Pacific Subtropical Research Articles

A Continuing Increase of the Impact of the Spring North Pacific Meridional Mode on the Following Winter El Niño and Southern Oscillation

Abstract This study reveals that the impact of the spring North Pacific meridional mode (PMM) on the following-winter El Niño–Southern Oscillation (ENSO) shows a continuing increase in the past. A comparative analysis is conducted for the high- and low-correlation periods to understand the factors for the strengthened impact of the PMM. The spring PMM-related sea surface temperature (SST) and atmospheric anomalies over the subtropical northeastern Pacific propagate southwestward to the tropical central Pacific via wind–evaporation–SST feedback in the high-correlation period. The tropical SST and atmospheric anomalies further develop to an ENSO-like pattern via positive air–sea interaction. In the low-correlation period, SST and atmospheric anomalies over the subtropical northeastern Pacific related to the PMM cannot extend to the deep tropics. Therefore, the spring PMM has a weak impact on ENSO. The extent to which the PMM-related SST and atmospheric anomalies extend toward the tropics is related to the background flow. The stronger mean trade winds in the high-correlation period lead to an increase in the air–sea coupling strength over the subtropical northeastern Pacific. As such, the spring PMM-related SST and atmospheric anomalies can more efficiently propagate southwestward to the tropical Pacific and exert stronger impacts on the succeeding ENSO. In addition, the southward shifted intertropical convergence zone in the high-correlation period also favors the southward extension of the PMM-related SST anomalies to the tropics and contributes to a stronger PMM–ENSO relation. The variation and its formation mechanism of the spring PMM–winter ENSO relationship appear in both the observations and the long historical simulation of Earth system models. Significance Statement The North Pacific meridional mode (PMM) is the leading atmosphere–ocean coupling pattern over the subtropical northeastern Pacific after removing the ENSO variability, with maximum variance during boreal spring. Previous studies indicated that the PMM plays an important role in relaying the impact of the atmosphere–ocean forcings over the extratropics on the tropical ENSO. This study reveals that the impact of the spring PMM on the following winter ENSO shows a continuing increase in the past 70 years. The physical mechanisms for this strengthened impact are further examined. Results obtained in this study have important implications for improving the prediction of the tropical ENSO variability.

Compound specific δ15N analysis of amino acids reveals unique sources and differential cycling of high and low molecular weight marine dissolved organic nitrogen

Marine dissolved organic nitrogen is the largest pool of reduced nitrogen (N) in the ocean. Yet, despite indications of a bacterial origin, few N-specific approaches have been developed to trace individual microbial N production and degradation mechanisms. Stable nitrogen isotope ratios (δ15N) are one of the most powerful tools to understand sources and cycling of organic N. However, δ15N measurements of total DON can only be made in the surface ocean and interpretations are complicated by the many factors influencing bulk δ15N values. Compound-specific isotope analysis of amino acids (CSI-AA) on isolated DON fractions represents a new approach for DON study throughout the water column with significantly greater information potential. Here, we compare for the first time δ15N amino acid (δ15N-AA) data on baseline inorganic N source and microbial loop processing in coupled ultrafiltered high molecular weight (HMW) and solid-phase extracted low molecular weight (LMW SPE-) DON from the surface to 2500 m deep in the Atlantic and Pacific Subtropical Gyres.δ15N-AA patterns were very different in HMW versus LMW SPE-DON, indicating unique formation and bacterial alteration mechanisms. HMW DON data supported a prior hypothesis that AA in surface HMW DON originate from heterotrophic bacterial production near the nitracline, but also suggests protozoan heterotrophy as an additional major process. In the mesopelagic, distinct HMW DON δ15N-AA patterns suggest heterotrophic bacterial resynthesis of suspended PON. In contrast, the first LMW DON δ15N-AA patterns were surprisingly similar to those of autotrophs, indicating limited resynthesis at any depth. Taken together with prior biomarker data, we hypothesize a direct bacterial surface source of LMW AA-containing molecules which are inherently refractory. Finally, within each size fraction, δ15N-AA patterns were remarkably similar between Station ALOHA and the BATS site, suggesting our findings can be extended to marine DON at least throughout global subtropical gyres.These findings demonstrate CSI-AA as a novel δ15N tool to understand sources and cycling of marine DON at a new level of detail and suggest a significant shift in our understanding of DON source and cycling. Specifically, we hypothesize that the production and degradation mechanisms of LMW AA-containing DON is fundamentally distinct from HMW DON, and that the majority of LMW refractory DON in the sea derives not from degradation of HMW material, but rather from surface production of intrinsically more refractory LMW molecules.

