Intertropical Convergence Zone Width Research Articles

Northward Shift and Narrowing of the ITCZ in 20 Years of AIRS Data

AbstractA time series analysis of Deep Convective Clouds (DCC) in Atmospheric InfraRed Sounder data between 2002 and 2023 shows that the mean latitude of the Inter Tropical Convergence Zone (ITCZ) has shifted northward for land at a rate of about 30 km/decade. The shift for ocean is north as well at about half of the rate for land. Comparisons of our results with the existing literature are complicated by the use of different methodologies and data sources: We used the daily observations of DCCs, which by their association with extreme rain rates are indicators of locally extreme vertical velocity on 15 km horizontal scales, while most of the literature used model or reanalysis gridded data with typically 100 km bins. The southern boundary of the ITCZ shifts north faster than the northern boundary, as qualitatively stated in the literature. The resulting contraction of the ITCZ width with global warming was qualitatively predicted based on theoretical consideration. Our observation of the rate of decrease in the width for ocean is statistically consistent with the contraction deduced from European Centre for Medium Range Forecast Reanalysis and Modern‐Era Retrospective analysis for Research and Applications data. The observed northward shift of the ITCZ was not predicted and is not seen in climate models in the 21st century.

Assessment of the Intertropical Convergence Zone over the Atlantic Ocean through an Algorithm Based on Precipitation

The Intertropical Convergence Zone (ITCZ) is a key atmospheric system on a global scale, primarily driven by trade wind convergence near the equator. The ITCZ plays a crucial role in modulating the climate of the borders of tropical continental areas. For instance, Northeastern Brazil experiences a climate influenced by the ITCZ over the Atlantic Ocean. In some periods, the ITCZ exhibits double bands, known as the double ITCZ. While the features of the ITCZ have been described using various approaches and atmospheric variables, there is still a lack of regional studies focusing on the ITCZ and double ITCZ in the Atlantic Ocean. In this context, the main goals of this study are (1) to describe a simple algorithm based on precipitation to identify the ITCZ and double ITCZ, (2) to present a climatology (1997–2022) of the position, width, and intensity of these two convective bands, and (3) to investigate variabilities in the ITCZ characteristics associated with anomalies of sea surface temperature (SST) in the tropical Pacific and Atlantic oceans. The double ITCZ typically occurs southward of the main cloud band, and between February and April, both bands are more distant (~4.5°). In the western sector of the Atlantic Ocean, the ITCZ and its double band extend to more southerly latitudes in austral autumn. Considering the entire Atlantic basin, the annual mean of the latitudinal position, width, and intensity of the ITCZ is 4.9°N, 4.2°, and 11 mm/day, respectively, while for the double ITCZ, it is 0.4°N, 2.6°, 10.3 mm/day, respectively. While the SST anomalies in the Pacific Ocean (El Niño and La Niña episodes) affect more the ITCZ width, the SST anomalies in the Tropical South Atlantic affect both its position and width.

Assessment of the Intertropical Convergence Zone over the Atlantic Ocean through an Algorithm Based on Precipitation

The Intertropical Convergence Zone (ITCZ) is a key atmospheric system on a global scale, primarily driven by trade wind convergence near the equator. The ITCZ plays a crucial role in modulating the climate of the borders of tropical continental areas. For instance, Northeastern Brazil experiences a climate influenced by the ITCZ over the Atlantic Ocean. In some periods, the ITCZ exhibits double bands, known as the double ITCZ. While the features of the ITCZ have been described using various approaches and atmospheric variables, there is still a lack of regional studies focusing on the ITCZ and double ITCZ in the Atlantic Ocean. In this context, the main goals of this study are (1) to describe a simple algorithm based on precipitation to identify the ITCZ and double ITCZ, (2) to present a climatology (1997–2022) of the position, width, and intensity of these two convective bands, and (3) to investigate variabilities in the ITCZ characteristics associated with anomalies of sea surface temperature (SST) in the tropical Pacific and Atlantic oceans. The double ITCZ typically occurs southward of the main cloud band, and between February and April, both bands are more distant (~4.5°). In the western sector of the Atlantic Ocean, the ITCZ and its double band extend to more southerly latitudes in austral autumn. Considering the entire Atlantic basin, the annual mean of the latitudinal position, width, and intensity of the ITCZ is 4.9°N, 4.2°, and 11 mm/day, respectively, while for the double ITCZ, it is 0.4°N, 2.6°, 10.3 mm/day, respectively. While the SST anomalies in the Pacific Ocean (El Niño and La Niña episodes) affect more the ITCZ width, the SST anomalies in the Tropical South Atlantic affect both its position and width.

