Central Equatorial Pacific Research Articles

Source and distribution characteristics of 239, 240, 241Pu, 237Np and 134, 137Cs in sediments in the Northwest and Central Equatorial Pacific after the Fukushima nuclear accident

To understand the possible influence of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on the deep sea, as well as the geochemical behavior and transport of radionuclides, 134Cs, 137Cs, 239, 240Pu, 241Pu, and 237Np were measured in the abyssal sediments of the Northwest Pacific (NWP) and Central Equatorial Pacific (CEP) Ocean. Data on the characteristics of these sediments obtained after the FDNPP accident are extremely rare, especially in the NWP subtropical gyre (NPSG) region. FDNPP-derived radio-Cs (134Cs, 137Cs) arrived at the open sea floor of the NWP before 2018 but was only found in the Kuroshio-Oyashio Extension (KOE) region. No FDNPP-derived Pu was detected in the abyssal sediments of the NWP or CEP. Pu in the NWP mainly originated from global fallout and the Pacific Proving Ground (PPG) close-in fallout, except for at station WP1 (39°N in the KOE region), where an abnormal but non-FDNPP-derived Pu signal was detected. Pu in the eastern CEP sediment was less affected by the PPG close-in fallout from the Marshall Islands and was mainly derived from global fallout, with some close-in fallout from the Johnston Atoll test. The KOE region was the area most affected by PPG close-in fallout Pu via Kuroshio transport, while the lowest inventories of 239+240Pu and 237Np were found in the NPSG region due to its oligotrophic environment. The 237Np originated from the same source as Pu, and the latitudinal pattern of 237Np was consistent with that of Pu. Station SS (in the marginal sea of the NWP) contained high 237Np/239Pu atom ratios in the deeper layers of sediment and had a 237Np depth profile opposite that of the 239+240Pu profile, compared to other stations; these differences are mainly attributed to differences in the behaviors of 237Np and 239Pu.

Interdecadal Modulation of ENSO‐Related Anomalous Equatorial Intermediate Currents in the Western Pacific by the PDO

AbstractAs the most prominent decadal variability in the Pacific Ocean, the Pacific Decadal Oscillation (PDO) can influence the El Niño‐Southern Oscillation‐related atmospheric and upper oceanic motions. Based on results from ocean reanalysis products and linear continuously stratified model experiments, we reveal that the PDO can further extend its interdecadal modulation effect to the equatorial intermediate currents below the thermocline. For the maximum El Niño‐related intermediate eastward transport anomaly in the western Pacific, its magnitude during the warm PDO is nearly twice as large as that during the cold PDO, and its occurrence time in the warm PDO leads that in the cold PDO by about 2 months. Such modulation is achieved through westward and downward propagation of the reflected Kelvin‐to‐Rossby waves, triggered by enhanced and eastward‐displaced anomalous westerly wind. The decisive wind forcing region for the modulation is located over the equatorial central Pacific of 5°S–5°N and 180°–140°W.

Long‐Lead Predictability of Western North Pacific Subtropical High

AbstractWe present a new assembly of three steppingstones in thorough inquiry of the complex chain reaction from the diverse equatorial sea surface temperature (SST) pattern two‐season ahead to the East Asia summer weather condition, by regarding the western North Pacific subtropical high (WNPSH) as an anchor. We identified three predictors that are discordant with conventional climatic indices and achieve a good skill in the summer WNPSH prediction (temporal correlation coefficient of 0.78) with a two‐season long lead in the leave‐1‐year‐out cross‐validation during 1981–2020, despite the diversity of El Niño‐Southern Oscillation (ENSO) and Indian ocean dipole (IOD) patterns. Specifically, the first predictor captures the initiation and development of both IOD and El Niño via the SST warming over central equatorial Pacific in autumn; the second one serves as a better delegate of IOD by capturing the easterly acceleration over tropical Indian ocean; the third one illustrates the unprecedented informative role of upper‐level high pressure over the entire tropics in the WNPSH prediction with 5months lead time. They mediate the discordance between conventional climate indices and the diverse WNPSH response and caution people to revisit and rethink the complex atmosphere‐ocean coupled response from the diverse tropical SST patterns two seasons ahead to EA summer weather condition.

