Tropical North Pacific Research Articles

Relationship between the South China Sea summer monsoon withdrawal and September–October rainfall over southern China

This study reveals a significant positive connection between the interannual variation in the South China Sea (SCS) summer monsoon (SCSSM) withdrawal and September–October rainfall over southern China. Specifically, pronounced positive rainfall anomalies can be found around southern China when the withdrawal of the SCSSM is later than normal. The occurrence of the significant SCSSM withdrawal-southern China rainfall relation may be attributed to both the atmospheric internal dynamics and the sea surface temperature (SST) anomalies over the tropical Pacific. In particular, SST warming over the tropical western North Pacific, which resembles the developing phase of a La Nina, could induce a significant anomalous low-level cyclone over the northern SCS via a Rossby wave-type atmospheric response. The southwesterly wind anomalies on the south flank of this anomalous cyclone favor late SCSSM withdrawal and contribute to more rainfall over southern China via the northward transport of water vapor. In addition, the enhanced quasi-biweekly oscillation and synoptic-scale variation (such as tropical cyclones) during late withdrawal years are also conducive to increased rainfall over southern China. Further analyses found notable interdecadal changes and asymmetry in the connection of the SCSSM withdrawal with southern China rainfall. The factors contributing to the nonstationary and asymmetric nature of this monsoon withdrawal-rainfall relationship are also briefly discussed.

N2 fixation rate and diazotroph community structure in the western tropical North Pacific Ocean

In the present study, we report N2 fixation rate (15N isotope tracer assay) and the diazotroph community structure (using the molecular method) in the western tropical North Pacific Ocean (WTNP) (13°–20°N, 120°–160°E). Our independent evidence on the basis of both in situ N2 fixation activity and diazotroph community structure showed the dominance of unicellular N2 fixation over majority of the WTNP surface waters during the sampling periods. Moreover, a shift in the diazotrophic composition from unicellular cyanobacteria group B-dominated to Trichodesmium spp.-dominated toward the western boundary current (Kuroshio) was also observed in 2013. We hypothesize that nutrient availability may have played a major role in regulating the biogeography of N2 fixation. In surface waters, volumetric N2 fixation rate (calculated by nitrogen) ranged between 0.6 and 2.6 nmol/(L·d) and averaged (1.2±0.5) nmol/(L·d), with <10 μm size fraction contributed predominantly (88%±6%) to the total rate between 135°E and 160°E. Depth-integrated N2 fixation rate over the upper 200 m ranged between 150 μmol/(m2·d) and 480 μmol/(m2·d) (average (225±105) μmol/(m2·d). N2 fixation can account for 6.2%±3.7% of the depth-integrated primary production, suggesting that N2 fixation is a significant N source sustaining new and export production in the WTNP. The role of N2 fixation in biogeochemical cycling in this climate change-vulnerable region calls for further investigations.

Differential Distribution and Determinants of Ammonia Oxidizing Archaea Sublineages in the Oxygen Minimum Zone off Costa Rica.

Ammonia oxidizing archaea (AOA) are microbes that are widely distributed in the ocean that convert ammonia to nitrite for energy acquisition in the presence of oxygen. Recent study has unraveled highly diverse sublineages within the previously defined AOA ecotypes (i.e., water column A (WCA) and water column B (WCB)), although the eco-physiology and environmental determinants of WCB subclades remain largely unclear. In this study, we examined the AOA communities along the water columns (40–3000 m depth) in the Costa Rica Dome (CRD) upwelling region in the eastern tropical North Pacific Ocean. Highly diverse AOA communities that were significantly different from those in oxygenated water layers were observed in the core layer of the oxygen minimum zone (OMZ), where the dissolved oxygen (DO) concentration was < 2μM. Moreover, a number of AOA phylotypes were found to be enriched in the OMZ core. Most of them were negatively correlated with DO and were also detected in other OMZs in the Arabian Sea and Gulf of California, which suggests low oxygen adaptation. This study provided the first insight into the differential niche partitioning and environmental determinants of various subclades within the ecotype WCB. Our results indicated that the ecotype WCB did indeed consist of various sublineages with different eco-physiologies, which should be further explored.

