Central Equatorial Pacific Ocean Research Articles

The importance of the soluble and colloidal pools for trace metal cycling in deep-sea pore waters

Physical and chemical trace metal speciation are important for our understanding of metal cycling and potential toxicity to marine life. Trace metals can behave differently in diffusion processes or particle-solution interactions and have different bioavailabilities depending on their physical and chemical forms, which often depend on redox conditions. Here we investigated dissolved (< 0.2 µm) and soluble (< 0.02 µm) concentrations of Mn, Co, Ni, Fe, Cu, V, Mo, U, Cd, and As in oxic and suboxic deep-sea sediments of the central equatorial Pacific Ocean. Vanadium, Mo, U, As, and Cd showed no significant concentration differences between their dissolved and soluble forms, suggesting that they are present as inorganic ionic species or organic complexes in the truly dissolved or small colloidal fraction. In contrast, the colloidal fraction (> 0.02 µm < 0.2 µm) of Mn, Co, Ni, and Cu increased with depth in oxic pore waters and Fe had the largest but variable colloidal pool. Soluble Mn, Co, and Ni were released in the uppermost 2-4 cm in the sediment because of reductive dissolution. The increasing colloidal fraction with depth suggests a decrease in the concentration of small organic ligands with depth, that are abundant in the surface sediment pore waters, and instead an increasing importance of larger (> 0.02 µm) inorganic nanoparticles and colloids such as Mn and Fe (oxyhydr)oxides that control Mn, Fe, and Co cycling at depths > 10 cm. The distribution of Ni and Cu cannot be exclusively explained by inorganic nanoparticles and a shift from low to larger high molecular weight organic ligands might occur. These findings provide new insights into trace metal distributions in the dissolved phase, highlighting the diversity of metal complexes and the need to incorporate these in future calculations of benthic metal fluxes and ecotoxicity assessments, especially in oxic pore waters.

Quantifying the Strength of the El Niño Southern Oscillation and the Indian Ocean Dipole in Influencing the OND Rainfall Season in Tanzania

The current paper examines the strength of variability between the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) in influencing October to December (OND) rainfall over the bimodal rainfall regime of Tanzania. The empirical orthogonal function (EOF), correlation analysis, and composite analysis were used during the data analysis. The results show more rainfall distribution over the western part of the Lake Victoria basin and to the peripheral of the northern coast of the country, thus suggesting that, during the OND rainfall season, the onset starts in the western part of the Lake Victoria basin, then spreads to the rest of the areas under investigation as the season progresses. Furthermore, on the spatial scale, the findings revealed that there is a strong correlation between IOD and ENSO indices and OND rains in the northeastern highlands. Furthermore, a robust temporal correlation is revealed between the mean OND rains over the bimodal rainfall areas and IOD (r = 0.70) compared to ENSO (r = 0.62). The anomalous warming over the western Indian Ocean (positive IOD) has a faster response to OND rains over the bimodal rainfall regime of Tanzania compared to the remote influence induced by anomalous warming from the central equatorial Pacific Ocean (warm phase of ENSO). Meanwhile, dry years are associated with negative IOD and the cold phase of ENSO conditions. The findings offer valuable insights on strategies for mitigating the effects associated with extreme weather events and improving resilience in Tanzania.

Numerical model generation of test frames for pre-launch studies of EarthCARE's retrieval algorithms and data management system

Abstract. The Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) satellite consists of active and passive sensors whose observations will be acted on by an array of retrieval algorithms. EarthCARE's retrieval algorithms have undergone pre-launch verifications within a virtual observing system that consists of 3D atmosphere–surface data produced by the Global Environmental Multiscale (GEM) numerical weather prediction (NWP) model, as well as instrument simulators that when applied to NWP data yield synthetic observations for EarthCARE's four sensors. Retrieval algorithms operate on the synthetic observations, and their estimates go into radiative transfer models that produce top-of-atmosphere solar and thermal broadband radiative quantities, which are compared to synthetic broadband measurements, thus mimicking EarthCARE's radiative closure assessment. Three high-resolution test frames were simulated; each measures ∼6200 km along-track by 200 km across-track. Horizontal grid spacing is 250 m, and there are 57 atmospheric layers up to 10 mbar. The frames span wide ranges of conditions and extend over (i) Greenland to the Caribbean, crossing a cold front off Nova Scotia; (ii) Nunavut to Baja California, crossing over Colorado's Rocky Mountains; and (iii) the central equatorial Pacific Ocean, which includes a mesoscale convective system. This report discusses how the test frames were produced and presents their key geophysical features. All data are publicly available and, owing to their high-resolution, could be used to simulate observations for other measurement systems.

