Central Pacific Sea Surface Temperature Research Articles

Equatorial Western–Central Pacific SST Responsible for the North Pacific Oscillation–ENSO Sequence

Abstract El Niño–Southern Oscillation (ENSO), the dominant mode of interannual variability in the tropical Pacific, is well known to affect the extratropical climate via atmospheric teleconnections. Extratropical atmospheric variability may in turn influence the occurrence of ENSO events. The winter North Pacific Oscillation (NPO), as the secondary dominant mode of atmospheric variability over the North Pacific, has been recognized as a potential precursor for ENSO development. This study demonstrates that the preexisting winter NPO signal is primarily excited by sea surface temperature (SST) anomalies in the equatorial western–central Pacific. During ENSO years with a preceding winter NPO signal, which accounts for approximately 60% of ENSO events observed in 1979–2021, significant SST anomalies emerge in the equatorial western–central Pacific in the preceding autumn and winter. The concurrent presence of local convection anomalies can act as a catalyst for NPO-like atmospheric circulation anomalies. In contrast, during other ENSO years, significant SST anomalies are not observed in the equatorial western–central Pacific during the preceding winter, and correspondingly, the NPO signal is absent. Ensemble simulations using an atmospheric general circulation model driven by observed SST anomalies in the tropical western–central Pacific can well reproduce the interannual variability of observed NPO. Therefore, an alternative explanation for the observed NPO–ENSO relationship is that the preceding winter NPO is a companion to ENSO development, driven by the precursory SST signal in the equatorial western–central Pacific. Our results suggest that the lagged relationship between ENSO and the NPO involves a tropical–extratropical two-way coupling rather than a purely stochastic forcing of the extratropical atmosphere on ENSO.

Enhanced North Pacific Victoria mode in a warming climate

The Pacific Decadal Oscillation (PDO), renowned as the dominant sea surface temperature (SST) fluctuation in the North Pacific and extensively scrutinized for its extensive influence on global climate patterns, stands in stark contrast to the Victoria mode (VM). Traditionally, the VM, representing the second most prominent SST pattern in the North Pacific, has not garnered comparable attention. However, our investigation unveils a remarkable surge in the low-frequency VM variability, spanning periods greater than 8 years, over the course of a century. Astonishingly, this enhanced VM variability now surpasses the PDO’s variability in recent decades, signifying a notable shift. Consequently, the heightened VM variability assumes newfound significance in shaping climate systems across the entire North Pacific region and in distant locales. This intensified VM behavior could be attributed to amplified atmospheric variability in the Hawaiian region, primarily stemming from the reinforced variability in the tropical central Pacific (CP) SST in recent decades. As greenhouse warming escalates CP SST variability, the VM’s enhanced variability may further intensify, yielding broader and more profound repercussions in the future.

Importance of realistic zonal currents in depicting the evolution of tropical central Pacific sea surface temperature

Classical El Niño–Southern Oscillation (ENSO) theories mainly consider the vertical process-related Ekman and thermocline feedbacks. However, the zonal current-related zonal advective feedback has been suggested to play a crucial role in the evolution of central Pacific (CP) El Niño and La Niña events. Also, the simulation of a realistic current is complex and not the focus of the classical ENSO theories. Using reanalysis datasets and a statistical model, this study emphasizes the importance of the zonal currents in the sea surface temperature anomaly (SSTA) evolution in the Niño4 region (160° E–150° W, 5° S–5° N). Specifically, in addition to the widely used predictors for the ENSO evolution, i.e. the equatorial Pacific mean thermocline depth anomaly (D20) and the zonal wind stress anomaly (ZWS), the zonal current anomaly (ZCA) averaged in the CP is first extracted to construct a statistical model to predict the SSTA of the Niño4 region. The results show that this model has improved overall prediction skill and accuracy for several CP El Niño and La Niña events during 1980–2020, compared with the benchmark linear regression model based on D20 and ZWS. By further removing the components related to the equatorial Kelvin and first symmetric meridional () Rossby waves (namely, the principal part of the traditional ENSO mechanism) from the ZCA, the remainder, which contains higher-order Rossby waves and other nonlinear components and is called the zonal current anomaly residual (ZCA_RSD), is found to be the key part of the improvements in the prediction skill. This suggests that to better simulate and predict the complex ENSO events, more vertical and meridional modes of the tropical Pacific need to be included to obtain a realistic anomalous zonal current.