Linking the Subsurface Indian Ocean Dipole to Central Pacific ENSO

AbstractWe identified a remote linkage from the Central Pacific (CP) El Niño–Southern Oscillation (ENSO) to the subsurface Indian Ocean dipole (Sub‐IOD) occurring in winter‐to‐spring. Although most Sub‐IOD events co‐occur with a surface Indian Ocean dipole (IOD) in autumn, some Sub‐IOD events develop independently in winter‐to‐spring. These spring Sub‐IODs are usually accompanied by a CP ENSO event that develops in the winter, peaks in early spring, and has a more westward‐located sea surface temperature anomaly over the central equatorial Pacific (around 180°E). The sea surface temperature anomaly and the accompanying surface zonal wind anomalie during these western CP El Niño (La Niña) events contributes to an anomalous descending (ascending) branch of the Walker circulation over the Indo‐Pacific region and easterly (westerly) anomalies over the equatorial Indian Ocean from winter to the following spring. This helps to trigger a positive (negative) Sub‐IOD event.

Key Processes on Triggering the Moderate 2020/21 La Niña Event as Depicted by the Clustering Approach

The 2020/21 La Niña was not well predicted by most climate models when it started in early-mid 2020. This paper adopted an El Niño-Southern Oscillation (ENSO) ensemble prediction system to evaluate the key physical processes in the development of this cold event by performing a clustering analysis of 100 ensemble member predictions 1 year in advance. The abilities of two clustering approaches were first examined in regard to capturing the development of the 2020/21 La Niña event. One approach was index clustering, which adopted only the 12-month Niño3.4 indices in 2020 as an indicator, and the other was pattern clustering through contrasting the evolution of sea surface temperature (SST) anomalies over the tropical Pacific in 2020 for clustering. Pattern clustering surpasses index clustering in better describing the evolution over the off-equatorial and equatorial regions during the 2020/21 La Niña. Consequently, based on the pattern clustering approach, a comparison of the selected most (five best) and least (five worst) representative ensemble members illustrated that the predominance of anomalous southeasterly winds over the central equatorial Pacific in spring 2020 played a crucial role in initiating the moderate La Niña event in 2020/21, by preventing the development of westerly winds over the warm pool. Moreover, the inherent spring predictability barrier (SPB) was still a major challenge for improving the prediction skill of the 2020/21 La Niña event when the prediction occurred across the spring season.

IOD, ENSO, and seasonal precipitation variation over Eastern China

This study investigates seasonal precipitation variation over eastern China associated with Indian Ocean Dipole (IOD) forcing and emphasizes the distinction of such responses to preceding IOD events compared with responses to El Niño–Southern Oscillation (ENSO). Precipitation evolution patterns in response to the IOD from autumn to the ensuing summer are derived from singular value decomposition analysis, revealing that the IOD causes a large portion of the seasonal variation in precipitation over eastern China in the simultaneous autumn and time-lagged summer. The difference in the impacts associated with the IOD and ENSO revealed by multiple linear regression and partial correlation analysis is that the IOD contributes mainly to abnormal precipitation in South China during autumn and in the region between the Yangtze River and Yellow River during the ensuing summer; while ENSO primarily boosts precipitation over eastern China during winter and spring. The distinctive effects of the IOD on the ensuing summer precipitation contrast with the less significant signals related to ENSO during the ensuing summer. Such precipitation responses correspond to an anomalous anticyclonic circulation pattern around the South China Sea, which is sustained by direct IOD forcing during autumn and winter and an SST cooling pattern triggered by the IOD over the central equatorial Pacific during the ensuing spring and summer. The calculation of wave activity flux and anomalous AGCM model experiments further confirm the importance of the direct heating of the IOD during autumn and winter and the indirect heating sink over the central Pacific during the ensuing spring and summer for modulating the seasonal variation in precipitation over eastern China.

