Tropical Mean Research Articles

Evolution of the Double‐ITCZ Bias Through CESM2 Development

AbstractThe structure of the east Pacific Intertropical Convergence Zone (ITCZ) as simulated in the Community Earth System Model version 2 (CESM2) is greatly improved as compared to its previous version, CESM version 1. Examination of intermediate model versions created as part of the development process for CESM2 shows the improvement in the ITCZ is well correlated with a reduction in the relative warmth of southeast Pacific sea surface temperatures (SSTs) as compared to the broader tropical mean. Cooling SST in this region enhances the zonal SST and surface pressure gradients and reduces the anomalously southward SST gradient present in boreal spring in early version of CESM2. The improvements in southeast Pacific SST are attributed to increases in low cloud cover and the associated shortwave cloud forcing over the southeast. Sensitivity tests using fixed SST simulations demonstrate the increase in cloud cover between two intermediate model versions, 119 and 125, to be driven by removal of the dependence of autoconversion and accretion rates on cloud water variance as well as the removal of a secondary condensation scheme. Both of these changes reduce drizzle rates in warm clouds increasing cloud lifetime and cloud fraction in the stratocumulus to trade cumulus transition region. The improvements in southeast Pacific shortwave cloud forcing and ITCZ climatology persist through subsequent changes to the cloud microphysics parameterizations. Despite improvements in the east Pacific ITCZ, the global mean ITCZ position and Pacific cold tongue bias strength do not exhibit a systematic improvement across the development simulations.

Controls on the Diversity in Climate Model Projections of Early Summer Drying over Southern Africa

Abstract Ninety-five percent of climate models contributing to phase 5 of the Coupled Model Intercomparison Project (CMIP5) project early summer [October–December (OND)] rainfall declines over subtropical southern Africa by the end of the century, under all emissions forcing pathways. The intermodel consensus underlies the Intergovernmental Panel on Climate Change (IPCC) assessment that rainfall declines are “likely” and implies that significant climate change adaptation is needed. However, model consensus is not necessarily a good indicator of confidence, especially given that there is an order of magnitude difference in the scale of rainfall decline among models in OND (from <10 mm season−1 to ~100 mm season−1), and that the CMIP5 ensemble systematically overestimates present-day OND precipitation over subtropical southern Africa (in some models by a factor of 2). In this paper we investigate the uncertainty in the OND drying signal by evaluating the climate mechanisms that underlie the diversity in model rainfall projections. Models projecting the highest-magnitude drying simulate the largest increases in tropospheric stability over subtropical southern Africa associated with anomalous upper-level subsidence, reduced evaporation, and amplified surface temperature change. Intermodel differences in rainfall projections are in turn related to the large-scale adjustment of the tropical atmosphere to emissions forcing: models with the strongest relative warming of the northern tropical sea surface temperatures compared to the tropical mean warming simulate the largest rainfall declines. The models with extreme rainfall declines also tend to simulate large present-day biases in rainfall and in atmospheric stability, leading the authors to suggest that projections of high-magnitude drying require further critical attention.

Variability and Predictability of North Atlantic Hurricane Frequency in a Large Ensemble of High-Resolution Atmospheric Simulations

AbstractVariability of North Atlantic annual hurricane frequency during 1951–2010 is studied using a 100-member ensemble of climate simulations by a 60-km atmospheric general circulation model that is forced by observed sea surface temperatures (SSTs). The ensemble mean results well capture the interannual-to-decadal variability of hurricane frequency in best track data since 1970, and suggest that the current best track data might underestimate hurricane frequency prior to 1966 when satellite measurements were unavailable. A genesis potential index (GPI) averaged over the main development region (MDR) accounts for more than 80% of the SST-forced variations in hurricane frequency, with potential intensity and vertical wind shear being the dominant factors. In line with previous studies, the difference between MDR SST and tropical mean SST is a useful predictor; a 1°C increase in this SST difference produces 7.05 ± 1.39 more hurricanes. The hurricane frequency also exhibits strong internal variability that is systematically larger in the model than observations. The seasonal-mean environment is highly correlated among ensemble members and contributes to less than 10% of the ensemble spread in hurricane frequency. The strong internal variability is suggested to originate from weather to intraseasonal variability and nonlinearity. In practice, a 20-member ensemble is sufficient to capture the SST-forced variability.

