Eastern Subtropical North Pacific Research Articles

Projected Increase in Probability of East Asian Heavy Rainy Summer in the 21st Century by CMIP5 Models

In the summer of 1998, heavy rainfall persisted throughout the summer and resulted in a severe prolonged flood event. Will a similar rainy summer happen again? To date, many studies have investigated the projected changes in seasonality or daily extreme precipitation events over East Asia; however, few studies have focused on the changes in extreme summer-averaged East Asian rainfall. We referred to this type of summer as “heavy rainy summer (HRS)” and investigated future changes in its probability, by analyzing CMIP5 model outputs in historical simulation (HIST) and under RCP4.5 and RCP8.5. All models projected an increased probability of HRS by a factor of two to three. Both the projected East Asian summer rainfall (EASR), i.e. EASRRCPs−EASRHIST, in climatology and HRS would intensify significantly. The increased EASR could be attributed to significantly intensified water vapor transport (WVT) originating from the tropical Indian Ocean (TIO) and the eastern subtropical North Pacific (SNP), which result from thermodynamic component. The WVT from TIO would supply more moisture for EASR because its stronger intensity and faster rate of increase. Meanwhile, the EASR anomaly in HRS relative to climatology, i.e. EASRHRS−EASRCLM, would increase by approximately 11%–33%. In HIST, the associated WVT anomaly, caused by only dynamic component, converges moisture from adjacent land and ocean. However, under RCPs, the WVT anomaly from TIO, resulted from thermodynamic component, would appear and increase triple to be comparable to the one from the eastern SNP. The latter would result from dynamic component but increase by only half.

Role of the eastern subtropical North Pacific Ocean on the El Niño’s transition processes

We examine the transition processes of El Nino occurring in 1 year after using observations, a multi-century model simulation and atmospheric general circulation model (AGCM) experiments. One type is characterized by a quick transition from an El Nino event into a La Nina event in the following winter, and the other type involves a slow decay to an almost neutral state or a continuous El Nino event. While both the Indian and the Atlantic Oceans contribute to the El Nino’s transition processes, we further find that sea surface temperature (SST) conditions in the eastern subtropical North Pacific and the associated surface wind anomalies play important roles in determining the different types of El Nino’s transition processes through atmosphere–ocean coupled processes. In particular, northeasterly wind anomalies in the central subtropical North Pacific during the early spring, which is in consequence of atmosphere–ocean coupled processes, contribute to decreasing ocean heat content (OHC) anomalies and strengthening easterly wind anomalies in the central tropical Pacific. These anomalies lead up to a transition into a La Nina event in the following winter. On the contrary, weakening of the northeasterly wind and warm SST anomalies in the eastern subtropical North Pacific during the early spring are conducive to a neutral state or a continuous El Nino event in the subsequent winter. Similar transition processes are also found in a multi-century model simulation. By conducting idealized AGCM experiments, we also show that the anomalous SST in the eastern subtropical North Pacific during El Nino peak season may induce surface wind anomalies in the central tropical Pacific during El Nino onset season. These results provide potential precursors for predicting the occurrence of a La Nina event, a neutral state and an El Nino event 1 year after the occurrence of an El Nino event.

Annual net community production in the subtropical Pacific Ocean from in situ oxygen measurements on profiling floats

AbstractAnnual net community production (ANCP) in the subtropical Pacific Ocean was determined by using annual oxygen measurements from Argo profiling floats with an upper water column oxygen mass balance model. ANCP was determined to be from 2.0 to 2.4 mol C m−2 yr−1 in the western subtropical North Pacific, 2.4 mol C m−2 yr−1 in the eastern subtropical North Pacific, and near zero in the subtropical South Pacific. Error analysis with the main sources of uncertainty being the accuracy of oxygen measurements and the parameterization of bubble fluxes in winter suggested an uncertainty of ~0.3 mol C m−2 yr−1 in subtropical Pacific. The results are in good agreement with previous observations in locations where ANCP has been determined before. These are the first results from the western subtropical North Pacific and subtropical South Pacific where ANCP have not been evaluated before. ANCP for the subtropical South Pacific is significantly lower than in all other open ocean locations where it has been determined by mass balance. Comparison of our observations with net biological carbon export estimated from remote sensing algorithms indicates that observations from the subtropical North Pacific are higher than the satellite estimates, but those in the subtropical South Pacific are lower than satellite‐determined carbon export.

