ITCZ shift and extratropical teleconnections drive ENSO response to volcanic eruptions

The mechanisms through which volcanic eruptions affect the El Niño-Southern Oscillation (ENSO) state are still controversial. Previous studies have invoked direct radiative forcing, an ocean dynamical thermostat (ODT) mechanism, and shifts of the Intertropical Convergence Zone (ITCZ), among others, to explain the ENSO response to tropical eruptions. Here, these mechanisms are tested using ensemble simulations with an Earth system model in which volcanic aerosols from a Tambora-like eruption are confined either in the Northern or the Southern Hemisphere. We show that the primary drivers of the ENSO response are the shifts of the ITCZ together with extratropical circulation changes, which affect the tropics; the ODT mechanism does not operate in our simulations. Our study highlights the importance of initial conditions in the ENSO response to tropical volcanic eruptions and provides explanations for the predominance of posteruption El Niño events and for the occasional posteruption La Niña in observations and reconstructions

Evolution of Eastern Equatorial Pacific Seasonal and Interannual Variability in response to orbital forcing during the Holocene and Eemian from Model Simulations

Characteristics of the seasonal and interannual sea surface temperature (SST) variability in the eastern equatorial Pacific (EEP) over last two interglacials, the Holocene and Eemian, are analyzed using transient climate simulations with the Kiel Climate Model (KCM). There is a tendency towards a strengthening of the seasonal as well as the El Niño/Southern Oscillation‐ (ENSO) related variability from the early to the late interglacials. The weaker EEP SST annual cycle during the early interglacials is mainly result of insolation‐forced cooling during its warm phase and dynamically‐induced warming during its cold phase. Enhanced convection over northern South America weakens northeasterlies in the EEP leading to weaker equatorial upwelling, deeper thermocline and subsequent warming in this region. We show that a negative ENSO modulation of the annual cycle operates only on short timescales and does not affect their evolution on orbital time scales where both ENSO and annual cycle show similar tendencies to increase

Northen hemisphere meridional and zonal temperature gradients and their relation to hydrologic extremes at mid-latitude: trends, variability and link to climate modes in observation and simulations

The jet stream dynamics and the associated mid-latitude storm track are modulated by large scale ocean-land boundary conditions, which depend on both the evolution of the known interannual and multi-decadal natural variability and on changes in meridional and zonal surface temperature gradients due to anthropogenic forcing. Here, within the framework of the Lorenz (1984) low-order atmospheric model, the Equator-to-Pole temperature Gradient (EPG) and the Ocean-Land temperature Contrast (OLC) are considered as drivers of mid-latitudinal circulation. The historical trends of the seasonal NH Equator-to-Pole temperature Gradient (EPG) and the Ocean-Land temperature Contrast (OLC) are explored, as well as their probability structure, and their potential relation to anthropogenic warming. The connection between dierent combinations of EPG and OLC and precipitation patterns at mid-latitudes are shown. Then, these variables and the relations described above are examined in two CGCM simulations of the 20th century. The results show that there exist systematic biases in the temporal simulation of the gradients at hand, which is likely propagated to the simulation of precipitation. However, we show that if a model is able to reasonably capture the observed relationship between the gradients and precipitation, then it produces large-scale spatial distributions of rainfall that are consistent with observed patterns. Therefore, an eort to address the biases in simulating EPG and OLC could lead to improved temporal and spatial simulations of precipitation in the models

A theoretical model of strong and moderate El Nino regimes

The existence of two regimes for El Nino (EN) events, moderate and strong, has been previously shown in the GFDL CM2.1 climate model and also suggested in observations. The two regimes have been proposed to originate from the nonlinearity in the Bjerknes feedback, associated with a threshold in sea surface temperature (Tc) that needs to be exceeded for deep atmospheric convection to occur in the eastern Pacific. However, although the recent 2015-16 EN event provides a new data point consistent with the sparse strong EN regime, it is not enough to statistically reject the null hypothesis of a unimodal distribution based on observations alone. Nevertheless, we consider the possibility suggestive enough to explore it with a simple theoretical model based on the nonlinear Bjerknes feedback. In this study, we implemented this nonlinear mechanism in the recharge-discharge (RD) ENSO model and show that it is sufficient to produce the two EN regimes, i.e. a bimodal distribution in peak surface temperature (T) during EN events. The only modification introduced to the original RD model is that the net damping is suppressed when T exceeds Tc, resulting in a weak nonlinearity in the system. Due to the damping, the model is globally stable and it requires stochastic forcing to maintain the variability. The sustained low-frequency component of the stochastic forcing plays a key role for the onset of strong EN events (i.e. for T>Tc), at least as important as the precursor positive heat content anomaly (h). High-frequency forcing helps some EN events to exceed Tc, increasing the number of strong events, but the rectification effect is small and the overall number of EN events is little affected by this forcing. Using the Fokker-Planck equation, we show how the bimodal probability distribution of EN events arises from the nonlinear Bjerknes feedback and also propose that the increase in the net feedback with increasing T is a necessary condition for bimodality in the RD model. We also show that the damping strength determines both the adjustment time-scale and equilibrium value of the ensemble spread associated with the stochastic forcing