Different Influencing Mechanisms of Two ENSO Types on the Interannual Variation in Diurnal SST over the Niño-3 and Niño-4 Regions

AbstractIn this paper, the different effects of the eastern equatorial Pacific (EP) and central equatorial Pacific (CP) Ocean El Niño–Southern Oscillation (ENSO) events on interannual variation in the diurnal sea surface temperature (SST) are explored in both the Niño-3 and Niño-4 regions. In the Niño-3 region, the diurnal SST anomaly (DSSTA) is negative during both EP and CP El Niño events and becomes positive during both EP and CP La Niña events. However, the DSSTA in the Niño-4 region is positive in El Niño years and negative in La Niña years, which is opposite to that in the Niño-3 region. Further analysis indicates that the incident shortwave radiation (SWR), wind stress (WS), and upward latent heat flux (LHF) are the main factors causing the different interannual variations in the DSST. In the Niño-3 region, decreased SWR and increased LHF lead to a negative DSSTA in EP El Niño years, and enhanced WS and increased LHF cause a negative DSSTA in CP El Niño years. Conversely, in that same region, increased SWR and decreased LHF lead to a positive DSSTA in EP La Niña years, and reduced WS and decreased LHF cause a positive DSSTA in CP La Niña years. In the Niño-4 region, the reduced trade wind plays a key role in producing the positive DSSTA, whereas the decreased SWR has an opposite effect that reduces the range of the DSSTA during both EP and CP El Niño events, and conversely the enhanced trade wind plays a key role in producing the negative DSSTA, whereas the increased SWR has an opposite effect that increases the range of the DSSTA during both EP and CP La Niña events.

Inter‐model spreading of changes in East Asian winter monsoon circulation under 1.5 and 2.0°C global warming targets

AbstractThis study investigates inter‐model spreading of changes in East Asian winter monsoon circulation and the underlying reasons under 1.5 and 2.0°C global warming for Representative Concentration Pathway 8.5. Results show that the spatial patterns of multi‐model ensemble and inter‐model spreading of changes in 850‐hPa zonal and meridional winds over East Asia are similar under the two global warming targets. The multi‐model ensemble changes in 850‐hPa meridional wind (V850) over southern China are near zero under the two targets. Meanwhile, the V850 changes over southern China exhibit large inter‐model spreading, which is closely related to those of regional differences in sea surface temperature changes (DSSTC) in the tropical Pacific. Models with positive V850 anomalies over southern China tend to project weak (strong) surface warming in the western North Pacific (South China Sea and the equatorial central and eastern Pacific). Such a configuration results in decreased precipitation over the east of the Philippines and hence an anomalous lower‐level high to its northwest. Accordingly, southwesterly anomalies prevail over southern China. The southwesterly anomalies are further enhanced by related positive feedback processes. Models with negative V850 anomalies tend to project strong (weak) surface warming in the western North Pacific (South China Sea). The projected surface warming in the equatorial central and eastern Pacific is also strong, but weaker than that of models with positive V850 anomalies. Therefore, the induced precipitation and circulation changes over the East Asia–western North Pacific are basically opposite to those described above. Besides, the inter‐model spreading of V850 changes over southern China decrease by about 36.2% from 1.5 to 2.0°C global warming, which can be partly attributed to the decrease in the inter‐model spreading of DSSTC in the tropical Pacific. Numerical experiments confirm the role of the sea surface temperature changes in the tropical Pacific.

The role of El Niño in the extreme Mei-yu rainfall in 2020

Mei-yu is a wide-spread rain band occurring in early summer over the East Asia, especially over the middle and lower Yangtze River valley (YRV). Devastating floods in the YRV were often associated with the heavy Mei-yu in the decaying phase of extremely strong El Niño events. However, the unusually heavy Mei-yu and flood in 2020 were preceded only by a weak El Niño in the central equatorial Pacific, and thus caught many forecasters by surprise. Here we try to find the cause of this peculiarity through data analysis and a set of model experiments. Our results indicate that the positive Indian Ocean basin mode (IOBM) and the sudden decay of the CP El Niño in mid-2020 significantly contributed to ascending anomaly over the Maritime Continent, which enhanced the subtropical high over the South China Sea and the convergence over the YRV. It is the zonal temperature gradient across the tropical Indo-Pacific Ocean before and during the mei-yu season, rather than the magnitude of the preceding El Niño or Indian Ocean warming, that determines the strength and duration of the Mei-yu. The results highlight the combined effects of the weak CP El Niño and positive IOBM on extreme Mei-yu event.

