Background Mean State Research Articles

Moist Baroclinic Instability of Mid–High-Latitude Intraseasonal Oscillations

Abstract The origin of mid–high-latitude intraseasonal oscillations (ISOs) was investigated through both observational and theoretical studies. It was found that maximum intraseasonal variability centers appear near 60°N, which is at odds with maximum synoptic variability centers that are located along the upper-tropospheric jet stream (∼45°N). The ISO along 60°N is characterized by a typical zonal wavelength of 7700 km and a westward phase speed of −3 m s−1. A marked feature of the mid–high-latitude ISO is a tight coupling among moisture and precipitation and circulation. Motivated by this observational discovery, a moist baroclinic theoretical model was constructed. The analysis of this model indicates that under a realistic background mean state, the model generates the most unstable mode along the reference latitude (60°N), which has a preferred zonal wavelength of 7000 km, a westward phase speed of about −3 m s−1, a westward tilted vertical structure, and a zonal structure of perturbation moisture/precipitation being located to the east of the low-level trough, all of which resemble the observed. The cause of the instability arises primarily from the moisture–precipitation–circulation feedback under a moderate background vertical shear.

Anthropogenic influence on the occurrence of extreme drought like that in eastern China in 2019

Eastern China (EC) experienced extreme drought during August–November 2019, with notable features of a particularly prolonged duration and record-breaking intensity, leading to huge agricultural and economic losses. In the context of anthropogenic climate change, the authors investigated the anthropogenic impacts on the occurrence of extreme drought like that in 2019 (2019Drought-like event) and the related physical mechanism. The authors first selected relatively better-performing models by evaluating their historical simulations, and found that the occurrence probability of a 2019Drought-like event increased by eight times due to anthropogenic forcing. It seems that anthropogenic forcing induces background mean state changes over EC, including a decline in mean moisture content and enhanced mean descending motion. These two mean state changes are favorable for the formation of extreme drought. Diagnosis of the moisture budget showed that the decline in mean moisture mainly stems from the enhanced mean descending motion via the term −δω¯·∂q¯∂p (vertical advection of mean moisture by the change in mean vertical motion). The enhanced descent is linked to the anthropogenic forcing–induced mean sea surface temperature warming pattern, which exhibits a maximum near the equatorial region. This reinforces the mean ascending motion over the Maritime Continent, as in “the rich-get-richer” mechanism, and further causes strengthened subsidence over the EC region through a “regional Hadley circulation”. This study suggests that anthropogenic forcing can sharply increase the occurrence probability of a 2019Drought-like event by modulating large-scale circulations.摘要2019年夏秋季中国东部遭遇了七十年一遇的强持续性极端干旱事件. 本文借助于CMIP6检测归因模式比较计划 (DAMIP) 数据, 量化了人为强迫对类似2019年极端干旱事件发生概率的影响. 通过比较不同外强迫因子作用下此类极端干旱事件的发生概率, 发现人为强迫可使此类极端干旱事件的发生概率提高8倍左右. 进一步研究发现人为强迫引起的海表温度增暖是经向不均匀的, 对应海洋大陆附近存在一个增暖大值中心, 从而增强海洋大陆地区平均上升环流, 通过局地Hadley环流作用导致中国东部地区平均垂直环流出现异常下沉, 最终有利于中国东部极端干旱事件发生概率增加. 该研究表明, 人为强迫通过调制气候平均背景场从而引起极端事件发生频次的变化是人类活动影响极端气候事件的重要途径之一.

Responses of Tropical Background State and ENSO Behaviors to Mid-Holocene Forcing Simulated by PMIP3 and PMIP4 Models

