Why is the itcz associated with high rainfall

This area is the intertropical convergence zone ITCZ. The ITCZ is also called the "climate equator"—it lies near the geographic equator, and divides the global air circulation patterns into two mirror images to the north and south. Air Masses. As these winds converge, moist air is forced upward, forming one portion of the Hadley cell. The air cools and rises see image below , causing water vapor to be "squeezed" out as rain, resulting in a band of heavy precipitation around the globe.

This reliable circulation feeds the lush rain forests of central Africa, and also defines the limits of the Sahara desert. The ITCZ has been called the doldrums by sailors because there is essentially no horizontal air movement, that is, no wind the air simply rises. Hadley Circulation. The position of the ITCZ varies predictably throughout the year. The sun crosses the equator twice a year in March and September, and consequently makes for two wet seasons each year.

In December and July, when the sun is at its greatest extent north or south of the equator makes for two dry seasons. Further away from the equator, the two wet seasons merge into one, and the climate becomes more monsoonal, with one wet season and one dry season.

Because of its location just north of the equator, Nigeria's climate is characterized by the hot and wet conditions associated with the movement of the Inter-Tropical Convergence Zone ITCZ north and south of the equator. This is easily seen in the normal monthly rainfall for two cities, Kano and Lagos, separated by miles km.

When the ITCZ is to the south of the equator, the north-east winds prevail over Nigeria, producing the dry-season conditions. When the ITCZ moves into the Northern Hemisphere, the south westerly wind prevails as far inland to bring rain fall during the wet season. The implication is that there is a prolonged rainy season in the far south of Nigeria, while the far north undergoes long dry periods annually.

What controls the sensitivity of the ITCZ to remote forcings? And how do the model biases in the ITCZ arise? Paleoclimate studies e. So when the northern hemisphere warms, for example, because northern ice cover and with it the polar albedo are reduced, the ITCZ shifts northward. This can be rationalized as follows: When the atmosphere receives additional energy in the northern hemisphere, it attempts to rectify this imbalance by transporting energy across the equator from the north to the south.

Most atmospheric energy transport near the equator is accomplished by the Hadley circulation, the mean tropical overturning circulation. The ITCZ lies at the foot of the ascending branch of the Hadley circulation, and the circulation transports energy in the direction of its upper branch, because energy or, more precisely, moist static energy usually increases with height in the atmosphere.

Southward energy transport across the equator then requires an ITCZ north of the equator, so the upper branch of the Hadley circulation can cross the equator going from the north to the south. The energy balance states that the atmosphere transports energy away from regions of net energy input e.

Broccoli et al. Focusing on the zonal mean e. The figure shows the atmospheric moist static energy flux in the zonal and annual mean in the present climate red line. If the northward cross-equatorial energy flux strengthens indicated schematically by the blue line , but the slope remains fixed, the energy flux equator moves southward.

Similarly, if increases, the energy flux equator moves toward the equator. Several previous studies had pointed out that the ITCZ position is proportional to the cross-equatorial energy flux e. That the net atmospheric energy input modulates the sensitivity of the ITCZ position to the cross-equatorial flux was pointed out in Bischoff and Schneider What are some implications of these insights from the energy balance? The analysis draws attention to the importance for the ITCZ of the atmospheric energy balance near the equator.

It is feasible that the ITCZ shifts in different ways over land and ocean because of the contrasting aerosol forcings, which could reconcile the negligible oceanic ITCZ shifts in Figure S1 with the previous studies using land data that found a southward shift.

A further consideration is the variation of sulfur emissions over time and between regions. The period shown in Figure S1 — had declining sulfur emissions in North America and Western Europe but periods of increasing and subsequent decreasing emissions in Asia and Eastern Europe [ 67 ]. Contrasting trends in emissions and scattering aerosol concentrations in these regions likely affected the ITCZ location in contrasting ways, and could help to explain why we do not observe a clear ITCZ shift over oceans in the last 39 years.