Thermocline variability in the subtropical northwestern Pacific since the last deglaciation

Thermocline conditions in the tropical western Pacific are closely linked to regional- and basin-scale hydrological variation and may have played a critical role in global climate evolution on various timescales. However, long-term changes in thermocline hydrology in the subtropical northwestern Pacific and their connection to the tropical Pacific remain unclear. Based on planktonic foraminiferal Mg/Ca and δ18O, we reconstructed the temperature and salinity of thermocline water since the last deglaciation, using core OKI02 from the Okinawa Trough (OKT) in the subtropical northwestern Pacific. The results indicated a discrepancy in evolution of both temperature and δ18O between the thermocline and surface water over the past 19 ka. Unlike for surface water, changes in thermocline hydrology correlated with changing conditions in the tropical western Pacific, indicating a close relationship between these regions since the last deglaciation. Thermocline temperature and salinity gradients between the OKT and the tropical western Pacific increased from ∼16.5 to 10 ka BP, reached their maximum values at approximately 10 ka BP, and then decreased from the Early Holocene to present. Variations in temperature and salinity gradients correlated with changes in the El Niño–Southern Oscillation (ENSO) over the past 19 ka, which were coupled with equatorial summer insolation variations. We suggest that the ENSO-like process, forced by equatorial summer insolation, regulated thermocline dynamics in the subtropical northwestern Pacific Ocean since the last deglaciation, by influencing the strength of the Kuroshio Current.

Attribution of the Subsurface Temperature Change in the Southern Hemisphere

Abstract The Southern Hemisphere temperature has experienced obvious changes with great spatial differences over the past several decades. Most regions show extreme warming, especially those located at 35°–55°S. In contrast, subsurface cooling exists between 15° and 35°S in the Indian and Pacific basins. The subsurface temperature and salinity change can be divided into spiciness change and heave components. The results show the warming due to isopycnal movement being largely offset by significant spiciness cooling at middepth. Surface warming and subduction into the interior ocean account for subsurface spiciness cooling near 45°S, while surface freshening and penetration along isopycnals are more important to the subsurface spiciness cooling farther north. The isobaric temperature change is associated with pure warming and pure heaving, and the subsurface cooling observed in the Indian and Pacific subtropics is predominantly attributed to pure heaving. This study provides a quantitative estimate of the relative contribution of surface temperature, salinity change, and circulation adjustment in subsurface temperature change, highlighting the importance of circulation change in producing subsurface cooling. Further research is needed to understand why different processes dominate in different ocean sections. Significance Statement While the global ocean is warming, the subsurface temperature change exhibits a significant regional disparity. This paper attempts to explain the deep-reaching warming at 35°–55°S and cooling at 15°–35°S based on three historical observation datasets. We find that the cooling mostly occurs between 400 and 1000 m in the south Indian and Pacific subtropics (15°–35°S), which is attributed to pure heaving, indicating the importance of circulation change in these regions. The midlatitude warming (35°–55°S) is mainly caused by the pure warming process, which is related to heat uptake at the subpolar surface and northward and downward heat transport. The spiciness cooling near 45°S is mainly driven by the subduction of the surface warming signal while the freshening process has a stronger impact on spiciness cooling farther north.

Impacts of moisture supply from the subtropical western Pacific on the subtropical high and the atmospheric river during the heavy rain of 2020 in Japan