Relative contributions of interdecadal and interannual SST variations to tropical precipitation decadal mean change in the late 1990s

A prominent precipitation decrease occurred over the equatorial central Pacific in the late 1990s, accompanied by precipitation increase around the Maritime Continent and over the equatorial America. Previous studies attributed the above change to La Nina-like decadal mean sea surface temperature (SST) cooling associated with a positive to negative phase switch of the Pacific Decadal Oscillation (PDO). Results of numerical experiments with an atmospheric general circulation model reveal that both the interdecadal and interannual components of SST variations contribute to the late 1990s’ precipitation reduction over the equatorial central Pacific in all the four seasons and the precipitation increase around the Maritime Continent in winter and summer. The accompanying precipitation increase over the Central America is mainly induced by the interdecadal components of SST variations. The contribution of interannual SST variations to the equatorial central Pacific precipitation decrease mostly stems from a larger rate of precipitation change with SST in positive than negative SST anomaly years, which leads to a residual decadal mean precipitation being larger during the period before than after the late 1990s. The moisture budget decomposition demonstrates that the dynamic effect associated with the vertical motion change dominates the tropical decadal mean precipitation changes in all the four seasons and the thermodynamic effect associated with the moisture change is small. This applies to the equatorial central Pacific, the Maritime Continent, and the Central America in both interdecadal and interannual SST forced simulations.

Role of wind stress in driving SST biases in the Tropical Atlantic

Coupled climate models used for long-term future climate projections and seasonal or decadal predictions share a systematic and persistent warm sea surface temperature (SST) bias in the tropical Atlantic. This study attempts to better understand the physical mechanisms responsible for the development of systematic biases in the tropical Atlantic using the so-called Transpose-CMIP protocol in a multi-model context. Six global climate models have been used to perform seasonal forecasts starting both in May and February over the period 2000–2009. In all models, the growth of SST biases is rapid. Significant biases are seen in the first month of forecast and, by 6 months, the root-mean-square SST bias is 80% of the climatological bias. These control experiments show that the equatorial warm SST bias is not driven by surface heat flux biases in all models, whereas in the south-eastern Atlantic the solar heat flux could explain the setup of an initial warm bias in the first few days. A set of sensitivity experiments with prescribed wind stress confirm the leading role of wind stress biases in driving the equatorial SST bias, even if the amplitude of the SST bias is model dependent. A reduced SST bias leads to a reduced precipitation bias locally, but there is no robust remote effect on West African Monsoon rainfall. Over the south-eastern part of the basin, local wind biases tend to have an impact on the local SST bias (except in the high resolution model). However, there is also a non-local effect of equatorial wind correction in two models. This can be explained by sub-surface advection of water from the equator, which is colder when the bias in equatorial wind stress is corrected. In terms of variability, it is also shown that improving the mean state in the equatorial Atlantic leads to a beneficial intensification of the Bjerknes feedback loop. In conclusion, we show a robust effect of wind stress biases on tropical mean climate and variability in multiple climate models.

