The effect of variation in sea-surface temperature and its meridional gradient on the equatorial and off-equatorial ITCZ in an Aquaplanet General Circulation model

Summary The factors that control the strength of the ITCZ (Inter Tropical Convergence Zone) in an aquaplanet GCM (General Circulation Model) have been investigated. The strength of the equatorial ITCZ was found to increase rapidly with increase in meridional gradient of SST. On the other hand, the strength of the off-equatorial ITCZ does not increase rapidly with increase in meridional gradient of SST. This unusual difference in behavior between off-equatorial and equatorial ITCZ has been interpreted with a diagnostic model. The diagnostic model is based on budgets of moisture and dry static energy in the ITCZ. The diagnostic model indicates that the variations in the strength of the ITCZ are related to changes in the net energy convergence and vertical moist static stability. It was found that the net energy convergence in the off-equatorial ITCZ increases much less rapidly with meridional SST gradient than the equatorial counterpart. This difference in the behavior of net energy convergence is related to the surface wind speed which in the off-equatorial ITCZ simulation is largely insensitive to changes in the meridional SST gradient. Thus the primary difference between the equatorial and off-equatorial ITCZ is on account of the fact that wind speeds were lower in the former (on account of the constraint that zonal wind has to be zero at the equator). The impact of increasing the SST maximum on the strength of the ITCZ has also been studied. It was found that the strength of ITCZ increases with an increase in SST maximum. This increase in the strength of the ITCZ with the maximum SST is governed by the increase in boundary layer specific humidity and its impact on vertical moist static stability.     

Global energy budget changes to high latitude North Atlantic cooling and the tropical ITCZ response

The intertropical convergence zone (ITCZ) in atmospheric general circulation models (coupled to slab ocean) shift southwards in response to northern extratropical cooling. Previous studies have demonstrated the utility of diagnosing the atmospheric energy fluxes in interpreting this teleconnection. This study investigates the nature of global energy flux changes in response to North Atlantic high latitude cooling applied to the Community Atmosphere Model version 3 coupled to a slab ocean, focusing on key local and remote feedbacks that collectively act to alter the energy budget and atmospheric energy transport. We also investigate the relative roles of tropical sea surface temperature (SST) and energy flux changes in the ITCZ response to North Atlantic cooling. Using a radiative kernel technique, we quantify the effects of key feedbacks—temperature, cloud and water vapor, to the top-of-the-atmosphere radiative flux changes. The results show only partial local energy flux compensation to the initial perturbation in the high latitudes, originating from the negative temperature feedback and opposed by positive shortwave albedo and longwave water vapor feedbacks. Thus, an increase in the atmospheric energy transport to the Northern extratropics is required to close the energy budget. The additional energy flux providing this increase comes from top-of-the-atmosphere radiative flux increase over the southern tropics, primarily from cloud, temperature and longwave water vapor feedbacks, and largely as a consequence of increased deep convection. It has been previously argued that the role of tropical SST changes was secondary to the role played by the atmospheric energy flux requirements in controlling the ITCZ shifts, proposing that the SST response is a result of the surface energy budget and not a driver of the precipitation response. Using a set of idealized simulations with the fixed tropical SSTs, we demonstrate that the ITCZ shifts are not possible without the tropical SST changes and suggest that the tropical SSTs are a more suitable driver of tropical precipitation shifts compared to the atmospheric energy fluxes. In our simulations, the ITCZ shifts are influenced mainly by the local (tropical) SST forcing, apparently independent of the actual high latitude energy demand.     

Sensitivity of ITCZ configuration to cumulus convective parameterizations on an aqua planet

This paper examines the performances of various cumulus convective parameterization schemes in the tropical atmosphere using an aqua-planet atmospheric General Circulation Model forced by zonally symmetric but latitudinally varying sea surface temperature (SST) and solar angle. The intertropical convergence zone (ITCZ) is represented by intense precipitation. The assigned Control experiment with a specific SST distribution, as designated by the Aqua Planet Experiment, yields a single ITCZ when Zhang’s scheme or Manabe’s scheme is employed, whereas a double ITCZ occurs when Tiedtke’s scheme is used. The key to the occurrence of a double ITCZ is latitudinal variation in evaporation within the boundary layer. Such variation is induced mainly by latitudinal variation in the zonal wind speed, with the existence of a calm belt at the equator and a maximum wind speed located off the equator, arising from the evaporation–wind feedback (EWF) mechanism. The latitudinal distribution of evaporation results in a decrease in the height of the lifting condensation level in areas off the equator and an increase at the equator. The occurrence of a single ITCZ in Zhang’s scheme is attributed to the use of a Convective Available Potential Energy criterion by which convection occurs more readily at the equator. As a result, a precipitation maximum is maintained at the equator via a prevailing Conditional Instability of the Second Kind mechanism.     

