Tim Woollings

Abstract:

The Atlantic multidecadal oscillation (AMO) in sea surface temperature (SST) has been shown to influence the climate of the surrounding continents. However, it is unclear to what extent the observed impact of the AMO is related to the thermodynamical influence of the SST variability or the changes in large-scale atmospheric circulation. Here, an analog method is used to decompose the observed impact of the AMO into dynamical and residual components of surface air temperature (SAT) and precipitation over the adjacent continents. Over Europe the influence of the AMO is clearest during the summer, when the warm SAT anomalies are interpreted to be primarily thermodynamically driven by warm upstream SST anomalies but also amplified by the anomalous atmospheric circulation. The overall precipitation response to the AMO in summer is generally less significant than the SAT but is mostly dynamically driven. The decomposition is also applied to the North American summer and the Sahel rainy season. Both dynamical and residual influences on the anomalous precipitation over the Sahel are substantial, with the former dominating over the western Sahel region and the latter being largest over the eastern Sahel region. The results have potential implications for understanding the spread in AMO variability in coupled climate models and decadal prediction systems.

Abstract:

©2017. American Geophysical Union. All Rights Reserved. Seasonal hindcast experiments, using prescribed sea surface temperatures (SSTs), are analyzed for Northern Hemisphere winters from 1900 to 2010. Ensemble mean Pacific/North American index (PNA) skill varies dramatically, dropping toward zero during the mid-twentieth century, with similar variability in North Atlantic Oscillation (NAO) hindcast skill. The PNA skill closely follows the correlation between the observed PNA index and tropical Pacific SST anomalies. During the mid-century period the PNA and NAO hindcast errors are closely related. The drop in PNA predictability is due to mid-century negative PNA events, which were not forced in a predictable manner by tropical Pacific SST anomalies. Overall, negative PNA events are less predictable and seem likely to arise more from internal atmospheric variability than positive PNA events. Our results suggest that seasonal forecasting systems assessed over the recent 30 year period may be less skillful in periods, such as the mid-twentieth century, with relatively weak forcing from tropical Pacific SST anomalies.

Abstract:

The eddy-driven jet is studied using a dry idealized model to determine its sensitivity to thermal forcings. The jet latitude, speed and variability are investigated under a series of Gaussian patch thermal forcing simulations applied systematically on a latitude-sigma grid in the troposphere. This work builds on previous studies by isolating the responses of the jet speed and latitude as opposed to combining them into a single annular mode index. It also explores the sensitivity of the jet to much smaller spatial heatings rather than applying forcing patterns to simulate anthropogenic climate change, as the size and magnitude of the forcings due to anthropogenic climate change are uncertain. The jet speed and latitude are found to have different sensitivity distributions from each other, which also vary between summer and winter. A simple mechanistic understanding of these sensitivities is presented by considering how the individual thermal forcings modify mean isentropic surfaces. In the cases analysed, the jet response to forcing scales approximately linearly with the strength of the forcing, and when forcings are applied in combination. The findings show a rich latitude-pressure distribution of jet sensitivities to thermal forcings, which will aid interpretation of jet responses in a changing climate. Furthermore, they highlight the areas where uncertainty needs to be reduced in the size and position of expected anthropogenic forcings, in order that the uncertainty in changes of the eddy-driven jet can be reduced.

Abstract:

The extra-tropical atmosphere is characterized by robust circulations which have time scales longer than that associated with developing baroclinic systems but shorter than a season. Such low frequency variability is governed to a large extent by non-linear dynamics, and hence is chaotic. A useful aspect of this low-frequency circulation is that it can often be described by just a few quasi-stationary regime states, broadly defined as recurrent or persistent large scale structures, that exert a significant impact on the probability of experiencing extreme surface weather conditions. We review a variety of techniques for identifying circulation regimes from reanalysis and numerical model output. While various techniques often yield similar regime circulation patterns, they offer different perspectives on the regimes. The regimes themselves are manifest in planetary scale patterns. They affect the structure of synoptic scale patterns. Extra-tropical flow regimes have been identified in simplified atmospheric models and comprehensive coupled climate models and in reanalysis data sets. It is an ongoing challenge to accurately model these regime states and high horizontal resolutions are often needed to accurately reproduce them. The regime paradigm helps to understand the response to external forcing on a variety of time scales, has been helpful in categorizing a large number of weather types and their effect on local conditions, and is useful in downscaling. Despite their usefulness, there is a debate on the \non-equivocal" and systematic existence of these nonlinear circulation regimes. We review our current understanding of the nonlinear and regime paradigms and suggest future research.

Abstract:

Abstract This study investigates atmospheric blocking over eastern South America in austral summer for the period of 1979–2014. The results show that blocking over this area is a consequence of propagating Rossby waves that grow to large amplitudes and eventually break anticyclonically over subtropical South America (SSA). The SSA blocking can prevent the establishment of the South Atlantic convergence zone (SACZ). As such, years with more blocking days coincide with years with fewer SACZ days and reduced precipitation. Convection mainly over the Indian Ocean associated with Madden–Julian oscillation (MJO) phases 1 and 2 can trigger the wave train that leads to SSA blocking whereas convection over the western/central Pacific associated with phases 6 and 7 is more likely to lead to SACZ events. It is found that the MJO is a key source of long-term variability in SSA blocking frequency. The wave packets associated with SSA blocking and SACZ episodes differ not only in their origin but also in their phase and refraction pattern. The tropopause-based methodology used here is proven to reliably identify events that lead to extremes of surface temperature and precipitation over SSA. Up to 80% of warm surface air temperature extremes occur simultaneously with SSA blocking events. The frequency of SSA blocking days is highly anticorrelated with the rainfall over southeast Brazil. The worst droughts in this area, during the summers of 1984, 2001, and 2014, are linked to record high numbers of SSA blocking days. The persistence of these events is also important in generating the extreme impacts.