Efficiency of Cultural Practices of Mulching and Nipping in the Management of Eggplant Infestation and Damage by Leucinodes orbonalis Gueéne (Lepidoptera: Pyralidae)

The objective of the study was to evaluate the effectiveness of cultural practices of mulching and nipping in the management of eggplant infestation and damage by Leucinodes orbonalis Guenée (Lepidoptera: Pyralidae). Therefore, a field experiment using six eggplant varieties was carried out under tropical mean ambient temperature (30.4 oC) and relative humidity (73.3 %). The experimental site was Teaching and Research Farm of Faculty of Agriculture, University of Port Harcourt, Nigeria. Each bed measured 2 m x 1.3 m and eggplant varieties were planted at a distance of 0.3 m x 0.4 m. The experiment was arranged in a randomized complete block design, left to natural infestation and had three replications. The eggplants were mulched and nipped every two weeks in the study. The results revealed that all the test varieties were susceptible to attack by the insect pest under the two management techniques. Fruit weight of all varieties increased progressively (25.06 g) up to the third week after planting and declined thereafter (18.09 g) probably due to damaging effect of L. orbonalis. Mulching and nipping supported good fruit weight but were not highly effective in protecting eggplants against L. orbonalis infestation and damage. There was no significant difference (P > 0.05) in their individual ability to manage L. orbonalis attack irrespective of resistance status except at second-week after maturity in the number of exit holes/fruit and number of larvae/fruit (P < 0.001).

Light absorption by marine cyanobacteria affects tropical climate mean state and variability

Abstract. Observations indicate that positively buoyant marine cyanobacteria, which are abundant throughout the tropical and subtropical ocean, have a strong local heating effect due to light absorption at the ocean surface. How these local changes in radiative heating affect the climate system on the large scale is unclear. We use the Max Planck Institute Earth System Model (MPI-ESM), include light absorption by cyanobacteria, and find a considerable cooling effect on tropical sea surface temperature (SST) in the order of 0.5 K on a climatological timescale. This cooling is caused by local shading of subtropical subsurface water by cyanobacteria that is upwelled at the Equator and in eastern boundary upwelling systems. Implications for the climate system include a westward shift of the Walker circulation and a weakening of the Hadley circulation. The amplitude of the seasonal cycle of SST is increased in large parts of the tropical ocean by up to 25 %, and the tropical Pacific interannual variability is enhanced by approx. 20 %. This study emphasizes the sensitivity of the tropical climate system to light absorption by cyanobacteria due to its regulative effect on tropical SST. Generally, including phytoplankton-dependent light attenuation instead of a globally uniform attenuation depth improves some of the major model temperature biases, indicating the relevance of taking this biophysical feedback into account in climate models.

Prediction of the Madden–Julian Oscillation: A Review

AbstractThere has been an accelerating interest in forecasting the weather and climate within the subseasonal time range. The Madden–Julian oscillation (MJO), an organized envelope of tropical convection, is recognized as one of the leading sources of subseasonal predictability. This review synthesizes the latest progress regarding the MJO predictability and prediction. During the past decade, the MJO prediction skill in dynamical prediction systems has exceeded the skill of empirical predictions. Such improvement has been mainly attributed to more observations and computer resources, advances in theoretical understanding, and improved numerical models aided in part by multinational efforts through field campaigns and multimodel experiments. The state-of-the-art dynamical forecasts have shown MJO prediction skill up to 5 weeks. Prediction skill can be extended by improving the ensemble generation approach tailored for MJO prediction and by averaging multiensembles or multimodels. MJO prediction skill can be influenced by the tropical mean state and low-frequency climate mode variations, as well as by the extratropical circulation. MJO prediction skill is proven to be sensitive to model physics, ocean–atmosphere coupling, and quality of initial conditions, while the impact of the model resolution seems to be marginal. Remaining challenges and recommendations on new research avenues to fully realize the predictability of the MJO are discussed.