Our recent study suggested that moisture from the subtropical Western Pacific (WP) contributed the most to an atmospheric river (AR) event and the related heavy rainfall during the heavy rain of 2020 in Japan based on a Lagrangian approach. However, the actual role of moisture from the subtropical WP region in the AR and heavy rain formations remains unclear. To evaluate that, we conducted a set of numerical sensitivity experiments by adjusting the surface moisture supply over the subtropical WP region with factors of 0%, 50%, and 200%. The sensitivity experiments suggest that the reduced surface evaporation over the subtropical WP suppressed the local convective activity and decreased moisture content in the whole troposphere, leading to shallow and weak positive geopotential height anomalies. Although the slightly strengthened WP subtropical High (WPSH) and related anomalous anticyclonic circulation enhanced the southwesterly wind, convective activities along the Meiyu-Baiu front were still weakened due to the largely reduced moisture supply, resulting in another anomalous anticyclonic circulation over Japan but had much stronger and deeper structures. These two anomalous circulations and the reduced moisture modulated the AR over Japan, which eventually caused the weakened rainfalls and the northward migration of the rainband. By contrast, larger surface evaporation enhanced the local convective activities and weakened the dominant WPSH, resulting in the weakening and the southward migration of the AR. Overall, this study confirmed the large contributions of moisture supply from the subtropical WP region to the AR and related precipitation over Japan during the record-breaking Meiyu-Baiu season in 2020 via both dynamic and thermodynamic influences. In addition, it reveals that, although larger evaporation over the WP region would increase the total rainfalls but would not have led to more several rainfalls over certain regions, such as the relatively small Kyushu Island.

Role of Pacific preconditioning in modulating the relationship between the spring North Tropical Atlantic SST and the ensuing El Niño

Previous literature has indicated that a spring cold sea surface temperature (SST) anomaly in the North Tropical Atlantic (NTA) could induce an ensuing El Niño event. However, observations show that this relationship is not always robust. For example, the spring cold NTA SST anomalies are not necessarily accompanied by El Niño events in the ensuing winter. Two types of cases are further classified for comparison: a spring cold NTA SST anomaly accompanied by an El Niño (tagged as NTA-El Niño) and a spring cold NTA SST anomaly accompanied by no El Niño (tagged as NTA-Normal). Statistical analysis indicates that Pacific preconditioning acts an important role in modulating the NTA SST and El Niño relationship. The zonal wind anomaly over the eastern subtropical Pacific serves as an important medium linking the spring NTA SST anomaly to the subsequent development of El Niño, and this part of the zonal wind anomaly in spring and summer is largely associated with the preceding winter tropical Pacific SST anomaly. The proposed mechanism is verified by two groups of Coupled General Circulation Models (CGCMs) runs with a weak and strong La Niña condition. Compared with the strong La Niña case, under a weak La Niña case in the previous winter, the spring cold NTA SST anomaly is more capable of modulating the development of the ensuing El Niño event. The present study suggests the importance of Pacific preconditioning while forecasting ENSO with the precursor of the spring NTA SST anomaly.

A deep learning model for forecasting global monthly mean sea surface temperature anomalies

Sea surface temperature (SST) variability plays a key role in the global weather and climate system, with phenomena such as El Niño-Southern Oscillation (ENSO) regarded as a major source of interannual climate variability at the global scale. The ability to make long-range forecasts of SST variations and extreme marine heatwave events have potentially significant economic and societal benefits, especially in a warming climate. We have developed a deep learning time series prediction model (Unet-LSTM), based on more than 70 years (1950–2021) of ECMWF ERA5 monthly mean SST and 2-m air temperature data, to predict global 2-dimensional SSTs up to a 24-month lead. Model prediction skills are high in the equatorial and subtropical Pacific. We have assessed the ability of the model to predict SST anomalies in the Niño3.4 region, an ENSO index in the equatorial Pacific, and the Blob marine heatwave events in the northeast Pacific in detail. An assessment of the predictions of the 2019–2020 El Niño and the 2016–2017 and 2017–2018 La Niña show that the model has skill up to 18 months in advance. The prediction of the 2015–2016 extreme El Niño is less satisfactory, which suggests that subsurface ocean information may be crucial for the evolution of this event. Note that the model makes predictions of the 2-d monthly SST field and Nino 3.4 is just one region embedded in the global field. The model also shows long lead prediction skills for the northeast Pacific marine heatwave, the Blob. However, the prediction of the marine heatwaves in the southeast Indian Ocean, the Ningaloo Niño, shows a short lead prediction. These results indicate the significant potential of data-driven methods to yield long-range predictions of SST anomalies.

