties starting around 1992, and continuing until now. This trend is as-sociated with large-scale rainfall variations in the North Pacific (Lukas et al. ,oral presentation OS44C-03).

Time series of salinity anomaly versus potential density (upper panel). The scale along the right axis indicates the mean depth (m) of the corresponding isopycnal surface. The mean salinity profile is given along the left. Time series of layer-averaged salinity for the salinity maximum layer and the main thermocline (lower panel). Light lines connect individual cruise values. Heavy lines are smoothing cubic splines. Closed circles are annual averages. Tic marks along the time axis indicate the timing of individual HOT cruises.

Enhanced levels of turbulence caused by abyssal water flows

Time series of potential temperature versus depth. Tic marks along the time axis indicate the timing of individual HOT cruises.

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The role of eddies in the distribution of ocean properties and biogeochemical changes

Mesoscale eddies are found north of the Hawaiian Islands, appearing in the Hawaii Ocean Time-series observations as anomalies in salinity, potential vorticity, dissolved oxygen, nitrate, phosphate, silicate and fluorescence. Principal component analysis of these properties over a range of isopycnal surfaces in the pycnocline extracts a coherent signal in time, where mesoscale eddies are seen as distinct, extreme events (Figure 1). The most plausible source location of these anoma-lies determined from Levitus climatology (Levitus and Boyer, 1994) is a small region 1,500 km east of ALOHA (Santiago-Mandujano and Lukas, in preparation).

One particularly extreme water mass anomaly was observed in the ther-mocline during January 2001 (Figures 2 and 3). Climatological property distribution maps were used to trace the source of this anomalous water to off-shore of Mexico near Baja California (Lukas and Santiago-Mandujano, 2001).

An eddy related surface plankton bloom was recently observed and sampled in July 2005 (Sadler et al., poster OS36F-03). This bloom, partly composed of diatoms and supported in part by nitrogen-fixing bacteria was associated with a southwestward traveling anticyclonic eddy.

Eddy related anomalies have also been identified in a secondary station of the HOT program, about 40 km south of ALOHA (Letelier et al., 2000).

Cold events are produced when near-bottom, relatively cold and salty water from the Maui Basin "overflows" into the Kauai Deep, where Station ALOHA is located (Lukas et al., 2001). The figure indi-cates that these events became colder and more frequent after 1996.

Enhanced levels of turbulence near the bottom are associated with the occurrence of the cold events. During the 1994, 1995-1996, 1998, and 1999 events, peak values of vertical diffusivity averaged 4 X 10-3 m2/s near the depth of the sill separating the Maui Basin from the Kauai Deep. Enhanced levels of turbulence near the bottom at the ALOHA station have also been inferred from Thorpe scale analysis of overturns (Finnigan et al., 2002), and are attributed to these overflow events.

Zonal sections of dissolved oxygen (top) and salinity (bottom) versus pressure from CTD profiles during the January 2001 HOT cruise. Stations are indicated by tic marks along the top axis. Dis-tances from Station ALOHA at 158 W are given along the x-axis.

Potential tempera-ture (theta), salinity (S), dissolved oxygen (O2), buoyancy fre-quency (N) and fluo-rescence (Fl) profiles from Station ALOHA during the January 2001 HOT cruise when the water mass anomaly was present (colored lines), and from the cruise con-ducted one month earlier (black lines).

Dominant principal component of ALOHA oxygen anomalies versus potential density. Stations are indicated by tic marks along the bottom axis. The scale along the right axis indicates the mean depth (m) of the corresponding isopycnal surface.

Past and present Hawaii-based research vessels used during the time-series: Kaimalino (top left), Ka’ imikai-O-Kanaloa (top right), Moana Wave (center) and Kilo Moana (bottom)

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Vertical concentrations of chlorophyll a are consis-tently greatest in the low light regions of the eupho-tic zone. Chlorophyll a inventories have varied ~2-fold over the time series program.

In contrast to the relative stability of chlorophyll a, depth integrated concentrations of selected pig-ments demonstrate remarkable temporal variability.

Similar to the apparent interdecadal changes in pig-ment concentrations and presumed time-dependent changes in selected groups of photoautotrophic plank-ton, picoplankton abundances have undergone large (~4-fold) changes. In particular, the abundances of pro-chlorococcus increased in the initial period of observa-tion , then declined, while abundances of picoeukary-otes have gradually increased over the latter period of the time-series observations.

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Station ALOHA: An outpost for assessing variability in oceanic carbon inventories

The Hawaii Ocean Time-series (HOT) program has conducted monthly measurements of oceanic dis-solved inorganic carbon, total alkalinity and pH since 1988, providing one of the longest records of time-dependent changes in oceanic inventories of CO2.

The upper ocean at Station ALOHA has been a net sink for atmospheric CO2 throughout most of the time-series observations. However, the increase in pCO2 in the upper ocean is greater than increasing atmospheric pCO2, suggesting Station ALOHA may soon become a net source of CO2 to the atmosphere.

To identify the role of biology on ocean carbon cy-cling, the HOT program has conducted monthly measurements of primary production and carbon export.

Despite a paucity of inorganic nutrients in the upper ocean, rates of primary production are consistently greatest in the well-lit portion of the euphotic zone.

Depth-integrated rates of primary production at Sta-tion ALOHA average 522.5 ± 163.6 mg/m2/d while carbon fluxes out of the upper ocean average 28.9 ± 9.7 mg/m2/d, indicating efficient remineralization of carbon production in the upper ocean.

Despite interdecadal variability in rates of primary pro-duction and carbon export, the e-ratio (export/primary production) appears largely unchanged, suggesting carbon production and export may be uncoupled in time.

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Non-steady state behavior in nutrient inventories at Station ALOHA

Upper ocean inventories of PO43- at Sta-

tion ALOHA have undergone pronounced inter-decadal changes potentially linked to regional-scale climate variability (see Karl, this meeting, oral presentation OS33B-01 and Letelier et al., this meeting, oral presen-tation OS33B-03).

The resulting N:P ratio of inorganic nutri-ent pools has also shown dramatic time-dependent variability.

Coincident with declining availability of PO4

3- inventories, particulate P fluxes have shown a systematic decline over the 17 years of time-series observations.

Systematic variations in PO43- availability

may be linked to basin-scale climate forc-ing by processes such as the El Niño South-ern Oscillation or phase changes in the Pa-cific Decadal Oscillation.

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