The Relationship Between Cloud Radiative Effect and Surface Temperature Variability at El Niño‐Southern Oscillation Frequencies in CMIP5 Models

AbstractThe relationship between the tropical cloud radiative effect (CRE) and tropical surface temperature variability on El Niño–Southern Oscillation (ENSO) time scales is investigated in preindustrial control simulations from the fifth Climate Model Intercomparison Project (CMIP5) archive. The tropical CRE is binned according to midtropospheric vertical velocities and then regressed in frequency space versus tropical mean surface temperatures. Low clouds play a leading role in the relationship between clouds and surface temperature variability, amplifying ENSO‐induced surface temperature anomalies through thermodynamically driven changes in the shortwave CRE. Changes in CRE driven by changes in the large‐scale dynamics have a minor influence on surface temperature variability. It is shown that the regression coefficients at ENSO frequencies between the CRE in regions of moderate subsidence and of weak ascent, and tropical mean surface temperatures are well correlated with models' climate sensitivities, constituting a potential emergent constraint on climate sensitivity.

On the Time of Emergence of Tropical Width Change

AbstractThe tropical belt is expected to expand in response to global warming, although most of the observed tropical widening since 1980, especially in the Northern Hemisphere, is believed to have mainly originated from natural variability. The view is of a small global warming signal relative to natural variability. Here we focus on the question whether and, if so when, the anthropogenic signal of tropical widening will become detectable. Analysis of two large ensemble climate simulations reveals that the forced signal of tropical width is strongly constrained by the forced signal of global mean temperature. Under a representative concentration pathway 8.5 (RCP8.5) emissions scenario, the aggregate of the two models indicates a regression of about 0.5° lat °C−1 during 1980–2080. The models also reveal that interannual variability in tropical width, a measure of noise used herein, is insensitive to global warming. Reanalysis data are therefore used to constrain the interannual variability, whose magnitude is estimated to be 1.1° latitude. Defining the time of emergence (ToE) for tropical width change as the first year (post-1980) when the forced signal exceeds the magnitude of interannual variability, the multimodel simulations of CMIP5 are used to estimate ToE and its confidence interval. The aforementioned strong constraint between the signal of tropical width change and global mean temperature change motivates using CMIP5-simulated global mean temperature changes to infer ToE. Our best estimate for the probable year for ToE, under an RCP8.5 emissions scenario, is 2058 with 10th–90th percentile confidence of 2047–68. Various sources of uncertainty in estimating the ToE are discussed.

Cross-equatorial winds control El Niño diversity and change

Over the past two decades, El Nino events have weakened on average and their sea surface temperature (SST) anomalies shifted westward towards the central Pacific. Moreover, the intertropical convergence zone (ITCZ), which typically migrates southward from its northerly position during El Nino events, has not crossed the Equator since 1998. The causes of these changes remain under debate1–5. Here, using in situ, satellite and atmospheric reanalysis data, we show they can be related to a multidecadal strengthening of cross-equatorial winds in the eastern Pacific. This gradual strengthening of meridional winds is unlikely to be caused by El Nino/Southern Oscillation (ENSO) changes, and contains signals forced both locally and from outside the tropical Pacific, probably from the tropical North Atlantic. Coupled model simulations in which the observed cross-equatorial wind strengthening is superimposed successfully reproduce the key features of the recent changes in tropical climate. In particular, the tropical mean state experiences a ‘La Nina-like’ change, the ENSO amplitude weakens by about 20%, the centre of the SST anomalies shifts westward and the ITCZ now rarely crosses the Equator. Thus, cross-equatorial winds are found to modulate tropical Pacific mean state and variability, with implications for quantifying projected changes in ENSO under anthropogenic warming. In recent years, El Nino sea surface temperature anomalies have weakened and shifted westward. Observational and model analyses reveal these changes can be related to a multidecadal strengthening of cross-equatorial winds, forced both locally and from the tropical Atlantic.