Interhemispheric antiphasing of neotropical precipitation during the past millennium

Uncertainty about the influence of anthropogenic radiative forcing on the position and strength of convective rainfall in the Intertropical Convergence Zone (ITCZ) inhibits our ability to project future tropical hydroclimate change in a warmer world. Paleoclimatic and modeling data inform on the timescales and mechanisms of ITCZ variability; yet a comprehensive, long-term perspective remains elusive. Here, we quantify the evolution of neotropical hydroclimate over the preindustrial past millennium (850 to 1850 CE) using a synthesis of 48 paleo-records, accounting for uncertainties in paleo-archive age models. We show that an interhemispheric pattern of precipitation antiphasing occurred on multicentury timescales in response to changes in natural radiative forcing. The conventionally defined “Little Ice Age” (1450 to 1850 CE) was marked by a clear shift toward wetter conditions in the southern neotropics and a less distinct and spatiotemporally complex transition toward drier conditions in the northern neotropics. This pattern of hydroclimatic change is consistent with results from climate model simulations indicating that a relative cooling of the Northern Hemisphere caused a southward shift in the thermal equator across the Atlantic basin and a southerly displacement of the ITCZ in the tropical Americas, with volcanic forcing as the principal driver. These findings are at odds with proxy-based reconstructions of ITCZ behavior in the western Pacific basin, where changes in ITCZ width and intensity, rather than mean position, appear to have driven hydroclimate transitions over the last millennium. This reinforces the idea that ITCZ responses to external forcing are region specific, complicating projections of the tropical precipitation response to global warming.

Weaker Links Between Zonal Convective Clustering and ITCZ Width in Climate Models Than in Observations.

Strong links are seen in observations between convective clustering and several properties of the Intertropical Convergence Zone (ITCZ). These links suggest that biases in how climate models simulate the ITCZ may be related to model biases in convective clustering or that there may be biases in how models represent the relationship between clustering and the ITCZ. We investigate these issues by analyzing convective clustering, and the link between clustering and ITCZ properties in 18 climate models. We find that the links between variability in convective clustering and variability of ITCZ properties are generally weaker and less robust in models than in observations. By contrast, model biases in the climatological convective clustering explain a substantial fraction of the climatological double‐ITCZ bias, though they do not explain biases in the climatological ITCZ width.

Variations in Precipitating Convective Feature Populations with ITCZ Width in the Pacific Ocean

AbstractMany recent studies have aimed to better understand changes in the characteristics of the intertropical convergence zone (ITCZ), including ITCZ location, width, and precipitation intensity. However, very few studies have looked at the relationship between characteristics of convection within the ITCZ and ITCZ width. The present work uses information from an ITCZ identification database and Tropical Rainfall Measuring Mission (TRMM) precipitation feature (PF) database to quantify variations in convective characteristics across the ITCZ in the Pacific Ocean. Data are partitioned into wide and narrow ITCZ regimes to quantify differences in convection between different ITCZ regimes. Under the wide regime, convection deeper than 5 km, with areas greater than 100 km2, or stratiform rain fractions greater than 0.5 is, on average, 24%, 23%, and 12% more frequent, respectively. In the narrow regime, the signal is reversed, with average increases in the frequency of convection with heights below 5 km, areas less than 100 km2, or stratiform rain fractions less than 0.5 of 15%, 4%, and 6%, respectively. Positive and negative anomalies in columnar water vapor (CWV) and sea surface temperature (SST) across the ITCZ are observed in the wide and narrow regimes, respectively. There is also a strong positive correlation between an El Niño–Southern Oscillation (ENSO) index and ITCZ width anomalies, with wide (narrow) ITCZs occurring during warm (cold) phases of ENSO. This implies that the strengthening and weakening of the Walker circulation associated with ENSO may play a role in modulating the convective populations that contribute to the Pacific ITCZ width variations.

