Little evidence of reduced global tropical cyclone activity following recent volcanic eruptions

The impact of volcanic aerosols on recent global tropical cyclone (TC) activity is examined in observations, reanalysis, and models (the Coupled Model Intercomparison Project phase 5 - CMIP5 multi-model, and one single model large ensemble). In observations, we find a reduction of TC activity only in the North Atlantic following the last three strong volcanic eruptions; that signal, however, cannot be clearly attributed to volcanoes, as all three eruptions were simultaneous with El Niño events. In reanalyses, we find no robust impact of volcanic eruptions on potential intensity (PI) and genesis indices. In models, we find a reduction in PI after volcanic eruptions in the historical simulations, but this effect is significantly reduced when differences between the model environment and observations are accounted for. Morever, the CMIP5 multi-model historical ensemble shows no effect of volcanic eruptions on a TC genesis index. Finally, there is no robust and consistent reduction in recent TC activity following recent volcanic eruptions in a large set of synthetic TCs downscaled from these simulations. Taken together, these results show that in recent eruptions volcanic aerosols did not reduce global TC activity

Tropical cyclones and climate change: recent results and uncertainties

In the first part of the talk I’ll give an overview of the current understanding and recent progress on the influence of climate changes on tropical cyclones. I’ll discuss both attribution of current trends, as well as projections for the end of the century. In the second part of the talk, I’ll describe the hazard model developed at Columbia University to study tropical cyclones risk, namely CHAZ (Columbia Hazard Model) and discuss its main properties in the current climate. Then I’ll discuss two applications of this model: first, analyzing the current risk of tropical cyclones to Mumbai; second, describing how CHAZ can be used to study the influence of climate change on tropical cyclones and our recent results on this topic. In particular, I’ll discuss the uncertainties in tropical cyclone frequency projections based on CHAZ and compare with the current state-of-the-art knowledge on this topic

Global and Regional Aspects of Tropical Cyclone Activity in the CMIP5 Models

Tropical cyclone (TC) activity is analyzed in 14 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The global TC activity in the historical runs is compared with observations. The simulation of TC activity in the CMIP5 models is not as good as in higher-resolution simulations. The CMIP5 global TC frequency is much lower than observed, and there is significant deficiency in the geographical patterns of TC tracks and formation. Although all of the models underestimate the global frequency of TCs, the models present a wide range of global TC frequency. The models with the highest horizontal resolution have the highest level of global TC activity, though resolution is not the only factor that determines model TC activity. A cold SST bias could potentially contribute to the low number of TCs in the models. The models show no consensus regarding the difference of TC activity in two warming scenarios [representative concentration pathway 4.5 (RCP4.5) and RCP8.5] and the historical simulation. The author examined in more detail North Atlantic and eastern North Pacific TC activity in a subset of models and found no robust changes across models in TC frequency. Therefore, there is no robust signal across the CMIP5 models in global and regional TC changes in activity for future scenarios. The future changes in various large-scale environmental fields associated with TC activity were also examined globally: genesis potential index, potential intensity, vertical wind shear, and sea level pressure. The multimodel mean changes of these variables in the CMIP5 models are consistent with the changes obtained in the CMIP3 models

Hottest summers the new normal

With the rise in temperature due to anthropogenic climate change, the occurrence of hot summers, temperature extremes and heat waves is increasing globally. Projections for the coming decades to century indicate increases in the occurrence, magnitude and duration of these events. In a recent paper, Mueller et al (2016 Environ. Res. Lett. 11 044011) showed that half of summers are expected to be 'hot' (warmer than the warmest on record) across much of the world in one or two decades. While these results are consistent with earlier work, what is new here includes (i) an earlier timing of emergence of the hot summer signal and (ii) additional confidence due to the rigorous statistical examination of the observations and the analyses of the latest improved suite of model experiments. The potential impacts of these projections on society are extremely serious