The potential influence of falling ice radiative effects on Central-Pacific El Niño variability under progressive global warming

The impacts of falling ice (snow) radiative effects (FIREs) on simulated surface wind stress and sea surface temperature (SST) in Central Pacific El Niño (CP-El Niño) under a progressive warming climate are examined. Using controlled simulations with the CESM1 model, it is shown that the exclusion of FIREs (no snow: NOS) generates persistent westerly anomalies in surface wind stress relative to that with FIREs (snow on: SON). These anomalies subsequently lead to a weakening of the easterly trade winds associated with warmer SST anomalies in modeled life cycle. Results over three separated 40 year intervals (P1: 21–60 years; P2: 61–100 years; P3: 101–140 years) are compared with Coupled Model Intercomparison Project phase 5 (CMIP5) models without FIREs. Both NOS configuration and CMIP5 models simulate longer life cycles of CP-El Niño events with weakening easterlies and warmer SST anomalies on the equator, persistently propagating eastward from the mature to dissipating phases. Compared to NOS, SON, on the other hand, produces a shorter CP-El Niño life cycle together with stronger easterlies and colder SSTs over the eastern to central equatorial Pacific. The magnitudes of the simulated westerlies and warm SST anomalies tend to diminish without eastward shifting following the peak of the CP-El Niño activity. There are substantial differences in CP-El Niño characteristics from P1 to P3 between NOS and SON. During P1, both SON and NOS show patterns which are consistent with their present-day counterparts. In P2 and P3, SON exhibits a prolonged CP-El Niño life cycle, while NOS develops a double-peak El Niño evolution at the mature and decaying phases. Regarding El Niño diversity and the projections, the CMIP5 models have not reached a consensus. The inclusion of the FIREs would increase the confidence in simulating El Niño future behavior.

Association between Antarctic Sea ice, Pacific SST and the Indian summer monsoon: An observational study

In this paper, results of diagnostic analysis for the period 1983–2015, demonstrate a robust relationship between Indian Summer Monsoon Rainfall (ISMR) and sea ice area (SIA) over the two sectors of Antarctica namely, Western Pacific Ocean (WPO) and Bellingshausen and Amundsen Seas (BAS). A significant direct (inverse) relationship is observed between ISMR and SIA over the WPO (BAS) sector. Further, plausible physical mechanism governing this relationship is proposed. El Niño Modoki exhibit direct (inverse) relation with SIA over BAS (WPO) sector during summer monsoon season, which has prompted investigation of SIA-El Niño Modoki-ISMR connection. The linkage between southern polar sea ice and the Indian summer monsoon has been demonstrated through the Pacific Ocean, wherein above normal SIA over the BAS (WPO) sector is associated with concurrent warm equatorial central (western) Pacific Sea Surface Temperature (SST) anomalies. Associated meridional circulations supplemented by above normal BAS (WPO) SIA extremes is accompanied by an ascending motion over the equatorial central (western) Pacific. In turn, through large-scale zonal circulation, ascending branch of circulation over the central (western) Pacific Ocean contemporaneously impacts ISMR adversely (favourably). Large-scale atmospheric circulation modulated by equatorial Pacific SST signature is suggested as a possible link between Antarctic sea ice and ISMR.

Interaction between the Tropical Atlantic and Pacific Oceans on an Interannual Time Scale

ABSTRACT Based on the fifth-generation reanalysis of the European Centre for Medium-range Weather Forecasts (ECMWF), the Hadley Centre sea surface temperature (SST) data, and the Global Precipitation Climatology Project (GPCP) precipitation data, the interaction between the tropical Atlantic and Pacific Oceans on an interannual time scale is investigated using correlation, regression, and composite analyses, and the quantitative estimates of the relative and collective contribution of the tropical North Atlantic (NTA) and equatorial Atlantic (ETA) SST anomalies to the El Niño–Southern Oscillation (ENSO) are presented. An El Niño or La Niña event can cause a same-sign SST anomaly in the NTA in the following boreal spring, accounting for 33% of the interannual variability of the NTA SST. The NTA SST anomaly may excite a same-sign SST anomaly of the ETA in boreal summer with an ETA SST anomaly of 0.44°C per 1°C NTA SST anomaly. Moreover, an SST anomaly in the NTA and ETA can cause an opposite-sign SST anomaly in the equatorial Pacific during the following winter. The NTA SST anomaly influences the central equatorial Pacific with a Niño 3.4 SST anomaly of −0.85°C per 1°C NTA SST anomaly, while the ETA SST anomaly exerts an impact on the central-eastern equatorial Pacific with a Niño 3.4 SST anomaly of −1.85°C per 1°C ETA SST anomaly based on a multiple regression analysis. The composite analyses based on Ensemble-Based Predictions of Climate Changes and Their Impacts (ENSEMBLES) hindcasts show that the ETA SST anomaly in summer acts as a primary contributor to the interaction between the tropical Atlantic and Pacific Oceans. The ETA impact can relay and amplify the NTA SST anomaly such that it affects the equatorial Pacific. The collective impact of spring NTA and summer ETA SST anomalies can explain 33% of the total variance of the winter Niño 3.4 index. This study deepens our understanding of the interannual interaction between the tropical Atlantic and Pacific Oceans and highlights the role of the tropical Atlantic Ocean, especially the ETA region, in the pan-tropical air–sea interaction.