North Pacific Eastern Subtropical Mode Water simulation and future projection

The present climate simulation and future projection of the Eastern Subtropical Mode Water (ESTMW) in the North Pacific are investigated based on the Geophysical Fluid Dynamics Laboratory Earth System Model (GFDL-ESM2M). Spatial patterns of the mixed layer depth (MLD) in the eastern subtropical North Pacific and the ESTMW are well simulated using this model. Compared with historical simulation, the ESTMW is produced at lighter isopycnal surfaces and its total volume is decreased in the RCP8.5 runs, because the subduction rate of the ESTMW decreases by 0.82×10−6 m/s during February-March. In addition, it is found that the lateral induction decreasing is approximately four times more than the Ekman pumping, and thus it plays a dominant role in the decreased subduction rate associated with global warming. Moreover, the MLD during February-March is banded shoaling in response to global warming, extending northeastward from the east of the Hawaii Islands (20°N, 155°W) to the west coast of North America (30°N, 125°W), with a maximum shoaling of 50 m, and then leads to the lateral induction reduction. Meanwhile, the increased northeastward surface warm current to the east of Hawaii helps strengthen of the local upper ocean stratification and induces the banded shoaling MLD under warmer climate. This new finding indicates that the ocean surface currents play an important role in the response of the MLD and the ESTMW to global warming.

Nitrous oxide cycling in the water column and sediments of the oxygen minimum zone, eastern subtropical North Pacific, Southern California, and Northern Mexico (23°N-34°N)

Identifying sources and sinks of N2O can illuminate N cycling processes in marine systems, particularly where changes in dissolved O2 can lead to changes in N cycling pathways (i.e., nitrification versus denitrification). We measured N2O and concentration and their stable isotope ratios (δ15N and δ18O) in the water column and sediments of the oxygen minimum zone in the nearshore eastern subtropical North Pacific (23°N–34°N). Atmospheric efflux of N2O ranged from 2.2 to 17.9 μmol m−2 d−1 or about 2–20 times higher than in oxygenated regions of the North Pacific. Surface waters were a source of 15N-depleted and 18O-enriched N2O to the atmosphere, indicating a bacterial, not archaeal, nitrification N2O source. Stable isotopes indicated that nitrification in both surface and intermediate waters (∼0–200 m) was the major source of N2O in this study area, with denitrification acting as a small N2O sink in strongly O2-depleted waters. Denitrification had a larger impact on observed patterns of N2O and concentrations and isotope ratios in the southern oxygen minimum zone. Sediments were generally neutral or a weak sink for N2O, with only one site (Soledad basin) showing a positive efflux of +3.5 ± 1.0 μmol N2O-N m−2 d−1. Sediment fluxes of N2O at all sites were several orders of magnitude smaller than fluxes of dinitrogen, nitrate, and ammonium measured in previous studies and did not appear to impact water column N2O concentrations. N2O was less than 0.1% of the N2 efflux from sedimentary denitrification.

Interpreting Energy and Tracer Spectra of Upper-Ocean Turbulence in the Submesoscale Range (1–200 km)

AbstractSubmesoscale (1–200 km) wavenumber spectra of kinetic and potential energy and tracer variance are obtained from in situ observations in the Gulf Stream region and in the eastern subtropical North Pacific. In the Gulf Stream region, steep kinetic energy spectra at scales between 200 and 20 km are consistent with predictions of interior quasigeostrophic–turbulence theory, both in the mixed layer and in the thermocline. At scales below 20 km, the spectra flatten out, consistent with a growing contribution of internal-wave energy at small scales. In the subtropical North Pacific, the energy spectra are flatter and inconsistent with predictions of interior quasigeostrophic–turbulence theory. The observed spectra and their dependence on depth are also inconsistent with predictions of surface quasigeostrophic–turbulence theory for the observed ocean stratification. It appears that unbalanced motions, most likely internal tides at large scales and the internal-wave continuum at small scales, dominate the energy spectrum throughout the submesoscale range. Spectra of temperature variance along density surfaces, which are not affected by internal tides, are also inconsistent with predictions of geostrophic-turbulence theories. Reasons for this inconsistency could be the injection of energy in the submesoscale range by small-scale baroclinic instabilities or modifications of the spectra by coupling between surface and interior dynamics or by ageostrophic frontal effects.