Comprehensive Representation of Tropical–Extratropical Teleconnections Obstructed by Tropical Pacific Convection Biases in CMIP6

Abstract The central role of tropical sea surface temperature (SST) variability in modulating Northern Hemisphere (NH) extratropical climate has long been known. However, the prevailing pathways of teleconnections in observations and the ability of climate models to replicate these observed linkages remain elusive. Here, we apply maximum covariance analysis between atmospheric circulation and tropical SST to reveal two coexisting tropical–extratropical teleconnections albeit with distinctive spatiotemporal characteristics. The first mode, resembling the Pacific–North American (PNA) pattern, favors a tropical–Arctic in-phase (warm Pacific–warm Arctic) teleconnection in boreal spring and winter. However, the second mode, with a slight seasonal preference of summer, is manifested as an elongated Rossby wave train emanating from the tropical eastern Pacific that features an out-of-phase relationship (cold Pacific–warm Arctic) between tropical central Pacific SSTs and temperature variability over the Arctic (referred to as the PARC mode). While climate models participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6) appear to successfully simulate the PNA mode and its temporal characteristics, the majority of models’ skill in reproducing the PARC mode is obstructed to some extent by biases in simulating low-frequency SST and rainfall variability over the tropical eastern Pacific and the climatological mean flow over the North Pacific during boreal summer. Considering the contribution of the PARC mode in shaping low-frequency climate variations over the past 42 years from the tropics to the Arctic, improving models’ capability to capture the PARC mode is essential to reduce uncertainties associated with decadal prediction and climate change projection over the NH. Significance Statement This study focuses on the skill of models in phase 6 of the Coupled Model Intercomparison Project (CMIP6) in simulating two leading observed Northern Hemisphere (NH) teleconnections that show distinctive spatial and temporal characteristics. The first one, the Pacific–North American (PNA) mode, exhibits a warm Pacific–warm Arctic pattern in boreal spring and winter, and the second one, the Pacific–Arctic (PARC) mode, features a cold Pacific–warm Arctic out-of-phase relationship. We find that models are skillful in simulating the PNA mode but not the PARC mode. This limitation may be rooted in unrealistic simulations of the mean state of winds and the low-frequency sea surface temperature variability in the tropical eastern Pacific. These biases call for caution when interpreting current models’ projections of extratropical circulations on multidecadal time scales.

A Zonal See‐Saw Variation of Tropical Cyclogenesis Over the Arabian Sea and Bay of Bengal‐South China Sea

AbstractThe variation of tropical cyclone (TC) genesis over the North Indian Ocean (NIO) and South China Sea (SCS) during the post‐monsoon season is explored during 1979–2020. A zonal see‐saw variation of TC genesis is detected between the Arabian Sea (AS) and Bay of Bengal (BOB) –SCS (BOBSCS for brevity). More (less) TCs over the AS tend to concur with fewer (more) TCs over the BOBSCS. Analyses of both the observations and the Coupled Model Intercomparison Project Phase 6 outputs show that tripole sea surface temperature (SST) anomalies in the tropical Indian and Pacific Oceans play a crucial role in the zonal see‐saw variation of TC genesis. Positive western Indian (WI) Ocean and tropical central and eastern Pacific (CEP) Ocean SST anomalies and negative western Pacific (WP) SST anomalies induce anomalous ascending and descending motions and high and low humidity transportation to the middle troposphere over the AS and BOBSCS, respectively. Meantime, anticyclonic vorticity anomalies appear in the BOBSCS region and negative anomalies of vertical wind shear are induced over the NIO and SCS. The combined positive contributions of upward motion, mid‐level humidity increase, and low vertical wind shear favor higher TC genesis over the AS, whereas the joint negative effects of downward motion, mid‐level water vapor decrease, and lower‐level anticyclonic vorticity anomalies result in less TC genesis over the BOBSCS. Numerical experiments show a critical role of the WI SST anomalies in the dipole pattern of vertical motion, which is crucial for the zonal dipole variation of TC genesis.

