Impact of tropical Atlantic sea-surface temperature biases on the simulated atmospheric circulation and precipitation over the Atlantic region: An ECHAM6 model study
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Abstract:
As many coupled atmosphere-ocean general circulation models, the coupled Earth System Model developed at the Max Planck Institute for Meteorology suffers from severe sea-surface temperature (SST) biases in the tropical Atlantic. We performed a set of SST sensitivity experiments with its atmospheric model component ECHAM6 to understand the impact of tropical Atlantic SST biases on atmospheric circulation and precipitation. The model was forced by a climatology of observed global SSTs to focus on simulated seasonal and annual mean state climate. Through the superposition of varying tropical Atlantic bias patterns extracted from the MPI-ESM on top of the control field, this study investigates the relevance of the seasonal variation and spatial structure of tropical Atlantic biases for the simulated response. Results show that the position and structure of the Intertropical Convergence Zone (ITCZ) across the Atlantic is significantly affected, exhibiting a dynamically forced shift of annual mean precipitation maximum to the east of the Atlantic basin as well as a southward shift of the oceanic rain belt. The SST-induced changes in the ITCZ in turn affect seasonal rainfall over adjacent continents. However not only the ITCZ position but also other effects arising from biases in tropical Atlantic SSTs, e.g. variations in the wind field, change the simulation of precipitation over land. The seasonal variation and spatial pattern of tropical Atlantic SST biases turns out to be crucial for the simulated atmospheric response and is essential for analyzing the contribution of SST biases to coupled model mean state biases. Our experiments show that MPI-ESM mean-state biases in the Atlantic sector are mainly driven by SST biases in the tropical Atlantic while teleconnections from other basins seem to play a minor role.Keywords:
Intertropical Convergence Zone
Tropical Atlantic
Atlantic Equatorial mode
Atmospheric Circulation
Walker circulation
Atmospheric models
Convergence zone
We investigate causes of interannual variability in Atlantic Intertropical Convergence Zone (ITCZ) convection using a monthly mean global precipitation data set spanning 1979–1999. Starting from the hypothesis of two dominant influences on the ITCZ, namely, the cross‐equatorial gradient in tropical Atlantic sea surface temperature (SST) and the anomalous Walker circulation due to the rearrangement of tropical Pacific convection associated with the El Niño–Southern Oscillation, we analyze anomaly composites over the 1979–1999 period that best isolate the effects of each mechanism. Our results suggest that to first order, a strong anomalous Walker circulation suppresses precipitation over the tropical Atlantic, whereas an anomalous warm north/cool south SST gradient shifts the meridional location of maximum ITCZ convection anomalously north. We examined the processes underlying each of the two mechanisms. For the anomalous Walker circulation we find consistency with the idea of suppression of convection through warming of the tropical troposphere brought about by anomalous convective heating in the eastern equatorial Pacific. For the SST gradient mechanism our results confirm previous studies that link convection to cross‐equatorial winds forced by meridional SST gradients. We find that positive surface flux feedback brought about through the cross‐equatorial winds is weak and confined to the deep tropics. On the basis of the results of this and other studies we propose an expanded physical picture that explains key features of Atlantic ITCZ variability, including its seasonal preference, its sensitivity to small anomalous SST gradients, and its role in the context of tropical Atlantic SST gradient variability.
Intertropical Convergence Zone
Tropical Atlantic
Walker circulation
Anomaly (physics)
Atlantic Equatorial mode
Forcing (mathematics)
Madden–Julian oscillation
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During some El Niño events Rossby wave trains (RWT) are observed to strongly modulate the seasonal atmospheric circulation of the South Pacific extratropics in the austral winter and spring. Here it is shown that seasonally intensified deep tropical convection is confined to the Pacific Intertropical Convergence Zone (ITCZ) close to the dateline in those events with strong RWT modulation but not in other cases. In other cases deep convection either withdraws from the ITCZ or the ITCZ strongly interacts with the South Pacific Convergence Zone (SPCZ) lying at 5–10°S. In both cases this points to a weakening of the local tropical Hadley circulation that may be crucial for Rossby wave generation. It is also found that the expected RWT response in the high latitudes can fail to occur even when SST in the central tropical Pacific are very high.