Greenhouse gas concentrations were also changing over —, and it is well known that the tropical circulation responds differently to aerosol versus greenhouse gas forcings [ 68 , 69 ]. The relative influences of these forcings on historical trends not only in ITCZ location but also in ITCZ width and strength could be disentangled using single-forcing simulations following Xie et al. In this section, we will consider projected changes in ITCZ location, width and strength over the twenty-first century.

Numerous studies have examined ITCZ location and its variability but few have focused on its width and strength. As a consequence, metrics for ITCZ width and strength are less established in the literature. The discussion of observed ITCZ characteristics in the previous section focused on observable quantities, primarily precipitation.

An alternative and intuitive set of metrics for characterizing the ITCZ is based on the atmospheric mass circulation. Characterizing the large-scale tropical circulation directly in observations is not feasible. In models, however, we can apply consistent circulation-based definitions and assess how the ITCZ structure responds to climate change across models. Below we briefly review existing definitions of ITCZ structure before presenting the metrics, based on the mass streamfunction, that we will use to analyze model projections of the ITCZ.

Metrics that have been used to define the ITCZ fall into two broad categories: those based on precipitation and those based on atmospheric mass and energy fluxes. The ITCZ location has been defined as the latitude i where the mid-tropospheric atmospheric mass streamfunction is zero [ 71 ], ii where the poleward flux of moist static energy by the atmosphere vanishes [ 19 ], iii where tropical precipitation has a maximum [ 13 ], and iv where the centroid of tropical precipitation lies [ 17 ].

The strength of the ITCZ is often left unquantified though obvious metrics include the strength of upward motion averaged over the ITCZ or the maximum zonal-mean precipitation rate. The magnitudes of the location, width and strength of the ITCZ depend on the metrics chosen to quantify them.

To be clear, for this section on model projections, we discuss changes in the ITCZ structure defined in terms of the annual zonal-mean Eulerian-mean meridional streamfunction [ 73 ]. For example, the well-established energetic theory for ITCZ location was largely developed in a zonal-mean framework [ 13 , 14 , 19 ], and later the mechanistic insights gained from this simplified setup have inspired a more comprehensive theory that takes into account the zonal structure [ 22 , 74 ].

Our lack of basic understanding of the width and strength of the ITCZ implies that there is still much to learn from the zonal-mean perspective before we proceed to tackling the more complex problem of understanding longitudinal variations. The metrics for the ITCZ location, width and total mass transport are indicated on a plot of the mid-tropospheric streamfunction for one climate model Fig.

The ITCZ location, defined in this way, represents the latitude of the boundary between the northern and southern Hadley cells. Equivalently, these are the latitudes at which the time-mean circulation transitions from ascending to descending. We also define a bulk vertical pressure velocity for the ITCZ that is proportional to the total mass transport divided by area:. This bulk vertical velocity is defined to be the ITCZ strength. Vertically averaged annual- and zonal-mean meridional streamfunction to hPa with mass weighting in the historical simulation — for the CNRM-CM5 model.

Observational analyses of circulation-based metrics for the ITCZ [ 1 — 4 ] are restricted by difficulties in directly observing the large-scale circulation. Consequently, in our analysis of recent ITCZ trends Figure S1 , we use metrics largely derived from satellite observations of precipitation intensity. Climate model trends in ITCZ location [ 21 ] and width [ 30 ] based on circulation versus precipitation metrics are qualitatively similar, but quantitative comparisons are more challenging.

However, for trends in ITCZ strength it is less clear that the circulation and precipitation metrics should scale together as climate changes. Precipitation can be approximated as the product of atmospheric moisture content and circulation strength. In our analyses below of projected changes in ITCZ strength we focus on the uncertain dynamic component and use the circulation-based definition 4. We note, however, that a direct comparison between modeled changes in ITCZ strength defined in terms of circulation and observed changes defined in terms of precipitation intensity is not straightforward given that observed changes in precipitation are due to both changes in circulation and atmospheric moisture content [ 76 ].

We examine annual-mean changes between the historical — and RCP8. A limited number of studies have assessed projected changes in ITCZ location [ 26 ], width [ 30 ] and strength [ 29 ] but these studies have used different sets of simulations and different metrics for the ITCZ. Examining the ITCZ location, width and strength changes side-by-side enables comparison of the robustness in the respective responses.