ENSO skewness hysteresis and associated changes in strong El Niño under a CO2 removal scenario

El Niño-Southern Oscillation (ENSO) sea surface temperature (SST) anomaly skewness encapsulates the nonlinear processes of strong ENSO events and affects future climate projections. Yet, its response to CO2 forcing remains not well understood. Here, we find ENSO skewness hysteresis in a large ensemble CO2 removal simulation. The positive SST skewness in the central-to-eastern tropical Pacific gradually weakens (most pronounced near the dateline) in response to increasing CO2, but weakens even further once CO2 is ramped down. Further analyses reveal that hysteresis of the Intertropical Convergence Zone migration leads to more active and farther eastward-located strong eastern Pacific El Niño events, thus decreasing central Pacific ENSO skewness by reducing the amplitude of the central Pacific positive SST anomalies and increasing the scaling effect of the eastern Pacific skewness denominator, i.e., ENSO intensity, respectively. The reduction of eastern Pacific El Niño maximum intensity, which is constrained by the SST zonal gradient of the projected background El Niño-like warming pattern, also contributes to a reduction of eastern Pacific SST skewness around the CO2 peak phase. This study highlights the divergent responses of different strong El Niño regimes in response to climate change.

Combined effect of the solar activity and ENSO on the tropical cyclone genesis frequency in the southeastern part of the western North Pacific

This study analyzes the frequency characteristics of tropical cyclone (TC) genesis in the southeastern part of the western North Pacific (SEWNP) during June–November from 1965 to 2019 and investigates the possible combined effect of the solar activity and El Niño-Southern Oscillation (ENSO). Results suggest that TCs generated in the SEWNP have the longest lifetime and greatest strength, and its frequency has apparent interannual and decadal variations, which is jointly affected by the solar activity and ENSO. In El Niño years during declining phases of solar cycle (1–3 years following the solar maximum), positive TC genesis frequency anomalies in the SEWNP are significantly strong and tend to occur in extremes. While the opposite is true for La Niña during ascending phases of solar cycle (1–3 years following the solar minimum). However, there exists no significant feature in the combined effect of La Niña (El Niño) and declining (ascending) phases of solar cycle. When declining (ascending) phases of solar cycle and El Niño (La Niña) are combined, the overlapping effect leads to apparently warmer (colder) sea surface temperature (SST) anomalies in the central equatorial Pacific and colder (warmer) SST anomalies in the western Pacific, so the SST anomalies gradient are stronger. It enhances low-level westerly (easterly) wind anomalies and upper-level easterly (westerly) wind anomalies, which is favorable for the further decrease (increase) of the vertical wind shear in the eastern (most) part of SEWNP. Moreover, the stronger and more westward low-level convergence (divergence) center appears in the Pacific, causing stronger low-level convergent (divergent) flow and upper-level divergent (convergent) flow anomalies, and strengthened (suppressed) ascending movement anomalies in the SEWNP. As a result, the TC genesis frequency in the SEWNP is much higher (lower). Further diagnoses show that absolute vorticity plays a leading role in El Niño years during declining phases of solar cycle, and its contribution is obviously much greater than other environmental factors. It is not perfectly symmetrical in La Niña years during ascending phases of solar cycle when the contribution of absolute vorticity is the greatest, but vertical wind shear is also important.