POSSIBLE INFLUENCE OF FEBRUARY-APRIL ARCTIC OSCILLATION ON THE ITCZ ACTIVITY OF WESTERN-CENTRAL PACIFIC

The daily patterns and activity of Intertropical Convergence Zone(ITCZ) in the Western-Central Pacific Ocean are analyzed using NOAA interpolated Outgoing Longwave Radiation dataset during the period from 1979 to 2008, and the relationships between ITCZ patterns and Arctic Oscillation(AO) is investigated in this paper. In accordance with the central activity region the daily ITCZ can be divided into six patterns—north, south, equator, double, full and weak pattern, respectively. The statistic result shows that the north(accounting for 30.98% of the total observations), south(31.11%) and weak(24.05%) ITCZ patterns are the most active daily patterns within a 30-year period, while the other three ITCZ patterns occur infrequently. Results show that the February-April AO index has a significant positive(negative) correlation with the frequency of the north(weak) ITCZ pattern from March-May to August-October, with the strongest correlation in April-June(March-May). At the same time, the lower tropospheric atmosphere circulation(850-hPa wind field) and SST anomalies corresponding to the AO change significantly in the tropical Pacific. When AO is in the positive phase, there is an anomalous westerly from the equator to 15°N and warmer SST in the critical north ITCZ active region, while there is an anomalous easterly and insignificant change of SST from the equator to 15°S. The wind and SST anomalies share the same characteristics of the equatorial asymmetry and thus enlarge the gradient between the south and north of equator, which would help reinforce convection in the north of equator and result in more frequent occurrence of the northern type of ITCZ.     

Regimes of seasonal air–sea interaction and implications for performance of forced simulations

Sea surface temperature (SST) anomalies can induce anomalous convection through surface evaporation and low-level moisture convergence. This SST forcing of the atmosphere is indicated in a positive local rainfall–SST correlation. Anomalous convection can feedback on SST through cloud-radiation and wind-evaporation effects and wind-induced oceanic mixing and upwelling. These atmospheric feedbacks are reflected in a negative local rainfall–SST tendency correlation. As such, the simultaneous rainfall–SST and rainfall–SST tendency correlations can indicate the nature of local air–sea interactions. Based on the magnitude of simultaneous rainfall–SST and rainfall–SST tendency correlations, the present study identifies three distinct regimes of local air–sea interactions. The relative importance of SST forcing and atmospheric forcing differs in these regimes. In the equatorial central-eastern Pacific and, to a smaller degree, in the western equatorial Indian Ocean, SST forcing dominates throughout the year and the surface heat flux acts mainly as a damping term. In the tropical Indo-western Pacific Ocean regions, SST forcing and atmospheric forcing dominate alternatively in different seasons. Atmospheric forcing dominates in the local warm/rainy season. SST forcing dominates with a positive wind-evaporation feedback during the transition to the cold/dry season. SST forcing also dominates during the transition to the warm/rainy season but with a negative cloud-radiation feedback. The performance of atmospheric general circulation model simulations forced by observed SST is closely linked to the regime of air–sea interaction. The forced simulations have good performance when SST forcing dominates. The performance is low or poor when atmospheric forcing dominates.     

Impact of different convective cloud schemes on the simulation of the tropical seasonal cycle in a coupled ocean–atmosphere model

The simulation of the mean seasonal cycle of sea surface temperature (SST) remains a challenge for coupled ocean–atmosphere general circulation models (OAGCMs). Here we investigate how the numerical representation of clouds and convection affects the simulation of the seasonal variations of tropical SST. For this purpose, we compare simulations performed with two versions of the same OAGCM differing only by their convection and cloud schemes. Most of the atmospheric temperature and precipitation differences between the two simulations reflect differences found in atmosphere-alone simulations. They affect the ocean interior down to 1,000 m. Substantial differences are found between the two coupled simulations in the seasonal march of the Intertropical Convergence Zone in the eastern part of the Pacific and Atlantic basins, where the equatorial upwelling develops. The results confirm that the distribution of atmospheric convection between ocean and land during the American and African boreal summer monsoons plays a key role in maintaining a cross equatorial flow and a strong windstress along the equator, and thereby the equatorial upwelling. Feedbacks between convection, large-scale circulation, SST and clouds are highlighted from the differences between the two simulations. In one case, these feedbacks maintain the ITCZ in a quite realistic position, whereas in the other case the ITCZ is located too far south close to the equator.     