The mid-Holocene (MH), a period about 6,000 years ago, provides an opportunity to understand climate change in response to orbital forcing change. Numerical model simulation is an effective tool through which we can study the climate change in the MH, although the climate in the MH can be partly inferred from proxy data. As the Paleoclimate Model Inter-comparison Project phase 4 (PMIP4) recently released the latest simulations for different past climate scenarios, we investigated tropical climate changes, including both the basic state and interannual variability, and tried to find out whether the PMIP phase 3 (PMIP3) and PMIP4 results can be reconciled. Almost all the modelling results show that the inter-hemisphere contrast was enhanced over the tropical Pacific, with warmer and wetter condition on the northern side of the equator and an intensified cross-equatorial flow in the MH than at present, and the annual cycle of the sea-surface temperature (SST) during the MH was reduced. Such background mean state changes arose from the seasonal changes of the solar incident radiation in the MH. In addition to the consistent changes in background mean state changes, some El Niño-Southern Oscillation (ENSO) features, such as the seasonal phase locking feature and periodicity, show consistent changes across the PMIP3 and PMIP4 models, that is, both suites of models exhibit no marked difference in the MH with respect to present-day simulations. In contrast, the modeling results show only agreement on the sign of the ENSO amplitude change (i.e., decrease in the ENSO amplitude in the MH), while the range of reduction varied with model and region. Additionally, the occurrence probability of central Pacific El Niño events increases in the MH, whereas the significance is quite marginal. The modeled changes in the mean state and ENSO serve as a test bed for studying tropical climate system’s response to natural warming, which may provide some insights into understanding climate changes in response to the current anthropogenic warming.

Role of the climatological intertropical convergence zone in the seasonal footprinting mechanism of the El Niño-Southern Oscillation

AbstractThe North Pacific Oscillation (NPO), a primary atmospheric mode over the North Pacific in boreal winter, is known to trigger the El Niño-Southern Oscillation (ENSO) in the following winter, the process of which is recognized as the seasonal footprinting mechanism (SFM). Based on the analysis of model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5), we found that the SFM acts differently among models, and the correlation between the NPO and subsequent ENSO events, called the SFM efficiency, depends on the background mean state of the model. That is, SFM efficiency becomes stronger as the climatological position of the Pacific Intertropical Convergence Zone (ITCZ) moves poleward, representing an intensification of the northern branch of the ITCZ. When the Pacific ITCZ is located poleward, the wind-evaporation-sea surface temperature (SST) feedback becomes stronger as the precipitation response to the SST anomaly is stronger in higher latitudes compared to that of lower latitudes. In addition, such active ocean-atmosphere interactions enhance NPO variability, favoring the SFM to operate efficiently and trigger an ENSO event. Consistent with the model results, the observed SFM efficiency increased during the decades in which the northern branch of the climatological ITCZ was intensified, supporting the importance of the tropical mean state of precipitation around the Pacific ITCZ.

Changes in the relationship between ENSO and the East Asian winter monsoon under global warming

Changes in the relationship between El Niño-Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM) at various global warming levels during the 21st century are examined using the Max Planck Institute Grand Ensemble Representative Concentration Pathway 8.5 experiments. The externally forced component of this relationship (i.e. forced by greenhouse gases and anthropogenic aerosols emissions) strengthens from present-day to +1.5 °C, and then weakens until +3 °C. These changes are characterized by variations in strength and location of the core of El Niño-related warming and associated deep convection anomalies over the equatorial Pacific leading to circulation anomalies across the Asian-Pacific region. Under global warming, the ENSO–EAWM relationship is strongly related to the background mean state of both the EAWM and ENSO, through changes in the EAWM strength and the shift of the ENSO pattern. Anthropogenic aerosols play a key role in influencing the ENSO–EAWM relationship under moderate warming (up to 1.5 °C).

A New Ensemble‐Based Approach to Correct the Systematic Ocean Temperature Bias of CAS‐ESM‐C to Improve Its Simulation and Data Assimilation Abilities

AbstractOver the past several decades, many efforts have been devoted to increasing the simulation performance of climate models, but significant biases remain that hinder the performance of coupled systems. Hence, bias correction is regarded not only as a useful tool for improving climate simulations but also as an important step before data assimilation, which depends on the hypothesis of unbiasedness. In this study, using sea temperature climatological data, a new ensemble‐based approach is proposed for correcting the biases of the sea temperature in CAS‐ESM‐C. Through analyzing the results of the proposed bias correction method with various intensities and time windows, its performance in suppressing the simulation biases of ocean fields is evaluated. The simulation biases of atmospheric variables are also reduced via air‐sea interactions, which will improve the ocean simulation performance. Additional benefits can be realized by applying the bias correction method. For example, a superior simulation of climate variabilities in a coupled model, such as ENSO (El Niño‐Southern Oscillation), is realized due to the improvement of climatological fields. The ability to assimilate various ocean observations is also significantly improved with a better background mean state.