Scenarios of Deoxygenation of the Eastern Tropical North Pacific During the Past Millennium as a Window Into the Future of Oxygen Minimum Zones

Diverse studies predict global expansion of Oxygen Minimum Zones (OMZs) as a consequence of anthropogenic global warming. While the observed dissolved oxygen concentrations in many coastal regions are slowly decreasing, sediment core paleorecords often show contradictory trends. This is the case for numerous high-resolution reconstructions of oxygenation in the Eastern Tropical North Pacific (ETNP). While major shifts in redox conditions of the ETNP are dominated by glacial-interglacial cycling, important fluctuations also occur in response to minor climatic and oceanographic perturbations. It is important to understand these scenarios of past redox variation, as they are the closest analogues for near future climate and oceanographic change. We present recently collected sediment core proxy records from the Gulf of California in which we reproduce the variability of productivity and oxygenation of the ETNP OMZ during the past millennium. We emphasize paleoproductivity (Corg, Ni, Baexcess) and paleoredox indicators (Mo, V, Uauth) in sediment cores collected in Alfonso and La Paz basins and compare these OMZ records with other archives of the Eastern Pacific. Our findings indicate that the OMZ expanded in response to increased upwelling and productivity during cold intervals around 1830, 1680, and 1500 years AD (evidenced by higher P, Ni, Mo, V, and Uauth). The most hypoxic times corresponded to the beginning of the Little Ice Age (expressed in elevated Mo). Significant OMZ contractions occurred around 1300, 1650, and 1900s AD after reoxygenation events that were instigated by low productivity (lower Ni, Mo, V, and Uauth). The mechanisms that control decadal-to-centennial oxygen variability in the ETNP remain unidentified but are likely influenced by solar forcing not only driving migrations of the Intertropical Convergence Zone (ITCZ) but more importantly changes in the intensity of the Pacific Walker Circulation (PWC). During the Little Ice Age solar irradiance was at its lowest for the past millennium, which strengthened the PWC. This pattern contributed to more frequent La Niña-like conditions, which enhanced upwelling of nutrient-rich waters in the west coast of North America, driving productivity and reducing bottom oxygen levels, as seen in our ETNP records.

Changing dynamical control of early Asian summer monsoon in the mid-1990s

The seasonal evolution of the upper tropospheric South Asian high follows and influences underlying summer monsoon advancement. A strong connection between the South Asian high and westerly perturbation to the north suggests further planetary-scale dynamical control of the monsoon. In the mid-1990s, a clear location shift of the South Asian high in May–June was noted and was observed in fewer (more) frequencies of the high centers over the Indochina Peninsula (Iranian Plateau). Continental confinement of monsoonal circulation and precipitation was observed during 1995–2010, as opposed to larger-scale development in the Asia–Pacific region during 1979–1994. In view of early-summer monsoon evolution, a westward shifting and faster migration of the South Asian high may imply increased control of the midlatitude dynamics. By contrast, the convection over the tropical Western North Pacific (WNP) has an opposite and delayed contribution to monsoon advancement. After the mid-1990s than it had been previously, the midlatitude jet stream largely weakened over northern Africa and the East Asia–Pacific region, corresponding to an increase in the upper tropospheric geopotential heights north of the jet stream. Climate model experiments further reveal that the warming over Europe–Asia and temperature change in the North Atlantic can result in the change in midlatitude perturbations and the monsoon evolution in the mid-1990s, suggesting large-scale and dynamic impact on monsoon climatology.

Nitrogen assimilation in picocyanobacteria inhabiting the oxygen‐deficient waters of the eastern tropical North and South Pacific

AbstractProchlorococcus and Synechococcus are the most abundant free‐living photosynthetic microorganisms in the ocean. Uncultivated lineages of these picocyanobacteria also thrive in the dimly illuminated upper part of oxygen‐deficient zones (ODZs), where an important portion of ocean nitrogen (N) loss takes place via denitrification and anaerobic ammonium oxidation. Recent metagenomic studies revealed that ODZ Prochlorococcus have the genetic potential for using different N forms, including nitrate and nitrite, uncommon N sources for Prochlorococcus, but common for Synechococcus. To determine which N sources ODZ picocyanobacteria are actually using in nature, the cellular 15N natural abundance (δ15N) and assimilation rates of different N compounds were determined using cell sorting by flow cytometry and mass spectrometry. The natural δ15N of the ODZ Prochlorococcus varied from −4.0‰ to 13.0‰ (n = 9), with 50% of the values in the range of −2.1–2.6‰. While the highest values suggest nitrate use, most observations indicate the use of nitrite, ammonium, or a mixture of N sources. Meanwhile, incubation experiments revealed potential assimilation rates of ammonium and urea in the same order of magnitude as that expected for total N in several environments including ODZs, whereas rates of nitrite and nitrate assimilation were very low. Our results thus indicate that reduced forms of N and nitrite are the dominant sources for ODZ picocyanobacteria, although nitrate might be important on some occasions. ODZ picocyanobacteria might thus represent potential competitors with anammox bacteria for ammonium and nitrite, with ammonia‐oxidizing archaea for ammonium, and with nitrite‐oxidizing bacteria for nitrite.