Remote tropical central Pacific influence on driving sea surface temperature variability in the Northeast Pacific

The Northeast Pacific (NEP) had two record-breaking marine heatwave events in the winters of 2013–2015 and summer of 2019, which had a detrimental impact on the fisheries, marine ecosystems, and climate in North America. Here, we investigated the cause of sea surface temperature (SST) variability in NEP during late spring–summer of 1981–2020. The regression circulation anomalies to the principal component of leading empirical orthogonal function mode suggested that the warm NEP SST were characterized by a cyclonic circulation anomaly in the midlatitude North Pacific and a warming SST center in the Gulf of Alaska. We noted that this cyclonic circulation anomaly, attributable to a barotropic atmospheric wave originating from the tropical central Pacific (CP) in the preceding spring, reduced the surface heat flux loss from the ocean to the atmosphere in the NEP and led to the warm SST anomalies in summer. This finding was confirmed by not only empirical diagnosis but also long-term numerical simulations forced by the observed SST perturbations in the tropical CP. Our results highlight the role of the tropical CP SST in driving the summertime North Pacific SST variability through the atmospheric bridge in recent decades.

Spatiotemporal variability of soil moisture over Ethiopia and its teleconnections with remote and local drivers

Soil moisture is one of the essential climate variables with a potential impact on local climate variability. Despite the importance of soil moisture, studies on soil moisture characteristics in Ethiopia are less documented. In this study, the spatiotemporal variability of Ethiopian soil moisture (SM) has been characterized, and its local and remote influential driving factors are investigated. An empirical orthogonal function (EOF) and KMeans clustering algorithm have been employed to classify the large domain into homogeneous zones. Complex maximum covariance analysis (CMCA) is applied to evaluate the covariability between SM and selected local and remote variables such as rainfall (RF), evapotranspiration (ET), and sea surface temperature (SST). Inter-comparison among SM datasets highlight that the FLDAS dataset better depicts the country’s SM spatial and temporal distribution (i.e., a correlation coefficient r=0.95\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$r=0.95$$\\end{document}, rmsd=0.04m3m-3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$rmsd=0.04{\\mathrm{m}}^{3}{\\mathrm{m}}^{-3}$$\\end{document} with observations). Results also indicate that regions located in northeastern Ethiopia are drier irrespective of the season (JJAS, MAM, and OND) considered. In contrast, the western part of the country consistently depicted a wetter condition in all seasons. During summer (JJAS), the soil moisture variability is characterized by a strong east–west spatial contrast. The highest and lowest soil moisture values were observed across the country’s central western and eastern parts, respectively. Furthermore, analyses indicate that interannual variability of SM is dictated substantially by RF, though the impact on some regions is weaker. It is also found that ET likely drives the SM in the eastern part of Ethiopia due to a higher atmospheric moisture demand that ultimately invokes changes in surface humidity and rainfall. A composite analysis based on the extreme five wettest and driest SM years revealed a similar spatial distribution of wet SM with positive anomalies of RF across the country and ET over the southern regions. Remote SSTs are also found to have a significant influence on SM distribution. In particular, equatorial central Pacific and western Indian oceans SST anomalies are predominant factors for spatiotemporal SM variations over the country. Major global oceanic indices: Oceanic Nino Index (ONI), Indian Ocean Dipole (IOD), Pacific warm pool (PACWARMPOOL), and Pacific Decadal Oscillations (PDO) are found to be closely associated with the SM anomalies in various parts of the country. The associationship between these remote SST anomalies and local soil moisture is via large-scale atmospheric circulations that are linked to regional factors such as precipitation and temperature anomalies.