The Dependence of Internal Multidecadal Variability in the Southern Ocean on the Ocean Background Mean State

AbstractPrevious studies have shown the existence of internal multidecadal variability in the Southern Ocean using multiple climate models. This variability, associated with deep ocean convection, can have significant climate impacts. In this work, we use sensitivity studies based on Geophysical Fluid Dynamics Laboratory (GFDL) models to investigate the linkage of this internal variability with the background ocean mean state. We find that mean ocean stratification in the subpolar region that is dominated by mean salinity influences whether this variability occurs, as well as its time scale. The weakening of background stratification favors the occurrence of deep convection. For background stratification states in which the low-frequency variability occurs, weaker ocean stratification corresponds to shorter periods of variability and vice versa. The amplitude of convection variability is largely determined by the amount of heat that can accumulate in the subsurface ocean during periods of the oscillation without deep convection. A larger accumulation of heat in the subsurface reservoir corresponds to a larger amplitude of variability. The subsurface heat buildup is a balance between advection that supplies heat to the reservoir and vertical mixing/convection that depletes it. Subsurface heat accumulation can be intensified both by an enhanced horizontal temperature advection by the Weddell Gyre and by an enhanced ocean stratification leading to reduced vertical mixing and surface heat loss. The paleoclimate records over Antarctica indicate that this multidecadal variability has very likely happened in past climates and that the period of this variability may shift with different climate background mean state.

Exploring the Factors Influencing the Strength and Variability of Convectively Coupled Mixed Rossby–Gravity Waves

AbstractThe three-dimensional structure, horizontal and vertical propagation characteristics, and convection–circulation coupling of the convectively coupled westward-propagating mixed Rossby–gravity (MRG) waves are examined by classifying the waves based on their amplitude. Convective signals of the MRG waves were identified and isolated using empirical orthogonal function analysis of wavenumber–frequency-filtered outgoing longwave radiation (OLR) data. It was found that about 50% of the MRG waves occur during the August–November months, and this strong seasonality was considered while characterizing the MRG waves. Five strong and five weak MRG wave seasons were identified during 1979–2019, based on seasonal wave amplitude, and through this classification, significant differences in the strength of convection–circulation coupling, zonal scale of circulation, vertical structure, and propagation characteristics of MRG waves were brought out. It was also found that the seasonal mean background state is significantly different during strong and weak MRG wave seasons. While a La Niña–like background state was found to favor enhanced MRG wave activity, the MRG wave activity is mostly suppressed during an El Niño–like background state. The presence of extratropical wave intrusions is another factor that distinguishes the strong MRG wave seasons from the weak ones. Eastward- and northeastward-propagating extratropical wave trains from the South Atlantic to the east Indian Ocean were observed during strong MRG wave seasons.

Local Ensemble Transform Kalman Filter with Cross Validation

Abstract Many ensemble data assimilation (DA) approaches suffer from the so-called inbreeding problem. As a consequence, there is an excessive reduction in ensemble spread by the DA procedure, causing the analysis ensemble spread to systematically underestimate the uncertainty of the ensemble mean analysis. The stochastic EnKF used for operational NWP in Canada largely avoids this problem by applying cross validation, that is, using an independent subset of ensemble members for updating each member. The goal of the present study is to evaluate two new variations of the local ensemble transform Kalman filter (LETKF) that also incorporate cross validation. In idealized numerical experiments with Gaussian-distributed background ensembles, the two new LETKF approaches are shown to produce reliable analysis ensembles such that the ensemble spread closely matches the uncertainty of the ensemble mean, without any ensemble inflation. In ensemble DA experiments with highly nonlinear idealized forecast models, the deterministic version of the LETKF with cross validation quickly diverges, but the stochastic version produces better results, nearly identical to the stochastic EnKF with cross validation. In the context of a regional NWP system, ensemble DA experiments are performed with the two new LETKF-based approaches with cross validation, the standard LETKF, and the stochastic EnKF. All approaches with cross validation produce similar ensemble spread at the first analysis time, though the amplitude of the changes to the individual members is larger with the stochastic approaches. Over the 10-day period of the experiments, the fit of the ensemble mean background state to radiosonde observations is statistically indistinguishable for all approaches evaluated.