Molecular and isotopic evidence of the distribution of nitrogen-cycling microbial communities in the oxygen minimum zone of the Tropical Mexican Pacific.

Microbial communities play a central role in the N cycle of oceanic oxygen minimum zones (OMZs), such as in the Eastern Tropical North Pacific (ETNP). We explored the spatial distribution of the genetic potential for ammonia oxidation (amoA gene for bacteria and archaea), denitrification (nirS and nirK), anaerobic ammonium oxidation (anammox) (hzo) and dissimilarity nitrate reduction to ammonia (DNRA) (nrfA) and their relationships with the hydrological variables and isotopic composition of nitrate in the ETNP off Mexico. Oxygen concentration, the availability of N chemical forms and the upwelling shaped the distribution of N cycling genes along the water column. The high abundance of N genes and the isotopic composition of nitrate suggest the N cycling is very dynamic in the OMZ core. The accumulation of nitrite, the high abundance of archaeal amoA genes, and the deviation of the N and O isotopes of nitrate from the expected 1:1 ratio for nitrate reduction in the upper portion of this OMZ indicate that nitrification is a relevant process that fuels the denitrifier community. Conversely, the high abundances of nitrate, ammonium and nrfA genes in the deeper layer indicate that DNRA is a crucial process enhancing anammox there. These results show the need for more detailed studies of the N processes in OMZs.

Dinitrogen Fixation Across Physico‐Chemical Gradients of the Eastern Tropical North Pacific Oxygen Deficient Zone

AbstractThe Eastern Tropical North Pacific Ocean hosts one of the world's largest oceanic oxygen deficient zones (ODZs). Hot spots for reactive nitrogen (Nr) removal processes, ODZs generate conditions proposed to promote Nr inputs via dinitrogen (N2) fixation. In this study, we quantified N2 fixation rates by 15N tracer bioassay across oxygen, nutrient, and light gradients within and adjacent to the ODZ. Within subeuphotic oxygen‐deplete waters, N2 fixation was largely undetectable; however, addition of dissolved organic carbon stimulated N2 fixation in suboxic (&lt;20 μmol/kg O2) waters, suggesting that diazotroph communities are likely energy limited or carbon limited and able to fix N2 despite high ambient concentrations of dissolved inorganic nitrogen. Elevated rates (&gt;9 nmol N·L−1·day−1) were also observed in suboxic waters near volcanic islands where N2 fixation was quantifiable to 3,000 m. Within the overlying euphotic waters, N2 fixation rates were highest near the continent, exceeding 500 μmol N·m−2·day−1 at one third of inshore stations. These findings support the expansion of the known range of diazotrophs to deep, cold, and dissolved inorganic nitrogen‐replete waters. Additionally, this work bolsters calls for the reconsideration of ocean margins as important sources of Nr. Despite high rates at some inshore stations, regional N2 fixation appears insufficient to compensate for Nr loss locally as observed previously in the Eastern Tropical South Pacific ODZ.

Ongoing Increase in Eastern Tropical North Pacific Denitrification as Interpreted Through the Santa Barbara Basin Sedimentary δ15N Record

AbstractDecades of observations show that the world's oceans have been losing oxygen, with far‐reaching consequences for ecosystems and biogeochemical cycling. To reconstruct oxygenation beyond the limited scope of instrumental records, proxy records are needed, such as sedimentary δ15N. We combine two δ15N records from the Santa Barbara Basin (SBB), a 24‐year‐long, biweekly sediment trap time series, and a 114‐year, high‐resolution sediment core together spanning the years 1892–2017. These records allow for the examination of δ15N variability on seasonal to centennial timescales. Seasonal variability in SBB δ15N is consistent in timing with the poleward advection of a high δ15N signal from the Eastern Tropical North Pacific in the summer and fall. Strong El Niño events result in variable δ15N signatures, reflective of local rainfall, and neither the Pacific Decadal Oscillation nor North Pacific Gyre Oscillation impose strong controls on bulk sedimentary δ15N. Seasonal and interannual variability in sediment trap δ13Corg is consistent with local productivity as a driver; however, this signal is not retained in the sediment core. The time series from the sediment trap and core show that bulk sedimentary δ15N in SBB has now exceeded that measured for the past 2,000 years. We hypothesize that the change in δ15N reflects the increasing influence of denitrified waters from the Eastern Tropical North Pacific and ongoing deoxygenation of the Eastern Pacific. When juxtaposed with other regional δ15N records our results further suggest that SBB is uniquely situated to record long‐term change in the Eastern Tropical North Pacific.