Decadal changes of the intraseasonal oscillation during 1979–2016

The Intraseasonal Oscillation (ISO) is the cornerstone for 2–8-week subseasonal prediction. Understanding the decadal variation of the ISO is important for improving subseasonal prediction; however, there is still a gap in our knowledge of ISO dynamics. Here, we presented a method, an Empirical Orthogonal Function (EOF) analysis of 11-year-sliding ISO evolution, to objectively detect decadal variation of the ISO originated from the equatorial Indian Ocean (EIO) during 1979–2016. The results show that the properties of ISO have a notable decadal change since 1998 for both boreal summer and boreal winter seasons, mainly in its evolution rather than in its intensity at origin. During the pre-1998 epoch, the boreal summer intraseasonal oscillation (BSISO), was confined to the Indian Ocean; since 1998, however, it propagated northeastward across the Maritime Continent (MC) and the intraseasonal variability over the western North Pacific was significant enhanced. On the other hand, the boreal-winter ISO, usually known as Madden-Julian Oscillation (MJO) shows minor changes in MC ‘barrier effect’ between the two epochs, and continuously propagates eastward across the MC. The MJO only shows suppressed activity over the central equatorial Pacific in the post-1998 epoch. These decadal changes are related to the eastern Pacific cooling during the ‘global warming hiatus’ period rather than to the four-decade global warming. Results here provide a set of potential precursors for foreseeing ISO propagation under different mean states.

Three-dimensional climatology, trends, and meteorological drivers of global and regional tropospheric type-dependent aerosols: insights from 13 years (2007–2019) of CALIOP observations

Abstract. Globally gridded aerosol extinction data from the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) during 2007–2019 are utilized to investigate the three-dimensional (3D) climatological distribution of tropospheric type-dependent aerosols and to identify the trends in column aerosol optical depth (AOD), partitioned within different altitude regimes, and their meteorological drivers. Using detection samples of layer aerosols, we also yield a 3D distribution of the frequency of occurrence (FoO) of aerosol subtypes classified by CALIOP. The results show that the aerosol extinction coefficient (AEC) shows contrasting vertical distribution patterns over land and ocean, with the former possessing significant geographical dependence, while the enhancement of AEC in the latter is mainly located below 1 km. The vertical structures of the type-dependent AECs, however, are strongly dependent on altitude. When the total AOD (TAOD) is partitioned into the planetary boundary layer (PBL) and the free troposphere (FT), results demonstrate that the PBL and FT contribute 62.08 % and 37.92 %, respectively, of the global tropospheric TAOD averaged over daytime and nighttime. Yet this CALIOP-based partitioning of the different aerosol subtypes in the PBL and FT varies significantly. Among all 12 typical regions of interest analyzed, more than 50 % of TAOD is located in the lower troposphere (0–2 km), while the contribution is less than 2 % above 6 km. In global average terms, we found the aerosol FoO averaged over all layers is 4.45 %, with the largest contribution from “clean marine” (1.79 %) and the smallest from “clean continental” (0.05 %). Overall, the FoO vertical structures of the aerosol layer exhibit a distribution pattern similar to that of AEC. The resulting trend analyses show that CALIOP accurately captures significant regional anomalies in TAOD, as observed in other satellite measurements and aerosol reanalysis. Our correlation analysis between meteorological factors and TAOD suggests the interannual variability of TAOD is related to the variability of precipitation (PPT), volumetric soil moisture (VSM), and wind speed (WS) in the particular regions. For instance, the positive TAOD trend over the equatorial central Pacific is mainly attributable to the increased PPT and decreased WS. In contrast, in dry convective regions dominated by dust and smoke, the interannual variability/trend in TAOD is largely modified by the VSM driven by the PPT. Additionally, we further found that these significant regional correlations are more robust within the PBL and significantly weakened or even reversed within the FT. This highlights the superiority of using the TAOD partitioned within the PBL as a proxy variable for the widely applied TAOD to explore the relationships between atmospheric pollution and meteorology.