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.

Theories for Past and Future Monsoon Rainfall Changes

Long-standing biases in simulations of past and present climate states and climate model disagreement even in sign of future monsoon rainfall changes evince limitations in our theoretical understanding. The dominant theoretical paradigms for understanding monsoon rainfall—convective-quasi equilibrium (CQE), the moist static energy (MSE) budget, and monsoons as local Intertropical Convergence Zone (ITCZ) shifts—all jettison the traditional “land-sea breeze” paradigm. Summer monsoon precipitation falls when the assumptions of CQE are most satisfied but those of the ITCZ shift framework are least satisfied. Zonal asymmetries, changes in ITCZ width and strength, hydrology-vegetation-CO2 coupling, and timescale-dependent responses complicate inferences of monsoon rainfall from paleoclimate proxy records. The MSE budget framework applied to deliberately designed simulations can illuminate key mechanisms underlying monsoon responses to external forcings, presenting a path toward falsifying model projections. Sustained, rapid progress in monsoon rainfall theory is urgently needed by society and is plausible based on recent advances.

Increasing Frequency of Midtropospheric Dry Layers in the Pacific Intertropical Convergence Zone

AbstractRecent long‐term characterization of the Pacific Intertropical Convergence Zone (ITCZ) shows that the ITCZ has narrowed, which may be due to the enhanced transport of dry air layers from the subtropics. In this study, we examine the relationship between the frequency of midtropospheric dry layers and ITCZ width. European Centre for Medium‐Range Weather Forecasts Reanalysis Interim midtroposphere (400 to 600 hPa) relative humidity is used to create a 36‐year (1979–2014) dry layer climatology for the Pacific ITCZ region. Analysis shows an inverse relationship between ITCZ width and dry layer frequency, with ITCZ width decreasing as the frequency of dry layers increases. Moreover, findings show that the region of greatest long‐term increase in dry layer frequency occurs in the central Pacific, where the greatest narrowing of the ITCZ is also observed.

Cloud and circulation feedbacks in a near‐global aquaplanet cloud‐resolving model

AbstractA near‐global aquaplanet cloud‐resolving model (NGAqua) with fixed meridionally varying sea‐surface temperature (SST) is used to investigate cloud feedbacks due to three climate perturbations: a uniform 4 K SST increase, a quadrupled‐CO2 concentration, and both combined. NGAqua has a horizontal resolution of 4 km with no cumulus parameterization. Its domain is a zonally periodic 20,480 km‐long tropical channel, spanning 46°S–N. It produces plausible mean distributions of clouds, rainfall, and winds. After spin‐up, 80 days are analyzed for the control and increased‐SST simulations, and 40 days for those with quadrupled CO2. The Intertropical Convergence Zone width and tropical cloud cover are not strongly affected by SST warming or CO2 increase, except for the expected upward shift in high clouds with warming, but both perturbations weaken the Hadley circulation. Increased SST induces a statistically significant increase in subtropical low cloud fraction and in‐cloud liquid water content but decreases midlatitude cloud, yielding slightly positive domain‐mean shortwave cloud feedbacks. CO2 quadrupling causes a slight shallowing and a statistically insignificant reduction of subtropical low cloud fraction. Warming‐induced low cloud changes are strongly correlated with changes in estimated inversion strength, which increases modestly in the subtropics but decreases in the midlatitudes. Enhanced clear‐sky boundary layer radiative cooling in the warmer climate accompanies the robust subtropical low cloud increase. The probability distribution of column relative humidity across the tropics and subtropics is compared between the control and increased‐SST simulations. It shows no evidence of bimodality or increased convective aggregation in a warmer climate.