Forecasting tropical ENSO-induced drought conditions using sea surface height in the Western Pacific

The interannual variability of rainfall caused by the El Niño-Southern Oscillation (ENSO) results in significant changes in hydrologic conditions. Forecasting ENSO and its impacts are mainly based on Central Pacific Sea surface temperature (SST) anomalies which satisfactorily correlate with timing and, to a lesser extent, the intensity of drought conditions in the Philippines and the Western Pacific during the El Niño phase. Changes in sea surface height (SSH) are also brought upon by ENSO through seawater density changes with temperature and oceanographic processes. Here, we report that the associative nature of SSH and drought, as measured by surface runoff, has a better correlation (r > 0.693, p < 0.05) in terms of the expected timing (1 to 3 month lag) and intensity compared to using SST indices. Furthermore, since SSH is co-located with its corresponding forecasted decrease in runoff, a localised prediction can be made which further increases the accuracy of this predictive tool and can be used in tandem with SST-based ENSO indices. In the wake of a changing climate, the ability to forecast the timing and volume of rainfall and surface water availability is of utmost importance, especially within the context of food and water security.

Recent interdecadal changes in the Tropospheric Biennial Oscillation of the East Asian summer monsoon

This paper focuses on the interdecadal changes in the biennial East Asian summer monsoon (EASM) transition related to the Tropospheric Biennial Oscillation (TBO), which has major impacts on East Asian climate. During 1958–2020, the TBO signal in the EASM exhibits noticeable interdecadal changes around the late 1970s and late 1990s. Among the three periods [1958–1979 (P1), 1980–2000 (P2), 2001–2020 (P3)], the TBO signal is the strongest in P2 and the weakest in P1. Such changes are closely associated with changes in the amplitude and position of wind anomalies over the western North Pacific (WNP). Further analysis indicates that the regime shifts in ENSO properties, including intensity, period, and location of ENSO-related sea surface temperature (SST) anomalies, play a crucial role. Compared with P1, the warm SST anomalies in the central Pacific and North Indian Ocean SST associated with the slowly decaying ENSO together induce the stronger WNP wind anomalies in both P2 and P3. However, such SST warming is relatively weaker in P3 compared with those in P2, which is consistent with the weak intensity and short period of ENSO in P3. In comparison, the anomalous circulation over the WNP is thus relatively weaker in P3 than in P2. Therefore, these anomalous SSTs associated with the shift in the ENSO regime are suggested to induce circulation anomalies over the WNP that subsequently contributed to the changes in the EASM TBO around the late 1970s and late 1990s.

The Role of Seasonality and the ENSO Mode in Central and East Pacific ENSO Growth and Evolution

Abstract A cyclostationary linear inverse model (CSLIM) is used to investigate the seasonal growth of tropical Pacific Ocean El Niño–Southern Oscillation (ENSO) events with canonical, central Pacific (CP), or eastern Pacific (EP) sea surface temperature (SST) characteristics. Analysis shows that all types of ENSO events experience maximum growth toward final states occurring in November and December. ENSO events with EP characteristics also experience growth into May and June, but CP events do not. A single dominant “ENSO mode,” growing from an equatorial heat content anomaly into a characteristic ENSO-type SST pattern in about 9 months (consistent with the delayed/recharge oscillator model of ENSO), is essential for the predictable development of all ENSO events. Notably, its seasonality is responsible for the late-calendar-year maximum in ENSO amplification. However, this ENSO mode alone does not capture the observed growth and evolution of diverse ENSO events, which additionally involve the seasonal evolution of other nonorthogonal Floquet modes. EP event growth occurs when the ENSO mode is initially “covered up” in combination with other Floquet modes. The ENSO mode’s slow seasonal evolution allows it to emerge while the other modes rapidly evolve and/or decay, leading to strongly amplifying and more predictable EP events. CP events develop when the initial state has a substantial contribution from Floquet modes with meridional mode–like SST structures. Thus, while nearly all ENSO events involve the seasonally varying ENSO-mode dynamics, the diversity and predictability of ENSO events cannot be understood without identifying contributions from the remaining Floquet modes. Significance Statement The purpose of this study is to identify structures that lead to seasonal growth of diverse types of El Niño–Southern Oscillation (ENSO) events. An important contribution from this study is that it uses an observationally constrained, empirically derived seasonal model. We find that processes affecting the evolution of diverse ENSO events are strongly seasonally dependent. ENSO events with eastern equatorial Pacific sea surface temperature (SST) characteristics are closely related to a single “ENSO mode” that resembles theoretical models of ENSO variability. ENSO events that have central equatorial Pacific SST characteristics include contributions from additional “meridional mode” structures that evolve via different physical processes. These findings are an important step in evaluating the seasonal predictability of ENSO diversity.