Prediction of La Niña Events Based on BP Neural Network Model

As an extreme weather phenomenon, La Niña event will cause low water temperature in the equatorial eastern Pacific Ocean and abnormal longitudinal circulation in East Asia, resulting in widespread and continuous abnormal cooling of sea surface temperature in the eastern and central equatorial Pacific Ocean, which will cause frequent precipitation and flooding in the north and high temperature and low rainfall and frequent droughts in the south[1]. Therefore, it is particularly important to assess the damage and propose response strategies[2]. For the main countries involved in the global triple La Niña event, this paper selects four representative countries located in different regions to analyze, through descriptive analysis and graphical visualization, and selects the relevant index data of the last three years: monthly average maximum and minimum temperatures, monthly average precipitation rate, monthly average precipitation days, monthly average wind speed and monthly average water temperature, and finds that New York City and Brazil were affected by the triple La Niña event at the same time and were affected to a higher degree, and the temperature changes in Malasila and Jakara were stable, and the temperature effects brought by the triple La Niña event were smaller[3]. Then, a BP neural network model was established to predict the probability of future La Niña events in each region by modeling pressure, temperature, and salinity data, and it was found that a total of 110 regions would experience La Niña events in the future, with a general probability of 55%-60%..

Zonal Temperature Gradients in the Tropical Free Troposphere

Abstract The horizontal temperature gradients in the tropical free troposphere are generally assumed to be weak. We show with ERA5 data that substantial zonal virtual temperature (Tυ) gradients persist climatologically in the tropical free troposphere and investigate their causes. The gradients change seasonally: Tυ at 500 hPa over the equatorial western Pacific Ocean (EWP) is usually much warmer (up to 3 K) than that over the equatorial central Pacific Ocean (ECP) during December–February (DJF), while the temperature differences between EWP and ECP are much smaller during June–August (JJA). During DJF, Tυ gradients over the Pacific prevail throughout the entire free troposphere, especially in the upper troposphere near 300 hPa. We find that the associated hydrostatic pressure gradients are mainly balanced by the nonlinear terms in the momentum equation, in particular via zonal wind advection. Strong zonal winds occur near the equator in boreal winter, transporting zonal momentum so as to balance the pressure gradient force. The zonal winds are due to large-scale equatorial waves, excited by a heating pattern that is relatively symmetric about the equator. In boreal summer, the large-scale equatorial waves are less active in the Pacific region due to a more asymmetric temperature pattern, so the zonal momentum advection and Tυ gradients are both much weaker. The results point to an important role of the nonlinear terms in the tropical balanced dynamics, stressing the need for an improved theoretical understanding and modeling framework of the tropical atmosphere that includes these nonlinear terms, or their net effect.

The effects of redistributed heat flux on ocean climate change in FAFMIP heat flux anomaly experiments