The seasonally-varying influence of ENSO on rainfall and tropical cyclone activity in the Philippines

An observational study covering the period 1950–2002 examines a seasonal reversal in the ENSO rainfall signal in the north-central Philippines. In boreal Summer of El Niño (La Niña) events, above (below) average rainfall typically occurs in this area. Rainfall anomalies of opposite sign develop across the country in the subsequent fall. This study investigates the seasonal evolution of the anomalous atmospheric circulation over the western North Pacific (WNP) during both El Niño and La Niña and places these features in the context of the large-scale evolution of ENSO events, including an analysis of changes in tropical cyclone activity affecting the Philippines. The results show that during boreal summer of El Niño (La Niña) events, a relatively narrow, zonally elongated band of enhanced (reduced) low-level westerlies develops across the WNP which serves to increase (decrease) the summer monsoon flow and moisture flux over the north-central Philippines and is associated with an increase (decrease) in the strength of the WNP monsoon trough via the anomalous relative vorticity. Tropical cyclone activity is shown to be enhanced (reduced) in the study region during boreal summer of El Niño (La Niña) events, which is related to the increase (decrease) of mid-level atmospheric moisture, as diagnosed using a genesis potential index. The subsequent evolution shows development of an anomalous anticyclone (cyclone) over the WNP in El Niño (La Niña) and the well-known tendency for below (above) average rainfall in the fall. Prolonged ENSO events also exhibit seasonal rainfall sign reversals in the Philippines with a similar evolution in atmospheric circulation

Description and Skill Evaluation of Experimental Dynamical Seasonal Forecasts of Tropical Cyclone Activity at IRI

The International Research Institute for Climate and Society has been issuing experimental seasonal tropical cyclone activity forecasts for several ocean basins since early 2003. In this paper we describe the method used to obtain these forecasts, and evaluate their performance. The forecasts are based on tropical cyclone-like features detected and tracked in a low-resolution climate model, namely ECHAM4.5. The simulation skill of the model using historical observed sea surface temperatures (SSTs) over several decades, as well as with SST anomalies persisted from the month ending at the forecast start time, is discussed. These simulation skills are compared with skills of purely statistically based hindcasts using as predictors observed SSTs preceding the forecast start time. For the recent 6-year period during which real-time forecasts have been made, the skill of the raw model output is compared with that of the subjectively modified probabilistic forecasts actually issued. Despite variations from one basin to another, the hindcast skills of the dynamical and statistical forecast approaches are found, overall, to be approximately equivalent. The dynamical forecasts require statistical post-prossessing (calibration) to be competitive with, and in some circumstances superior to, the statistical models. Hence, during the recent period of real-time forecasts, the subjective forecasts are found to have resulted in probabilistic skill better than that of the raw model output, primarily because of the forecasters' elimination of the systematic bias of "overconfidence" in the model's forecasts. Prospects for the future improvement of dynamical tropical cyclone prediction are considered

Workshop on Tropical Cyclones and Climate, March 27-29, 2006

In March 2006, the International Research Institute for Climate and Society (IRI) hosted a two and a half day workshop on "Tropical Cyclones and Climate". The centerpiece of the workshop was a set of invited lectures, with a modest number of contributed oral presentations and a small poster session. Relatively lengthy discussion periods were built into the schedule, allowing in-depth discussion of the presentations and related issues