Convective Influence on the Humidity and Clouds in the Tropical Tropopause Layer During Boreal Summer

AbstractThe impact of convection on the humidity and clouds in the tropical tropopause layer (TTL) during boreal summer 2007 is investigated in simulations of detailed cloud microphysical processes and their effects on the water vapor (H2O) profile along backward trajectories from the 379 K potential temperature (100‐hPa pressure) surface. Convective influence is determined by tracing the trajectories through time‐dependent fields of satellite‐based convective cloud top height. The simulated H2O mixing ratios at the 100‐hPa level and cloud occurrence fractions in the middle to upper (16–18 km) TTL exhibit a pronounced maximum over the Asian monsoon region as in observations; these local enhancements are virtually absent in the simulation without convection, indicating that convection is the dominant driver of the localized H2O and cloud maxima in the Asian summer monsoon region. Convection moistens the 100‐hPa level by 0.6 ppmv (~15%) averaged over the 10°S–50°N domain and increases tropical (10°S–30°N) mean cloud occurrence in the middle to upper TTL by ~170%. Nearly all of the convective enhancements in H2O and clouds are due to the effect of convective saturation; convectively detrained ice crystals have negligible impact. Parcels are most frequently hydrated by deep convection in the southern sector of the Asian monsoon anticyclone and subsequently dehydrated downstream of convection to the west, shifting the locations of final dehydration northwest of the cold temperature region in the northern Tropics. Infrequent, extreme deep convective systems (cloud tops exceeding 380 K) have a disproportionately large effect on TTL humidity and clouds.

The Effect of Hydrometeors on MSU/AMSU Temperature Observations over the Tropical Ocean

AbstractThe temperature of Earth’s atmosphere has been monitored continuously since late 1978 by the Microwave Sounding Unit (MSU) and the Advanced Microwave Sounding Unit (AMSU) flown on polar-orbiting weather satellites. It is well known that these measurements are affected by the scattering and emission from hydrometeors, including cloud water, precipitation, and ice particles. In this study the hydrometeor effects on MSU/AMSU temperature observations are investigated by comparing satellite-observed temperature of the middle troposphere (TMT) with synthetic TMT constructed using temperature fields from ECMWF Interim [ERA-Interim (ERA-I)]. Precipitation data have been used to estimate how much of the difference between these two TMT fields is due to hydrometeor contamination effects. It is shown that there exists a robust linear proportionality between TMT deficit (i.e., the measured TMT minus the synthetic TMT) and precipitation at individual grid points in monthly mean fields. The linear correlation is even stronger in the annual mean and seasonally varying climatology and also in the spatial pattern of ENSO-related anomalies. The linear regression coefficient obtained in all of these analyses is virtually identical: −0.042 K (mm day−1)−1. The channel that senses lower-tropospheric temperature (TLT) is more sensitive to precipitation than the TMT channel: the regression coefficient is −0.059 K (mm day−1)−1. It is shown that correcting the TMT or TLT monthly anomalies by removing the hydrometeor contamination does not significantly influence estimates of tropical mean temperature trends, but it could affect the pattern of temperature trend over the tropical oceans.

Spatially Resolved Isotopic Source Signatures of Wetland Methane Emissions

AbstractWe present the first spatially resolved wetland δ13C(CH4) source signature map based on data characterizing wetland ecosystems and demonstrate good agreement with wetland signatures derived from atmospheric observations. The source signature map resolves a latitudinal difference of ~10‰ between northern high‐latitude (mean −67.8‰) and tropical (mean −56.7‰) wetlands and shows significant regional variations on top of the latitudinal gradient. We assess the errors in inverse modeling studies aiming to separate CH4 sources and sinks by comparing atmospheric δ13C(CH4) derived using our spatially resolved map against the common assumption of globally uniform wetland δ13C(CH4) signature. We find a larger interhemispheric gradient, a larger high‐latitude seasonal cycle, and smaller trend over the period 2000–2012. The implication is that erroneous CH4 fluxes would be derived to compensate for the biases imposed by not utilizing spatially resolved signatures for the largest source of CH4 emissions. These biases are significant when compared to the size of observed signals.