Equatorial Atlantic variability and its relation to mean state biases in CMIP5

Coupled general circulation model (GCM) simulations participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are analyzed with respect to their performance in the equatorial Atlantic. In terms of the mean state, 29 out of 33 models examined continue to suffer from serious biases including an annual mean zonal equatorial SST gradient whose sign is opposite to observations. Westerly surface wind biases in boreal spring play an important role in the reversed SST gradient by deepening the thermocline in the eastern equatorial Atlantic and thus reducing upwelling efficiency and SST cooling in the following months. Both magnitude and seasonal evolution of the biases are very similar to what was found previously for CMIP3 models, indicating that improvements have only been modest. The weaker than observed equatorial easterlies are also simulated by atmospheric GCMs forced with observed SST. They are related to both continental convection and the latitudinal position of the intertropical convergence zone (ITCZ). Particularly the latter has a strong influence on equatorial zonal winds in both the seasonal cycle and interannual variability. The dependence of equatorial easterlies on ITCZ latitude shows a marked asymmetry. From the equator to 15°N, the equatorial easterlies intensify approximately linearly with ITCZ latitude. When the ITCZ is south of the equator, on the other hand, the equatorial easterlies are uniformly weak. Despite serious mean state biases, several models are able to capture some aspects of the equatorial mode of interannual SST variability, including amplitude, pattern, phase locking to boreal summer, and duration of events. The latitudinal position of the boreal spring ITCZ, through its influence on equatorial surface winds, appears to play an important role in initiating warm events.     

气候系统模式FGOALS模拟的南亚夏季风：偏差和原因分析

本文通过与观测和再分析资料的对比,评估了LASG/IAP发展的气候系统模式FGOALS的两个版本FGOALS-g2和FGOALS-s2对南亚夏季风的气候态和年际变率的模拟能力,并使用水汽收支方程诊断,研究了造成降水模拟偏差的原因。结果表明,两个模式夏季气候态降水均在陆地季风槽内偏少,印度半岛附近海域偏多,在降水年循环中表现为夏季北侧辐合带北推范围不足。FGOALS-g2中赤道印度洋"东西型"海温偏差导致模拟的东赤道印度洋海上辐合带偏弱,而FGOALS-s2中印度洋"南北型"海温偏差导致模拟的海上辐合带偏向西南。水汽收支分析表明,两个模式中气候态夏季风降水的模拟偏差主要来自于整层积分的水汽通量,尤其是垂直动力平流项的模拟偏差。一方面,夏季阿拉伯海和孟加拉湾的海温偏冷而赤道西印度洋海温偏暖,造成向印度半岛的水汽输送偏少;另一方面,对流层温度偏冷,冷中心位于印度半岛北部对流层上层,同时季风槽内总云量偏少,云长波辐射效应偏弱,对流层经向温度梯度偏弱以及大气湿静力稳定度偏强引起的下沉异常造成陆地季风槽内降水偏少。在年际变率上,观测中南亚夏季风环流和降水指数与Ni?o3.4指数存在负相关关系,但FGOALS两个版本模式均存在较大偏差。两个模式中与ENSO暖事件相关的沃克环流异常下沉支和对应的负降水异常西移至赤道以南的热带中西印度洋,沿赤道非对称的加热异常令两个模式中越赤道环流季风增强,导致印度半岛南部产生正降水异常。ENSO相关的沃克环流异常下沉支及其对应的负降水异常偏西与两个模式对热带南印度洋气候态降水的模拟偏差有关。研究结果表明,若要提高FGOALS两个版本模式对南亚夏季风气候态模拟技巧,需减小耦合模式对印度洋海温、对流层温度及云的模拟偏差;若要提高南亚夏季风和ENSO相关性模拟技巧需要提高模式对热带印度洋气候态降水以及与ENSO相关的环流异常的模拟能力。     

A comparison of low-latitude cloud properties and their response to climate change in three AGCMs sorted into regimes using mid-tropospheric vertical velocity

