Abstract: Here we explore the relationship between the global climatological characteristics of tropical cyclones (TCs) in climate models and the modeled large-scale environment across a large number of models. We consider the climatology of TCs in 30 climate models with a wide range of horizontal resolutions. We examine if there is a systematic relationship between the climatological diagnostics for the TC activity [number of tropical cyclones (NTC) and accumulated cyclone energy (ACE)] by hemisphere in the models and the environmental fields usually associated with TC activity, when examined across a large number of models. For low-resolution models, there is no association between a conducive environment and TC activity, when integrated over space (tropical hemisphere) and time (all years of the simulation). As the model resolution increases, for a couple of variables, in particular vertical wind shear, there is a statistically significant relationship in between the models' TC characteristics and the environmental characteristics, but in most cases the relationship is either nonexistent or the opposite of what is expected based on observations. It is important to stress that these results do not imply that there is no relationship between individual models' environmental fields and their TC activity by basin with respect to intraseasonal or interannual variability or due to climate change. However, it is clear that when examined across many models, the models' mean state does not have a consistent relationship with the models' mean TC activity. Therefore, other processes associated with the model physics, dynamical core, and resolution determine the climatological TC activity in climate models.

Abstract: We investigate the interannual variability of Agulhas leakage in an ocean-eddy-resolving coupled simulation and characterize its influence on regional climate. Many observational leakage estimates are based on the study of Agulhas rings, whereas recent model studies suggest that rings and eddies carry less than half of leakage transport. While leakage variability is dominated by eddies at seasonal time scales, the noneddy leakage transport is likely to be constrained by large-scale forcing at longer time scales. To investigate this, leakage transport is quantified using an offline Lagrangian particle tracking approach. We decompose the velocity field into eddying and large-scale fields and then recreate a number of total velocity fields by modifying the eddying component to assess the dependence of leakage variability on the eddies. We find that the resulting leakage time series show strong coherence at periods longer than 1000 days and that 50% of the variance at interannual time scales is linked to the smoothed, large-scale field. As shown previously in ocean models, we find Agulhas leakage variability to be related to a meridional shift and/or strengthening of the westerlies. High leakage periods are associated with east-west contrasting patterns of sea surface temperature, surface heat fluxes, and convective rainfall, with positive anomalies over the retroflection region and negative anomalies within the Indian Ocean to the east. High leakage periods are also related to reduced inland convective rainfall over southeastern Africa in austral summer.

Abstract: A realistic representation of the North Atlantic tropical cyclone tracks is crucial as it allows, for example, explaining potential changes in U.S. landfalling systems. Here, the authors present a tentative study that examines the ability of recent climate models to represent North Atlantic tropical cyclone tracks. Tracks from two types of climate models are evaluated: explicit tracks are obtained from tropical cyclones simulated in regional or global climate models with moderate to high horizontal resolution (1°–0.25°), and downscaled tracks are obtained using a downscaling technique with large-scale environmental fields from a subset of these models. For both configurations, tracks are objectively separated into four groups using a cluster technique, leading to a zonal and a meridional separation of the tracks. The meridional separation largely captures the separation between deep tropical and subtropical, hybrid or baroclinic cyclones, while the zonal separation segregates Gulf of Mexico and Cape Verde storms. The properties of the tracks’ seasonality, intensity, and power dissipation index in each cluster are documented for both configurations. The authors’ results show that, except for the seasonality, the downscaled tracks better capture the observed characteristics of the clusters. The authors also use three different idealized scenarios to examine the possible future changes of tropical cyclone tracks under 1) warming sea surface temperature, 2) increasing carbon dioxide, and 3) a combination of the two. The response to each scenario is highly variable depending on the simulation considered. Finally, the authors examine the role of each cluster in these future changes and find no preponderant contribution of any single cluster over the others.