Westward migration of tropical cyclone rapid-intensification over the Northwestern Pacific during short duration El Ni\xf1o

The El Niño-Southern Oscillation (ENSO) can significantly affect the rapid intensification of tropical cyclones over the western North Pacific (WNP). However, ENSO events have various durations, which can lead to different atmospheric and oceanic conditions. Here we show that during short duration El Niño events, the WNP tropical cyclone rapid-intensification mean occurrence position migrates westward by ~8.0° longitude, which is caused by reduced vertical wind shear, increased mid-tropospheric humidity, and enhanced tropical cyclone heat potential over the westernmost WNP. The changes in these factors are caused by westward advected upper ocean heat during the decaying phase of a short duration El Niño. As super El Niño events tend to have short durations and their frequency is projected to increase under global warming, our findings have important implications for future projections of WNP tropical cyclone activity.

Tropical cyclogenesis in warm climates simulated by a cloud-system resolving model

Here we investigate tropical cyclogenesis in warm climates, focusing on the effect of reduced equator-to-pole temperature gradient relevant to past equable climates and, potentially, to future climate change. Using a cloud-system resolving model that explicitly represents moist convection, we conduct idealized experiments on a zonally periodic equatorial β-plane stretching from nearly pole-to-pole and covering roughly one-fifth of Earth’s circumference. To improve the representation of tropical cyclogenesis and mean climate at a horizontal resolution that would otherwise be too coarse for a cloud-system resolving model (15 km), we use the hypohydrostatic rescaling of the equations of motion, also called reduced acceleration in the vertical. The simulations simultaneously represent the Hadley circulation and the intertropical convergence zone, baroclinic waves in mid-latitudes, and a realistic distribution of tropical cyclones (TCs), all without use of a convective parameterization. Using this model, we study the dependence of TCs on the meridional sea surface temperature gradient. When this gradient is significantly reduced, we find a substantial increase in the number of TCs, including a several-fold increase in the strongest storms of Saffir–Simpson categories 4 and 5. This increase occurs as the mid-latitudes become a new active region of TC formation and growth. When the climate warms we also see convergence between the physical properties and genesis locations of tropical and warm-core extra-tropical cyclones. While end-members of these types of storms remain very distinct, a large distribution of cyclones forming in the subtropics and mid-latitudes share properties of the two.

Impact of chlorophyll bias on the tropical Pacific mean climate in an earth system model

Climate modeling groups nowadays develop earth system models (ESMs) by incorporating biogeochemical processes in their climate models. The ESMs, however, often show substantial bias in simulated marine biogeochemistry which can potentially introduce an undesirable bias in physical ocean fields through biogeophysical interactions. This study examines how and how much the chlorophyll bias in a state-of-the-art ESM affects the mean and seasonal cycle of tropical Pacific sea-surface temperature (SST). The ESM used in the present study shows a sizeable positive bias in the simulated tropical chlorophyll. We found that the correction of the chlorophyll bias can reduce the ESM’s intrinsic cold SST mean bias in the equatorial Pacific. The biologically-induced cold SST bias is strongly affected by seasonally-dependent air–sea coupling strength. In addition, the correction of chlorophyll bias can improve the annual cycle of SST by up to 25%. This result suggests a possible modeling approach in understanding the two-way interactions between physical and chlorophyll biases by biogeophysical effects.

Diagnosing ENSO and Global Warming Tropical Precipitation Shifts Using Surface Relative Humidity and Temperature