The Flux–Anomaly-Forced Model Inter-comparison Project (FAFMIP) aims to study the response of the ocean to changes in prescribed surface flux (heat, freshwater, and wind) caused by atmospheric CO2 increase. Among these surface flux perturbations, the heat flux has the greatest effect on ocean heat uptake (OHU), the Atlantic Meridional Circulation (AMOC), and sea level changes. This occurs in part because of the redistributed heat flux (Qr’), which is the component of the heat flux driven by ocean circulation changes and almost nearly doubles the initial heat flux perturbation (F) prescribed by FAFMIP in the North Atlantic (NA). In this study, we used a standalone ocean model to quantitatively investigate the impact of Qr’ on OHU and the AMOC in prescribed heat anomaly experiments (faf-heat experiment in FAFMIP). We also showed that whether the other two FAFMIP heat flux anomaly experiments (faf-heat-NA50pct and faf-heat-NA0pct) can reasonably simulate the ocean’s response to prescribed surface heat flux, especially OHU and AMOC. Our findings show that faf-heat-NA50pct is better because the redistribution-feedback (Qr’) nearly equals the initial perturbation (50%F) in the NA and thus recovers the total perturbation F in faf-heat. Although direct comparisons cannot be drawn, both faf-heat-NA50pct and faf-heat experiments with no effect of Qr’ cause AMOC weakening that is roughly comparable with the 1pctCO 2 experiments. The magnitude of simulated AMOC weakening in faf-heat-NA50pct is 3.9Sv, accounting for about 29% of the AMOC intensity in unperturbed experiment, which is closer to the faf-heat experiment (about 31%) with no effect of Qr’ relative to the faf-heat experiment. Because of negative Qr’ in all regions except the NA region especially for the Central and Eastern Equatorial Pacific and Southern Ocean, the simulated global OHU in faf-heat-NA50pct is reduced by 10% as compared to the added OHU in the faf-heat experiment. The Qr’ in the faf-heat experiment increases OHU by approximately 13% of the total OHU and further reduces AMOC intensity by 3.4 Sv, accounting for 26% of the AMOC strength in an unperturbed experiment. Faf-heat-NA0pct results in a significant reduction in OHU (approximately 26%), and the magnitude of AMOC weakening is only approximately 1.9 Sv, despite no F being added in the NA.

Reduction of Equatorial Obduction by Atmospheric Intraseasonal Oscillations in the Western and Central Pacific Ocean

AbstractObduction is the large‐scale, irreversible upward water transport from the permanent pycnocline to the surface mixed layer. Using Hybrid Coordinate Ocean Model (HYCOM), this study explores the impact of atmospheric intraseasonal oscillations (ISOs) on the Pacific equatorial obduction that plays a fundamental role in regulating the tropical Pacific climate. Parallel HYCOM experiments forced by atmospheric fields with and without ISO variability are compared to assess the effect of ISOs on the ocean. The results suggest that the ISOs can reduce the annual obduction rate of the western and central equatorial Pacific Ocean (WCEPO; 150°E−160°W, 5°S–5°N) by ∼12%, and their impact on the eastern Pacific is rather limited. The ISOs cause prominent intraseasonal variability of mixed layer depth (MLD) in the WCEPO primarily through surface wind forcing, which in turn affects the two key processes controlling the annual obduction rate. First, the intraseasonal MLD deepening events narrow down the total time windows for obduction by ∼30% and thereby give rise to the decrease in obduction rate; second, it leads to high‐frequency entrainment events and enhances the entrainment rate by ∼18%. The increased entrainment rate is overwhelmed by the reduced time windows for effective entrainment, and the net effect of ISOs is to reduce the obduction rate and modify its seasonal cycle. This work highlights the importance of atmospheric ISOs in the equatorial ocean dynamics, with implications for tropical surface temperature and climate variabilities and biogeochemical cycle.

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.

Boreal Summer Negative Correlation Relationship Between Interannual SST and Precipitation Anomalies in the Tropical and Subtropical Western North Pacific

AbstractWestern North Pacific (WNP) is a key region for ENSO development and its effect on East Asian summer monsoon. Different from most other tropical oceanic areas, sea surface temperature (SST) and precipitation anomalies over the WNP are negatively correlated in boreal summer, indicating the ocean is mainly driven by the atmosphere rather than the opposite over there. Two interannual variability modes of such negative correlation relationship are revealed with SVD method, called tropical and subtropical WNP air‐sea coupled modes (WNP‐TCM and WNP‐STCM) in terms of their locations accordingly. Their core regions are both consistent with the near‐zero line of low‐level zonal wind, corresponding to the Northwest Pacific monsoon trough and the west ridge of WNP subtropical high, respectively. In WNP‐TCM, precipitation anomaly is caused by local vorticity anomaly, which is the result of Matsuno‐Gill response to SST anomalies with the reversed signs in central equatorial Pacific and eastern equatorial Indian Ocean. In WNP‐STCM, it is produced by local divergence anomaly, which is remotely forced by SST anomalies with the same signs in equatorial Pacific and Indian Ocean through anomalous Hadley circulation. SST anomalies in the two modes are both controlled by ocean thermal process, especially the solar radiation anomaly. The key factors determining such unique WNP air‐sea interaction are the weak latent heat flux and horizontal SST advection anomalies, which are both closely related to the near‐zero low‐level zonal wind. In addition, WNP‐TCM often happens in ENSO developing summer whereas WNP‐STCM usually appears in ENSO decaying summer.