Intertropical Convergence Zone
Extratropical cyclone
Atmospheric Circulation
Walker circulation
Hadley cell
Convergence zone
Atmospheric convection
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The Intertropical Convergence Zone (ITCZ) in the tropical eastern Indian Ocean exhibits strong interannual variability, often co-occurring with positive Indian Ocean Dipole (pIOD) events. During what we identify as an extreme ITCZ event, a drastic northward shift of atmospheric convection coincides with an anomalously strong north-minus-south sea surface temperature (SST) gradient over the eastern equatorial Indian Ocean. Such shifts lead to severe droughts over the maritime continent and surrounding islands but also devastating floods in southern parts of the Indian subcontinent. Understanding future changes of the ITCZ is therefore of major scientific and socioeconomic interest. Here we find a more-than-doubling in the frequency of extreme ITCZ events under greenhouse warming, estimated from climate models participating in the Coupled Model Intercomparison Project phase 5 that are able to simulate such events. The increase is due to a mean state change with an enhanced north-minus-south SST gradient and a weakened Walker Circulation, facilitating smaller perturbations to shift the ITCZ northwards.
Intertropical Convergence Zone
Convergence zone
Walker circulation
Indian subcontinent
Tropical climate
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During periods of reduced Atlantic meridional overturning circulation (AMOC) associated with a freshening of northern North Atlantic surface water, paleo proxy records indicate a corresponding surface salinity increase over the entire tropical Atlantic. Although latitudinal‐shifts in the mean position of the Atlantic Intertropical Convergence Zone (ITCZ) can explain certain features of the paleo salinity reconstructions, this mechanism does not offer an explanation for the reconstructed basin‐wide paleo‐salinity response to AMOC change. Here, we present new results from general circulation model simulations that suggest the sea surface salinity (SSS) increase in the tropical north Atlantic during periods of weakened AMOC is mainly controlled by the atmospheric response to the North Atlantic cooling, while the oceanic teleconnection contributes to increased SSS over the equatorial and south tropical Atlantic Ocean.
Intertropical Convergence Zone
Tropical Atlantic
Atlantic Equatorial mode
Teleconnection
Convergence zone
Atlantic hurricane
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We investigate how air‐sea interaction affects an Intertropical Convergence Zone (ITCZ) simulation in the SAMIL2.08 atmospheric general circulation model (AGCM). In a control experiment (Exp1) with the observed sea surface temperature (SST) prescribed in the AGCM, there exists a problem of excessive precipitation over much of the Tropics and insufficient precipitation over the equatorial Indian Ocean and the Pacific. The equatorial drought belt arises from the compensatory descending motion associated with exaggerated deep convection over the tropics in both hemispheres. A double ITCZ disappears in a coupled experiment (Exp2) with the same AGCM as used in Exp1 coupled to an interactive ocean mixed layer within the great warm pool. This finding demonstrates that local air‐sea interaction can modify the SST pattern, thereby regulating the climate mean state via the following processes. Local air‐sea flux exchanges in tropical convective regions such as the ITCZ tend to cool SST via negative cloud‐radiation and wind‐evaporation feedbacks. Such changes further modify the tropical atmospheric circulation structure such that the equatorial compensatory descent in Exp1 is replaced by the equatorial convergence zone, as seen in nature. A third sensitivity experiment (Exp3), with the AGCM driven by the monthly SST field derived from the coupled experiment, yielded similar results to those obtained in Exp2. Overall, the results indicate that a reasonable depiction of the air‐sea coupling process is important to successfully simulating the tropical precipitation pattern, as the atmosphere is closely coupled with the ocean over the tropics.
