Intertropical Convergence Zone Bias Research Articles

On the Role of Wind–Evaporation–SST Feedbacks in the Subseasonal Variability of the East Pacific ITCZ

Abstract The intertropical convergence zone (ITCZ) is a zonally elongated band of near-surface convergence and precipitation near the equator. During boreal spring, the eastern Pacific ITCZ migrates latitudinally on daily to subseasonal time scales, and climate models exhibit the greatest ITCZ biases during this time of the year. In this work, we investigate the air–sea interactions associated with the variability in the eastern Pacific ITCZ’s latitudinal location for consecutive days when the ITCZ is only located north of the equator (nITCZ events) compared to when the ITCZ is on both sides of the equator or south of the equator (dsITCZ events) during February–April. The distribution of sea surface temperature (SST) anomalies and surface latent heat flux (SLHF) anomalies during the nITCZ and dsITCZ events follow the classic wind–evaporation–SST (WES) positive feedback mechanism. However, an alternative mechanism, embracing the effect of SST anomalies on vertical stratification and momentum mixing, gives rise to a negative WES feedback. Our results show that in the surface layer, there is a general progression of positive WES feedbacks happening in the weeks leading to the events followed by negative WES feedbacks occurring after the ITCZ events, with an alternate mechanism involving air–sea humidity differences limiting evaporation occurring in between. Additionally, the spatial structures of the components of the feedbacks are nearly mirror images for these opposite ITCZ events over the east Pacific during boreal spring. In closing, we find that understanding the air–sea interactions during daily to weekly varying ITCZ events (nITCZ and dsITCZ) helps to pinpoint how fundamental processes differ for ITCZs in different hemispheres.

Application of the Pseudo‐Global Warming Approach in a Kilometer‐Resolution Climate Simulation of the Tropics

AbstractClouds over tropical oceans are an important factor in the Earth's response to increased greenhouse gas concentrations, but their representation in climate models is challenging due to the small‐scale nature of the involved convective processes. We perform two 4‐year‐long simulations at kilometer‐resolution (3.3 km horizontal grid spacing) with the limited‐area model COSMO over the tropical Atlantic on a 9,000 × 7,000 km2 domain: A control simulation under current climate conditions driven by the ERA5 reanalysis, and a climate change scenario simulation using the Pseudo‐Global Warming approach. We compare these results to the changes projected in the CMIP6 scenario ensemble. Validation shows a good representation of the annual cycle of albedo, in particular for trade‐wind clouds, even compared to the ERA5 reanalysis. Also, the vertical structure and annual cycle of the intertropical convergence zone (ITCZ) is accurately simulated, and the simulation does not suffer from the double ITCZ problem commonly present in global climate models (GCMs). The response to global warming differs between the COSMO simulation and the analyzed GCMs. While both exhibit an overall weakening of the Hadley circulation, the narrowing of the ITCZ (known as the deep‐tropics squeeze) is not so pronounced in the kilometer‐resolution simulation, likely due to the absence of a double ITCZ bias. Also, there is a more pronounced intensification of the ITCZ at the equator in the kilometer‐resolution COSMO simulation, and a stronger associated increase in the anvil cloud fraction.

Double Intertropical Convergence Zones in Coupled Ocean‐Atmosphere Models: Progress in CMIP6

AbstractReducing the double Intertropical Convergence Zone (ITCZ) bias has been the subject of intensive research in the climate modeling community. Recent studies have shown small to little progress in the ensemble of Coupled Model Intercomparison Project Phase 6 (CMIP6) models compared to Phase 3 (CMIP3) and Phase 5 (CMIP5) models in simulating the ITCZ. This study distinguishes the performances of the ensemble of CMIP models and the sub‐ensemble of top five models in reference to the double ITCZ bias by using the spatial correlation as the metric. We show that while there was little progress in the whole CMIP6 ensemble, there is progress in the top five models from CMIP5 to CMIP6. These models have largely eliminated the bias. This improvement is masked in the whole ensemble by the larger inter‐model spread. Results point to promising directions to which coupled models are heading to eradicate the chronic double ITCZ bias.