GCM projection of precipitation extremes in the mediterranean: changes ansd low frequency characteristic

Precipitation extremes simulated a medium by resolution GCM ( INMCM3.0) as are analyzed for the Mediterranean region. A structured analysis of low frequency variability in the control and forced ( corresponding to the IPCC scenarios) model runs is performed. The preliminary results of the analysis of rainfall patterns under global warming conditions, during the extended winter ONDJFM season, show an increase of rainfall extremes in both frequency and intensity in northern Europe and a decrease in the most part of Mediterranean. In the latter region an increase of dry conditions is also observed. The change in the rainfall patterns can be explained by a northward shift of the North Atlantic winter storm track. This shift is related changes in meridional and zonal surface temperature gradients ( Equator- Pole and Ocean-Land contrast, respectively) due to anthropogenic forcing. Changes in the inter-annual and multi-decadal natural variability are also noted

21st Projections of precipitation extremes in the Mediterranean from a medium resolution GCM

Precipitation extremes simulated by a medium resolution GCM ( INMCM3.0) are analyzed for the Mediterranean region. A structured analysis of low frequency variability in the control and forced ( corresponding to the IPCC scenarios) model runs is performed. The preliminary results of the analysis of rainfall patterns under global warming conditions, during the extended winter ONDJFM season, show an increase of rainfall extremes in both frequency and intensity in northern Europe and a decrease in the most part of Mediterranean. In the latter region an increase of dry conditions is also observed. The change in the rainfall patterns can be explained by a northward shift of the North Atlantic winter storm track. This shift is related changes in meridional and zonal surface temperature gradients ( Equator- Pole and Ocean-Land contrast, respectively) due to anthropogenic forcing. Changes in the inter-annual and multi-decadal natural variability are also noted

Mid Latitude Extreme Precipitation under future changed climate

Precipitation patterns under global warming scenario are statistically analyzed for the Mediterranean and North Europe areas. Simulation data from the global coupled atmosphere-ocean model INMCM.3 are used and compared with obervations. Changes in intensity, frequency, duration and amount of precipitation due to different IPCC scenarios are investigated. Furthermore we analyze the precipitation patterns for the better understanding of the hydrologic cycle, including a statistical analysis of precipitation extreme events

Extreme precipitation in the south and south-east Mediterranean climate structure and predictability

As part of a Global Flood project, we assess the conditions that lead to the changing frequency and spatial structure of extreme daily precipitation events in this region for the 6 month winter period from Oct-March. Spatial and temporal trends in the ECA&D data for this region are analyzed in the precipitation frequency and intensity for events exceeding the 99th percentile of daily precipitation. The associated climatic conditions (SST, atmospheric circulation patterns, canonical moisture sources and moisture transport patterns) are analyzed using re-analysis data to establish the concurrent and season ahead conditions associated with the leading space and time patterns. Analogs of these patterns are investigated for GCM integrations for future climate (seasonal forecasts as well as IPCC scenarios) to assess potential predictability and outcomes

Insights from low order model of increasing complexity: probability and temporal structure of climate extreme

This study provides insight to changes in the probability and temporal structure of mid-latitude circulation features under possible shift of average conditions to a more El Ni˜ no-like or La Ni˜ na-like state. I Analysis shows that the response to dierent El Ni˜no events in mid-latitudes is highly variable, depending on the corresponding changes to the combination of Equator-to-Pole Gradient and Ocean-Land Contrast. I Types of ENSO events and their potential impacts could be classified based on corresponding fF,Gg and the associated pdf’s of energy (X2 + Y2 + Z2), as well as the attractor’s properties. I The general impact of El Ni˜no conditions is dissipation and organization of the eddies via enhancement of the jet stream. I La Ni˜na conditions cannot be considered as the opposite of El Ni˜no conditions. Similar responses in mid-latitudes are possible. I The question whether ENSO information is transmitted to mid-latitudes via EPG or OLC changes, or both (ultimately seen as fF,Gg combinations) needs further investigation. I Seasonality eects on ENSO impact in mid-latitudes are important. I Many impacts identified in the present study are qualitatively consistent with high-order GCM results, thus illustrating the ability of low order models to represent atmospheric processes in a correct manner. I Hence, introducing low-frequency modes interactively in a plausible way may be useful in understanding the behavior and potential outcomes associated with low frequency forcings in the coupled ocean-atmosphere system