Low-latitude cloud distributions and cloud responses to climate perturbations are compared in near-current versions of three leading U.S. AGCMs, the NCAR CAM 3.0, the GFDL AM2.12b, and the NASA GMAO NSIPP-2 model. The analysis technique of Bony et al. (Clim Dyn 22:71–86, 2004) is used to sort cloud variables by dynamical regime using the monthly mean pressure velocity ω at 500 hPa from 30S to 30N. All models simulate the climatological monthly mean top-of-atmosphere longwave and shortwave cloud radiative forcing (CRF) adequately in all ω-regimes. However, they disagree with each other and with ISCCP satellite observations in regime-sorted cloud fraction, condensate amount, and cloud-top height. All models have too little cloud with tops in the middle troposphere and too much thin cirrus in ascent regimes. In subsidence regimes one model simulates cloud condensate to be too near the surface, while another generates condensate over an excessively deep layer of the lower troposphere. Standardized climate perturbation experiments of the three models are also compared, including uniform SST increase, patterned SST increase, and doubled CO₂ over a mixed layer ocean. The regime-sorted cloud and CRF perturbations are very different between models, and show lesser, but still significant, differences between the same model simulating different types of imposed climate perturbation. There is a negative correlation across all general circulation models (GCMs) and climate perturbations between changes in tropical low cloud cover and changes in net CRF, suggesting a dominant role for boundary layer cloud in these changes. For some of the cases presented, upper-level clouds in deep convection regimes are also important, and changes in such regimes can either reinforce or partially cancel the net CRF response from the boundary layer cloud in subsidence regimes. This study highlights the continuing uncertainty in both low and high cloud feedbacks simulated by GCMs.     

耦合海-气环流模式中双热带辐合带现象及其热收支分析

文中研究了耦合海-气环流模式中的双热带辐合带(Double ITCZ)现象,并对这一现象的成因从海洋热量收支的角度进行了初步分析。Double ITCZ现象是在热带太平洋赤道南北两侧各出现一条ITCZ的现象,这是耦合海-气环流模式中的较为普遍的一种异常现象,与实际气候中出现的Double ITCZ现象并非指同一问题。文中对比观测和模式结果,指出了Double ITCZ现象的主要特征,针对它的出现过程进行细致分析,再利用模式输出的热量收支各项进行统计,得到了从海洋热收支分析得到的海表温度变化原因。与观测到的正常模态相比,Double ITCZ是一个异常的模态,它的特征突出地表现为西太平洋暖池区的降温和东南太平洋10°S附近的升温。海洋热量收支分析表明,暖池区的降温主要是受到扩散的作用,而表层平流的异常增暖在决定异常辐合带区升温过程中占第一位的作用。     

Oscillations of the intertropical convergence zone and the genesis of easterly waves. Part I: diagnostics and theory

We examine the mean and transient state of the intertropical convergence zone (ITCZ) by analyzing data and using simple theory. We concentrate on the tropical eastern Pacific Ocean noting that there exists in this region a well-developed mean ITCZ. Furthermore, it is a region where there has been considerable discussion in the literature of whether easterly waves develop in situ or propagate westwards from the Atlantic Ocean. The region is typical of tropical regions where there is a strong cross-equatorial pressure gradient (CEPG): mean convection well removed from the equator but located equatorward of the maximum sea-surface temperature (SST) and minimum sea level pressure (MSLP). Further to the west, near the dateline where the CEPG is much smaller, convection is weaker and collocated with SST and MSLP extrema. It is argued that in regions of significant CEPG that the near-equatorial tropical system is inertially unstable and that the rectification of the instability for a given CEPG determines the location and intensity of the climatological ITCZ. Using simple theoretical arguments, we develop an expression for the mean latitude of the ITCZ as a function of the CEPG. We note on a day-by-day basis that the ITCZ is highly transient state with variability occurring on 3–8 day time scales. Transients with amplitudes about half of the mean ITCZ, propagate northwards from the near-equatorial southern hemisphere as anomalous meridional oscillations, eventually amplifying convection in the vicinity of the mean ITCZ. It is argued that in these longitudes of strong CEPG the mean ITCZ is continually inertially unstable with advections of anticylonic vorticity across the equator resulting in the creation of an oscillating divergence–convergence doublet. The low-level convergence produces convection and the resultant vortex tube stretching generates cyclonic vorticity which counteracts the northward advection of anticylonic vorticity. During a cycle, the mid-troposphere heating near 10oN oscillates between 6 and 12 K/day at the inertial frequency of the latitude of the mean convection. As a result, there exists an anomalous and shallower, oscillating meridional circulation with a magnitude about 50% of the mean ITCZ associated with the stable state following the generation of anticylonic vorticity. Further, it is argued that the instabilities of the ITCZ are directly associated with in situ development of easterly waves which occur with the inertial period of the latitude of the mean ITCZ. The dynamical sequences and the genesis of easterly waves are absent in the regions further to the east where the CEPG is much weaker or absent altogether. In a companion study (Part II), numerical experiments are conducted to test the hypothesis raised in the present study.     