Prediction Challenges From Errors in Tropical Pacific Sea Surface Temperature Trends

Models in the North American Multi-Model Ensemble (NMME) predict sea surface temperature (SST) trends in the central and eastern equatorial Pacific Ocean which are more positive than those observed over the period 1982–2020. These trend errors are accompanied by linear trends in the squared error of SST forecasts whose sign is determined by the mean model bias (cold equatorial bias is linked to negative trends in squared error and vice versa). The reason for this behavior is that the overly positive trend reduces the bias of models that are too cold and increases the bias of models that are too warm. The excessive positive SST trends in the models are also linked with overly positive trends in tropical precipitation anomalies. Larger (smaller) SST trend errors are associated with lower (higher) skill in predicting precipitation anomalies over the central Pacific Ocean. Errors in the linear SST trend do not explain a large percentage of variability in precipitation anomaly errors, but do account for large errors in amplitude. The predictions toward a too warm and wet tropical Pacific, especially since 2000, are strongly correlated with an increase in El Niño false alarms. These results may be relevant for interpreting the behavior of uninitialized CMIP5/6 models, which project SST trends that resemble the NMME trend errors.

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.

Whether the CMIP5 Models Can Reproduce the Long-Range Correlation of Daily Precipitation?

In this study, we investigated the performance of nine CMIP5 models for global daily precipitation by comparing with NCEP data from 1960 to 2005 based on the detrended fluctuation analysis (DFA) method. We found that NCEP daily precipitation exhibits long-range correlation (LRC) characteristics in most regions of the world. The LRC of daily precipitation over the central of North American continent is the strongest in summer, while the LRC of precipitation is the weakest for the equatorial central Pacific Ocean. The zonal average scaling exponents of NCEP daily precipitation are smaller in middle and high latitudes than those in the tropics. The scaling exponents are above 0.9 over the tropical middle and east Pacific Ocean for the year and four seasons. Most CMIP5 models can capture the characteristic that zonal mean scaling exponents of daily precipitation reach the peak in the tropics, and then decrease rapidly with the latitude increasing. The zonal mean scaling exponents simulated by CMCC-CMS, GFDL-ESM2G and IPSL-CM5A-MR show consistencies with those of NCEP, while BCC_CSM1.1(m) and FGOALS-g2 cannot capture the seasonal variations of daily precipitation’s LRC. The biases of scaling exponents between CMIP5 models and NCEP are smaller in the high latitudes, and even less than the absolute value of 0.05 in some regions, including Arctic Ocean, Siberian, Southern Ocean and Antarctic. However, for Western Africa, Eastern Africa, Tropical Eastern Pacific and Northern South America, the simulated biases of scaling exponents are greater than the absolute value of 0.05 for the year and all four seasons. In general, the spatial biases of LRC simulated by GFDL-ESM2G, HadGEM2-AO and INM-CM4 are relatively small, which indicating that the LRC characteristics of daily precipitation are well simulated by these models.

Prediction of dominant daily modes of the Indian summer monsoon in the NCEP GFS model

The prediction capability of dominant daily monsoon modes of Indian summer monsoon in the forecast of the Global Forecast System Version 2 (GFSv2) model is scrutinized. The dominant monsoon modes are procured by performing the multichannel singular spectrum analysis (MSSA) on daily precipitation anomalies of the Indian summer monsoon region (60–100°E, Eq.-35°N) during 2001–2014. The observation has one seasonally persistent mode and two intraseasonal oscillations with periods around 42 and 26 days, and the model has correctly simulated these modes. The spatial structure of the phase composites of the precipitation anomalies of the intraseasonal modes of the model is almost similar to the observed spatial pattern with slightly less magnitude of the precipitation anomalies over Western India and the core monsoon zone. The spatial structure of the 26-day mode is similar to the spatial structure of the 42-day mode with less magnitude of the precipitation anomalies all over the study domain. The lead forecast of the model demonstrates the robust predictive skill of intraseasonal modes. The variation of the active and break spells of monsoon precipitation over the Indian subcontinent is captured accurately by the contribution of both intraseasonal modes. The observed eastward and northward propagation features of the Indian summer monsoon have been accurately simulated by the model. The model has weak seasonally persistent signals over Western India, northeast India, and eastern land regions adjacent to the Western Ghats. The seasonally persistent mode shows a strong relationship with the equatorial central Pacific Ocean SSTs and a moderate correlation with the Indian and Atlantic Ocean SSTs. The seasonally persistent mode contributes largely to the seasonal precipitation anomalies over the Indian monsoon region.