Two Aspects of Decadal ENSO Variability Modulating the Long‐Term Global Carbon Cycle

AbstractThe El Niño–Southern Oscillation (ENSO) drives variations in terrestrial carbon fluxes by affecting the terrestrial ecosystem via atmospheric teleconnections and thus plays an important role in interannual variability of the global carbon cycle. However, we lack such knowledge on decadal time scales, that is, how the carbon cycle can be affected by decadal variations of ENSO characteristics. Here we examine how, and by how much, decadal ENSO variability affects decadal variability of the global carbon cycle by analyzing a 1,801‐year preindustrial control simulation. We identify two different aspects, together explaining ~36% of the decadal variations in the global carbon cycle. First, climate variations induced by decadal ENSO‐like variability regulate terrestrial carbon flux and hence atmospheric CO2 on decadal time scales. Second, decadal changes in the asymmetrical response of the terrestrial ecosystem, resulting from decadal modulation of ENSO amplitude and asymmetry, rectify the background mean state, thereby generating decadal variability of land carbon fluxes.

Impacts of North Pacific Subtropical and Subarctic Oceanic Frontal Zones on the Wintertime Atmospheric Large-Scale Circulations

AbstractAn oceanic frontal zone is a confluent region of warm and cool ocean currents, characterized by a strong meridional gradient of sea surface temperature (SST). High-resolution SST observations show that the wintertime North Pacific exhibits a unique double-oceanic-front structure, with a subtropical frontal zone (STFZ) and a subarctic frontal zone (SAFZ), whose impacts on the weather and climate over the East Asia–North Pacific–North American region need further investigation. In this study, we conduct groups of multiyear and ensemble simulations using a WRF high-resolution regional climate model, through which the different impacts of the STFZ and SAFZ on the wintertime atmospheric circulations are identified and compared. Our multiyear simulations show that the STFZ, although with weaker intensity, exerts evident and consistent impacts on the storm track and westerly jet in the North Pacific by enhancing and elongating the eddy activity, zonal wind, and Aleutian low. The SAFZ exhibits coherent impacts on the low-level atmospheric baroclinicity and storm track; however, its impacts on the upper-level storm track and atmospheric circulations are divergent, exhibiting strong year-by-year difference. Our study suggests that the SAFZ’s impacts on the atmospheric circulations strongly depend on the background mean state, which contributes to the divergent impacts of the SAFZ. Furthermore, our results highlight the role of diabatic heating for the above different impacts of the STFZ and SAFZ on the atmosphere. We argue that the much deeper diabatic heating induced by the STFZ, via affecting the baroclinicity through the whole troposphere, can exert consistent influence on eddy activities and atmospheric circulations.

A Modeling- and Process-Oriented Study to Investigate the Projected Change of ENSO-Forced Wintertime Teleconnectivity in a Warmer World

Abstract Considerable uncertainties remain about the expected changes of ENSO and associated teleconnectivity as the climate is warming. Two ensembles of pacemaker experiments using the CNRM-CM5 coupled model are designed in a perfect model framework to contrast ENSO-forced teleconnectivity between the preindustrial period versus a warmer background state (obtained from a long stabilized simulation under late-twenty-first-century RCP8.5 constant forcing). The most notable sensitivity to the mean background state is found over the North Atlantic, where the ENSO–NAO teleconnection is considerably reinforced in a warmer world. We attribute this change to (i) a stronger and eastward-extended mean upper-level jet over the North Pacific, (ii) an eastward-shifted ENSO teleconnection over the North Pacific, and (iii) an equatorward-shifted and reinforced mean jet over the North Atlantic. These altogether act as a more efficient waveguide, leading to a better penetration of synoptic storms coming from the Pacific into the Atlantic. This downstream penetration into the North Atlantic basin forces more systematically the NAO through wave breaking. The reinforcement in the teleconnection is asymmetrical with respect to the ENSO phase and is mainly sensitive to La Niña events. Even though the Pacific jet tends to retract westward and move northward during cold events, mean changes are such that both Pacific and Atlantic jets remain connected in a warmer climate by contrast to the preindustrial period, thus ensuring preferred anticyclonic wave breaking downstream over the North Atlantic leading ultimately to NAO+ events.