Energetic Constraints on the Width of the Intertropical Convergence Zone

Abstract The intertropical convergence zone (ITCZ) has been the focus of considerable research in recent years, with much of this work concerned with how the latitude of maximum tropical precipitation responds to natural climate variability and to radiative forcing. The width of the ITCZ, however, has received little attention despite its importance for regional climate and for understanding the general circulation of the atmosphere. This paper investigates the ITCZ width in simulations with an idealized general circulation model over a wide range of climates. The ITCZ, defined as the tropical region where there is time-mean ascent, displays rich behavior as the climate varies, widening with warming in cool climates, narrowing in temperate climates, and maintaining a relatively constant width in hot climates. The mass and energy budgets of the Hadley circulation are used to derive expressions for the area of the ITCZ relative to the area of the neighboring descent region, and for the sensitivity of the ITCZ area to changes in climate. The ITCZ width depends primarily on four quantities: the net energy input to the tropical atmosphere, the advection of moist static energy by the Hadley circulation, the transport of moist static energy by transient eddies, and the gross moist stability. Different processes are important for the ITCZ width in different climates, with changes in gross moist stability generally having a weak influence relative to the other processes. The results are likely to be useful for analyzing the ITCZ width in complex climate models and for understanding past and future climate change in the tropics.

Long‐term characterization of the Pacific ITCZ using TRMM, GPCP, and ERA‐Interim

AbstractAn objective, automated Intertropical Convergence Zone (ITCZ) identification and characterization algorithm is developed and applied to European Centre for Medium‐Range Weather Forecasts Reanalysis Interim (ERA‐Interim) variables, and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and Global Precipitation Climatology Project (GPCP) rain rates (RRs) to create 15 and 36 year climatologies, respectively, in the tropical Pacific ocean. Characteristics derived from the algorithm include ITCZ latitude, northern and southern ITCZ convection boundaries and extent, and precipitation intensity. The climatological location of the ITCZ was found near 8°N, consistent with previous studies, with a preferred southern boundary location of 4°N. The northern ITCZ boundary did not exhibit a preferred boundary with locations between 7° and 15°N having similar frequencies of occurrence. The northern and southern extents of the ITCZ were symmetric in the central Pacific and asymmetric in the east Pacific. Long‐term trends in the ITCZ width and precipitation intensity showed significant narrowing and intensifying. Separation of characteristics by season and location (i.e., central and eastern Pacific) revealed negative trends in ITCZ width in both domains, with trends in the east Pacific weaker than those in the central Pacific. Trends in precipitation intensity near the center of the ITCZ indicate a strong intensification, with slightly stronger trends in the central Pacific than in the east Pacific. These findings show that while the location of the ITCZ has not changed significantly over the past three decades, the ITCZ has narrowed and convection has intensified.

Variabilidade da largura e intensidade da Zona de Convergência Intertropical atlântica: aspectos observacionais

Este trabalho apresenta um estudo observacional das características da Zona de Convergência Intertropical (ZCIT) na porção central do Oceano Atlântico. A variabilidade da posição, largura e intensidade (precipitação média) da ZCIT Atlântica durante 10 anos (1999-2008) foi obtida objetivamente de dados de precipitação pentadais. O método dos compostos foi utilizado para obter a distribuição espacial de precipitação, temperatura da superfície do mar (TSM) e ventos à superfície a fim de verificar os fatores associados à variação de largura da ZCIT. Os resultados confirmaram os de trabalhos anteriores sobre a variabilidade da posição e apresentaram informações adicionais sobre a variabilidade da intensidade e largura. Na escala mensal, a largura e intensidade da ZCIT tendem a diminuir ao atingir as suas posições mais austrais (fevereiro, março e abril). A amplitude da variação anual foi de aproximadamente 7° de latitude para a posição média (de 1°N em abril a 8°N em agosto), de 3° para a largura (de 3° em março a 6° em outubro) e, quanto à intensidade da precipitação, foi de 3 mm.dia-1 (de 10 mm.dia-1 em março a 13 mm.dia-1 em julho). Na análise dos compostos da precipitação, TSM e divergência dos ventos à superfície na região do Atlântico Tropical, encontraram-se indícios de que, em condições de posição da ZCIT próxima à média, anomalias positivas de largura estariam associadas às de negativas de TSM e às positivas de convergência dos ventos à superfície, mas não houve significância estatística das diferenças.