Changes in ENSO-monsoon relations from early to recent decades during onset, peak and withdrawal phases of Indian summer monsoon

The El Nino Southern Oscillation (ENSO) is an important coupled ocean–atmosphere phenomenon in the tropical Pacific and an important modulator of the spatio-temporal variability of Indian summer monsoon rainfall (ISMR). Here, we explore the impact of ENSO during onset (June), peak (July–August) and withdrawal (September) phases of ISMR for the period 1951–2015, by studying the changes in the ENSO-monsoon relationship from early decades (1951–1980) to recent decades (1986–2015). We observe noticeable changes in the ENSO-monsoon relationship from the early to recent decades during all the three phases. During El Nino events, rainfall over most of the Indian regions is significantly increased in recent decades during onset phase, but a significant decrease in rainfall is observed during peak and withdrawal phases. On the other hand, the rainfall during La Nina events is significantly decreased over the monsoon core zone of India during all the three phases. A significant increase (decrease) in sea surface temperature (SST) is observed over the central equatorial Pacific and Indian Ocean during El Nino (La Nina) events in recent decades. These changes differ from one phase to another and related to the observed rainfall patterns over the Indian region. Apart from these, the changes in the anomalies of low level circulation and Walker circulation over the Indo-Pacific domain are also linked to the changes in rainfall during the ENSO events, with most significant relationship during the onset phase.

The Tropical Easterly Jet over Africa, its representation in six reanalysis products, and its association with Sahel rainfall

AbstractThis paper compares the characteristics of the Tropical Easterly Jet (TEJ) and upper‐level winds in six reanalysis products, compares them with soundings at seven West African locations, examines the relationship between Sahel rainfall and the TEJ, and examines factors influencing the TEJ. The jet characteristics assessed by MERRA2, NCEP 1, JRA 55, and ERA 5 are similar. CFSR and 20th Century Reanalysis are outliers in nearly every analysis, overestimating wind speeds by as much as 25 to 40% compared to other reanalyses. Over the period 1948 to 2014, the correlation between rainfall and TEJ magnitude is .72. Arguments based on observations and modelling studies provide evidence that on interannual scales changes in the TEJ are not forced by rainfall, that large‐scale factors drive the TEJ. Potential mechanisms are discussed for a causal relationship such that a strong jet leads to high rainfall. However, further modelling efforts are needed to conclusively determine whether the TEJ/Sahel rainfall link is a result of common forcing factors. The factors that appear to control jet strength include sea‐surface temperature (SST) contrast between the central equatorial Pacific and central equatorial Indian Ocean (correlation of −.64), SST contrast between the central equatorial and the southern subtropical Indian Ocean (correlation of −.39), the latitude of the shift between upper‐tropospheric easterlies and westerlies in the Southern Hemisphere (correlation of −.84 at 150 hPa), and the intensity of the Southern Hemisphere westerlies (correlation of +.52 at 200 hPa). This suggests considerable control on the TEJ by extra‐tropical circulation in the Southern Hemisphere.