Outsize Influence of Central American Orography on Global Climate

AbstractGlobal Climate Models (GCMs) exhibit substantial biases in their simulation of tropical climate. One particularly problematic bias exists in GCMs' simulation of the tropical rainband known as the Intertropical Convergence Zone (ITCZ). Much of the precipitation on Earth falls within the ITCZ, which plays a key role in setting Earth's temperature by affecting global energy transports, and partially dictates dynamics of the largest interannual mode of climate variability: The El Niño‐Southern Oscillation (ENSO). Most GCMs fail to simulate the mean state of the ITCZ correctly, often exhibiting a “double ITCZ bias,” with rainbands both north and south rather than just north of the equator. These tropical mean state biases limit confidence in climate models' simulation of projected future and paleoclimate states, and reduce the utility of these models for understanding present climate dynamics. Adjusting GCM parameterizations of cloud processes and atmospheric convection can reduce tropical biases, as can artificially correcting sea surface temperatures through modifications to air‐sea fluxes (i.e., “flux adjustment”). Here, we argue that a significant portion of these rainfall and circulation biases are rooted in orographic height being biased low due to assumptions made in fitting observed orography onto GCM grids. We demonstrate that making different, and physically defensible, assumptions that raise the orographic height significantly improves model simulation of climatological features such as the ITCZ and North American rainfall as well as the simulation of ENSO. These findings suggest a simple, physically based, and computationally inexpensive method that can improve climate models and projections of future climate.

Inter-model spread of the climatological annual mean Hadley circulation and its relationship with the double ITCZ bias in CMIP5

In the present paper, we study the climatological annual mean Hadley circulation simulated by 47 coupled atmospheric and ocean general circulation models (AOGCMs) of the phase 5 of the Coupled Model Intercomparison Project (CMIP5). Our results show that the climatological annual mean states of the Hadley circulation, i.e., strength, central line, and poleward boundaries of the Hadley cells, have a large inter-model spread. The strength of the Hadley cells, i.e., the maximum value of mean meridional streamfunction (MMS), varies from 7.0 × 1010 to 11.1 × 1010 kg s− 1. The variation range is about half of the multi-model ensemble mean strength of about 8.7 × 1010 kg s− 1. The central line location between the two Hadley cells varies from 3.7° S to 7.8° N, and the poleward boundary latitudes have a variation range of 5.5° in latitude. In contrast, inter-model spreads of the Hadley circulation are much smaller in reanalyses. Using a method of inter-model empirical orthogonal function (EOF) analysis of annual-mean MMS, we show that the first principal component (PC1) is closely correlated with the central line latitudes of the Hadley cells, and that the second principal component (PC2) has a high correlation with the poleward boundaries. Both PC1 and PC2 are significantly correlated with the strength of the Hadley cells. Regressions of sea surface temperatures (SSTs) and precipitation on PC1 and PC2 show that EOF1 is associated with the well-known double intertropical convergence zone (ITCZ) problem in AOGCMs, and that EOF2 is related to the cold tongue bias. The results suggest that the large inter-model spread of the climatological mean Hadley circulation is largely due to the double ITCZ and cold tongue biases, and that the key for improving the simulation performance of the Hadley circulation is to reduce the double ITCZ and cold tongue bias in AOGCMs.

Excessive ITCZ but Negative SST Biases in the Tropical Pacific Simulated by CMIP5/6 Models: The Role of the Meridional Pattern of SST Bias

AbstractIn phases 5 and 6 of the state-of-the-art Coupled Model Intercomparison Project (CMIP5 and CMIP6, respectively) models, there is an apparent excessive rainfall bias with a negative SST bias in the tropical Pacific intertropical convergence zone (ITCZ). The regime of the excessive ITCZ but negative SST bias is inconsistent with the common positive rainfall–SST correlation of climate anomalies over the tropics. Using a two-mode model, we decomposed the rainfall bias into two components and found that the surface convergence (SC) bias is the key factor forming the excessive ITCZ bias in the historical runs of 25 CMIP5 models and 23 CMIP6 models. A mixed layer model was further applied to connect the formation of the SC bias with the SST pattern bias. The results suggest that the meridional pattern of the SST bias plays a key role in forming the SC bias. In the CMIP5 and CMIP6 models, the overall negative SST bias has two apparent meridional troughs at around 10°S and 10°N, respectively. The two meridional troughs in the SST bias drive two convergence centers in the SC bias favoring the excessive ITCZ, even though the local SST bias is negative.