Analysis of the tropical climate variability in a two-column framework

This work examines the relevance of a classical two-column modeling framework of the tropical climate in terms of observed natural variability. A method is developed to analyze the observed tropical climate in a simple framework that features a moist, ascending column and a dry, subsiding one. This method is used to analyze the natural variability of the tropical climate in the ERA40 reanalysis and in ISCCP satellite data. It appears that the seasonal cycle of the tropic-wide sea surface temperature (SST) is almost linearly linked to the seasonal cycle of the relative area of the moist regions, as predicted by the sensitivity of the two-column models. A more detailed analysis shows that this link is the product of a complex interaction and adjustments between the moist and dry regions. The seasonal cycle of low-cloud cover in the dry regions also appears to interact with the SST seasonal cycle: the low-cloud cover influences the tropic-wide SST via its direct radiative forcing on the local SST and it appears to be controlled by the SST difference between moist and dry regions. By contrast, the SST interannual variability appears to be driven by the El Ni?o Southern Oscillation (ENSO), with no significant impact from the changes in the relative area of the moist regions or in the low-cloud cover in the dry regions independently of the ENSO. ENSO-related changes in the area of moist regions and low-cloud cover constitute negative feedbacks on the ENSO-related SST variability.     

The ocean’s role in setting the mean position of the Inter-Tropical Convergence Zone

Through study of observations and coupled climate simulations, it is argued that the mean position of the Inter-Tropical Convergence Zone (ITCZ) north of the equator is a consequence of a northwards heat transport across the equator by ocean circulation. Observations suggest that the hemispheric net radiative forcing of climate at the top of the atmosphere is almost perfectly symmetric about the equator, and so the total (atmosphere plus ocean) heat transport across the equator is small (order 0.2 PW northwards). Due to the Atlantic ocean’s meridional overturning circulation, however, the ocean carries significantly more heat northwards across the equator (order 0.4 PW) than does the coupled system. There are two primary consequences. First, atmospheric heat transport is southwards across the equator to compensate (0.2 PW southwards), resulting in the ITCZ being displaced north of the equator. Second, the atmosphere, and indeed the ocean, is slightly warmer (by perhaps 2 °C) in the northern hemisphere than in the southern hemisphere. This leads to the northern hemisphere emitting slightly more outgoing longwave radiation than the southern hemisphere by virtue of its relative warmth, supporting the small northward heat transport by the coupled system across the equator. To conclude, the coupled nature of the problem is illustrated through study of atmosphere–ocean–ice simulations in the idealized setting of an aquaplanet, resolving the key processes at work.     

Impact of resolving the diurnal cycle in an ocean–atmosphere GCM. Part 2: A diurnally coupled CGCM

