ABSTRACT:In 2016 and continuing into 2017, Kenya experienced drought conditions, with over 3 million people in need of food aid due to low rainfall during 2016. Whenever extreme events like this happen, questions are raised about the role of climate change and how natural variability such as the El Niño -Southern Oscillation influenced the likelihood and intensity of the event. Here we aim to quantify the relative contributions of different climate drivers to this drought by applying three independent methodologies of extreme event attribution. Analysing precipitation data for the South East and North West of Kenya we found no consistent signal from human-induced climate change and thus conclude that it has not greatly affected the likelihood of low rainfall such as in 2016. However, 2016 was a La Niña year and we show that this event was indeed more likely because of the specific sea surface temperatures. There is a trend in temperatures in the region due to climate change that may have exacerbated the effects of this drought. By analysing precipitation minus evaporation and soil moisture, simulated by one climate model only, we did not see a reduction in moisture in simulations in the current climate compared with simulations without climate change. However, there are expected effects of higher temperatures that our simulations do not cover, such as increased demand on water resources and stress on livestock. Although we find no significant influence of climate change on precipitation, we cannot rule out that temperature-related impacts of drought are linked to human-induced climate change.

Quantifying the width of the tropics has important implications for understanding climate variability and the atmospheric response to anthropogenic forcing. Considerable effort has been placed on quantifying the width of the tropics at tropospheric levels, but substantially less effort has been placed on quantifying the width at stratospheric levels. Here we probe tropical width in the stratosphere using chemical tracers, which are accessible by direct measurement. Two new tracer-based width metrics are developed, denoted here as the "1σ method" and the gradient weighted latitude (GWL) method. We evaluate widths from three tracers, CH 4 , N 2 O, and SF 6. We demonstrate that unlike previously proposed stratospheric width methods using tracers, these metrics perform consistently throughout the depth of the stratosphere, at all times of year and on coarse temporal data. The GWL tracer-based widths correlate well with the turnaround latitude and the critical level, where wave dissipation occurs, in the upper and midstratosphere during certain months of the year. In the lower stratosphere, the deseasonalized tracer-based widths near the tropical tropopause correlate with the deseasonalized tropopause-height based metrics. We also find that tracer-tracer width correlations are strongest at pressure levels where their chemical lifetimes are similar. These metrics represent another useful way to estimate stratospheric tropical width and explore any changes under anthropogenic forcing. Plain Language Summary Studies of the expansion of the Earth's tropical belt have focused on the surface and the troposphere, with limited consideration to changes in the stratosphere. Further, these studies largely use measures of tropical width that depend on dynamical quantities like vertical and longitudinal velocities, stream function, and tropopause height. Changes to the position of the subtropical jet and to tropopause height have implications for stratospheric transport, mixing and upwelling strength as well as for long-lived trace gas variability in the lower stratosphere, motivating study of the width of the stratospheric tropics and potential metrics here. In this study, we argue that chemical tracers form a useful basis for stratospheric tropical width measurements, particularly for species that are directly measured. We develop two new tracer-based metrics that we call the 1σ method and the gradient weighted latitude method. These metrics advance opportunities to measure changes in stratospheric tropical width and to assess relationships between tropospheric and stratospheric width changes.

We demonstrate that identification of stratospheric ozone changes attributable to ozone depleting substances and actions taken under the Montreal Protocol requires evaluation of confounding influences from volcanic eruptions. Using a state‐of‐the‐art chemistry‐climate model, we show that increased stratospheric aerosol loading from volcanic eruptions after 2004 impeded the rate of ozone recovery post‐2000. In contrast, eruptions increased ozone loss rates over the depletion era from 1980 to 1998. We also present calculations without any aerosol chemistry to isolate contributions from gas‐phase chemistry alone. This study reinforces the need for accurate information regarding stratospheric aerosol loading when modeling ozone changes, particularly for the challenging task of accurately identifying the early signs of ozone healing distinct from other sources of variability.

Abstract

Open questions about the modulation of near‐surface trace gas variability by stratosphere‐troposphere tracer transport complicate efforts to identify anthropogenic sources of gases such as CFC‐11 and N2O and disentangle them from dynamical influences. In this study, we explore one model's modulation of lower stratospheric tracer advection by the quasi‐biennial oscillation (QBO) of stratospheric equatorial zonal‐mean zonal winds at 50 hPa. We assess instances of coherent modulation versus disruption through phase unlocking with the seasonal cycle in the model and in observations. We quantify modeled advective contributions to the temporal rate of change of stratospheric CFC‐11 and N2O at extratropical and high‐latitudes by calculating a transformed Eulerian mean (TEM) budget across isentropic surfaces from a 10‐member WACCM4 ensemble simulation. We find that positive interannual variability in seasonal tracer advection generally occurs in the easterly QBO phase, as in previous work, and briefly discuss physical mechanisms. Individual simulations of the 10‐member ensemble display phase‐unlocking disruptions from this general pattern due to seasonally varying synchronizations between the model's repeating 28‐month QBO cycle and the 12‐month seasonal cycle. We find that phase locking and unlocking patterns of tracer advection calculations inferred from observations fall within the envelope of the ensemble member results. Our study bolsters evidence for variability in the interannual stratospheric dynamical influence of CFC‐11 near‐surface concentrations by assessing the QBO modulation of lower stratospheric advection via synchronization with the annual cycle. It identifies a likely cause of variations in the QBO influence on tropospheric abundances.