Andrew Hoell

Does El Niño intensity matter for California precipitation

The sensitivity of California precipitation to El Nino intensity is investigated by applying a multimodel ensemble of historical climate simulations to estimate how November–April precipitation probability distributions vary across three categorizations of El Nino intensity. Weak and moderate El Nino events fail to appreciably alter wet or dry risks across northern and central California, though odds for wet conditions increase across southern California during moderate El Nino. Significant increases in wet probabilities occur during strong El Nino events across the entire state. In Californias main northern watershed regions, simulations indicate an 85% chance of greater than normal precipitation and a 50% probability of at least 125% of normal. Our results indicate that both the statewide average and the spatial distribution of California precipitation are sensitive to El Nino intensity. Forecasts of El Nino intensity would thus contribute to improved situational awareness for California water planning and related water resource impacts.

Coral record of southeast Indian Ocean marine heatwaves with intensified Western Pacific temperature gradient.

Increasing intensity of marine heatwaves has caused widespread mass coral bleaching events, threatening the integrity and functional diversity of coral reefs. Here we demonstrate the role of inter-ocean coupling in amplifying thermal stress on reefs in the poorly studied southeast Indian Ocean (SEIO), through a robust 215-year (1795–2010) geochemical coral proxy sea surface temperature (SST) record. We show that marine heatwaves affecting the SEIO are linked to the behaviour of the Western Pacific Warm Pool on decadal to centennial timescales, and are most pronounced when an anomalously strong zonal SST gradient between the western and central Pacific co-occurs with strong La Niñas. This SST gradient forces large-scale changes in heat flux that exacerbate SEIO heatwaves. Better understanding of the zonal SST gradient in the Western Pacific is expected to improve projections of the frequency of extreme SEIO heatwaves and their ecological impacts on the important coral reef ecosystems off Western Australia.

Modulation of the Southern Africa precipitation response to the El Niño Southern Oscillation by the subtropical Indian Ocean Dipole

The climate of Southern Africa, defined as the land area bound by the region 15°S–35°S; 12.5°E–42.5°E, during the December–March rainy season is driven by Indo-Pacific sea surface temperature (SST) anomalies associated with the El Niño Southern Oscillation (ENSO) and the Subtropical Indian Ocean Dipole (SIOD). The observed December–March 1979–2014 Southern Africa precipitation during the four ENSO and SIOD phase combinations suggests that the phase of the SIOD can disrupt or enhance the Southern Africa precipitation response to ENSO. Here, we use a large ensemble of model simulations driven by global SST and ENSO-only SST to test whether the SIOD modifies the relationship between Southern Africa precipitation and ENSO. Since ENSO-based precipitation forecasts are used extensively over Southern Africa, an improved understanding of how other modes of SST variability modulate the regional response to ENSO is important. ENSO, in the absence of the SIOD, forces an equivalent barotropic Rossby wave over Southern Africa that modifies the regional mid-tropospheric vertical motions and precipitation anomalies. El Niño (La Niña) is related with high (low) pressure over Southern Africa that produces anomalous mid-tropospheric descent (ascent) and decreases (increases) in precipitation relative to average. When the SIOD and ENSO are in opposite phases, the SIOD compliments the ENSO-related atmospheric response over Southern Africa by strengthening the regional equivalent barotropic Rossby wave, anomalous mid-tropospheric vertical motions and anomalous precipitation. By contrast, when the SIOD and ENSO are in the same phase, the SIOD disrupts the ENSO-related atmospheric response over Southern Africa by weakening the regional equivalent barotropic Rossby wave, anomalous mid-tropospheric vertical motions and anomalous precipitation.

The influence of tropical forcing on extreme winter precipitation in the western Himalaya