Water, Water Everywhere, nor Any Drop to Drink? Options for Improving the Resilience of a Subtropical Island to Drought and the Sensitivity of Their Yield under a Projected Drier Future Climate

Norfolk Island, like many islands in the subtropical Pacific, is at increasing risk of drought. To test the hypothesis that desalination is the most cost-effective technology for providing potable water during ‘water emergencies’, this study calculated and compared the levelised cost on Norfolk Island of seven ‘centralised’ intervention options and two ‘diffuse’ options for increasing groundwater accessions. Gully dams were the ‘centralised’ intervention option found to have the lowest levelised cost but had the largest percentage reductions in yield (i.e., 39% for 1 ML storage) under a projected drier future climate, greater than twice the percentage reduction in rainfall (16%) but less than the percentage reduction in runoff (44%). Cluster-scale roof-harvested rainwater systems with ‘turkey nest’ earth embankment had the second lowest levelised cost but are probably socially unacceptable due to siting on the community’s premier sporting oval. Desalination had the third lowest levelised cost, followed by the use of existing deep groundwater bores to pump and store water exclusively for use during a water emergency. Although desalination was not the most cost-effective technology, it is likely to be more socially acceptable than the two more cost-effective options. After gully dams, rainwater harvesting intervention options had the second highest proportional reductions in yield under a projected drier future climate (~10%); however, the reductions were less than the percentage reductions in rainfall. The yield from desalination and groundwater were unaffected under a drier future climate projected for ~2060.

Effects of cyanate enrichment on growth of natural phytoplankton populations in the subtropical Pacific

Effects of cyanate enrichment on growth of natural phytoplankton populations in the subtropical Pacific

Strengthening Amplitude and Impact of the Pacific Meridional Mode on ENSO in the Warming Climate Depicted by CMIP6 Models

Abstract The Pacific meridional mode (PMM) has been suggested to play an important role in modulating the development of El Niño–Southern Oscillation (ENSO). In this study, we examine the projected changes in the PMM and its impact on ENSO under greenhouse gas forcing using the models of phase 6 of the Coupled Model Intercomparison Project. These models can properly reproduce the characteristics of PMM patterns but reveal discrepant PMM–ENSO relationships owing to different wind–evaporation–sea surface temperature (SST) (WES) feedback efficiency and different magnitude of atmospheric convection response to SST anomalies. We select the models that show good performance in simulating the PMM and its impact on ENSO for investigation of future projections. Results show potential increases in both PMM amplitude and its impact on ENSO under the shared socioeconomic pathway (SSP) 585 (SSP585) warming scenario with great intermodel consensus. Diagnosis of the WES feedback indicates increasing sensitivity of latent heat flux to zonal wind speed in a warming climate, which seems to be the main reason for the projected strengthening PMM and its impact on ENSO. In addition, a slightly intensified response of atmospheric convection to SST anomalies in the subtropical Pacific may partially contribute to a stronger PMM–ENSO relationship. The results from this study highlight the increasing importance of the PMM for ENSO development, which calls for more attention to be paid to the PMM for ENSO prediction. Significance Statement Variability of the sea surface temperature in the equatorial Pacific related to El Niño–Southern Oscillation (ENSO) can exert a great impact on global climate. The development of ENSO is partially modulated by the dominant mode of ocean–atmospheric variation in the subtropical North Pacific, namely, the Pacific meridional mode (PMM). This study is aimed to understand the change in the amplitude of the PMM and its impact on ENSO due to climate change. Multimodel projections suggest that the PMM will likely become stronger and exert a greater impact on ENSO since the future warmer climate is favorable for the growth of the PMM. These results call for more attention to be paid to the PMM for ENSO prediction.

Response of East Asian Summer Precipitation to Intermediate SST Anomalies while El Niño Decays and Dependence on Type of Events

Abstract Impacts of El Niño on the following summer East Asian precipitation are dependent on its peak-time characteristics. Three types are identified and denoted as eastern Pacific (EP), mixed-type Pacific (MP), and central Pacific (CP) El Niño. For EP, excessive rainfall occurs in the Yangtze River valley, in relation to an anomalous anticyclone over the western tropical Pacific. For MP, a dipolar precipitation with anomalous wet conditions in northern China and dry conditions in east-central China is mainly due to an anomalous cyclone over Northeast Asia and a weak anticyclone over the Sea of Japan. For the case of CP El Niño, wetness along the Yangtze–Huaihe River valley and dryness in Southeast China are ascribed to an anomalous cyclone in Northeast China and a strong anticyclone over the western tropical Pacific. It is also revealed that the precipitation anomalies are fundamentally caused by different intermediate sea surface temperature (SST) anomalies from other basins while El Niño decays in the following summer. In the case of EP El Niño, the joint action from warm SST in the tropical Indian Ocean, the Niño-1.2 region, and the North Atlantic is responsible for the delayed effect of El Niño. For MP, the main delayed effect comes from warm SST in the northeast subtropical Pacific, developing La Niña, and cold SST in the southern tropical Atlantic. Finally, CP El Niño exerts its delayed influence through warm SST in the northern tropical Atlantic.