Abstract: On the basis of the total energy balance within an atmosphere-surface column, an attribution analysis is conducted for the Northern Hemisphere (NH) atmospheric and surface temperature response to the northern annular mode (NAM) in boreal winter. The local temperature anomaly in the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) is decomposed into partial temperature anomalies because of changes in atmospheric dynamics, water vapor, clouds, ozone, surface albedo, and surface dynamics with the coupled atmosphere-surface climate feedback-response analysis method (CFRAM). Large-scale ascent/descent as part of the NAM-related mean meridional circulation anomaly adiabatically drives the main portion of the observed zonally averaged atmospheric temperature response, particularly the tropospheric cooling/warming over northern extratropics. Contributions from diabatic processes are generally small but could be locally important, especially at lower latitudes where radiatively active substances such as clouds and water vapor are more abundant. For example, in the tropical upper troposphere and stratosphere, both cloud and ozone forcings are critical in leading to the observed NAM-related temperature anomalies. Radiative forcing due to changes in water vapor acts as the main driver of the surface warming of southern North America during a positive phase of NAM, with atmospheric dynamics providing additional warming. In the negative phase of NAM, surface albedo change drives the surface cooling of southern North America, with atmospheric dynamics providing additional cooling. Over the subpolar North Atlantic and northern Eurasia, atmospheric dynamical processes again become the largest contributor to the NAM-related surface temperature anomalies, although changes in water vapor and clouds also contribute positively to the observed surface temperature anomalies while change in surface dynamics contributes negatively to the observed temperature anomalies.

Abstract: Changes in the gradients in sea level pressure (SLP) and sea surface temperature (SST) along the equatorial Pacific are analyzed in observations and 101 numerical experiments performed with 37 climate models participating the Fifth Phase of the Coupled Model Intercomparison Project (CMIP5). The ensemble of numerical experiments simulates changes in the Earth's climate during the 1870-2004 period in response to changes in natural (solar variations, volcanoes) and anthropogenic (well-mixed greenhouse gases, ozone, direct aerosol forcing, and land use) radiative forcings. A reduction in the zonal SLP gradient is present in observational records, and is the typical response of the ensemble; yet only 26 out of the 101 “all forcing” historical experiments are able to simulate a reduced SLP gradient within 95% statistical confidence of the observed value. The multi-model response indicates a reduction of the Walker circulation to 'historical' forcings an order of magnitude smaller than the observed value. There are multiple, non-exclusive interpretations of these results: i) the observed trend may not be entirely forced, and includes a substantial component from internal variability; ii) there are problems with the observational record that lead to a spuriously large trend; iii) the strength of the Walker circulation, as measured by the zonal SLP gradient, may be less sensitive to external forcing in models than in the real climate system. Analysis of a subset of experiments suggests that greenhouse gases act to weaken the circulation, but aerosol forcing drives a strengthening of the circulation, which appears to be overestimated by the models, resulting in a muted response to the combined anthropogenic forcings.

Abstract: A statistical model for the intensity of the strongest hurricanes has been developed and a new methodology introduced for estimating the sensitivity of the strongest hurricanes to changes in sea-surface temperature. Here we use this methodology on observed hurricanes and hurricanes generated from two global climate models (GCMs). Hurricanes over the North Atlantic during the period 1981�2010 show a sensitivity of 7.9 ± 1.19 m s−1 K−1 (standard error) when over seas warmer than 25°C. In contrast, hurricanes over the same region and period generated from the GFDL HiRAM show a significantly lower sensitivity with the highest at 1.8 ± 0.42 m s−1 K−1 (s.e.). Similar weaker sensitivity is found using hurricanes generated from the FSU COAPS model with the highest at 2.9 ± 2.64 m s−1 K−1 (s.e.). A statistical refinement of HiRAM-generated hurricane intensities heightens the sensitivity to a maximum of 6.9 ± 3.33 m s−1 K−1 (s.e.), but the increase is offset by additional uncertainty associated with the refinement. Results suggest that the caution that should be exercised when interpreting GCM scenarios of future hurricane intensity stems from the low sensitivity of limiting GCM-generated hurricane intensity to ocean temperature.