Within the Karakoram and western Himalaya (KH), snowfall from winter westerly disturbances (WD) maintains the region’s snowpack and glaciers, which melt seasonally to sustain water resources for downstream populations. WD activity and subsequent precipitation are influenced by global atmospheric variability and tropical-extratropical interactions. On interannual time-scales, El Niño related changes in tropical diabatic heating induce a Rossby wave response over southwest Asia that is linked with enhanced dynamical forcing of WD and available moisture. Consequently, extreme orographic precipitation events are more frequent during El Niño than La Niña or neutral conditions. A similar spatial pattern of tropical diabatic heating is produced by the MJO at intraseasonal scales. In comparison to El Niño, the Rossby wave response to MJO activity is less spatially uniform over southwest Asia and varies on shorter time-scales. This study finds that the MJO’s relationship with WD and KH precipitation is more complex than that of ENSO. Phases of the MJO propagation cycle that favor the dynamical enhancement of WD simultaneously suppress available moisture over southwest Asia, and vice versa. As a result, extreme precipitation events in the KH occur with similar frequency in most phases of the MJO, however, there is a transition in the relative importance of dynamical forcing and moisture in WD to orographic precipitation in the KH as the MJO evolves. These findings give insight into the dynamics and predictability of extreme precipitation events in the KH through their relationship with global atmospheric variability, and are an important consideration in evaluating Asia’s water resources.

Assessing North American multimodel ensemble (NMME) seasonal forecast skill to assist in the early warning of anomalous hydrometeorological events over East Africa

The skill of North American multimodel ensemble (NMME) seasonal forecasts in East Africa (EA), which encompasses one of the most food and water insecure areas of the world, is evaluated using deterministic, categorical, and probabilistic evaluation methods. The skill is estimated for all three primary growing seasons: March–May (MAM), July–September (JAS), and October–December (OND). It is found that the precipitation forecast skill in this region is generally limited and statistically significant over only a small part of the domain. In the case of MAM (JAS) [OND] season it exceeds the skill of climatological forecasts in parts of equatorial EA (Northern Ethiopia) [equatorial EA] for up to 2 (5) [5] months lead. Temperature forecast skill is generally much higher than precipitation forecast skill (in terms of deterministic and probabilistic skill scores) and statistically significant over a majority of the region. Over the region as a whole, temperature forecasts also exhibit greater reliability than the precipitation forecasts. The NMME ensemble forecasts are found to be more skillful and reliable than the forecast from any individual model. The results also demonstrate that for some seasons (e.g. JAS), the predictability of precipitation signals varies and is higher during certain climate events (e.g. ENSO). Finally, potential room for improvement in forecast skill is identified in some models by comparing homogeneous predictability in individual NMME models with their respective forecast skill.

Advancing Science and Services during the 2015-16 El Niño: The NOAA El Niño Rapid Response Field Campaign

AbstractForecasts by mid-2015 for a strong El Nino during winter 2015/16 presented an exceptional scientific opportunity to accelerate advances in understanding and predictions of an extreme climat...

Austral Summer Southern Africa Precipitation Extremes Forced by the El Niño–Southern Oscillation and the Subtropical Indian Ocean Dipole

Southern Africa, defined here as the African continent south of 15°S latitude, is prone to seasonal precipitation extremes during December–March that have profound effects on large populations of people. The intensity of summertime precipitation extremes can be remarkable, with wet seasons experiencing up to a doubling of the seasonal average precipitation. Recognizing the importance of understanding the causes of Southern Africa precipitation extremes for the purpose of improved early warning, an 80-member ensemble of atmospheric model simulations forced by observed time-varying boundary conditions during 1979–2016 is used to examine the mechanisms by which December–March precipitation extremes are delivered to Southern Africa and whether the El Niño-Southern Oscillation (ENSO) and the Subtropical Indian Ocean Dipole (SIOD) modify the probabilities of extreme seasonal precipitation occurrences. The model simulations reveal that the synchronous ENSO and SIOD phasing conditions the probability of December–March extreme precipitation occurrences. The probability of extreme wet seasons is greatly increased by La Niña, especially so when combined with a positive SIOD, and greatly decreased by El Niño regardless of SIOD phasing. By contrast, the probability of extreme dry seasons is increased by El Niño and is decreased by La Niña. The mechanisms by which extreme precipitation are delivered are the same regardless of ENSO and SIOD phase. Extreme wet seasons are a result of an anomalous lower tropospheric cyclone over Southern Africa that increases convergence and moisture fluxes into the region while extreme dry seasons are a result of an anomalous lower tropospheric anticyclone that decreases convergence and moisture fluxes into the region.