The Influence of Interannual and Decadal Indo-Pacific Sea Surface Temperature Variability on Australian Monsoon Rainfall

Abstract Monsoonal rainfall in northern Australia (AUMR) varies substantially on interannual, decadal, and longer time scales, profoundly impacting natural systems and agricultural communities. Some of this variability arises in response to sea surface temperature (SST) variability in the Indo-Pacific linked to both El Niño–Southern Oscillation (ENSO) and the interdecadal Pacific oscillation (IPO). Here we use observations to investigate unresolved issues regarding the influence of the IPO and ENSO on AUMR. Specifically, we show that during negative IPO phases, central Pacific (CP) El Niño events are associated with below-average rainfall over northeast Australia, an anomalous anticyclonic pattern to the northwest of Australia, and eastward moisture advection toward the date line. In contrast, CP La Niña events (distinct from eastern Pacific La Niña events) during negative IPO phases drive significantly wet conditions over much of northern Australia, a strengthened Walker circulation, and large-scale moisture flux convergence. During positive IPO phases, the impact of CP El Niño and CP La Niña events on AUMR is weaker. The influence of central Pacific SSTs on AUMR has been stronger during the recent (post-1999) negative IPO phase. The extent to which this strengthening is associated with climate change or merely natural internal variability is not known.

Characterizing the highest tropical cyclone frequency in the Western North Pacific since 1984

The 2018 boreal summer in the Western North Pacific (WNP) is highlighted by 17 tropical cyclones (TC)—the highest record during the reported reliable years of TC observations. We contribute to the existing knowledge pool on this extreme TC frequency record by showing that the simultaneous highest recorded intensity of the WNP summer monsoon prompted the eastward extension of the monsoon trough and enhancement of tropical convective activities, which are both favorable for TC development. Such changes in the WNP summer monsoon environment led to the extreme TC frequency record during the 2018 boreal summer. Meanwhile, the highest record in TC frequency and the intensity of the WNP summer monsoon are both attributed with the combined increase in the anomalous westerlies originating from the cold tropical Indian Ocean sea surface temperature (SST) anomalies drawn towards the convective heat source that is associated with the warm central Pacific SST anomalies. Our results provide additional insights in characterizing above normal tropical cyclone and summer monsoon activities in the WNP in understanding seasonal predictable horizons in the WNP, and in support of disaster risk and impact reduction.

Role of equatorial central Pacific sea surface temperature in modulating rainfall over north India during Indian summer monsoon

AbstractThe summer monsoon season contributes about 80% of annual rainfall in the highly populated region of north India. The teleconnections moderating the variation of summer monsoon rainfall in this region are not satisfactorily understood. A pathway of the equatorial central Pacific sea surface temperature (PSST) influences the north India summer rainfall is revealed from the high‐resolution, more reliable, and state‐of‐the‐art 41‐year (1979–2019) observational data analysis and numerical experiment. The rise in PSST intensifies convection over the equatorial central Pacific with compensatory subsidence over the western equatorial Pacific. Consequently, a much broader and intense anomalous convergence is developed at the upper‐troposphere, which subsequently intensifies the Asian subtropical westerly jet‐stream. The Asian jet intensification strengthens the tropospheric wind shear at north India, as the low‐level monsoonal winds are easterlies over there. The strong wind shear restricts the convective activities in north India. These results are also confirmed using the CFSv2 model sensitivity experiment and a case study of the year 2010.