Distribution of nitrous oxide dissolved in water masses in the eastern subtropical North Pacific and its origin inferred from isotopomer analysis

N2O concentration and its isotopomer ratios were measured over a wide area from San Diego to Honolulu in the eastern subtropical North Pacific (ESNP). Waters in the study area had an N2O maximum (38.2–50.5 nmol kg−1) at 600–1000 m depth, which is similar to the profiles obtained previously in other areas in the North Pacific. We separated the seawater into five water masses (two for the surface layer, two for the middle layer, and one for the deep layer) and deduced N2O production–consumption mechanisms in each water body by use of N2O isotopomer ratios. The results showed that the mechanisms differ slightly among water masses. In the “coastal” surface layer, N2O is produced by nitrification (NH2OH oxidation). In the “open ocean” surface layer, it is produced mainly by nitrifier denitrification and to a lesser extent by nitrification under substrate-limited conditions. In both “upwelling” and “open ocean” middle layers it is produced mainly by denitrification and to a lesser extent by nitrifier denitrification. It is also partly reduced. In the deep layer, it is produced predominantly by denitrification with partial reduction. In this way, isotopomers aid elucidation of production–consumption mechanisms of N2O in the sea even though the mechanisms cannot always be ascertained.

A Far-Reaching Footprint of the Tropical Pacific Meridional Mode on the Summer Rainfall over the Yellow River Loop Valley

Abstract Over thousands of years, the vicissitudes of the Yellow River and surrounding geographic environment form a unique river loop in northwestern China, namely, the Yellow River Loop Valley. Results from both observations and a climate model indicate that the summer rainfall over the Yellow River Loop Valley region can be remotely controlled by sea surface temperature fluctuations in the eastern subtropical North Pacific. This far-reaching teleconnection is achieved by an atmospheric wave train emanating from the eastern subtropical North Pacific, with a long traveling journey from the western to the Eastern Hemisphere. Furthermore, it is found that the SST forcing pattern resembles the Tropical Pacific Meridional Mode, a mode characterized by an interhemispheric temperature gradient and shift of the intertropical convergence zone in the eastern tropical Pacific.

Local stratification control of marine productivity in the subtropical North Pacific

Strengthened stratification of the upper ocean due to global warming is generally expected to inhibit marine primary productivity in the subtropics, based on the supposition that increased water column stability will decrease vertical mixing and consequently the entrainment of deep nutrients into the euphotic zone. A recent analysis of observational data from the subtropical North Atlantic, however, demonstrates that productivity in this region is not correlated with stratification on interannual time scales over the modern observational record, but is instead impacted by other dynamics that affect vertical mixing and nutrient supply. Herein, we examine data from the Hawaiian Ocean Time series program's Station ALOHA (A Long‐Term Oligotrophic Habitat Assessment) in the subtropical North Pacific. We find that stratification and productivity are not strongly correlated at this location over the observational record. In contrast to the North Atlantic, the weakness of correlation observed at ALOHA may reflect the strongly stratified ecosystem of the eastern subtropical North Pacific and a lack of sufficiently strong interannual forcing in this region. Although basin‐wide climate processes (namely El Niño–Southern Oscillation and Pacific Decadel Oscillation) have previously been suggested to impact local stratification and vertical nutrient supply at ALOHA, we find no evidence of a strong or consistent linkage. Comparing local ecosystem variability to the recently identified North Pacific Gyre Oscillation, however, we observe a correlation with local subsurface productivity and salinity. The correlations have similar structure in both space (i.e., depth) and time and are possibly linked to dynamics associated with the formation and advection of water masses in the central gyre.