Our analysis shows that climate models predict no robust change in ITCZ location over the twenty-first century Fig. The median model shows a northward shift of 0. It has been suggested that interhemispheric radiative forcing asymmetries associated with declining Northern Hemisphere scattering aerosol concentrations over the twenty-first century may drive a northward shift of the ITCZ [ 16 , 85 ].

However, there are large differences in aerosol radiative forcing across climate models in both historical and future simulations [ 86 , 87 ], and it is likely that this forcing uncertainty contributes to the inter-model spread in projected ITCZ shifts.

The red lines indicate the median model changes, the boxes show the interquartile ranges, and the whiskers show the full model ranges. Although the zonal-mean ITCZ is our focus here, it should be noted that tropical precipitation at individual longitudes is only weakly related to the zonal-mean ITCZ at least in climate models [ 21 ] , and so it is plausible that regional changes in ITCZ location under global warming could be substantially larger than the median predicted zonal-mean change of 0.

By the small-angle approximation, fractional changes in ITCZ width and area are very similar. ITCZ narrowing contrasts with the overall widening of the Hadley circulation under global warming [ 39 , 41 , 42 , 88 , 89 ]. The predicted fractional changes in ITCZ width are smaller than fractional changes in the width of the descent region of the Hadley circulation, and the changes are strongly anti-correlated [ 30 ]: Climate models that predict a large widening of the dry, subtropical descent region tend to also predict a large narrowing of the wet, tropical ITCZ.

Changes in the width of the ITCZ defined using mass versus moisture convergence are well correlated [ 30 ], indicating that the predicted narrowing of tropical ascent will coincide with changes in the hydrological cycle near the edges of the ITCZ.

To directly compare simulated future changes in ITCZ width to the observed trends shown in Figure S1 , we apply the observational analysis technique of Wodzicki and Rapp [ 27 ] based on precipitation to the CMIP5 models.

The contrasting responses of the Atlantic and Pacific ITCZs to observed global warming over recent decades versus in model simulations of the future have multiple potentially valid interpretations.

Another is simply that internal variability, rather than a forced trend, in regional ITCZ width drives the observed variation. As is the case for changes in ITCZ width, this weakening is more robust across climate models than changes in ITCZ location, though there is considerable inter-model spread. Although the ITCZ weakens with warming on average, this weakening is the small residual between two larger quantities: strongly reduced ascent on the equatorward edges of the ITCZ and increased ascent in the core of the ITCZ [ 29 , 30 , 33 ] Fig.

A weakening of the ITCZ is consistent with the general weakening of the overturning atmospheric circulation with warming [ 28 , 93 , 94 , 95 , 96 , 97 ]. In the next section we discuss the relationship between changes in ITCZ width and strength.

The red shading indicates the interquartile range in vertical velocity changes across models at each latitude. The vertical velocities have been vertically averaged with mass weighting from hPa to hPa. The red vertical lines show the multimodel-median northern and southern edges of the ITCZ in the historical simulations, as defined using the mass streamfunction method.

The black dots indicate individual CMIP5 models and the red dot shows the median model changes. What physical mechanisms drive shifts in the ITCZ? The energetic theory for zonal-mean ITCZ location has been comprehensively reviewed in four recent articles [ 1 , 24 , 25 , 26 ]. Here, we summarize this body of work, briefly discuss a distinct but complementary dynamical theory for ITCZ location, and outline some outstanding questions. The energetic theory for zonal-mean ITCZ location has been developed over the last two decades through a combination of new conceptual insights and a wide array of idealized simulations.

This theory relates ITCZ location to interhemispheric contrasts in temperature and net radiative fluxes at TOA [ 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ].

The key idea is that warming or cooling of one hemisphere relative to the other necessitates an anomalous cross-equatorial flow of energy into the colder hemisphere and an ITCZ shift. Early versions of this energetic theory assumed a passive ocean and a cross-equatorial atmospheric energy flux that is associated entirely with the zonal-mean Hadley circulation, with negligible contributions from transient or stationary eddies.