The asymmetric influence of ocean heat content on ENSO predictability in the CNRM-CM5 coupled general circulation model

AbstractUnusually high western Pacific oceanic heat content often leads to El Niño about 1 year later, while unusually low heat content leads to La Niña. Here, we investigate if El Niño Southern Oscillation (ENSO) predictability also depends on the initial state recharge, and discuss the underlying mechanisms. To that end, we use the CNRM-CM5 model, which has a reasonable representation of the main observed ENSO characteristics, asymmetries and feedbacks. Observations and a 1007-years long CNRM-CM5 simulation indicate that discharged states evolve more systematically into La Niña events than recharged states into neutral states or El Niño events. We ran 70-members ensemble experiments in a perfect-model setting, initialized in boreal fall from either recharged or discharged western Pacific heat content, sampling the full range of corresponding ENSO phases. Predictability measures based both on spread and signal-to-noise ratio confirm that discharged states yield a more predictable ENSO outcome one year later than recharged states. As expected from recharge oscillator theory, recharged states evolve into positive central Pacific sea surface temperature anomalies in boreal spring, inducing stronger and more variable Westerly Wind Event activity and a fast growth of the ensemble spread during summer and fall. This also enhances the positive wind stress feedback in fall, but the effect is offset by changes in thermocline and heat flux feedbacks. The state-dependent component of westerly wind events is thus the most likely cause for the predictability asymmetry in CNRM-CM5, although changes in the low-frequency wind stress feedback may also contribute.

What Modulates the Intensity of Synoptic-Scale Variability over the Western North Pacific during Boreal Summer and Fall?

AbstractThe present study investigates the factors that affect the year-to-year change in the intensity of synoptic-scale variability (SSV) over the tropical western North Pacific (TWNP) during boreal summer and fall. It is found that the intensity of the TWNP SSV in summer is associated with the equatorial central-eastern Pacific sea surface temperature (SST) anomalies that modulate the background fields through a Rossby wave response both in the source region and along the propagation path of the synoptic-scale disturbances. In fall, the intensity of the TWNP SSV is related to an SST anomaly pattern with opposite anomalies in the equatorial central Pacific and TWNP that modulates the background fields from the equatorial central Pacific to TWNP. However, the equatorial central Pacific SST anomalies alone fail to change the intensity of the TWNP SSV as the induced background field changes are limited to the equatorial central Pacific. It is shown that tropical western Pacific SST anomalies may induce notable changes in the intensity of the TWNP SSV. The relation of the TWNP SSV to the equatorial eastern Pacific SST is weak due to opposite SST anomalies in different types of years. Both seasonal mean and intraseasonal flows provide sources of barotropic energy for the change in the intensity of the TWNP synoptic-scale disturbances in summer. Seasonal mean flow has a main contribution to the barotropic energy conversion for the change in the intensity of the TWNP synoptic-scale disturbances in fall.

Large‐Scale Reductions in Terrestrial Carbon Uptake Following Central Pacific El Niño

AbstractThe El Niño–Southern Oscillation (ENSO) affects global climate and ecosystems, but a recent shift toward more frequent central Pacific (CP) El Niño events could alter these relationships. Here, we show strong responses of the terrestrial carbon cycle to CP ENSO, exceeding even those to canonical eastern Pacific (EP) ENSO. Annual GPP of both global tropical forests and semiarid ecosystems were reduced by ∼0.3–0.5 Pg C yr−1 K−1 increase in CP sea surface temperatures (SSTs), which also reduced net ecosystem production of key tropical and semiarid regions like the Amazon and Australia, but with smaller (and generally not significant) responses to EP SSTs. Given these large negative responses of ecosystem production to CP SSTs, our results suggest that a recent shift toward CP‐dominated ENSO events could further alter Earth's terrestrial carbon cycle, especially when coupled with possible increases in ENSO amplitude with continued warming.