Geochemical and climate modeling evidence for Holocene aridification in Hawaii: dynamic response to a weakening equatorial cold tongue

A 13.5 m sequence of Holocene limnic sediments from a sinkhole on Oahu, Hawaii provides a valuable paleoclimatic record for the central subtropical Pacific. δ 13C analysis of plant leaf waxes ( n-alkanes: n-C 27, n-C 29, n-C 31 and n-C 33) is used to infer vegetative changes. Average δ 13C values of the suite of n-alkanes increase from approximately −31 ± 0.5‰ at 10 ca kyr BP (calibrated thousand years before present) to about −27 ± 0.5‰ by 6 ca kyr BP and then remain roughly constant until the Polynesian arrival (about 1.15 ca kyr BP). The increase in δ 13C values of n-alkanes is interpreted to indicate a shift in the local vegetation from C 3 to C 4-dominated flora. Based on mass-balance calculations, the observed increase in the δ 13C values translates to at least a doubling of the relative abundance of C 4 plants. We argue that the expansion of C 4 plants was a response to decreased overall water availability (aridification) due to reduced wintertime precipitation. Model simulations of an orbitally-induced increase in insolation along the equator during the Holocene provide evidence for a wintertime drying trend in the eastern subtropical North Pacific. This trend is associated with boreal fall to winter warming of the cold tongue in the eastern equatorial Pacific (EEP). These model results provide a conceptual framework to explain a dynamic link between the reconstructed Holocene drying trend in Hawaii and orbitally-forced climate change in the EEP that is analogous to the modern El Niño-Southern Oscillation teleconnection.

Observational evidence for propagation of decadal spiciness anomalies in the North Pacific

The propagation of density‐compensated (warm/salty or cool/fresh) spiciness anomalies in the North Pacific thermocline is investigated using Argo profiles for the period 2001–2008. A cool/fresh spiciness anomaly on 25 < σθ < 25.5 kg m−3 isopycnals appears in the eastern subtropical North Pacific at 120°W–150°W in 2003–2004 with a salinity anomaly of about −0.15 PSS‐78. This spiciness anomaly migrates southwestward, and arrives in the western tropical North Pacific at 145°E–175°W in 2008 with the salinity anomaly decreasing to about −0.043 PSS‐78. Two warm/salty anomalies are observed to propagate along the same path from 2003 to 2005, and after 2005. The propagation path and speed of the anomalies are in good agreement with advection by the mean geostrophic current. In the course of propagation, the anomalies are diffused and are subject to high frequency injection of spiciness anomalies, especially in the eastern subtropical North Pacific.

Oxygen minimum zones (OMZs) in the modern ocean

In the modern ocean, oxygen minimum zones (OMZs) are potential traces of a primitive ocean in which Archean bacteria lived and reduced chemical anomalies occurred. But OMZs are also keys to understanding the present unbalanced nitrogen cycle and the oceans’ role on atmospheric greenhouse control. OMZs are the main areas of nitrogen loss (as N 2, N 2O) to the atmosphere through denitrification and anammox, and could even indirectly mitigate the oceanic biological sequestration of CO 2. It was recently hypothesized that OMZs are going to spread in the coming decades as a consequence of global climate change. Despite an important OMZ role for the origin of marine life and for the biogeochemical cycles of carbon and nitrogen, there are some key questions on the structure of OMZs at a global scale. There is no agreement concerning the threshold in oxygen that defines an OMZ, and the extent of an OMZ is often evaluated by denitrification criteria which, at the same time, are O 2-dependent. Our work deals with the identification of each OMZ, the evaluation of its extent, volume and vertical structure, the determination of its seasonality or permanence and the comparison between OMZs and denitrification zones at a global scale. The co-existence in the OMZ of oxic (in its boundaries) and suboxic (even anoxic, in its core) conditions involves rather complex biogeochemical processes such as strong remineralization of the organic matter, removal of nitrate and release of nitrite. The quantitative OMZ analysis is focused on taking into account the whole water volume under the influence of an OMZ and adapted to the study of the specific low oxygen biogeochemical processes. A characterization of the entire structure for the main and most intense OMZs (O 2 < 20 μM reaching 1 μM in the core) is proposed based on a previously published CRIO criterion from the eastern South Pacific OMZ and including a large range of O 2 concentrations. Using the updated global WOA2005 O 2 climatology, the four known tropical OMZs in the open ocean have been described: the Eastern South Pacific and Eastern Tropical North Pacific, in the Pacific Ocean; the Arabian Sea and Bay of Bengal, in the Indian Ocean. Moreover, the Eastern Sub-Tropical North Pacific (25–52°N) has been identified as a lesser known permanent deep OMZ. Two additional seasonal OMZs at high latitude have also been identified: the West Bering Sea and the Gulf of Alaska. The total surface of the permanent OMZs is 30.4 millions of km 2 (∼8% of the total oceanic area), and the volume of the OMZ cores (10.3 millions of km 3) corresponds to a value ∼7 times higher than previous evaluations. The volume of the OMZ cores is about three times larger than that of the associated denitrification zone, here defined as NMZ (‘nitrate deficit or NDEF > 10 μM’ maximum zone). The larger OMZ, relative to the extent of denitrification zone, suggests that the unbalanced nitrogen cycle on the global scale could be more intense than previously recognized and that evaluation of the OMZ from denitrification could underestimate their extent.