Coupled ocean atmosphere general circulation models (GCM) are typically coupled once every 24 h, excluding the diurnal cycle from the upper ocean. Previous studies attempting to examine the role of the diurnal cycle of the upper ocean and particularly of diurnal SST variability have used models unable to resolve the processes of interest. In part 1 of this study a high vertical resolution ocean GCM configuration with modified physics was developed that could resolve the diurnal cycle in the upper ocean. In this study it is coupled every 3 h to atmospheric GCM to examine the sensitivity of the mean climate simulation and aspects of its variability to the inclusion of diurnal ocean-atmosphere coupling. The inclusion of the diurnal cycle leads to a tropics wide increase in mean sea surface temperature (SST), with the strongest signal being across the equatorial Pacific where the warming increases from 0.2°C in the central and western Pacific to over 0.3°C in the eastern equatorial Pacific. Much of this warming is shown to be a direct consequence of the rectification of daily mean SST by the diurnal variability of SST. The warming of the equatorial Pacific leads to a redistribution of precipitation from the Inter tropical convergence zone (ITCZ) toward the equator. In the western Pacific there is an increase in precipitation between Papa new guinea and 170°E of up to 1.2 mm/day, improving the simulation compared to climatology. Pacific sub tropical cells are increased in strength by about 10%, in line with results of part 1 of this study, due to the modification of the exchange of momentum between the equatorially divergent Ekman currents and the geostropic convergence at depth, effectively increasing the dynamical response of the tropical Pacific to zonal wind stresses. During the spring relaxation of the Pacific trade winds, a large diurnal cycle of SST increases the seasonal warming of the equatorial Pacific. When the trade winds then re-intensify, the increase in the dynamical response of the ocean leads to a stronger equatorial upwelling. These two processes both lead to stronger seasonal basin scale feedbacks in the coupled system, increasing the strength of the seasonal cycle of the tropical Pacific sector by around 10%. This means that the diurnal cycle in the upper ocean plays a part in the coupled feedbacks between ocean and atmosphere that maintain the basic state and the timing of the seasonal cycle of SST and trade winds in the tropical Pacific. The Madden–Julian Oscillation (MJO) is examined by use of a large scale MJO index, lag correlations and composites of events. The inclusion of the diurnal cycle leads to a reduction in overall MJO activity. Precipitation composites show that the MJO is stronger and more coherent when the diurnal cycle of coupling is resolved, with the propagation and different phases being far more distinct both locally and to larger lead times across the tropical Indo-Pacific. Part one of this study showed that that diurnal variability of SST is modulated by the MJO and therefore increases the intraseasonal SST response to the different phases of the MJO. Precipitation-based composites of SST variability confirm this increase in the coupled simulations. It is argued that including this has increased the thermodynamical coupling of the ocean and atmosphere on the timescale of the MJO (20–100 days), accounting for the improvement in the MJO strength and coherency seen in composites of precipitation and SST. These results show that the diurnal cycle of ocean–atmosphere interaction has profound impact on a range of up-scale variability in the tropical climate and as such, it is an important feature of the modelled climate system which is currently either neglected or poorly resolved in state of the art coupled models.     

Long-Term Integration of a Global Non-Hydrostatic Atmospheric Model on an Aqua Planet

A global non-hydrostatic atmospheric model, i.e., GRAPES_YY (Global/Regional Assimilation and Prediction System on the Yin–Yang grid), with a semi-implicit semi-Lagrangian (SISL) dynamical core developed on the Yin–Yang grid was coupled with the physical parameterization package of the operational version of GRAPES. A 3.5-yr integration was carried out on an aqua planet to assess the numerical performance of this non-hydrostatic model relative to other models. Specific aspects of precipitation and general circulation under two different sea surface temperature (SST) conditions (CONTROL and FLAT) were analyzed. The CONTROL SST peaked at the equator. The FLAT SST had its maximum gradient at about 20° latitude, giving a broad equatorial SST maximum in the tropics and flat profile approaching the equator. The tropical precipitation showed different propagation features in the CONTROL and FLAT simulations. The CONTROL showed tropical precipitation bands moving eastward with some envelopes of westward convective-scale disturbance. Less organized westward-propagating rainfall cells and bands were seen in the FLAT and the propagation of the tropical wave varied with the SST gradient. The Inter Tropical Convergence Zone (ITCZ), Hadley cell, and westerly jet core were weaker and more poleward as the SST profile flattened from the CONTROL to FLAT. The climatological structures simulated by GRAPES_YY, such as the distribution of precipitation and the large-scale circulation, fell within the bounds from other models. The stronger ITCZ precipitation, accompanied with stronger Hadley cells and convective heating in the CONTROL simulation, may be summed up as a result of stronger parameterized convection and the non-hydrostatic effects in GRAPES_YY. In addition, mechanism of the zonal mean circulation maintaining is analyzed for the different SST patterns referring the transient eddy flux.     