Characteristics of Western North Pacific Model Tropical Cyclogenesis

"Tropical cyclogenesis" in a low-resolution Atmospheric General Circulation model is studied, focusing on the Western North Pacific region during the June-October typhoon season. Time-dependent composites of the cyclones are formed and analyzed, with a focus on the temporal evolution of quantities averaged in space around the storm centers. Day zero of each composite corresponds to the time at which the cyclone passes the criteria for detection. Some variables whose magnitude is related to cyclone intensity (such as low-level vorticity and surface wind speed) show similar temporal evolution, with a slight decrease up to a few days before day zero, a weak local minimum at that point, and a strong increase after that for a week or more. The relative humidity at low levels has its minimum somewhat later, at about day zero. The mean composite environmental vertical wind shear lacks a minimum and increases monotonically through the entire genesis period until a week after day zero. This variation is mostly due to the mean cyclone track's moving through regions of different climatological shear, which varies monotonically from easterly to westerly, crossing zero shortly after day zero, and would be consistent with a controlling role of the shear on model cyclogenesis. A signal in the skewness of the lower-level relative humidity distribution over the ensemble suggests that a dry lower troposphere can prevent development of a model cyclone. The local minimum in many variables' time series suggests the presence of an initial disturbance that is suddenly enhanced, becoming a model tropical cyclone, as has been noted in observations

Influence of the western North Pacific tropical cyclones on their large-scale environment

The authors investigate the influence of western North Pacific (WNP) tropical cyclones (TCs) on their large-scale environment by lag regressing various large-scale climate variables [atmospheric temperature, winds, relative vorticity, outgoing longwave radiation (OLR), column water vapor, and sea surface temperature (SST)] on an index of TC activity [accumulated cyclone energy (ACE)] on a weekly time scale. At all leads and lags out to several months, persistent, slowly evolving signals indicative of the El Niño–Southern Oscillation (ENSO) phenomenon are seen in all the variables, reflecting the known seasonal relationship of TCs in the WNP to ENSO. Superimposed on this are more rapidly evolving signals, at leads and lags of one or two weeks, directly associated with the TCs themselves. These include anomalies of positive low-level vorticity, negative OLR, and high column water vapor associated with anomalously positive ACE, found in the region where TCs most commonly form and develop. In the same region, lagging ACE by a week or two and so presumably reflecting the influence of TCs on the local environment, signals are found that might be expected to negatively influence the environment for later cyclogenesis. These signals include an SST reduction in the primary region of TC activity, and a reduction in column water vapor and increase in OLR that may or may not be a result of the SST reduction. On the same short time scale, an increase in equatorial SST near and east of the date line is seen, presumably associated with equatorial surface westerly anomalies that are also found. This, combined with the correlation between ACE and ENSO indices on the seasonal time scale, suggests the possibility that TCs may play an active role in ENSO dynamics

Formation of tropical storms in an atmospheric general circulation model

The formation of tropical storms in a low-resolution Atmospheric General Circulation model is studied on the Western North Pacific region during the June-October season. The model simulates the mean annual cycle of storm number in this basin quite well. Time-dependent composites of the storms are formed and analyzed, with a focus on the temporal evolution of quantities averaged in space around the storm centers. Day zero of each composite corresponds to the time at which the disturbance passes criteria for detection. The composites depict the model storms as convectively-coupled, synoptic-scale vortices whose degree of coupling to convection increases at some point, leading to intensification. Variables related to disturbance intensity have significant anomalies at day -7, indicating a finite amplitude disturbance prior to "genesis''. Many of these variables show similar temporal evolution, with a local minimum two or three days before day zero, and a strong increase after that for several days, followed by an eventual decrease. The precipitation reaches its maximum on day 2, the net moist static energy forcing (surface fluxes minus net tropospheric radiative cooling, each of which has an anomaly of 20-30 Wm2 in the sense of warming the atmosphere) a day later, and dynamical variables such as vorticity and temperature still later, with broad plateaus centered around day 4 or 5. The vorticity increases at the surface at the same time as at midlevels, unlike in observed storms. The me! an composite environmental vertical wind shear has a maximum amplitude on day -2 and then decreases. This could indicate a causal role of shear in limiting development, but would also be consistent with a coincidental storm motion to regions of lower shear, with development controlled by other factors. A signal in the skewness of the lower-level relative humidity distribution over the ensemble suggests that a dry lower troposphere can prevent development of a model tropical disturbance