Circulation rate changes in the eastern subtropical North Pacific based on chlorofluorocarbon ages

A space‐time regression analysis of chlorofluorocarbon (CFC) data from the eastern North Pacific Ocean's subtropical thermocline (25.6–26.6 σθ) is used to extract age and velocity changes from a collection of 1980s and 1990s hydrographic sections. Results indicate substantial increases of CFC ages, from 0.2 yr yr−1 on 25.8 σθ increasing with depth up to 0.4 yr yr−1 on 26.6 σθ, and increases of meridional age gradients, implying a circulation slowdown. Using the regression‐derived spatial and temporal derivatives, an isopycnal balance of advection and diffusion, including the effects of mixing on CFC ages, was applied to estimate changes in flow. Southward velocities at 20°N, 145°W generally decreased by ∼ 0.03 ± 0.02 cm s−1 yr−1 on isopycnals 25.6 to 26.4 σθ during the 1980s and 1990s, consistent with overturning changes from historical hydrographic data and with concurrent increases in AOU. The deeper (26.6 σθ) age increase was consistent with mixing effects.

Intraseasonal Atmospheric Variability in the Extratropics and Its Relation to the Onset of Tropical Pacific Sea Surface Temperature Anomalies

Abstract Previous research has shown that seasonal-mean boreal winter variations in the subtropical/extratropical sea level pressure and wind stress fields over the central North Pacific are significantly related to the state of the El Niño–Southern Oscillation (ENSO) 12–15 months later. Results presented in this note indicate that boreal winter ENSO events are also preceded by increased intraseasonal variance in the antecedent boreal winter atmospheric circulation patterns over the extratropical central North Pacific as well. Low (high) surface pressure anomalies associated with intraseasonal variability in this region are related to intraseasonal wind stress anomalies that represent a weakening (strengthening) of the trade winds over both the north and south subtropical/tropical Pacific. There is also a concurrent increase (decrease) in the central and eastern subtropical North Pacific sea surface temperatures that projects onto the seasonal-mean SST anomalies that precede mature ENSO events by 9–12 months. Overall these results suggest that similar to seasonal-mean subtropical surface pressure and wind stress fields, enhanced transient variability in the midlatitudes can subsequently induce changes in the atmospheric and oceanic structure of the tropical Pacific that may serve as a precursor to ENSO variability.

Nitrogen cycling in the sediments of Santa Barbara basin and Eastern Subtropical North Pacific: Nitrogen isotopes, diagenesis and possible chemosymbiosis between two lithotrophs (Thioploca and Anammox)—“riding on a glider”