Toward an optimal observational array for improving two flavors of El Niño predictions in the whole Pacific

This paper investigates the optimal observational array for improving the El Niño-Southern Oscillation (ENSO) prediction by exploring sensitive areas for target observations of two types of El Niño events in the whole Pacific. A target observation method based on the particle filter and pre-industrial control runs from six coupled model outputs in Coupled Model Intercomparison Project Phase 5 (CMIP5) experiments are used to quantify the relative importance of the initial accuracy of sea surface temperature (SST) in different Pacific areas. The initial accuracy of the tropical Pacific, subtropical Pacific, and extratropical Pacific can all exert influences on both types of El Niño predictions. The relative importance of different areas changes along with different lead times of predictions. Tropical Pacific observations are crucial for decreasing the root mean square error of predictions of all lead times. Subtropical and extratropical observations play an important role in decreasing the prediction uncertainty, especially when the prediction is made before and throughout the boreal spring. To consider different El Niño types and different start months for predictions, a quantitative frequency method based on frequency distribution is applied to determine the optimal observations of ENSO predictions. The final optimal observational array contains 31 grid points, including 21 grid points in the equatorial Pacific and 10 grid points in the North Pacific, suggesting the importance of the initial SST conditions for ENSO predictions not only in the tropical Pacific but also in the area outside the tropics. Furthermore, the predictions made by assimilating SST in sensitive areas have better prediction skills in the verification experiment, which can indicate the validity of the optimal observational array designed in this study.

Systematic revision of the Indo-West Pacific bubble-snails of the genus

The genus Haloa includes dull-coloured species of haminoeid snails inhabiting tidal and shallow waters of the tropical and sub-tropical Indo-West Pacific. This paper reports on the diversity and systematics of Haloa based on the phylogenetic hypothesis generated by Oskars and Malaquias (2019) and on the morphological study of specimens. Shells, external features and anatomical characters from the jaws, radula, gizzard plates and male reproductive system were studied by optical and scanning electron microscopy. The molecular species delimitation method Automatic Barcode Gap Discovery, using DNA sequences of the cytochrome c oxidase subunit I gene, was implemented to aid delimiting candidate species. Thirteen species were recognised as valid, including two new to science, namely Haloa cobbi, sp. nov. and Haloa eora, sp. nov., and one unnamed species herein referred to as Haloa sp. 1. Species depict intraspecific variability in their colouration and little character displacement in their anatomy. Shells are of limited taxonomic use; only details of the male reproductive system (e.g. fundus) and the DNA were truly diagnostic. Three species have an Indo-West Pacific distribution and three are restricted to the Indian Ocean. The remaining seven species occur in the Western and Central Pacific Ocean, three of them having geographical ranges restricted to offshore islands.

Two Pathways of Subsurface Spiciness Anomalies in the Subtropical South Pacific

Subduction and migration of density-compensated (warm/salty or cool/fresh) temperature and salinity water-mass anomalies on isopycnals, referred to as spiciness anomalies, are examined in the subtropical gyre of the South Pacific using an observational dataset. The present results demonstrate that the spiciness anomalies are found to follow two pathways from the subtropical region to the tropical area on the 25–25.5σθ isopycnals. The water masses of one pathway subduct south of 20°S and mainly flow westward via the mean geostrophic current to the western boundary region. The water masses of this pathway correspond to the salinity maximum on these isopycnals as well as the bottom of the South Pacific Tropical Water. Positive temperature and salinity trends were prominent along this pathway during the study period. In the other pathway, the water masses subduct north of 20°S and go directly to the tropics through the interior region. Decadal variability of the spiciness anomalies is prominent along this pathway. In both pathways, sea surface salinity variability likely plays an important role in generating the spiciness anomalies on the isopycnals. A passive tracer experiment revealed that the advection by the South Equatorial Countercurrent (SECC) divides these two pathways. Hence, SECC plays a key role in determining whether a spiciness anomaly propagates through the interior region or the western boundary region.