Intertropical Convergence Zone
Walker circulation
Convergence zone
Hadley cell
Tropical Atlantic
Atmospheric models
Atmospheric Circulation
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The Atlantic multidecadal oscillation (AMO) has been shown to play a major role in the multidecadal variability of the Northern Hemisphere, impacting temperature and precipitation, including intertropical convergence zone (ITCZ)-driven precipitation across Africa and South America. Studies into the location of the intertropical convergence zone have suggested that it resides in the warmer hemisphere, with the poleward branch of the Hadley cell acting to transport energy from the warmer hemisphere to the cooler one. Given the impact of the Atlantic multidecadal oscillation on Northern Hemisphere temperatures, we expect the Atlantic multidecadal oscillation to have an impact on the location of the intertropical convergence zone. We find that the positive phase of the Atlantic multidecadal oscillation warms the Northern Hemisphere, resulting in a northward shift of the intertropical convergence zone, which is evident in the Pacific climate proxy record. Using a coupled climate model, we further find that the shift in the intertropical convergence zone is consistent with the surface energy imbalance generated by the Atlantic multidecadal oscillation. In this model, the Pacific changes are driven in large part by the warming of the tropical Atlantic and not the extratropical Atlantic.
Intertropical Convergence Zone
Extratropical cyclone
Atlantic Equatorial mode
Convergence zone
Tropical Atlantic
Forcing (mathematics)
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Abstract The tropical zonal-mean precipitation distribution varies between having single or double peaks, which are associated with intertropical convergence zones (ITCZs). Here, the effect of this meridional modality on the sensitivity of the ITCZ to hemispherically asymmetric heating is studied using an idealized GCM with parameterized Ekman ocean energy transport (OET). In the idealized GCM, transitions from unimodal to bimodal distributions are driven by equatorial ocean upwelling and cooling, which inhibits equatorial precipitation. For sufficiently strong equatorial cooling, the tropical circulation bifurcates to anti-Hadley circulation in the deep tropics, with a descending branch near the equator and off-equatorial double ITCZs. The intensity and extent of the anti-Hadley circulation is limited by a negative feedback: westerly geostrophic surface wind tendency in its poleward-flowing lower branches balances the easterly stress (and hence equatorial upwelling) required for its maintenance. For weak ocean stratification, which goes along with unimodal or weak bimodal tropical precipitation distribution, OET damps shifts of the tropical precipitation centroid but amplifies shifts of precipitation peaks. For strong ocean stratification, which goes along with pronounced double ITCZs, asymmetric heating leads to relative intensification of the precipitation peak in the warming hemisphere, but negligible meridional shifts. The dynamic feedbacks of the coupled system weaken the gradient of the atmospheric energy transport (AET) near the equator. This suggests that over a wide range of climates, the ITCZ position is proportional to the cubic root of the cross-equatorial AET, as opposed to the commonly used linear relation.
Intertropical Convergence Zone
Hadley cell
Walker circulation
Convergence zone
Tropical Atlantic
Stratification (seeds)
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Abstract This study investigated the Intertropical Convergence Zone (ITCZ) extreme shifts in the Southern Maritime Continent (SMC) region in austral spring during the period 1979–2015. The results have shown that the ITCZ Southern Boundary (SB) presents great correlations with a meridional Sea Surface Temperature (SST) gradient and Indian Ocean Dipole (IOD) events. However, besides these local forcing, the extreme ITCZ shifts are also influenced by El Niño‐Southern Oscillation (ENSO) events and an associated zonal SST gradient. The extreme shifts are due mainly to the combination of changes in Walker Circulation (WC) in the Pacific and Indian Ocean (IO) and local SST anomalies. Therefore, during ITCZ southward extreme shifts, the WC is stronger leading to the strengthening of the northwest equatorial winds in the IO, which pumps moist air and boosts the diabatic heating in the SMC due to the local positive SST anomalies present in this region. Otherwise, during northward extreme shifts, the WC is weaker, not allowing the performance of the northwest equatorial winds in the IO, which associated with the local negative SST anomalies, leads to drier conditions in the SMC region. Another interesting result found here is that a strong meridional or zonal index not necessarily implies a larger shift. The combination of very strong ENSO and IOD events can lead to greater ITCZ SB shifts. Besides, the ITCZ SB shifts can act as a bridge linking IOD and ENSO.
Intertropical Convergence Zone
Walker circulation
Convergence zone
Hadley cell
Forcing (mathematics)
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