Dynamical connection between the stratospheric Arctic vortex and sea surface temperatures in the North Atlantic

We used two long-term reanalysis datasets and time-slice simulations to examine the decadal relationship between the stratospheric Arctic vortex (SAV) and sea surface temperature anomalies (SSTAs) in the North Atlantic and the dynamic mechanisms involved in the linkage between the two. Our results show that there is a significant decadal linkage between SSTAs over the North Atlantic and the SAV, where warmed (cooled) SSTAs over the North Atlantic in association with its principal mode correspond to a weakened (strengthened) SAV. The warmed North Atlantic SSTAs tend to result in a weakened SAV via two dynamic processes: (1) constructive interference at high latitudes with a ridge in the Atlantic sector and a trough in the Pacific accompanied by a negative North Atlantic Oscillation-like pattern over the North Atlantic and a weakened Aleutian low over the North Pacific; and (2) more wavenumber-1 waves propagated into the Arctic stratosphere by modifying the baroclinic term of the zonal mean background state and altering the propagating conditions around the tropopause over the Arctic. Results from reanalysis and model simulations both suggest that a strengthening wave intensity in the high-latitude troposphere and more upward propagation of the planetary wavenumber-1 wave in response to the warmed North Atlantic SSTAs conjunctly contribute to the increased planetary wave flux in the Arctic stratosphere, facilitating a weakened SAV. These results provide a new understanding of what dynamic processes control the SAV, and will help to predict the stratosphere on decadal timescales.

Floodwater impact on Galveston Bay phytoplankton taxonomy, pigment composition and photo-physiological state following Hurricane Harvey from field and ocean color (Sentinel-3A OLCI) observations

Abstract. Phytoplankton taxonomy, pigment composition and photo-physiological state were studied in Galveston Bay (GB), Texas (USA), following the extreme flooding associated with Hurricane Harvey (25–29 August 2017) using field and satellite ocean color observations. The percentage of chlorophyll a (Chl a) in different phytoplankton groups was determined from a semi-analytical IOP (inherent optical property) inversion algorithm. The IOP inversion algorithm revealed the dominance of freshwater species (diatom, cyanobacteria and green algae) in the bay following the hurricane passage (29 September 2017) under low salinity conditions associated with the discharge of floodwaters into GB. Two months after the hurricane (29–30 October 2017), under more seasonal salinity conditions, the phytoplankton community transitioned to an increase in small-sized groups such as haptophytes and prochlorophytes. Sentinel-3A Ocean and Land Colour Instrument (OLCI)-derived Chl a obtained using a red ∕ NIR (near-infrared) band ratio algorithm for the turbid estuarine waters was highly correlated (R2>0.90) to the (high-performance liquid chromatography) HPLC-derived Chl a. Long-term observations of OLCI-derived Chl a (August 2016–December 2017) in GB revealed that hurricane-induced Chl a declined to background mean state in late October 2017. A non-negative least squares (NNLS) inversion model was then applied to OLCI-derived Chl a maps of GB to investigate spatiotemporal variations of phytoplankton diagnostic pigments pre- and post-hurricane; results appeared consistent with extracted phytoplankton taxonomic composition derived from the IOP inversion algorithm and microplankton pictures obtained from an Imaging FlowCytobot (IFCB). OLCI-derived diagnostic pigment distributions also exhibited good agreement with HPLC measurements during both surveys, with R2 ranging from 0.40 for diatoxanthin to 0.96 for Chl a. Environmental factors (e.g., floodwaters) combined with phytoplankton taxonomy also strongly modulated phytoplankton physiology in the bay as indicated by measurements of photosynthetic parameters with a fluorescence induction and relaxation (FIRe) system. Phytoplankton in well-mixed waters (mid-bay area) exhibited maximum PSII photochemical efficiency (Fv∕Fm) and a low effective absorption cross section (σPSII), while the areas adjacent to the shelf (likely nutrient-limited) showed low Fv∕Fm and elevated σPSII values. Overall, the approach using field and ocean color data combined with inversion models allowed, for the first time, an assessment of phytoplankton response to a large hurricane-related floodwater perturbation in a turbid estuarine environment based on its taxonomy, pigment composition and physiological state.