Impacts of heat stress on soft corals, an overlooked and highly vulnerable component of coral reef ecosystems, at a central equatorial Pacific atoll

Climate change threatens coral reefs, with recent heatwaves causing widespread coral bleaching and mortality. Soft corals (order Alcyonacea) provide reef structure and habitat, but most reef research has instead focused on reef-building hard corals (order Scleractinia). Reviewing the primary literature documenting the effects of recent (2014–2020) marine heatwaves on corals, a period including the third global coral bleaching event, we found that only 20% (19/94) of studies presented any results of heat stress effects on soft corals, and even fewer (5%) presented taxonomic-specific results, highlighting the need for further study. Using images (n = 6265) collected on 16 expeditions to the world's largest atoll (Kiritimati; central equatorial Pacific) between 2007 and 2019, we quantified soft coral cover around the atoll's shallow (10-12 m depth) forereefs before and after a prolonged heatwave (the 2015/2016 El Niño). Prior to the heatwave, soft coral accounted for 3.0% of the benthic community, with greater cover at more sheltered sites with lower net primary productivity; Lobophytum and Sinularia were the most common genera. We documented a complete loss of soft coral after the heatwave within our sites. New soft corals were detected in 2019, three years after the heatwave. Photos of individual colonies tracked over time revealed that soft coral structures can persist for several years post-mortality, and hard corals can recruit onto these structures. Our study exposes the vulnerability of soft corals to heat stress, which has received scant previous documentation. Future studies examining the effects of heatwaves on coral bleaching and mortality should include soft corals.

ENSO Dynamics in the E3SM-1-0, CESM2, and GFDL-CM4 Climate Models

AbstractThis study examines historical simulations of ENSO in the E3SM-1-0, CESM2, and GFDL-CM4 climate models, provided by three leading U.S. modeling centers as part of the Coupled Model Intercomparison Project phase 6 (CMIP6). These new models have made substantial progress in simulating ENSO’s key features, including: amplitude; timescale; spatial patterns; phase-locking; spring persistence barrier; and recharge oscillator dynamics. However, some important features of ENSO are still a challenge to simulate. In the central and eastern equatorial Pacific, the models’ weaker-than-observed subsurface zonal current anomalies and zonal temperature gradient anomalies serve to weaken the nonlinear zonal advection of subsurface temperatures, leading to insufficient warm/cold asymmetry of ENSO’s sea surface temperature anomalies (SSTA). In the western equatorial Pacific, the models’ excessive simulated zonal SST gradients amplify their zonal temperature advection, causing their SSTA to extend farther west than observed. The models underestimate both ENSO’s positive dynamic feedbacks (due to insufficient zonal wind stress responses to SSTA) and its thermodynamic damping (due to insufficient convective cloud shading of eastern Pacific SSTA during warm events); compensation between these biases leads to realistic linear growth rates for ENSO, but for somewhat unrealistic reasons. The models also exhibit stronger-than-observed feedbacks onto eastern equatorial Pacific SSTAs from thermocline depth anomalies, which accelerates the transitions between events and shortens the simulated ENSO period relative to observations. Implications for diagnosing and simulating ENSO in climate models are discussed.