A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models

Abstract. Modern coupled general circulation models produce systematic biases in the tropical Atlantic that hamper the reliability of long-range predictions. This study focuses on a common springtime westerly wind bias in the equatorial Atlantic in seasonal hindcasts from two coupled models – ECMWF System 4 and EC-Earth v2.3 – and in hindcasts also based on System 4, but with prescribed sea-surface temperatures. The development of the equatorial westerly bias in early April is marked by a rapid transition from a wintertime easterly, cold tongue bias to a springtime westerly bias regime that displays a marked double intertropical convergence zone (ITCZ). The transition is a seasonal feature of the model climatology (independent of initialisation date) and is associated with a seasonal increase in rainfall where a second branch of the ITCZ is produced south of the Equator. Excess off-equatorial convergence redirects the trade winds away from the Equator. Based on arguments of temporal coincidence, the results of our analysis contrast with those from previous work, and alleged causes hereto identified as the likely cause of the equatorial westerly bias in other models must be discarded. Quite in general, we find no evidence of remote influences on the development of the springtime equatorial bias in the Atlantic in the IFS-based models. Limited evidence however is presented that supports the hypothesis of an incorrect representation of the meridional equatorward flow in the marine boundary layer of the southern Atlantic as a contributing factor. Erroneous dynamical constraints on the flow upstream of the Equator may generate convergence and associated rainfall south of the Equator. This directs attention to the representation of the properties of the subtropical boundary layer as a potential source for the double ITCZ bias.

Dynamics of ITCZ Width: Ekman Processes, Non-Ekman Processes, and Links to Sea Surface Temperature

Abstract The dynamical processes controlling the width of the intertropical convergence zone (ITCZ) are investigated using idealized and CMIP5 simulations. ITCZ width is defined in terms of boundary layer vertical velocity. The tropical boundary layer is approximately in Ekman balance, suggesting that wind stress places a strong constraint on ITCZ width. A scaling based on Ekman balance predicts that ITCZ width is proportional to the wind stress and inversely proportional to its meridional gradient. A toy model of an Ekman boundary layer illustrates the effects of wind stress perturbations on ITCZ width. A westerly wind perturbation widens the ITCZ whereas an easterly perturbation narrows the ITCZ. Multiplying the wind stress by a constant factor does not shift the ITCZ edge, but ITCZ width is sensitive to the latitude of maximum wind stress. Scalings based on Ekman balance cannot fully capture the behavior of ITCZ width across simulations, suggesting that non-Ekman dynamical processes need to be accounted for. An alternative scaling based on the full momentum budget explains variations in ITCZ width and highlights the importance of horizontal and vertical momentum advection. Scalings are also introduced linking ITCZ width to surface temperature. An extension to Lindzen–Nigam theory predicts that ITCZ width scales with the latitude where the Laplacian of SST is zero. The supercriticality theory of Emanuel is also invoked to show that ITCZ width is dynamically linked to boundary layer moist entropy gradients. The results establish a dynamical understanding of ITCZ width that can be applied to interpret persistent ITCZ biases in climate models and the response of tropical precipitation to climate change.

Alleviated Double ITCZ Problem in the NCAR CESM1: A New Cloud Scheme and the Working Mechanisms

AbstractSimulation of tropical precipitation in global climate models remains a challenge, in which double Intertropical Convergence Zone (ITCZ) bias is a prominent manifest. Compared with the original cloud scheme, a new diagnostic statistical cloud macrophysics scheme alleviates the spurious southern ITCZ bias associated with warm sea surface temperature (SST) bias over the central Pacific in the National Center for Atmospheric Research (NCAR) Community Earth System Model, version 1 (CESM1). Owing to the reduced net surface shortwave radiation associated with the increased low cloud fraction and liquid water path over the southeastern Pacific Ocean, the new scheme reduced local SST and further suppressed convection and precipitation. Stronger Walker circulation and enhanced tropical easterlies lead to increased tropical ocean zonal currents, which further lead to reduced SST in the central Pacific due to increased oceanic cold advection. As a result, over the central Pacific, SST cooling suppresses precipitation and alleviates the double ITCZ bias. The study suggests that low clouds over subtropical eastern ocean regions can impact tropical circulation and precipitation via a strong coupling with SST and ocean dynamics.

The Roles of Convection Parameterization in the Formation of Double ITCZ Syndrome in the NCAR CESM: I. Atmospheric Processes

AbstractSeveral improvements are implemented in the Zhang‐McFarlane (ZM) convection scheme to investigate the roles of convection parameterization in the formation of double intertropical convergence zone (ITCZ) bias in the NCAR CESM1.2.1. It is shown that the prominent double ITCZ biases of precipitation, sea surface temperature (SST), and wind stress in the standard CESM1.2.1 are largely eliminated in all seasons with the use of these improvements in convection scheme. This study for the first time demonstrates that the modifications of convection scheme can eliminate the double ITCZ biases in all seasons, including boreal winter and spring. Further analysis shows that the elimination of the double ITCZ bias is achieved not by improving other possible contributors, such as stratus cloud bias off the west coast of South America and cloud/radiation biases over the Southern Ocean, but by modifying the convection scheme itself. This study demonstrates that convection scheme is the primary contributor to the double ITCZ bias in the CESM1.2.1, and provides a possible solution to the long‐standing double ITCZ problem. The atmospheric model simulations forced by observed SST show that the original ZM convection scheme tends to produce double ITCZ bias in high SST scenario, while the modified convection scheme does not. The impact of changes in each core component of convection scheme on the double ITCZ bias in atmospheric model is identified and further investigated.