Interannual relationship between intensity of rainfall intraseasonal oscillation and summer-mean rainfall over Yangtze River Basin in eastern China

The analysis of observational rainfall shows that the intensity of rainfall intraseasonal oscillation (ISO) and the summer-mean rainfall over the middle-lower reaches of the Yangtze River Basin (YRB) exhibit a significant positive correlation during 1979–2007. A stronger (weaker) ISO variability is often associated with wet (dry) summer in the YRB. The composite ISOs in both the wet and dry summers are further analyzed. In the wet summers, the rainfall ISO in YRB is primarily associated with the northward propagation of a low-level cyclone-anticyclone pair from the tropics. Cyclonic vorticity and associated boundary layer convergence strengthen the rainfall in situ. In contrast, the rainfall ISO in YRB in the dry summers is primarily associated with the westward propagation of an anomalous anticyclone. Southerly flow to the west of the anomalous anticyclone enhances rainfall in YRB through anomalous moisture advection. In addition to the difference in ISO propagation, the background mean state also shows a marked difference. The diagnosis of water vapor flux budget shows that the convergence and advection of seasonal mean moisture play a critical role in maintenance of the intraseasonal rainfall in the YRB. A greater mean ascending motion and associated higher mean moisture in YRB in the wet summers favor greater intraseasonal rainfall variability in situ. The mean state difference is responsible for distinctive vertical structures of boundary layer vertical velocity. A possible feedback of the ISO to the summer-mean rainfall over the YRB is also discussed.

Reducing TC Position Uncertainty in an Ensemble Data Assimilation and Prediction System: A Case Study of Typhoon Fanapi (2010)

Abstract Ensemble-based data assimilation (EDA) has been used for tropical cyclone (TC) analysis and prediction with some success. However, the TC position spread determines the structure of the TC-related background error covariance and affects the performance of EDA. With an idealized experiment and a real TC case study, it is demonstrated that observations in the core region cannot be optimally assimilated when the TC position spread is large. To minimize the negative impact from large position uncertainty, a TC-centered EDA approach is implemented in the Weather Research and Forecasting (WRF) Model–local ensemble transform Kalman filter (WRF-LETKF) assimilation system. The impact of TC-centered EDA on TC analysis and prediction of Typhoon Fanapi (2010) is evaluated. Using WRF Model nested grids with 4-km grid spacing in the innermost domain, the focus is on EDA using dropsonde data from the Impact of Typhoons on the Ocean in the Pacific field campaign. The results show that the TC structure in the background mean state is improved and that unrealistically large ensemble spread can be alleviated. The characteristic horizontal scale in the background error covariance is smaller and narrower compared to those derived from the conventional EDA approach. Storm-scale corrections are improved using dropsonde data, which is more favorable for TC development. The analysis using the TC-centered EDA is in better agreement with independent observations. The improved analysis ameliorates model shock and improves the track forecast during the first 12 h and landfall at 72 h. The impact on intensity prediction is mixed with a better minimum sea level pressure and overestimated peak winds.

The impact of the mean moisture bias on the key physics of MJO propagation in the ECMWF reforecast

AbstractThe impact of the seasonal mean background state on the physical processes responsible for better Madden‐Julian Oscillation (MJO) propagation and prediction was investigated using 20 year European Centre for Medium‐Range Weather Forecasts (ECMWF) reforecast data sets. While the reforecast captures the MJO propagating signal in the circulation field, the outgoing longwave radiation and column‐integrated moist static energy (MSE) anomalies show a weaker signal than the reanalysis when the MJO detours and propagates in the southern Maritime Continent. By comparing individual MSE budget terms, it is found that the predicted positive MSE tendency to the east of the MJO convection center is weaker than the reanalysis due to the weaker horizontal MSE advection. The dry bias in the seasonal mean lower tropospheric moisture is a key factor that weakens the horizontal MSE advection, and thus deteriorates the MJO propagation in the reforecasts. Therefore, improvements in the mean state could further extend the MJO prediction and associated subseasonal weather phenomena over the globe.