Glider Observations of a Mesoscale Oceanic Island Wake

AbstractIn this study, a 2-yr time series of velocity profiles to 1000 m from meridional glider surveys is used to characterize the wake in the lee of a large island in the western tropical North Pacific Ocean, Palau. Surveys were completed along sections to the east and west of the island to capture both upstream and downstream conditions. Objectively mapped in time and space, mean sections of velocity show the incident westward North Equatorial Current accelerating around the island of Palau, increasing from 0.1 to 0.2 m s−1 at the surface. Downstream of the island, elevated velocity variability and return flow in the lee are indicative of boundary layer separation. Isolating for periods of depth-average westward flow reveals a length scale in the wake that reflects local details of the topography. Eastward flow is shown to produce an asymmetric wake. Depth-average velocity time series indicate that energetic events (on time scales from weeks to months) are prevalent. These events are associated with mean vorticity values in the wake up to 0.3f near the surface and with instantaneous values that can exceed f (the local Coriolis frequency) during periods of sustained, anomalously strong westward flow. Thus, ageostrophic effects become important to first order.

Three-ocean interactions and climate variability: a review and perspective

Interactions among the Pacific, Atlantic and Indian Oceans through ocean–atmosphere coupling can initiate and/or modulate climate variability. The Pacific Ocean is home to ENSO which affects other oceans through atmospheric bridges and the oceanic Indonesian throughflow (ITF). A warm Indian Ocean can produce atmospheric Kelvin waves that propagate eastward and increase equatorial easterly wind anomalies in the western Pacific and thus cool eastern Pacific sea surface temperature (SST). A positive Indian Ocean dipole establishes a southwestward pressure gradient force in the ITF region which increases the ITF transport and decreases ocean heat content in the western Pacific and may cool eastern Pacific SST. The Indian Ocean can also influence the Atlantic by atmospheric bridge and the oceanic Agulhas leakage south of Africa. Midlatitude North Atlantic SSTs may affect Pacific climate variability: (1) The Atlantic multidecadal oscillation (AMO) influences North Pacific variability; (2) The warm AMO phase increases the occurrence of central Pacific (CP)-type El Niño; (3) The warm AMO phase helps induce anomalous cyclonic circulation in the tropical western North Pacific; and (4) A cold midlatitude North Atlantic Ocean in the summer may initiate an El Niño in subsequent year via the East Atlantic/West Russia teleconnection. A warm tropical North Atlantic in the spring can induce a CP-type La Niña in the subsequent winter, via two pathways of the tropical eastern North and South Pacific. Finally, the Atlantic Niño (Niña) in the summer, through the Walker circulation and ocean dynamics, helps induce an eastern Pacific-type La Niña (El Niño) in the subsequent winter. The Atlantic Niño can also warm the tropical western Indian Ocean and weaken Indian monsoon rainfall.

Projected near term changes in the East Asian summer monsoon and its uncertainty

Changes in the East Asian summer monsoon (EASM) during the mid-21st century relative to present day are simulated in two related models GOML1 and GOML2. Both models are the atmospheric components of two state-of-the-art climate models coupled to a multi-level mixed-layer ocean model, following the RCP 4.5 scenario. Both show that the EASM is enhanced due to the amplified land-sea thermal contrast. Summer precipitation over northern China is projected to increase by 5%–10% in both models mainly driven by enhancement of the monsoon circulation. Over south-eastern China the two models project different signs of precipitation change: a decrease in GOML1 with the maximum of about −1.0 mm d−1 and an increase in GOML2 with a maximum of around 1.0 mm d−1. Though the thermal effect of climate warming leads to a projected increase in precipitation over south-eastern China in both models, circulation changes are opposite and dominate the precipitation response. This indicates that uncertainty in changes in projected precipitation largely arises from uncertainly in projected circulation changes. The different circulation changes in the two models are likely related to differences in projected Sea Surface Temperature (SST) in the Western tropical Pacific and North Pacific. In GOML1, the SST warming in the tropical Pacific is associated with an anomalous local Hadley circulation, characterized by anomalous ascent in the tropics and southern subtropics, and anomalous descent with less precipitation over south-eastern China. In GOML2, the large decrease in the meridional SST gradient between the South China Sea and Western North Pacific is associated with an anomalous local Hadley circulation with anomalous ascent at 20°N–30°N and anomalous descent at 5°N–15°N, leading to an anti-cyclonic circulation anomaly over the South China Sea and increased precipitation over south-eastern China.