Processes Leading to Double Intertropical Convergence Zone Bias in CESM1/CAM5

Abstract The double intertropical convergence zone (ITCZ) bias in the eastern Pacific in the Community Earth System Model version 1 with Community Atmosphere Model version 5 (CESM1/CAM5) is diagnosed. In CAM5 standalone, the northern ITCZ is associated with inertial instability and the southern ITCZ is thermally forced. After air–sea coupling, the processes on both hemispheres are switched because the spatial pattern of sea surface temperature (SST) is changed. Biases occur during boreal spring in both CAM5 and the ocean model. In CAM5 alone, weaker-than-observed equatorial easterly in the tropical eastern South Pacific leads to weaker evaporation and an increase in local SST. The shallow meridional circulation overly converges in the same region in the CAM5 standalone simulation, the planetary boundary layer and middle troposphere are too humid, and the large-scale subsidence is too weak at the middle levels. These biases may result from excessive shallow convection behavior in CAM5. The extra moisture would then fuel stronger convection and a higher precipitation rate in the southeastern Pacific. In the ocean model, the South Equatorial Current is underestimated and the North Equatorial Countercurrent is located too close to the equator, causing a warm SST bias in the southeastern Pacific and a cold bias in the northeastern Pacific. These SST biases feed back to the atmosphere and further influence convection and the surface wind biases in the coupled simulation. When the convection in the tropical northeastern Pacific becomes thermally forced after coupling, the northern ITCZ is diminished due to colder SST, forming the so-called alternating ITCZ bias.

The double ITCZ bias in CMIP5 models: interaction between SST, large-scale circulation and precipitation

The double intertropical convergence zone (ITCZ) bias still affects all the models that participate to CMIP5 (Coupled Model Intercomparison Project, phase 5). As an ensemble, general circulation models have improved little between CMIP3 and CMIP5 as far as the double ITCZ is concerned. The present study proposes a new process-oriented metrics that provides a robust statistical relationship between atmospheric processes and the double ITCZ bias, additionally to the existing relationship between the sea surface temperature (SST) and the double ITCZ bias. The SST contribution is examined using the THR-MLT index (Bellucci et al. in J Clim 5:1127–1145, 2010), which combines biases on the representation of local SSTs and the SST threshold leading to the onset of ascent in the double ITCZ region. As a metrics of a model’s bias in simulating the interaction between circulation and precipitation, we propose to use the Combined Precipitation Circulation Error (CPCE). It is computed as the quadratic error on the contribution of each vertical regime to the total precipitation over the tropical oceans. CPCE is a global measure of the circulation-precipitation coupling that characterizes the model physical parameterizations rather than the regional characteristics of the eastern Pacific. A linear regression analysis shows that most of the double ITCZ spread among CMIP5 coupled ocean–atmosphere models is attributed to SST biases, and that the precipitation large-scale dynamics relationship explains a significant fraction of the bias in these models, as well as in the atmosphere-only models.

Effects of Pacific Intertropical Convergence Zone precipitation bias on ENSO phase transition

In this study, the effect of mean precipitation bias over the Pacific Intertropical Convergence Zone (ITCZ) on the El Niño southern oscillation (ENSO) transition is examined using CMIP3 and CMIP5 archives. It is found that the climate models with excessive mean precipitation over the central/eastern Pacific ITCZ tend to simulate slower phase transition of the ENSO. This is because a wetter climatology provides a favorable condition for anomalously strong convective activity; the El Niño-related convection anomaly tends to be increased over the central/eastern Pacific ITCZ with a local wet bias. This induces additional low-level westerlies over the central/eastern equatorial Pacific. As a result, the ENSO-related zonal wind stress anomaly over the central Pacific, which is south of the equator without the wet ITCZ bias during boreal winter, is shifted to the east, and its meridional width is expanded northward. It is found that both the eastward shift and northward expansion of ENSO-related wind stress can lead to slower ENSO phase transition as it takes longer time for the reflected Rossby waves to suppress the ENSO growth. This implies that the off-equatorial mean precipitation plays an important role in ENSO phase transition.