Regionally Variable Responses of Nitrogen Fixation to Iron and Phosphorus Enrichment in the Pacific Ocean

AbstractIron and phosphorus are major nutrients limiting marine nitrogen fixation. We directly assessed iron and phosphate enrichment using the surface water of the western and central North Pacific subtropical gyre (NPSG), central equatorial Pacific, and eastern South Pacific subtropical gyre (SPSG). Diazotrophic phylotypes showed regionally distinct distribution patterns. The abundances of nifH in UCYN‐A, Trichodesmium, Crocosphaera, and γ‐24774A11 and filaments of Richelia were significantly higher in the central and western NPSG than in the eastern SPSG. The surface dissolved iron concentration was <0.2 nmol l−1 in most of the regions; however, nitrogen fixation was enhanced by the addition of iron only in the western NPSG and equatorial upwelling. In the western NPSG, where surface phosphate was almost exhausted, and levels varied below 7 nmol l−1, diazotrophs also exhibited either phosphate limitation or colimitation by iron and phosphate. The present study demonstrated the limitation of nitrogen fixation by iron and phosphorus individually and colimitation by iron and phosphorus in the western NPSG for the first time, and the data showed that the nutrients limiting nitrogen fixation in the Pacific Ocean differed among the regions, indicating that the response of nitrogen fixation to environmental changes is not uniform.

Increasing Coral Reef Resilience Through Successive Marine Heatwaves

AbstractOcean warming is causing declines of coral reefs globally, raising critical questions about the potential for corals to adapt. In the central equatorial Pacific, reefs persisting through recurrent El Niño heatwaves hold important clues. Using an 18‐year record of coral cover spanning three major bleaching events, we show that the impact of thermal stress on coral mortality within the Phoenix Islands Protected Area (PIPA) has lessened over time. Disproportionate survival of extreme thermal stress during the 2009–2010 and 2015–2016 heatwaves, relative to that in 2002–2003, suggests that selective mortality through successive heatwaves may help shape coral community responses to future warming. Identifying and facilitating the conditions under which coral survival and recovery can keep pace with rates of warming are essential first steps toward successful stewardship of coral reefs under 21st century climate change.

Mechanism for Southward Shift of Zonal Wind Anomalies During the Mature Phase of ENSO

AbstractThe cause of southward shift of anomalous zonal wind in the central equatorial Pacific (CEP) during ENSO mature winter was investigated through observational analyses and numerical model experiments. Based on an antisymmetric zonal momentum budget diagnosis using daily ERA-Interim data, a two-step physical mechanism is proposed. The first step involves advection of the zonal wind anomaly by the climatological mean meridional wind. The second step involves the development of an antisymmetric mode in the CEP, which promotes a positive contribution to the observed zonal wind tendency by the pressure gradient and Coriolis forces. Two positive feedbacks are responsible for the growth of the antisymmetric mode. The first involves the moisture–convection–circulation feedback, and the second involves the wind–evaporation–SST feedback. General circulation model experiments further demonstrated that the boreal winter background state is critical in generating the southward shift, and a northward shift of the zonal wind anomaly is found when the same SST anomaly is specified in boreal summer background state.

How does the onset time of El Niño events affect tropical cyclone genesis and intensity over the western North Pacific?

AbstractPrevious studies have shown that the equatorial Pacific Ocean warming can be separated into spring (April~June; SP) and summer (July~October; SU) modes based on the onset time of El Niño events. Different onset times are caused by different atmospheric and oceanic conditions, which, in turn, should influence tropical cyclone (TC) activity. While the location of TC formation in the western North Pacific (WNP) shifts significantly eastward and equatorward in the SP type relative to climatology (1951–2017), only an eastward shift occurs in the SU type. Only intense TCs (category 1 + 2 + 3 and 4 + 5) have an obvious eastward and equatorward shift in the SP type, with no such shift in any TC category in the SU type. The number of category 4 + 5 TCs in the SP type increases significantly, but fewer category 1 + 2 + 3 TCs in the SU type. In the SP type, TCs are generally stronger, with more in category 4 + 5, and longer life spans occurring across all categories. However, no obvious change in TC intensity can be identified except for category 1 + 2 + 3 in the SU type. The eastward and equatorward shift of the mean genesis position in the developing phases of SP can be attributed to a southeastward shift of favourable atmospheric and oceanic conditions, which include weaker zonal vertical wind shear, stronger relative vorticity, higher mid‐level relative humidity, and enhanced upper‐ocean temperature (5–100 m). All these conditions are mainly related to the onset time of the westerly anomalies near the central equatorial Pacific. These results suggest that the onset time of El Niño events could serve as a good forecasting indicator for TC genesis position and intensity over the WNP.