Modulations of the Phase Speed of Convectively Coupled Kelvin Waves by the ITCZ

Abstract A number of studies suggest a two-way feedback between convectively coupled Kelvin waves (CCKWs) and the intertropical convergence zone (ITCZ). Viewed here as a proxy for deep convection, analysis of brightness temperature data reveals several aspects of these interdependencies. A wavenumber–frequency spectral analysis is applied to the satellite data in order to filter CCKWs. The ITCZ is characterized by a region of low brightness temperature and a proxy for both the ITCZ location and width are defined. The phase speed of CCKW data is determined using the Radon transform method. Linear regression techniques and probability density analysis are consistent with previous theoretical predictions and observational results. In particular, the fastest waves are found when the ITCZ is the farthest from the equator and the narrowest. Conversely, the slowest waves coincide with broad ITCZs that are located near the equator.

Convectively Coupled Waves Propagating along an Equatorial ITCZ

Abstract The dynamics of convectively coupled gravity waves traveling over a precipitating region are analyzed in an idealized model for the large-scale atmospheric circulation. The model is composed of a shallow water system coupled to an advection equation for moisture through the convection term, utilizing a quasi-equilibrium relaxation to moisture closure. Here the authors investigate the model in the strict quasi-equilibrium (SQE) of infinitely short relaxation time. This framework is applied to study the behavior of a disturbance propagating along a narrow precipitation band, similar to the intertropical convergence zone (ITCZ). For an ITCZ width on the order of the equatorial Rossby radius, Kelvin waves propagate at the moist gravity wave speed (about 15 m s−1), whereas for a narrow ITCZ, the propagation speed is comparable to the dry gravity wave (about 50 m s−1). It is also shown that a Kelvin wave propagating along a narrow precipitation region exhibits a meridional circulation that modulates the precipitation rate and affects the propagation speed of the wave.

Abrupt Seasonal Migration of the ITCZ into the Summer Hemisphere

AbstractAlthough the maximum of solar radiation at the top of the atmosphere moves gradually from one hemisphere to the other as part of the seasonal cycle, the intertropical convergence zone (ITCZ) moves abruptly into the summer hemisphere. An axisymmetric circulation model is developed to study this rapid transition. The model consists of an upper and lower layer of the Hadley circulation (HC), with the surface layer attached to a slab ocean and the lower layer connected to the upper layer by a constant lapse rate. The model is forced by solar heating, and the ITCZ is prescribed to coincide with the warmest sea surface temperature (SST). The collocation of tropical rainfall with warm SST allows the model ITCZ migration to be understood in terms of the relative influence of solar heating and atmospheric dynamics upon ocean temperature. Atmospheric dynamics allow the ITCZ to move off the equator by flattening the meridional temperature gradient that would exist in radiative–convective equilibrium. For the present-day tropical oceanic mixed layer depth and ITCZ width, the model exhibits an abrupt seasonal transition of the ITCZ across the equator. It is found that there are two determinative factors on the abrupt transition of the ITCZ: the nonlinear meridional advection of angular momentum by the circulation and ocean thermal inertia. As a result of nonlinear dynamics, angular momentum is well mixed, resulting in minimum atmospheric temperature at the equator and a similar equatorial minimum in SST. This inhibits convection over the equator while favoring a rapid seasonal transition of the ITCZ between the warmer surface water on either side of this latitude.