On the observational determination of climate sensitivity and its implications

We estimate climate sensitivity from observations, using the deseasonalized fluctuations in sea surface temperatures (SSTs) and the concurrent fluctuations in the top-of-atmosphere (TOA) outgoing radiation from the ERBE (1985–1999) and CERES (2000–2008) satellite instruments. Distinct periods of warming and cooling in the SSTs were used to evaluate feedbacks. An earlier study (Lindzen and Choi, 2009) was subject to significant criticisms. The present paper is an expansion of the earlier paper where the various criticisms are taken into account. The present analysis accounts for the 72 day precession period for the ERBE satellite in a more appropriate manner than in the earlier paper. We develop a method to distinguish noise in the outgoing radiation as well as radiation changes that are forcing SST changes from those radiation changes that constitute feedbacks to changes in SST. We demonstrate that our new method does moderately well in distinguishing positive from negative feedbacks and in quantifying negative feedbacks. In contrast, we show that simple regression methods used by several existing papers generally exaggerate positive feedbacks and even show positive feedbacks when actual feedbacks are negative. We argue that feedbacks are largely concentrated in the tropics, and the tropical feedbacks can be adjusted to account for their impact on the globe as a whole. Indeed, we show that including all CERES data (not just from the tropics) leads to results similar to what are obtained for the tropics alone — though with more noise. We again find that the outgoing radiation resulting from SST fluctuations exceeds the zerofeedback response thus implying negative feedback. In contrast to this, the calculated TOA outgoing radiation fluxes from 11 atmospheric models forced by the observed SST are less than the zerofeedback response, consistent with the positive feedbacks that characterize these models. The results imply that the models are exaggerating climate sensitivity.     

Single and double ITCZ in an aqua-planet model with constant sea surface temperature and solar angle

It has been known for more than a decade that an aqua-planet model with a globally- and temporally-uniform sea surface temperature and solar isolation angle can generate intertropical convergence zones (ITCZ). Employing such a model, previous studies have shown that one of several means can be used to change between a single ITCZ over the equator and a double ITCZ straddling the equator. These means include switching to a different cumulus parametrization scheme, making changes within the cumulus parametrization scheme, and changing other aspects of the model such as horizontal resolution. Here, an interpretation of these findings is offered. In an aqua-planet model with globally and temporally uniform sea surface temperature and solar isolation angle, the latitudinal location of an ITCZ is the latitude where a balance exists between two types of attraction, both resulting from the Earths rotation. The first attraction pulls the ITCZ towards the equator and is not sensitive to changes in model design. It is directly related to the Coriolis parameter, which provides stability to the atmosphere. The second ssattraction pulls the ITCZ poleward and is sensitive to changes in model design. It is related to the convective circulation, modified by the Coriolis force. A balance between the two types of attraction is reached either at the equator or more than 10° north and south of the equator, depending on the shape and magnitude of the attractions. A balance at the equator yields a single ITCZ over the equator, whereas a balance north and south of the equator yields a double ITCZ straddling the equator.     

Coupled ocean-atmosphere surface variability and its climate impacts in the tropical Atlantic region

 This study examines time evolution and statistical relationships involving the two leading ocean-atmosphere coupled modes of variability in the tropical Atlantic and some climate anomalies over the tropical 120 °W–60 °W region using selected historical files (75-y near global SSTs and precipitation over land), more recent observed data (30-y SST and pseudo wind stress in the tropical Atlantic) and reanalyses from the US National Centers for Environmental Prediction (NCEP/NCAR) reanalysis System on the period 1968–1997: surface air temperature, sea level pressure, moist static energy content at 850 hPa, precipitable water and precipitation. The first coupled mode detected through singular value decomposition of the SST and pseudo wind-stress data over the tropical Atlantic (30 °N–20 °S) expresses a modulation in the thermal transequatorial gradient of SST anomalies conducted by one month leading wind-stress anomalies mainly in the tropical north Atlantic during northern winter and fall. It features a slight dipole structure in the meridional plane. Its time variability is dominated by a quasi-decadal signal well observed in the last 20–30 ys and, when projected over longer-term SST data, in the 1920s and 1930s but with shorter periods. The second coupled mode is more confined to the south-equatorial tropical Atlantic in the northern summer and explains considerably less wind-stress/SST cross-covariance. Its time series features an interannual variability dominated by shorter frequencies with increased variance in the 1960s and 1970s before 1977. Correlations between these modes and the ENSO-like Nino3 index lead to decreasing amplitude of thermal anomalies in the tropical Atlantic during warm episodes in the Pacific. This could explain the nonstationarity of meridional anomaly gradients on seasonal and interannual time scales. Overall the relationships between the oceanic component of the coupled modes and the climate anomaly patterns denote thermodynamical processes at the ocean/atmosphere interface that create anomaly gradients in the meridional plane in a way which tends to alter the north–south movement of the seasonal cycle. This appears to be consistent with the intrinsic non-dipole character of the tropical Atlantic surface variability at the interannual time step and over the recent period, but produces abnormal amplitude and/or delayed excursions of the intertropical convergence zone (ITCZ). Connections with continental rainfall are approached through three (NCEP/NCAR and observed) rainfall indexes over the Nordeste region in Brazil, and the Guinea and Sahel zones in West Africa. These indices appear to be significantly linked to the SST component of the coupled modes only when the two Atlantic modes+the ENSO-like Nino3 index are taken into account in the regressions. This suggests that thermal forcing of continental rainfall is particularly sensitive to the linear combinations of some basic SST patterns, in particular to those that create meridional thermal gradients. The first mode in the Atlantic is associated with transequatorial pressure, moist static energy and precipitable water anomaly patterns which can explain abnormal location of the ITCZ particularly in northern winter, and hence rainfall variations in Nordeste. The second mode is more associated with in-phase variations of the same variables near the southern edge of the ITCZ, particularly in the Gulf of Guinea during the northern spring and winter. It is primarily linked to the amplitude and annual phase of the ITCZ excursions and thus to rainfall variations in Guinea. Connections with Sahel rainfall are less clear due to the difficulty for the model to correctly capture interannual variability over that region but the second Atlantic mode and the ENSO-like Pacific variability are clearly involved in the Sahel climate interannual fluctuations: anomalous dry (wet) situations tend to occur when warmer (cooler) waters are present in the eastern Pacific and the gulf of Guinea in northern summer which contribute to create a northward (southward) transequatorial anomaly gradient in sea level pressure over West Africa. Received: 14 April 1998 / Accepted: 24 December 1998     