Abstract: The process of obtaining key information on climate variability and change from large climate datasets often involves large computational costs and removal of noise from the data. In this study, the authors accelerate the computation of a newly developed, multidimensional temporal–spatial analysis method, namely multidimensional ensemble empirical mode decomposition (MEEMD), for climate studies. The original MEEMD uses ensemble empirical mode decomposition (EEMD) to decompose the time series at each grid point and then pieces together the temporal–spatial evolution of climate variability and change on naturally separated time scales, which is computationally expensive. To accelerate the algorithm, the original MEEMD is modified by 1) using principal component analysis (PCA) to transform the original temporal–spatial multidimensional climate data into principal components (PCs) and corresponding empirical orthogonal functions (EOFs); 2) retaining only a small fraction of PCs and EOFs that contain spatially and temporally coherent structures; 3) decomposing PCs into oscillatory components on naturally separated time scales; and 4) obtaining the original MEEMD components on naturally separated time scales by summing the contributions of the similar time scales from different pairs of EOFs and PCs. The study analyzes extended reconstructed sea surface temperature (ERSST) to validate the accelerated (fast) MEEMD. It is demonstrated that, for ERSST climate data, the fast MEEMD can 1) compress data with a compression rate of one to two orders and 2) increase the speed of the original MEEMD algorithm by one to two orders.

Abstract: There is a lack of consensus on the physical mechanisms that drive the anthropogenic weakening of tropical circulation. This study investigates the relative roles of direct CO2 forcing, mean SST warming, and the pattern of SST change on the weakening of the tropical circulation using an ensemble of AMIP and aquaplanet simulations. In terms of the mean weakening of the tropical circulation, the SST warming dominates over the direct CO2 forcing through its control over the tropical mean hydrological cycle and tropospheric stratification. In terms of the spatial pattern of circulation weakening, however, the three forcing agents are all important contributors, especially over the ocean. The increasing CO2 weakens convection over ocean directly by stabilizing the lower troposphere and indirectly via the land-sea warming contrast. The mean SST warming drives strong weakening over the centers and edges of convective zones. The pattern of SST warming plays a crucial role on the spatial pattern of circulation weakening over the tropical Pacific.The anthropogenic weakening of the Walker circulation is mostly driven by the mean SST warming. Increasing CO2 strengthens the Walker circulation through its indirect effect on land-sea warming contrast. Changes in the upper-level velocity potential indicate that the pattern of SST warming does not weaken the Walker circulation despite being El Nino-like. A weakening caused by the mean SST warming also dominates changes in the Hadley circulation in the AMIP simulations. However, this weakening is not simulated in the Southern Hemisphere in coupled simulations.

Abstract: In this paper, the global distribution of cloud water based on International Satellite Cloud Climatology Project (ISCCP), Moderate Resolution Imaging Spectroradiometer (MODIS), CloudSat Cloud Profiling Radar (CPR), European Center for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim), and Climate Forecast System Reanalysis (CFSR) datasets is presented, and the variability of cloud water from ISCCP, the Special Sensor Microwave Imager (SSM/I), ERA-Interim, and CFSR data over the time period of 1995 through 2009 is discussed. The results show noticeable differences in cloud water over land and over ocean, as well as latitudinal variations. Large values of cloud water are mainly distributed over the North Pacific and Atlantic Oceans, eastern ITCZ, regions off the west coast of the continents as well as tropical rain forest. Cloud water path (CWP), liquid water path (LWP), and ice water path (IWP) from these datasets show a relatively good agreement in distributions and zonal means. The results of trend analyzing show an increasing trend in CWP, and also a significant increasing trend of LWP can be found in the dataset of ISCCP, ERA-Interim, and CFSR over the ocean. Besides the long-term variation trend, rises of cloud water are found when temperature and water vapor exhibit a positive anomaly. EOF analyses are also applied to the anomalies of cloud water, the first dominate mode of CWP and IWP are similar, and a phase change can be found in the LWP time coefficient around 1999 in ISCCP and CFSR and around 2002 in ERA-Interim.