Modulation of Boreal Summer Intraseasonal Oscillations over the Western North Pacific by ENSO

Abstract The authors investigate the effects of El Niño and La Niña on the intraseasonal oscillation (ISO) in the boreal summer (May–October) over the western North Pacific (WNP). It is found that during El Niño summers, the ISO is dominated by a higher-frequency oscillation with a period of around 20–40 days, whereas during La Niña summers the ISO is dominated by a lower-frequency period of around 40–70 days. The former is characterized by northwestward-propagating convection anomalies in the WNP, and the latter is characterized by northward- and eastward-propagating convective signals over the tropical Indian Ocean/Maritime Continent. The possible mechanisms through which El Niño–Southern Oscillation (ENSO)-induced background mean state changes influence the ISO behavior are examined through idealized numerical experiments. It is found that enhanced (weakened) mean moisture and easterly (westerly) vertical wind shear in the WNP during El Niño (La Niña) are the main causes of the strengthened (weakened) 20–40-day northwestward-propagating ISO mode, whereas the 40–70-day ISO initiated from the Indian Ocean can only affect the WNP during La Niña years because the dry (moist) background moisture near the Maritime Continent during El Niño (La Niña) suppresses (enhances) the ISO over the Maritime Continent, and the ISO propagates less over the Maritime Continent during El Niño years than in La Niña years.

Relative Roles of Background Moisture and Vertical Shear in Regulating Interannual Variability of Boreal Summer Intraseasonal Oscillations

Abstract The interannual variability of the boreal summer intraseasonal oscillation (BSISO) is investigated using observed outgoing longwave radiation (OLR) and ERA-Interim data for the period of 1980–2012. It is found that the interannual variability of BSISO intensity is much stronger in the tropical western Pacific (TWP) than the tropical Indian Ocean (TIO). A BSISO intensity index is defined based on a multivariate EOF analysis in TWP. It is found that strong BSISO years are associated with El Niño–like sea surface temperature anomalies in the tropical Pacific, anomalous easterly shear, and enhanced background moisture condition in the region. Using a 2.5-layer atmospheric model with a specified idealized background mean state, the authors further examine the relative roles of background moisture and vertical shear fields in modulating the BSISO intensity. Sensitivity numerical experiments indicate that the background moisture change is most important in regulating the BSISO intensity, whereas the background vertical shear change also plays a role.

The Role of Tropical–Extratropical Interaction and Synoptic Variability in Maintaining the South Pacific Convergence Zone in CMIP5 Models

Abstract The South Pacific convergence zone (SPCZ) is simulated as too zonal a feature in the current generation of climate models, including those in phase 5 of the Coupled Model Intercomparison Project (CMIP5). This zonal bias induces errors in tropical convective heating, with subsequent effects on global circulation. The SPCZ structure, particularly in the subtropics, is governed by the tropical–extratropical interaction between transient synoptic systems and the mean background state. In this study, analysis of synoptic variability in the simulated subtropical SPCZ reveals that the basic mechanism of tropical–extratropical interaction is generally well simulated, with storms approaching the SPCZ along comparable trajectories to observations. However, there is a broad spread in mean precipitation and its variability across the CMIP5 ensemble. Intermodel spread appears to relate to a biased background state in which the synoptic waves propagate. In particular, the region of mean negative zonal stretching deformation or “storm graveyard” in the upper troposphere is displaced in CMIP5 models to the northeast of its position in reanalysis data, albeit with pronounced (≈25°) intermodel longitudinal spread. Precipitation along the eastern edge of the SPCZ shifts in accordance with a storm graveyard shift, and in general models with stronger storm graveyards show higher precipitation variability. Building on prior SPCZ research, it is suggested that SPCZs simulated by CMIP5 models are not simply too zonal; rather, in models the subtropical SPCZ manifests a diagonal tilt similar to observations while SST biases force an overly zonal tropical SPCZ, resulting in a more discontinuous SPCZ than observed.