中东太平洋ITCZ对两类厄尔尼诺SST异常的敏感性试验

利用MPAS-A(The Model for Prediction Across Scales-Atmosphere)模式设计了中东太平洋热带辐合带CEP-ITCZ(Intertropical Convergence Zone over Central and Eastern Pacific)对两类厄尔尼诺SST(Sea Surface Temperature)异常的敏感性试验,通过试验结果与两类厄尔尼诺年实际大气异常的对比,初步解释了CEP-ITCZ在两类厄尔尼诺年产生不同异常的可能原因。通过CP-EL试验发现,热带太平洋SST异常的第一模态会使中东太平洋低层风场辐合增强,但对辐合带的位置影响不大,与中部型厄尔尼诺对CEP-ITCZ的影响基本一致。通过EP-EL试验发现,热带太平洋SST异常的第二模态会使中东太平洋低层风场产生较大异常,辐合带中心向南移动,辐合带明显减弱增宽,与东部型厄尔尼诺对CEP-ITCZ的影响基本一致。     

Investigating the effect of the Tibetan Plateau on the ITCZ using a coupled Earth system model

The effect of the Tibetan Plateau (TP) on the Intertropical Convergence Zone (ITCZ) was investigated using a coupled Earth system model. The location of the ITCZ (in this work represented by the center of the tropical precipitation maximum) over the tropical Atlantic was found to be sensitive to the existence of the TP. Removing the TP led to a remarkable sea surface temperature (SST) cooling (warming) in the Northern (Southern) Hemisphere, which manifested clearly in the Atlantic rather than the Pacific. The locations of maximum precipitation and SST moved southwards clearly in the tropical Atlantic, forcing a southward shift of the atmospheric convection center, and thus the ITCZ. The shift in the ITCZ was also supported by the latitudinal change in the ascending branch of the tropical Hadley Cell, which moved southwards by about 2° in the boreal summer in response to the TP's removal. From the viewpoint of the energy balance between the two hemispheres, the cooling (warming) in the Northern (Southern) Hemisphere requires an enhanced northward atmospheric heat transport across the equator, which can be realized by the southward displacement of the ITCZ. This study suggests that the presence of the TP may have played an important role in the climatology of the ITCZ, particularly its location over the tropical Atlantic.摘要本文利用耦合地球气候系统模式研究了青藏高原对热带辐合带 (ITCZ) 的影响. 我们研究发现热带大西洋ITCZ的位置对青藏高原存在与否有明显的敏感性. 与目前真实情况相比, 移除青藏高原会导致北半球海面降温, 南半球海面升温. 这种海面温度变化在大西洋表现得尤为明显, 导致热带大西洋最大海温中心向南移动, 从而迫使大气对流中心向南移动, 即表现为ITCZ的南移. 相应地, 夏季热带大气Hadley环流的上升支也发生明显南移. 北 (南) 半球海洋变冷 (变暖) 这种态势要求增强跨赤道向北的大气经向热量输送, 从而维持各个半球的能量平衡, 而这需要ITCZ位置的南移才能实现. 本文研究表明, 青藏高原的存在在现今ITCZ气候态的形成中可能扮演了重要角色.