Abstract This article presents data on the isotopic composition of pore water ammonium and N org in the anoxic sediments of the Santa Barbara Basin and the Eastern Subtropical North Pacific (ESNP) region. Ammonium δ 15 N behaves differently in the two regions, reflecting variability in bacterial processes of sedimentary nitrogen cycling. In Santa Barbara Basin, stoichiometry of TCO 2 and NH 4 + production defined by pore water chemistry indicates that the only reaction influencing pore water ammonium concentration in these sediments appears to be organic matter decomposition. Pore water ammonium was found to be 1–3‰ heavier than decomposing organic matter. In contrast, the upper 15–30 cm of ESNP pore waters have ammonium with δ 15 N that is more than 10‰ heavier than decomposing organic matter. At greater depth, the δ 15 N of deeper pore waters approaches that of decomposing organic matter. Mass balance calculations indicate that the observed isotopic enrichment is not due to the preferential loss of isotopically enriched organic matter. Pore water profiles show that the zone with heavy isotopes is characterized by a TCO 2 /NH 4 + inputs ratio of 4 or less. We attribute this to the presence of Thioploca bacteria, who transport NO 3 − to depth and reduce it to NH 4 + while oxidizing sulfide. Based on pore water stoichiometry, and reaction–diffusion modeling of pore water isotopic composition we propose that the 15 N-enriched ammonium is produced as the result of chemosymbiosis between Thioploca and Anammox-like bacteria in these sediments. Our work provides the new insights into benthic microbial ecology, and the role of coupled individual metabolic pathways in sedimentary nitrogen cycling. In Santa Barbara Basin, factors which contribute to the isotopic fractionation of 1–3‰ between pore water ammonium and solid N org were also considered. In this sediments, ∼23% of the rain of organic nitrogen is lost to diagenesis, therefore, the isotopic effect of degradation on the residual N org is estimated to be small, − 0.7‰. The most likely cause of fractionation is preferential degradation of an isotopically heavier, more labile marine fraction relative to an isotopically lighter, more refractory terrestrial component of the organic matter, but other possibilities cannot be ruled out. When corrected for the observed fractionation, it appears that the δ 15 N of these sediments may represent non-steady state conditions.

Temporal changes in pCFC-12 ages and AOU along two hydrographic sections in the eastern subtropical North Pacific

Changes in chlorofluorocarbon-12 (CFC-12) derived ventilation ages (pCFC-12 ages) and in apparent oxygen utilization (AOU) are investigated in the eastern subtropical North Pacific, where a portion of a 1991 WOCE cruise along 152°W was repeated in 1997 and a portion of a 1985 WOCE cruise along 24°N was repeated in 2000. The effects of mixing biases and possible gas undersaturations at the isopycnal outcrops are carefully examined during the calculation of pCFC-12 ages and oxygen utilization rates ( OUR = AOU / age ). The most conspicuous and robust change observed in the data is an increase in pCFC-12 ages and in AOU that occurs at the subtropical–subpolar gyre boundary at 152°W and is centered at σ θ = 26.65 kg m - 3 . pCFC-12 ages (including a correction for mixing biases) and AOU increased by as much as 5 years and 40 μ mol kg - 1 from 1991 to 1997 here, which corresponds to a change of about 40%. These data are in agreement with previous studies suggesting a reduction of ventilation in the subpolar gyre. A proposed mechanism for these changes is a reduction or cessation of the outcropping of the core isopycnal ( σ θ = 26.65 kg m - 3 ) that most likely occurred prior to 1991. Since mixing-bias-corrected pCFC-12 ages and AOU changed in approximately equal proportions, OURs close to the core isopycnal remained roughly constant. In contrast, OURs in the 24°N sections appear to have decreased off the coast of California from 1985 to 2000. The average OUR difference here reduces to < 1 μ mol kg - 1 yr - 1 when a mixing-bias correction and gas saturations less than 100% (90% for CFC-12 and 95% for oxygen) are applied, but it remains significant at the 95% confidence level. A robust change in ventilation ages is not observed in the 24°N sections.

Using preformed nitrate to infer decadal changes in DOM remineralization in the subtropical North Pacific

The preformed nitrate distribution throughout the subtropical North Pacific is characterized by a negative anomaly between 100 m and the 25.4 σθ surface. Its presence indicates that nitrogen remineralization in the upper thermocline deviates significantly from Redfield stoichiometry. It has been previously suggested that this feature is created during the degradation of nitrogen‐poor dissolved organic matter (DOM) (Emerson and Hayward, 1995). Here we present evidence from two transects in the eastern subtropical North Pacific that degradation of DOM with a high C:N ratio is quantitatively responsible for negative preformed nitrate. We develop a simple isopycnal mass balance to determine preformed nitrate using apparent oxygen utilization (AOU), dissolved organic carbon (DOC), and dissolved organic nitrogen (DON) data. These results agree well with the traditional preformed nitrate calculation and indicate that the intensity of the anomaly is proportional to the magnitude of DOM remineralization and its C:N remineralization ratio. An analysis of preformed nitrate distributions in the eastern subtropical North Pacific between 1980 and 1997 reveals a significant increase in the negative anomaly. While a portion of this change may be attributable to a reduction in ventilation of the gyre, the majority of the signal is due to an increase in the magnitude and/or C:N ratio of DOM remineralization. This conclusion supports recent hypotheses that DOM production and cycling may have increased in this region in the past 2 decades as a result of changes in the phytoplankton community structure.