Analysis of summer monsoon rainfall over Maharashtra and its relation with SSTs

This paper is based on the spatio-temporal analysis of daily, monthly and seasonal rainfall variability over Maharashtra state of the Indian region, for the period 1901–2017 during summer monsoon, viz., June through September. This analysis is carried out over Maharashtra’s four sub-divisions, where mean rainfall received for JJAS (1901–2017) is more over Konkan and Goa and east Vidarbha and less over Madhya Maharashtra. Coefficient of variation is considered to understand spatial variability of total rainfall received during JJAS by the state. The percentage of rainy days is found to be more for July and August months compared to June and September months as per the advancement of monsoonal flow noticeable through this study. Hence, contribution to JJAS rainfall more from July month proceeded with August month is considerable. September month contribution is the least; nevertheless its contribution is prominent from south Madhya Maharashtra. Rainfall variability is linked with El Nino and La Nina, by calculating composites of standardized rainfall anomalies and % rainy days during El Nino and La Nina years, which show that monthly and seasonal rainfall, are modulated by variations in SSTs of equatorial central Pacific Ocean. Thus this study aims to analyze the changing pattern of rainfall over Maharashtra and its sub-division as linking El-Nino and La-Nina events with rainfall received over small domain is the need of the hour as each region behaves differently during El-Nino or La-Nina.

The Geomorphology of Submarine Channel Systems of the Northern Line Islands Ridge, Central Equatorial Pacific Ocean

More than 844,000 km² of the northern Line Islands Ridge mapped with multibeam bathymetry and backscatter provide unprecedented views of the geomorphology of this isolated area in the central equatorial Pacific Ocean. A compilation of all available multibeam data in the area reveals six extensive submarine dendritic channel systems that encompass a combined drainage area that exceeds 60,000 km². The channel systems occur in a predominately carbonate environment and are the longest calciclastic submarine channel systems mapped in the oceans to date. The channel systems occur in a carbonate-dominated region well above the carbonate compensation depth and have developed into the surface of basins that are surrounded by small guyots and seamounts that make up a discontinuous rim around the summit of the northern Line Island Ridge. The channels have mostly straight or gently curved well-developed tributaries and main reaches. Although the Line Island Ridge has been dated at 86 to 68 Ma old, the channels occur on the surface and are not buried by any significant sediment accumulations. Levees are very rare along the channel banks and no bathymetric expression of submarine fans was found where the channels exit onto the adjacent abyssal basins. There is sparse evidence of landslide deposits throughout the ridge although the flanks of the guyots exhibit numerous headwall scarps. The presence of plunge pools below the northwest escarpment, together with well-defined channels meters to hundreds of meters deep relative to the surrounding seafloor, suggests the channels might be relatively recent (perhaps late Neogene or even younger) features developed long after the ridge subsided more than a kilometer below sea level.

Interannual and Interdecadal Variabilities of Spring Rainfall over Northeast China and Their Associated Sea Surface Temperature Anomaly Forcings

AbstractAn empirical orthogonal function (EOF) analysis was conducted for spring precipitation gauge data over northeast China (NEC). The first EOF mode is characterized by a homogenous rainfall pattern throughout NEC. The corresponding principal component has both significant interannual and interdecadal variations. This leading mode explains a large portion of the total NEC spring rainfall (NECSR) variances and is statistically independent from other higher modes. The physical processes responsible for the interannual and interdecadal variabilities were investigated via observational diagnoses and numerical experiments. On the interannual time scale, NECSR is mainly affected by the SST anomalies (SSTAs) in the northern tropical Atlantic Ocean. When the SSTAs are positive, the subsequently induced positive precipitation and convection can stimulate two quasi-barotropic Rossby wave trains over the mid- to high latitudes. A cyclonic anomaly center of the Rossby wave train appears over northeastern Asia, leading to a positive rainfall anomaly in the region. On the interdecadal time scale, NECSR is mainly influenced by the SSTAs over the warm-pool region. Positive SSTAs in the warm-pool region result in enhanced convection (ascending motion) around the Maritime Continent and suppressed convection (descending motion) over the central equatorial Pacific Ocean. This zonal dipole convection pattern stimulates a quasi-barotropic circulation pattern with an anticyclonic anomaly over the Tibetan Plateau and a cyclonic anomaly over northeastern Asia. The cyclonic anomaly over northeastern Asia enhances the NECSR. Numerical experiments further suggested that the convective heating anomaly over the Maritime Continent, rather than cooling over the central equatorial Pacific, plays a more essential role in driving the interdecadal rainfall variability of NECSR.