Equatorial Origin of the Observed Tropical Pacific Quasi‐Decadal Variability From ENSO Nonlinearity

AbstractQuasi‐decadal (QD) climate variability is detected in the tropical Pacific based on the recent 70 years of observations. This QD variability is identified in equatorial sea surface temperatures (SSTs), the pattern of which resembles the Central Pacific (CP) El Niño‐Southern Oscillation (ENSO) but extends further meridionally to the northeastern subtropical Pacific. Whereas equatorial upper‐ocean heat content and SSTs are in quadrature for ENSO, these two quantities are almost in phase on the QD timescale. Further analysis shows that nonlinear dynamical heating, primarily originating from strong El Niño events, tends to lead QD SSTs by a quarter of its dominant period (approximately 30 months) and shapes the dominant QD periodicity in observations. Our results suggest that the observed QD variability largely originates from ENSO nonlinearity and thus is fundamentally different from ENSO's oscillatory nature.

Availability of vitamin B12 and its lower ligand intermediate α-ribazole impact prokaryotic and protist communities in oceanic systems

Genome analyses predict that the cofactor cobalamin (vitamin B12, called B12 herein) is produced by only one-third of all prokaryotes but almost all encode at least one B12-dependent enzyme, in most cases methionine synthase. This implies that the majority of prokaryotes relies on exogenous B12 supply and interacts with producers. B12 consists of a corrin ring centred around a cobalt ion and the lower ligand 5’6-dimethylbenzimidazole (DMB). It has never been tested whether availability of this pivotal cofactor, DMB or its intermediate α-ribazole affect growth and composition of prokaryotic microbial communities. Here we show that in the subtropical, equatorial and polar frontal Pacific Ocean supply of B12 and α-ribazole enhances heterotrophic prokaryotic production and alters the composition of prokaryotic and heterotrophic protist communities. In the polar frontal Pacific, the SAR11 clade and Oceanospirillales increased their relative abundances upon B12 supply. In the subtropical Pacific, Oceanospirillales increased their relative abundance upon B12 supply as well but also downregulated the transcription of the btuB gene, encoding the outer membrane permease for B12. Surprisingly, Prochlorococcus, known to produce pseudo-B12 and not B12, exhibited significant upregulation of genes encoding key proteins of photosystem I + II, carbon fixation and nitrate reduction upon B12 supply in the subtropical Pacific. These findings show that availability of B12 and α-ribazole affect growth and composition of prokaryotic and protist communities in oceanic systems thus revealing far-reaching consequences of methionine biosynthesis and other B12-dependent enzymatic reactions on a community level.

Joint Effect of West Pacific Warming and the Arctic Oscillation on the Bidecadal Variation and Trend of the East Asian Trough

AbstractThis study investigates bidecadal variation and trends in the winter East Asian trough (EAT) intensity for the period from 1958 to 2020. The EAT intensity index, calculated with the JRA55 reanalysis, demonstrates bidecadal variation, which is closely related to the intensity of cold advection in East Asia that affects northeast China, the Korean Peninsula, and Japan. In addition, it is noted that the positive phase of the Arctic Oscillation (AO) plays an important role in suppressing EAT intensity, particularly on a bidecadal time scale. On the other hand, a warm anomaly in the west Pacific can enhance the intensity of the EAT, which is also reproduced by numerical simulation. The influence of a significant warming trend in the west Pacific and a weak positive trend in the AO on the EAT intensity and the East Asian winter monsoon is investigated. Warming in the west Pacific supports a strengthening of EAT intensity, cooling in northeast Asia, and enhancement of easterly wind in the subtropical west Pacific. A weak positive AO trend plays an insignificant role in affecting the trend of EAT intensity, but it supports warming in northeast Asia.

Interannual Variations of Submesoscale Circulations in the Subtropical Northeastern Pacific

AbstractImpacts of oceanic submesoscale circulations on oceanic circulation as well as marine ecosystem and air‐sea heat flux have been reported by recent studies. Therefore, their regionality and variations on various timescales are important to understand their impacts. The submesoscale circulations are active in winter, which is mostly enhanced by mixed layer instability (MLI). However, their variations on timescales longer than interannual are not well understood. This study shows the interannual variations of active submesoscale circulations in winter and their mechanism in the subtropical northeastern Pacific using the output of a submesoscale permitting simulation. The submesoscale kinetic energy varies synchronously with the available potential energy release in a process of MLI. El Niño events appear to modulate the interannual variations. In addition, the similarities and differences between the variations in the eastern and western subtropics are highlighted.