Reproducibility of precipitation distribution over the tropical oceans in CMIP5 multi-climate models compared to CMIP3

Reproducibility of precipitation distribution over the tropical oceans for the recent dataset of the Coupled Model Intercomparison Project phase 5 (CMIP5) is investigated and compared to CMIP3. The Taylor skill score for the reproducibility of the CMIP5 multi-model ensemble mean (0.64) is slightly higher than that of CMIP3 (0.60), but the difference is not statistically significant. Still, there is some evidences that the double intertropical convergence zone (ITCZ) bias is mitigated from CMIP3 to CMIP5, whereas the cold tongue bias remains similar. An inter-model empirical orthogonal function analysis shows that these two biases are closely related to the dominant inter-model discrepancies of precipitation patterns. The two biases are attributed to two factors, respectively. In the CMIP5 models with the prominent double ITCZ, the deep convection is not sensitive enough to environmental air humidity at the lower-mid troposphere, as is in CMIP3. Thus, the deep convection is not suppressed even over the dry subsidence region of the southeastern Pacific, forming the double ITCZ bias. Conversely, models with the severe cold tongue bias have lower ocean model resolution with too strong equatorial trades. Therefore, proper representation of the sensitivity of deep convection to humidity and higher resolution of the ocean models with better equatorial trades are important for reducing the double ITCZ and the cold tongue biases.

Convective Entrainment and Large-Scale Organization of Tropical Precipitation: Sensitivity of the CNRM-CM5 Hierarchy of Models

Abstract The spurious double intertropical convergence zone (ITCZ) is a systematic bias affecting state-of-the-art coupled general circulation models (GCMs). Modeling studies show that the ITCZ structure is very sensitive to moist convection parameterization and in particular, to the vertical profile of convective heating and free-tropospheric moistening. To further explore this sensitivity, the authors focus in this study on the influence of lateral entrainment in convective plumes on the simulated tropical precipitation and large-scale circulation. Sensitivity studies to the entrainment parameter were performed in a hierarchy of models (coupled ocean–atmosphere GCM, atmospheric GCM, and aquaplanet GCM), in order to mitigate the double ITCZ problem in the Centre National de Recherches Météorologiques Coupled Global Climate Model, version 5 (CNRM-CM5). The sensitivity of the ITCZ structure to lateral entrainment is robust across our hierarchy of models. In response to increased entrainment, the realistic simulations exhibit a weakening of the southern side of the double ITCZ over the southeastern Pacific Ocean and a better representation of the South Pacific convergence zone (SPCZ). However, as a result of stronger moisture–convection feedbacks, precipitation is overestimated in the center of convergence zones. The change in ITCZ configuration is associated with a more realistic representation of the large-scale vertical regimes, explained by a decreased frequency of weak-to-moderate ascending regimes and an enhanced frequency of subsidence regimes. Mechanisms at play in this circulation change are examined by analyzing the vertically integrated dry static energy budget. This energetic analysis suggests that the feedback between large-scale dynamics and deep convection is crucial in controlling the probability distribution function (PDF) of midtropospheric vertical wind. This PDF, in turn, controls the precipitation distribution and, in particular, the double ITCZ bias.

Rain Reevaporation, Boundary Layer–Convection Interactions, and Pacific Rainfall Patterns in an AGCM

Sensitivity experiments with an atmospheric general circulation model (AGCM) show that parameterized rain reevaporation has a large impact on simulated precipitation patterns in the tropical Pacific, especially on the configuration of the model’s intertropical convergence zone (ITCZ). Weak reevaporation leads to the formation of a “double ITCZ” during the northern warm season. The double ITCZ is accompanied by strong correlation between precipitation and high-frequency vertical motion in the planetary boundary layer (PBL). Strong reevaporation leads to a better overall agreement of simulated precipitation with observations. The model’s double ITCZ bias is reduced. At the same time, correlation between high-frequency (periods < 15 days) vertical motion in the PBL and precipitation is reduced. Experiments with modified physics indicate that evaporative cooling by rain near the PBL top weakens the coupling between precipitation-related heating and vertical motion in high-frequency motions. The strength of high-frequency vertical motions in the PBL was also reduced directly through the introduction of a diffusive cumulus momentum transport (DCMT) parameterization. The DCMT had a visible impact on simulated precipitation in the Tropics but did not reduce the model’s double ITCZ bias in all cases. Further analyses of mass and water vapor budgets, as well as vertical motion statistics, in the ITCZ complex, show that time-mean moisture convergence in the southern ITCZ is largely dominated by high-frequency modes, while in the northern ITCZ time-mean moisture convergence contains large contributions from slower modes. This may explain why the simulated southern ITCZ is more susceptible to parameterization changes that alter high-frequency coupling between moist heating and PBL convergence.