According to this theory, an increase in the interhemispheric TOA radiation contrast as a result of, say, imposing ice in one hemisphere and increasing its albedo [ 10 ] requires an increase in cross-equatorial energy flux by the Hadley circulation and a shift of the ITCZ further into the hemisphere that is receiving additional radiation.

The relationship between the cross-equatorial energy flux and ITCZ location is central to the energetic theory, as it controls how far the ITCZ must shift in response to interhemispheric radiation asymmetries. Building on the work of Kang et al. Thus, a change in net energy input is a possible mechanism for shifting the ITCZ under global warming, and likely contributes to the inter-model spread in predicted ITCZ shifts Fig.

The energetic theory for ITCZ location was developed largely under the assumption of a passive ocean. However, new research [ 1 , 99 , , , , ] has demonstrated that the inclusion of a dynamic ocean strongly damps ITCZ shifts by a factor of approximately three according to one estimate [ ] and reduces the sensitivity of ITCZ location to interhemispheric radiation contrasts.

For a detailed discussion of the role of ocean coupling in ITCZ dynamics, see the recent review by Kang et al. An alternative dynamical theory for ITCZ location, less prominent in the literature than the energetic theory, is based on principles of tropical atmospheric dynamics [ , , , ]. Assuming convection is sufficiently active in the ITCZ such that the lapse rate is close to moist adiabatic [ ] and further assuming the Hadley circulation conserves angular momentum in the free troposphere, the ITCZ location is expected to lie just equatorward of the maximum in boundary-layer moist static energy.

Over oceans, assuming near-surface relative humidity is sufficiently constant in space, the dynamical theory implies that the ITCZ location is just equatorward of the maximum in sea-surface temperature SST. The dynamical theory is broadly verified by an observational analysis of monsoons though the presence of dry, shallow circulations can complicate the picture [ ]. Although this dynamical theory is diagnostic in the sense that the ITCZ location can be determined only if the distribution of boundary-layer moist static energy is known, it nevertheless provides a distinct framework to the energetic theory with which to understand ITCZ migrations.

The energetic theory, summarized by Eq. A priority for future work should be to use this theory and extensions to it to identify the processes driving the inter-model uncertainty in ITCZ shifts Fig. These processes are represented in climate models by a wide variety of components including those simulating cloud physics, radiative transfer, ocean dynamics and ice sheets.

Only after identifying the dominant contributors to the spread in projected ITCZ shifts can reducing this uncertainty become feasible. On the theoretical side, there are opportunities to further develop our physical interpretation of the processes controlling ITCZ location and build towards a complete, predictive theory. The energetic theory for ITCZ location assumes the cross-equatorial atmospheric energy flux is due entirely to the zonal-mean Hadley circulation and that energy transports by transient and stationary eddies are negligible.

However, transient and stationary eddies are not generally negligible in the energy and water budgets of the tropical atmosphere in both the observed climate and in models [ 18 , 21 , 30 , 36 , , , , ].

Such an analysis would inform as to whether a detailed knowledge of eddies at low latitudes is necessary for an understanding of ITCZ location. However, the impacts of climate change are experienced regionally and models suggest that local tropical precipitation changes are not tightly coupled to the zonal-mean ITCZ location [ 21 ]. Furthermore, if we seek to develop a robust understanding of tropical circulation and precipitation in past climates, we need to advance the theory for ITCZ location beyond the zonal mean and make hindcasts that can be compared to paleoclimate data which are inherently regional.

Thus far, this extended theory has been applied to explain continental rainfall shifts in the mid-Holocene [ 22 ] and the observed seasonal and interannual behavior of the zonally anomalous ITCZ over recent decades [ 74 ].

The response of regional tropical precipitation to climate change is highly uncertain [ 96 , ], and this new method should now be applied to identify the processes contributing to inter-model uncertainty on regional scales. Arguably these features are as important to understand and predict as ITCZ location, given that changes in ITCZ width and strength are likely to have important implications for hydroclimates in tropical regions. Here, we review the current state of knowledge on ITCZ width and strength, demonstrate that changes in these quantities are strongly anti-correlated across CMIP5 models, and outline our vision for how progress can be made to advance understanding of these key features of the ITCZ structure.