Assessment of the Evolution and Socio-Economic Impacts of Extreme Rainfall Events in October 2019 over the East Africa

This study aimed at establishing and quantifying the evolution and socio-economic impacts of extreme rainfall events in October 2019. The study also focused on ascertaining the extent to which the Indian Ocean Dipole (IOD) and the El Ni?o Southern Oscillation (ENSO) influenced anomalous rainfall over East Africa (EA) in October 2019. It employed Singular Value Decomposition (SVD) methods to analyze inter-annual variability of EA rainfall and the Sea Surface Temperature Anomalies (SSTA) over the Indian and Pacific Ocean with a focus on October to December 2019 rainfall season. The SVD analysis enabled the exploration of the leading modes from the mean monthly rainfall and SSTs leading to the determination of the likely influence of the IOD and ENSO respectively. The first SVD coupled modes, which dominate the co-variability between the October rainfall over the EA domain, and SSTA over the Indian and Pacific Oceans based on 1981 to 2010 climatology indicate the monopole positive co-variability with rainfall over the entire EA domain. The corresponding spatial pattern for the SSTA over the Indian Ocean (IO) recaptures the positive IOD event while the central equatorial Pacific Ocean (i.e., over Ni?o 3.4 region) reveals a monopole positive loading, a typical signal for the warm phase of ENSO. The positive rainfall anomaly over the EA during October is found to be associated with either the IOD event or ENSO condition events independently or in phase. However, the inter-annual variability between October rainfall over EA and ENSO reveals a moderate relationship (r = 0.4212) while a robust association (r = 0.7084) is revealed with IOD. Comparatively, the October 2019 rainfall anomaly peaks the highest in history over the EA and was found to be coupled with highest positive IOD event in record. Unlikely, the 1997 October rainfall (which peaked the second in history), was associated with the co-occurrence of the positive phase of ENSO and IOD events. The findings of this study suggest that the positive IOD coupled mode had large impact on the distribution and variability of the October 2019 rainfall over the EA region.

Indonesian throughflow controlled the westward extent of the Indo-Pacific Warm Pool during glacial-interglacial intervals

Abstract The tropical atmospheric circulation is strongly influenced by the sea surface temperature (SST) of the warmest region of the world ocean, the Indo-Pacific Warm Pool (IPWP). The effect of recent anomalous Indian Ocean warming on the extent of the IPWP remains uncertain. Past SST records from the tropical Indian Ocean, associated with different temporal boundary conditions, can help shed light on how climate change impacts the IPWP and associated monsoonal processes. Here, we reconstruct the seawater temperature and precipitation changes (from Mg/Ca and δ18O of surface-dwelling planktic foraminifera Globigerinoides ruber) of the last ~184 kyr BP from the westernmost margin of the IPWP. The data is then used to reconstruct the spatial extent and intensity of the IPWP structure as well as its relationship with other regional phenomena. A consistently warmer (>28 °C) central equatorial Pacific Ocean and significantly cooler equatorial western Pacific and the Indian Ocean suggests that the warm pool extent reduced considerably during MIS6 as well as the last glacial interval, leading to a strong La-Nina like condition. The most enriched seawater δ18O (δ18Osw) during MIS6, was in the westernmost IPWP, suggesting weakened precipitation, despite La-Lina like conditions. The westernmost part of the IPWP became warmer than the threshold 28 °C during MIS5e and the early Holocene, suggesting the westward extension of the IPWP. The eastern equatorial Pacific Ocean also warmed considerably during MIS5e, suggesting a weaker Walker Circulation and a stronger El-Nino. The difference in δ18Osw decreased considerably during MIS5e and is attributed to increased precipitation/freshwater runoff in the central equatorial Indian Ocean. The increase in δ18Osw (~1.5‰) between ~55 kyr until the last glacial maximum, in a majority of the Indian Ocean records, suggests progressively weakened precipitation. A majority of the records have a uniform SST anomaly throughout the Holocene. A warmer equatorial Indian Ocean during interglacials suggests a strong modulation of the IPWP extent by the transport of warm water through the Indonesian throughflow region. The large drop in δ18Osw despite the comparable change in SST during the last glacial interval suggests that monsoon forcing has been much stronger in altering the surface hydrography in the Indian Ocean than the central Pacific Ocean.