Local and Remote Forcing of Denitrification in the Northeast Pacific for the Last 2,000 Years

AbstractSedimentary δ15N (δ15Nsed) has been widely applied as a proxy for water column denitrification. When combined with additional productivity proxies, it provides insights into the driving forces behind long‐term changes in water column oxygenation. High‐resolution (~2 years) δ15Nsed and productivity proxy records (total organic carbon [TOC], Si/Ti, and Ca/Ti) from Santa Barbara Basin, California, were generated from a well‐dated Kasten core (SPR0901‐03KC). These records reveal the relationship between Southern California upwelling and oxygenation over the past 2,000 years. Inconsistencies between Si/Ti (coastal upwelling proxy) and TOC (total export productivity proxy) suggest wind curl upwelling influenced Southern California primary productivity, especially during intervals of weak coastal upwelling. Coherence between δ15Nsed, TOC, and drought indicators supports a local control of δ15Nsed by atmospheric circulation, as persistent northerly winds associated with an intensified North Pacific High pressure cell lead to enhanced coastal upwelling. In the northeast Pacific, δ15Nsed is used as a water mass tracer of denitrification signals transported north from the eastern tropical North Pacific (ETNP) via the California Undercurrent. A 1,200‐year δ15Nsed record from the Pescadero slope, Gulf of California, lies between denitrifying subsurface waters in the ETNP and Southern California. During the Medieval Climate Anomaly, coherence between Pescadero and Santa Barbara Basin δ15Nsed indicates connections between ETNP and Southern California on centennial timescales. Yet an out‐of‐phase relationship occurred when the Aleutian Low was anomalously strong during the Little Ice Age. We suggest intensified nutrient‐rich subarctic water advection might have transported high‐15N nitrate into Southern California when the California Undercurrent and ETNP denitrification weakened.

Cyanobacteria and cyanophage contributions to carbon and nitrogen cycling in an oligotrophic oxygen-deficient zone

Up to half of marine N losses occur in oxygen-deficient zones (ODZs). Organic matter flux from productive surface waters is considered a primary control on N2 production. Here we investigate the offshore Eastern Tropical North Pacific (ETNP) where a secondary chlorophyll a maximum resides within the ODZ. Rates of primary production and carbon export from the mixed layer and productivity in the primary chlorophyll a maximum were consistent with oligotrophic waters. However, sediment trap carbon and nitrogen fluxes increased between 105 and 150 m, indicating organic matter production within the ODZ. Metagenomic and metaproteomic characterization indicated that the secondary chlorophyll a maximum was attributable to the cyanobacterium Prochlorococcus, and numerous photosynthesis and carbon fixation proteins were detected. The presence of chemoautotrophic ammonia-oxidizing archaea and the nitrite oxidizer Nitrospina and detection of nitrate oxidoreductase was consistent with cyanobacterial oxygen production within the ODZ. Cyanobacteria and cyanophage were also present on large (>30 μm) particles and in sediment trap material. Particle cyanophage-to-host ratio exceeded 50, suggesting that viruses help convert cyanobacteria into sinking organic matter. Nitrate reduction and anammox proteins were detected, congruent with previously reported N2 production. We suggest that autochthonous organic matter production within the ODZ contributes to N2 production in the offshore ETNP.