Assessment of CMIP6 models' skill for tropical Indian Ocean sea surface temperature variability

AbstractThe present study examines the ability of Coupled Model Inter‐comparison Project phase 6 (CMIP6) models in representing the dominant modes of tropical Indian Ocean (TIO) sea surface temperature (SST) variability on the interannual and decadal time scale. Historical simulations from 27 CMIP6 models are assessed against Extended Reconstructed SST over the period of 1854 to 2014. Spectrum analysis reveals that many models reproduce interannual and decadal variability of TIO SST but underestimate the amplitude of variability with some disparity in the periodicity. All models can reproduce the dominant basin‐wide mode of interannual and decadal variability of TIO SST reasonably well. Skill score analysis of TIO SST variability reveals that KACE‐1‐0‐G has highest skill, followed by FGOALS‐f3‐L, EC‐Earth3‐Veg‐LR, ACCESS‐ESM1‐5, CanESM5‐CanOE on the interannual timescale and FGOALS‐f3‐L, CanESM5‐CanOE, KACE‐1‐0‐G and CanESM5, respectively, showed highest skills for decadal variability. It is found that variations in radiation and latent heat flux are primarily responsible for interannual variability in TIO SST, the basin‐wide warming, in the observations. Taylor diagram analysis reveals that all the models exhibit better skill for the radiative flux; however, skill for the latent heat and momentum flux varies from model to model. It is important to note that the models in which the latent heat flux and zonal wind are better represented have produced better TIO SST variability compared to other models. A higher discrepancy in latent heat and zonal momentum flux leads to improper wind‐evaporation‐SST and wind‐circulation‐SST feedback, which in turn restricts the model skill. Besides, model that has realistic central and eastern Pacific SST variability show better skill for TIO SST variability in both interannual and decadal time scales. The present study advocates that better representation of latent heat flux and zonal wind in coupled models is important for the accurate simulation of interannual and decadal variability in TIO SST.

Mechanisms of the decadal variability of monsoon rainfall in the southern Tibetan Plateau

The Tibetan Plateau (TP), as a whole, has undergone a moistening process since the late 1990s. However, the southern Tibetan Plateau (STP) is an exception, where summer monsoon precipitation amount has decreased, and lakes have shrunk. The cause for the precipitation decrease is not clear yet. Here we show that the monsoon (June to September) mean precipitation changes in the STP from 1979 to 2018 features a decadal variation component with a peak of around 10 years that is superposed on an upward ‘trend’ from 1979 to 1998 and a downward ‘trend’ afterward. We find that the decadal variation of the STP precipitation is associated with a large-scale dipolar sea surface temperature (SST) pattern between the equatorial central Pacific and the Indo-Pacific warm pool. A wet STP corresponds to negative SST anomaly in the equatorial central Pacific and positive SST anomaly in the Indo-Pacific warm pool. This equatorial SST gradient in the western Pacific generates pronounced easterly anomalies and a dipolar rainfall anomaly (i.e. a positive rainfall anomaly over the Maritime Continent and a negative anomaly in the equatorial western and central Pacific). Due to less precipitation over the equatorial western Pacific, the suppressed heat source appears to excite an anomalous anticyclonic band along 15–20° N extending from the Philippine Sea to the Bay of Bengal by emanating westward propagating descending transient Rossby waves. The low-level anticyclonic circulation over the Bay of Bengal further enhances northward moisture transport toward the STP and promote upward motion in the STP through changing local meridional circulation. Besides, the linearized atmospheric general circulation model experiments demonstrate that the dipole heating source can generate a high-pressure zone under the control of anticyclone over the western Pacific, which can extend westward to the Indian monsoon region.