Recent and Possible Future Variations in the North American Monsoon

The dynamics and recent and possible future changes of the June–September rainfall associated with the North American Monsoon (NAM) are reviewed in this chapter. Our analysis as well as previous analyses of the trend in June–September precipitation from 1948 until 2010 indicate significant precipitation increases over New Mexico and the core NAM region, and significant precipitation decreases over southwest Mexico. The trends in June–September precipitation have been forced by anomalous cyclonic circulation centered at 15°N latitude over the eastern Pacific Ocean. The anomalous cyclonic circulation is responsible for changes in the flux of moisture and the divergence of moisture flux within the core NAM region. Future climate projections using the Coupled Model Intercomparison Project Phase 5 (CMIP5) models, as part of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5), support the observed analyses of a later shift in the monsoon season in the presence of increased greenhouse gas concentrations in the atmosphere under the RCP8.5 scenario. The CMIP5 models under the RCP8.5 scenario predict significant NAM-related rainfall decreases during June and July and predict significant NAM-related rainfall increases during September and October.

Introduction to Explaining Extreme Events of 2015 from a Climate Perspective

In the first years of this report, we answered questions such as: “What is event attribution?” and “Is it even possible to address the effects of long-term changes on extreme events using event attribution?” The science has now advanced to the point that we can detect the effects of climate change on some events with high confidence (e.g., especially those linked to temperature), although results are necessarily probabilistic and not deterministic. The growing popular interest in event-attribution is feeding back to the science, for example by requiring it to more carefully consider the impacts of various interpretations and framings of the causation question. We thus now ask: “What is the confidence of the results?” and “How should the results be interpreted?” We are conscious of the importance of the precise question being asked, for instance “What are long-term contributions to event frequency?” versus “What are long-term contributions to event intensity?” (e.g., Dole et al. 2011). There remains an ongoing need to reconcile attribution results pertaining to different aspects of extreme event behavior (e.g., Otto et al. 2012). To state that event attribution is complex, especially for extreme rainfall and related storm systems including tropical cyclones, is obvious. Yet, such complexities mean that the analytic work to pull numerous pieces together to establish probable cause continues to require considerable time, even as computers become more powerful to aid the effort. Thus, the reliability and realism of “real time” attribution for which there is great public appetite, continues to be an open question. The scope of information demand is also multifaceted, not only to explain “why the event happened,” but also “how well the event was anticipated.” These new questions are far more challenging to address and are increasingly relevant to the concerns of society. Attribution science has made progress in answering these questions, though considerably more work needs to be done. This last year has been exciting for attribution science, as the U.S. National Academy of Sciences released its report on the topic (NAS 2016). To date, it is the most comprehensive look at the state of event attribution science, including how the framing of attribution questions impacts the results. For example, in a complex event such as drought, a study of precipitation versus a study of temperature may yield different results regarding the role of climate change. The report also addresses how attribution results are presented, interpreted, and communicated. It provides the most robust description to date of the various methodologies used in event attribution and addresses the issues around both the confidence of the results and the current capabilities of near-real time attribution. No single methodology exists for the entire field of event attribution, and each event type must be examined individually. Confidence in results of an attribution analysis depends on what has been referred to as the “three pillars” of event attribution: the quality of the observational record, the ability of models to simulate the event, and our understanding of the physical processes that drive the event and how they are being impacted by climate change. A recently published paper (Mitchell et al. 2016) marks the beginning of an important new undertaking for the event attribution field by providing an example of how to apply event attribution science to understanding and preparing for impacts. For many years, the scientific community has discussed linking event attribution to the impacts of these events and the role climate change has played in altering those impacts. This year, for the first time, attribution scientists partnered with public health officials to assess the role climate change played in increased mortality from a specific event—the 2003 European heatwave (Mitchell et al. 2016). Their results concluded that in the summer of 2003, “out of the estimated ~315 and ~735 summer deaths directly AFFILIATIONS: herring—NOAA/National Centers for Environmental Information, Boulder, Colorado; hoell and hoerling—NOAA/Earth System Research Laboratory, Physical Sciences Division, Boulder, Colorado; KoSSin—NOAA/National Centers for Environmental Information, Madison, Wisconsin; SChreCK—NOAA/National Centers for Environmental Information and Cooperative Institute for Climate and Satellites– North Carolina, North Carolina State University, Asheville, North Carolina; Stott—Met Office Hadley Centre and University of Exeter, Exeter, United Kingdom

Middle East and Southwest Asia Daily Precipitation Characteristics Associated with the Madden Julian Oscillation During Boreal Winter

AbstractThe spatial and temporal evolution of Middle East and southwest Asia (MESW) precipitation characteristics and the associated atmospheric circulation during times in which tropical eastern I...