A water mass history of the Southern California current system

AbstractThe California Current System represents a confluence of different water masses originating in the subarctic, subtropical, and tropical eastern Pacific. Variations in their relative influence can alter regional biogeochemistry and ecosystem structure. We perform an optimum multiparameter analysis on historical hydrographic data to quantify the spatiotemporal variability of water mass contributions to the California Current. Within the pycnocline, a strong cross‐shore gradient in the primary water mass source reflects the dominant advective pathways within the California Current and California Undercurrent. The El Niño Southern Oscillation imparts variability in the relative contributions and depth structure of source waters, allowing stronger upwelling during La Niña to more effectively tap nutrient‐rich, oxygen‐poor waters originating in the eastern tropical North Pacific. This regional water mass history provides context for understanding the drivers and pathways of biogeochemical variability in the California Current and demonstrates that oceanic changes occurring far afield can have regionally heterogeneous impacts.

Inter‐decadal variations in El Niño–Southern Oscillation impacts on the autumn precipitation in the eastern China

AbstractBy using the observation, reanalysis data and numerical simulation, the inter‐decadal variations in El Niño–Southern Oscillation (ENSO) impacts on the eastern China (EC) precipitation during its developing autumn in the past 65 years have been investigated. Results show that ENSO is related to the significant dipole precipitation anomalies in EC in the early decades (1951–1981); El Niño/La Niña introduces more/less precipitation in the southern EC, but less/more in the northern EC. However, the significant dipole pattern disappears in the recent decades (1985–2015), mainly owing to loss of the significant positive influences of La Niña on the northern EC precipitation. Comparison of the atmospheric circulation anomalies in East Asia related to La Niña in the two periods shows that there are anticyclonic circulation anomalies in the northeastern Asia near Japan in the first period, while they shift to Mongolia in the second period. Hence, in the first period, the northern EC is under the influences of anomalous southwesterlies along the southwestern flank of the anticyclonic circulation anomalies that advect more moisture from the south to the northern EC and lead to more precipitation there. In contrast, in the second period, the northern EC is under the influences of anomalous northeasterlies along the southeastern edge of the anticyclonic circulation anomalies near Mongolia that impede the northwards transport of moisture and are not conducive to surplus precipitation. The westwards shift of the anticyclonic circulation anomalies may be closely related to the higher SST and thus convective anomalies related to La Niña in the tropical western North Pacific (WNP) east of the Philippines in the second than the first periods. The sensitivity experiment forced by the positive SST anomalies in the tropical WNP comparable to the observed in an atmospheric general circulation model can successfully simulate the anticyclonic circulation anomalies around Mongolia.

Anaerobic methane oxidation is an important sink for methane in the ocean's largest oxygen minimum zone

AbstractWe investigated methane oxidation in the oxygen minimum zone (OMZ) of the eastern tropical North Pacific (ETNP) off central Mexico. Methane concentrations in the anoxic core of the OMZ reached ~ 20 nmol L−1 at off shelf sites and 34 nmol L−1 at a shelf site. Rates of methane oxidation were determined in ship‐board incubations with 3H‐labeled methane at O2 concentrations 0–75 nmol L−1. In vertical profiles at off‐shelf stations, highest rates were found between the secondary nitrite maximum at ~ 130 m and the methane maximum at 300–400 m in the anoxic core. Methane oxidation was inhibited by addition of 1 μmol L−1 oxygen, which, together with the depth distribution, indicated an anaerobic pathway. A coupling to nitrite reduction was further indicated by the inhibitory effect of the nitric oxide scavenger 2‐phenyl‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide (PTIO). Metatranscriptomes from the anoxic OMZ core supported the likely involvement of nitrite‐reducing bacteria of the NC10 clade in anaerobic methane oxidation, but also indicated a potential role for nitrate‐reducing euryarchaeotal methane oxidizers (ANME‐2d). Gammaproteobacteria of the Methanococcales were further detected in both 16S rRNA gene amplicons and metatranscriptomes, but the role of these presumed obligately aerobic methane oxidizers in the anoxic OMZ core is unclear. Given available estimates of water residence time, the measured rates and rate constants (up to ~ 1 yr−1) imply that anaerobic methane oxidation is a substantial methane sink in the ETNP OMZ and hence attenuates the emission of methane from this and possibly other OMZs.