AbstractLarge uncertainty remains in future projections of tropical precipitation change under global warming. A simplified method for diagnosing tropical precipitation change is tested here on present-day El Niño–Southern Oscillation (ENSO) precipitation shifts. This method, based on the weak temperature gradient approximation, assumes precipitation is associated with local surface relative humidity (RH) and surface air temperature (SAT), relative to the tropical mean. Observed and simulated changes in RH and SAT are subsequently used to diagnose changes in precipitation. Present-day ENSO precipitation shifts are successfully diagnosed using observations (correlationr= 0.69) and an ensemble of atmosphere-only (0.51 ≤r≤ 0.8) and coupled (0.5 ≤r≤ 0.87) climate model simulations. RH (r= 0.56) is much more influential than SAT (r= 0.27) in determining ENSO precipitation shifts for observations and climate model simulations over both land and ocean. Using intermodel differences, a significant relationship is demonstrated between method performance over ocean for present-day ENSO and projected global warming (r= 0.68). As a caveat, the authors note that mechanisms leading to ENSO-related precipitation changes are not a direct analog for global warming–related precipitation changes. The diagnosis method presented here demonstrates plausible mechanisms that relate changes in precipitation, RH, and SAT under different climate perturbations. Therefore, uncertainty in future tropical precipitation changes may be linked with uncertainty in future RH and SAT changes.

Cloud climatologies from the infrared sounders AIRS and IASI: strengths and applications

Abstract. Global cloud climatologies have been built from 13 years of Atmospheric Infrared Sounder (AIRS) and 8 years of Infrared Atmospheric Sounding Interferometer (IASI) observations, using an updated Clouds from Infrared Sounders (CIRS) retrieval. The CIRS software can handle any infrared (IR) sounder data. Compared to the original retrieval, it uses improved radiative transfer modelling, accounts for atmospheric spectral transmissivity changes associated with CO2 concentration and incorporates the latest ancillary data (atmospheric profiles, surface temperature and emissivities). The global cloud amount is estimated to be 0.67–0.70, for clouds with IR optical depth larger than about 0.1. The spread of 0.03 is associated with ancillary data. Cloud amount is partitioned into about 40 % high-level clouds, 40 % low-level clouds and 20 % mid-level clouds. The latter two categories are only detected in the absence of upper clouds. The A-Train active instruments, lidar and radar of the CALIPSO and CloudSat missions, provide a unique opportunity to evaluate the retrieved AIRS cloud properties. CIRS cloud height can be approximated either by the mean layer height (for optically thin clouds) or by the mean between cloud top and the height at which the cloud reaches opacity. This is valid for high-level as well as for low-level clouds identified by CIRS. IR sounders are particularly advantageous to retrieve upper-tropospheric cloud properties, with a reliable cirrus identification, day and night. These clouds are most abundant in the tropics, where high opaque clouds make up 7.5 %, thick cirrus 27.5 % and thin cirrus about 21.5 % of all clouds. The 5 % annual mean excess in high-level cloud amount in the Northern compared to the Southern Hemisphere has a pronounced seasonal cycle with a maximum of 25 % in boreal summer, in accordance with the moving of the ITCZ peak latitude, with annual mean of 4° N, to a maximum of 12° N. This suggests that this excess is mainly determined by the position of the ITCZ. Considering interannual variability, tropical cirrus are more frequent relative to all clouds when the global (or tropical) mean surface gets warmer. Changes in relative amount of tropical high opaque and thin cirrus with respect to mean surface temperature show different geographical patterns, suggesting that their response to climate change might differ.

Evaluating the Evidence of a Global Sea Surface Temperature Threshold for Tropical Cyclone Genesis

Recent studies have reaffirmed a global threshold sea surface temperature (SST) of 26°C for tropical cyclone (TC) genesis. However, it is well understood that other thermodynamic variables influence TC genesis and that high SST in isolation is not a sufficient criterion for genesis. Here, a basin-by-basin analysis of the SST distributions in the five most active ocean basins is performed, which shows that there is no global SST threshold for TC genesis. The distributions of genesis SST show substantial variations between basins. Furthermore, analysis of the conditional probability of genesis for a given TC season main development region SST suggests that the SST bounds for TC genesis are largely determined by the climatological bounds of the basin and that the SST values within this environmental range have similar probabilities of genesis. The distribution of relative SST (the difference between local and tropical mean) and tropical cyclone potential intensity at TC genesis are more distinct from those of the TC season environment, consistent with their utility in TC genesis indices.