Biodiversity and Biogeography of Abundant and Rare Microbial Assemblages in the Western Subtropical Pacific Ocean.

The levels of chlorophyll a and nutrient concentrations in the surface waters of the western subtropical Pacific Ocean are among the lowest globally. In addition, our knowledge of basin-scale diversity and biogeography of microbial communities in this vast extremely oligotrophic environment is still rather limited. Here, high-throughput sequencing was used to examine the biodiversity and biogeography of abundant and rare microbial assemblages throughout the water column from the surface to a depth of 3,000 m across a horizontal distance of 1,100 km in the western Pacific Ocean. Microbial alpha diversity in the 200-m layer was higher than at other depths, with Gammaproteobacteria, Alphaproteobacteria, and Clostridia as the dominant classes in all samples. Distinctly vertical distributions within the microbial communities were revealed, with no difference horizontally. Some microbes exhibited depth stratification. For example, the relative abundances of Cyanobacteria and Alphaproteobacteria decreased with depth, while Nitrososphaeria, Actinobacteria, and Gammaproteobacteria increased with depth in the aphotic layers. Furthermore, we found that environmental (selective process) and spatial (neutral process) factors had different effects on abundant and rare taxa. Geographical distance showed little effect on the dispersal of all and abundant taxa, while statistically significant distance–decay relationships were observed among the rare taxa. Temperature and chlorophyll a were strongly associated with all, abundant, and rare taxa in the photic layers, while total inorganic nitrogen was recognized as the crucial factor in the aphotic layers. Variance partitioning analysis indicated that environmental selection played a relatively important role in shaping all and abundant taxa, while the variation in rare taxa explained by environmental and spatial processes was relatively low, as more than 70% of the variation remained unexplained. This study provides novel knowledge related to microbial community diversity in the western subtropical Pacific Ocean, and the analyzes biogeographical patterns among abundant and rare taxa.

Impact of biomass burning and stratospheric intrusions in the remote South Pacific Ocean troposphere

Abstract. The ozone mixing ratio spatiotemporal variability in the pristine South Pacific Ocean is studied, for the first time, using 21-year-long ozone (O3) records from the entire southern tropical and subtropical Pacific between 1994 and 2014. The analysis considered regional O3 vertical observations from ozonesondes, surface carbon monoxide (CO) observations from flasks, and three-dimensional chemistry-transport model simulations of the global troposphere. Two 21-year-long numerical simulations, with and without biomass burning emissions, were performed to disentangle the importance of biomass burning relative to stratospheric intrusions for ambient ozone levels in the region. Tagged tracers of O3 from the stratosphere and CO from various biomass burning regions have been used to track the impact of these different regions on the southern tropical Pacific O3 and CO levels. Patterns have been analyzed based on atmospheric dynamics variability. Considering the interannual variability in the observations, the model can capture the observed ozone gradients in the troposphere with a positive bias of 7.5 % in the upper troposphere/lower stratosphere (UTLS) as well as near the surface. Remarkably, even the most pristine region of the global ocean is affected by distant biomass burning emissions by convective outflow through the mid and high troposphere and subsequent subsidence over the pristine oceanic region. Therefore, the biomass burning contribution to tropospheric CO levels maximizes in the UTLS. The Southeast Asian open fires have been identified as the major contributing source to CO from biomass burning in the tropical South Pacific, contributing on average for the study period about 8.5 and 13 ppbv of CO at Rapa Nui and Samoa, respectively, at an altitude of around 12 km during the burning season in the spring of the Southern Hemisphere. South America is the second-most important biomass burning source region that influences the study area. Its impact maximizes in the lower troposphere (6.5 ppbv for Rapa Nui and 3.8 ppbv for Samoa). All biomass burning sources contribute about 15–23 ppbv of CO at Rapa Nui and Samoa and account for about 25 % of the total CO in the entire troposphere of the tropical and subtropical South Pacific. This impact is also seen on tropospheric O3, to which biomass burning O3 precursor emissions contribute only a few ppbv during the burning period, while the stratosphere–troposphere exchange is the most important source of O3 for the mid troposphere of the South Pacific Ocean, contributing about 15–20 ppbv in the subtropics.