Interannual Variability in the Eastern Subtropical North Pacific Ocean

Abstract Interannual variability in the eastern subtropical North Pacific Ocean is investigated using the Naval Research Laboratory Layered Ocean Model (NLOM). Emphasis is placed on examining the nature of westward propagating Rossby waves and their interaction with the overlying atmosphere. Singular value decomposition (SVD), complex empirical orthogonal function (CEOF), and multichannel singular spectrum (M-SSA) analyses are used to isolate the standing and propagating response due to modeled Rossby waves and observed wind stress curl (WSC) anomalies as well as their dominant frequencies of oscillation, respectively. In addition to a large-scale interdecadal fluctuation, two distinct forms of Rossby waves are found to exist, one wind forced and the other freely propagating. SVD and CEOF analyses of NLOM upper layer thickness anomalies suggest a leading order source of variability stemming from a large-scale interdecadal fluctuation. M-SSA analysis also depicts this low frequency mode, but to a lesser ex...

Constant ventilation age of thermocline water in the eastern subtropical North Pacific Ocean from chlorofluorocarbon measurements over a 12‐year period

Northeastern Pacific chlorofluorocarbon (CFC) data collected between 1982 and 1994 near Geochemical Ocean Sections Study (GEOSECS) station 1 (28.5°N, 122.5°W) record decadal timescale ventilation processes of the subtropical thermocline in this region. The CFC‐12 concentration age field versus potential density has been remarkably constant over the 12‐year period, although CFC concentrations in the upper kilometer of the water column have increased with time. Results from a simple one‐dimensional advection‐diffusion model are consistent with an advection velocity of ca. 0.8–0.9 cm s−1 from the source area. The influence of the 1982–83 El Niño is noticeable in the 1983 observations. While the main stream of subarctic source water appears to spread southward at a constant rate, during El Niño years the influence of comparatively CFC‐free tropical thermocline waters is enhanced in this region, leading to reduced vertical inventories of CFCs, but without changing the apparent CFC ages as functions of potential density. Apparent oxygen utilization rates decrease with increasing CFC age, and also appear not to have changed significantly over the 12‐year measurement period.

Distributions of TOP, TON and TOC in the North Pacific subtropical gyre: Implications for nutrient supply in the surface ocean and remineralization in the upper thermocline

We report measurements of total organic phosphorus, nitrogen, and carbon (TOP, TON, and TOC) along a meridional transect in the eastern subtropical North Pacific. In the surface waters, TOP and TON comprise 83% and 98% of the total dissolved phosphorus and nitrogen pools, respectively. Between the equatorial region and the subtropical gyre, there is a distinct gradient in surface TOP of about 0.25 μM, but very little gradient in TON. TOC increases by nearly 30 μM across the same region. Meridional advection of TOP along its gradient via Ekman transport can provide 40-80% of the phosphorus necessary to supply present estimates of particulate phosphorus export from the subtropical gyre. Mass balance requires an additional input of nitrogen to the surface waters to fuel particulate nitrogen export. This input is likely supported by nitrogen fixation. As dissolved organic matter (DOM) is degraded along isopycnals that outcrop within the subtropical gyre, TOP and TOC are consumed more rapidly than TON. TOC and TON contribute 70% and 20% to organic matter remineralization along these isopycnals, respectively, and the C:N ratio for remineralization is 30 ± 10. In contrast, along isopycnals that outcrop to the north of the gyre, both TOC and TON contribute 30% to organic matter remineralization and the C:N ratio is 8 ± 1. Our results suggest that excess TOC, produced within the surface waters of the subtropical gyre during nitrogen fixation, is highly labile and dominates respiration along shallow isopycnals. Preferential remineralization of TOC relative to TON within the gyre results in a negative preformed nitrate signal in the upper thermocline, and may establish a feedback between upper thermocline DOM remineralization and surface ocean DOM production that maintains nitrogen fixation in the surface waters.