The ITCZ is narrower than the neighboring subsiding regions of the Hadley circulation [ 36 ] and is a region of widespread moist convection [ ]. Consequently, theories for the area fraction of moist convection have potential relevance for the ITCZ width.

Bjerknes [ ] used a thermodynamic argument based on dry and saturated moist adiabatic lapse rates to argue that moist convection tends to occupy an updraft region that is narrow relative to the downdraft region, although he neglected temperature tendencies due to radiative and surface fluxes. Later, analytical and idealized-modeling studies of the Walker circulation found that the area of ascending motion depends upon the SST gradient, the gross moist stability, Footnote 2 cloud-radiative feedbacks and atmosphere-ocean coupling [ 34 , ].

Following these studies that considered the convective area fraction quite generally, various idealized-modeling studies noted a dependence of ITCZ width on the dynamical core and model resolution [ ], the convective parameterization [ 14 ], the strength of horizontal diffusion of moisture [ 35 ], the radiative effects of clouds and water vapor [ 38 , ], and the longwave optical thickness [ 36 ].

It is clear from these idealized models, and indeed from observations [ 27 ] and comprehensive models [ 29 , 30 ], that the ITCZ width is influenced by a variety of climate processes. A number of mechanisms have been put forward to explain this narrowing.

This advective drying in the boundary layer, combined with enhanced upper-tropospheric warming, inhibits convection on the ITCZ edges and reduces precipitation. However, the upped-ante hypothesis is not a complete explanation; it does not account for processes such as the divergence of moist static energy out of the tropics by transient eddies and changes in net energy input to the atmosphere, which have been shown to affect ITCZ width in a warming climate [ 30 ].

An alternative hypothesis using two heuristic models linking moisture, vertical velocity, and rainfall distributions suggests that an increase in the skewness of the vertical velocity distribution under global warming could explain ITCZ narrowing [ 37 ]. This increase in skewness arises naturally from the asymmetric effect of latent heating on vertical motions, consistent with earlier work [ ]. However, the extent to which this conceptual model can explain the observed and projected ITCZ narrowing in more comprehensive models has yet to be fully explored.

Recently, a new diagnostic equation linking the ITCZ width to energy transports in the climate system has been derived [ 36 ] and applied to quantify the processes contributing to ITCZ narrowing under global warming [ 30 ] see Section 3 of Byrne and Schneider [ 36 ] for a derivation and discussion of this equation. Interestingly, because the Hadley circulation mass budget links area fractions and vertical velocities in the ITCZ and descent regions, processes in both the ITCZ and descent regions can change the ITCZ width though local processes within the ITCZ are typically dominant because the gross moist stability is small there [ 30 ].

The theory described above quantifies how changes in four distinct climate processes influence ITCZ width, and the physical interpretation for these influences is straightforward. Take net energy input to the atmosphere as an example. If climate is perturbed such that net energy input to the ITCZ increases, perhaps due to increased CO 2 concentrations and reduced longwave cooling, the ITCZ circulation must strengthen to transport this excess energy poleward assuming a thermally direct mean divergent circulation, i.

All else being equal in this thought experiment, the vertical mass flux in the descent region of the Hadley circulation remains unchanged, implying that the ITCZ narrows so as to maintain equal and opposite mass fluxes in the ITCZ and descent region Figure S2 , see Supplementary Material. Analogous physical arguments can be made to understand how the other three terms in the diagnostic equation [ 36 ] impact ITCZ width.

In CMIP5 simulations, ITCZ narrowing has been found to be driven by steepening of the meridional moist static energy gradient with global warming—this enhances cooling of the ITCZ by Hadley circulation advection and transient-eddy divergence, which affect the tropical vertical velocity and hence the ITCZ width [ 30 ]. Energy-transporting transient eddies originating in mid-latitudes are thus an important non-local influence on the ITCZ width.