Identifying the origin of nitrous oxide dissolved in deep ocean by concentration and isotopocule analyses

Nitrous oxide (N2O) contributes to global warming and stratospheric ozone depletion. Although its major sources are regarded as bacterial or archaeal nitrification and denitrification in soil and water, the origins of ubiquitous marine N2O maximum at depths of 100–800 m and N2O dissolved in deeper seawater have not been identified. We examined N2O production processes in the middle and deep sea by analyzing vertical profiles of N2O concentration and isotopocule ratios, abundance ratios of molecules substituted with rare stable isotopes 15N or 18O to common molecules 14N14N16O, in the Atlantic, Pacific, Indian, and Southern oceans. Isotopocule ratios suggest that the N2O concentration maxima is generated by in situ microbial processes rather than lateral advection or diffusion from biologically active sea areas such as the eastern tropical North Pacific. Major production process is nitrification by ammonia-oxidizing archaea (AOA) in the North Pacific although other processes such as bacterial nitrification/denitrification and nitrifier-denitrification also significantly contribute in the equatorial Pacific, eastern South Pacific, Southern Ocean/southeastern Indian Ocean, and tropical South Atlantic. Concentrations of N2O below 2000 m show significant correlation with the water mass age, which supports an earlier report suggesting production of N2O during deep water circulation. Furthermore, the isotopocule ratios suggest that AOA produce N2O in deep waters. These facts indicate that AOA have a more important role in marine N2O production than bacteria and that change in global deep water circulation could affect concentration and isotopocule ratios of atmospheric N2O in a millennium time scale.

An evaluation of East Asian summer monsoon forecast with the North American Multimodel Ensemble hindcast data

AbstractThe abilities of three models (Climate Forecast System version 2 [CFSv2], Canadian Coupled Climate Model version 3 [CanCM3] and Canadian Coupled Climate Model version 4 [CanCM4]) in the North American Multimodel Ensemble for the East Asian summer monsoon (EASM) forecast were evaluated with their 29‐year hindcast data (1982–2010). Many EASM features including monsoon precipitation centres, large‐scale monsoon circulations and monsoon onset and retreat are generally captured by the three models and their ensemble mean, and the multimodel ensemble has the best performance. Since the East Asian domain includes the tropical western North Pacific summer monsoon (WNPSM) and the subtropical continent monsoon, two well‐known monsoon indices, the WNPSM index (WNPSMI) and EASM index (EASMI), and their associated low‐level winds and precipitation anomalies are well forecasted by the models. However, the forecast performance generally decreases as the leads increase, and the performance of EASMI is not as good as that of WNPSMI. CFSv2 forecasts well at leads up to 6 months whereas the skill of CanCM3 (CanCM4) decreases rapidly when the lead increases to 2 months (3 months). The failure of CanCM3 is mainly attributed to the poor forecast of the relationship of EASMI with the El Niño‐Southern Oscillation and Northern Indian Ocean‐western North Pacific (WNP) sea surface temperature anomaly. However, the causes of the poor forecast of CanCM4 for EASMI require further investigation. Sources of the forecast error (FE), which is the difference between model and observation for monsoon precipitation, are more significant than those of the predictability error (PE), which originates from the initial condition error, indicating that model deficiency plays a dominant role in limiting the EASM precipitation forecast. However, the PE cannot be neglected over the tropical western Pacific in CFSv2, over the WNP in CanCM3 and over the Tibetan Plateau in CanCM4. As the lead time increases, the FE does not remarkably change whereas the PE decreases significantly.

Metabolic adaptations of the pelagic octopod Japetella diaphana to oxygen minimum zones

The pelagic octopod Japetella diaphana inhabits meso- and bathypelagic depths worldwide. Across its geographic and depth range, individuals encounter oxygen levels ranging from nearly air-saturated to nearly anoxic. In this study, we assessed the physiological adaptations of individuals from one of the largest and most extreme regions of ocean hypoxia, the Eastern Tropical North Pacific (ETNP) oxygen minimum zone (OMZ). Ship-board measurements of metabolic rate and hypoxia tolerance were conducted and a metabolic index was constructed to model suitable habitat for aerobic metabolism. We found that individuals from the ETNP had a low metabolic rate, yet higher than conspecifics from more oxygen-rich habitats. Despite their higher rates, hypoxia tolerance may be similar to or greater than in conspecifics from more oxygen-rich waters. Furthermore, hypoxia tolerance in J. diaphana has a reverse temperature-dependence from most marine ectotherms, a characteristic that uniquely suits the physical characteristics of the lower oxycline. Even with its high tolerance to hypoxia, J. diaphana is incapable of maintaining basal oxygen demand in the OMZ core, at a depth where populations are typically most abundant in more oxygenated regions. Despite the limited metabolically-suitable habitat, J. diaphana is abundant in the ETNP outside the OMZ core, suggesting that physiological and behavioral plasticity is sufficient to maintain species fitness in this region.