Southeastern United States summer rainfall framework and its implication for seasonal prediction

Li, L., and W. Li, 2013: “Southeastern United States summer rainfall framework and its implication for seasonal prediction.” Environmental Research Letters, v. 8, paper no. 044017, doi: 10.1088/1748-9326/8/4/044017.

A new rainfall framework is constructed to describe the complex probability distribution of southeastern United States (SE US) summer (June–July–August) rainfall, which cannot be well represented by traditional kernel fitting methods. The new framework is based on the configuration of a three-cluster finite normal mixture model and is realized by Bayesian inference and a Markov Chain Monte Carlo (MCMC) algorithm. The three rainfall clusters reflect the probability distribution of light, moderate, and heavy rainfall in summer, and are linked to different climate factors. The variation of light rainfall intensity is likely associated with the combined effects of La Niña and the tri-pole sea surface temperature anomaly (SSTA) over the North Atlantic. Heavy rainfall concurs with a ‘horseshoe-like’ SSTA over the North Atlantic. In contrast, moderate rainfall is less correlated with the SSTA and likely caused by atmospheric internal dynamics. Rainfall characteristics and their linkages with SSTAs help improve seasonal predictions of regional climate. Such a new framework has an important implication in understanding the response of regional hydrology to climate variability and climate change; and our study suggest that it can be extended to other regions and seasons with similar climate.

Open Access

Signatures of Tibetan Plateau heating on Indian summer monsoon rainfall variability

Rajagopalan, B., and P. Molnar, 2013: “Signatures of Tibetan Plateau heating on Indian summer monsoon rainfall variability.” Journal of Geophysical Research: Atmospheres, v. 118, pp. 1170-1178, doi: 10.1002/jgrd.50124.

Despite recent challenges, conventional wisdom has held that heating over the Tibetan Plateau leads to increased Indian summer monsoon rainfall via enhancement of cross-equatorial circulation aloft, and a concurrent strengthening of both the Somali Jet and westerly winds that bring moisture to southern India. We show that such heating, quantified by monthly estimates of moist static energy in the atmosphere just above the surface, correlates with summer monsoon rainfall, but only in the early (20 May to 15 June) and late (September 1 to 15 October) monsoon season. Correlations during the main monsoon season (15 June to 31 August) are small and insignificant. The positive correlations with early and late monsoon season, however, allow for heating over Tibet to modulate as much as ~30% of the total rainfall. Furthermore, we demonstrate that heating over Tibet is independent of the El Niño Southern Oscillation, so that together they explain a substantial portion of variability in the early and late season rainfall, providing potential predictability. These links may also explain the wet conditions over India during early Holocene time and provide a quantitative link between a rise of Tibet and stronger Somali Jet.

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Drought induces spruce beetle (Dendroctonus rufipennis) outbreaks across northwestern Colorado

Hart, S.J., T.T. Veblen, K.S. Eisenhart, D. Jarvis, and D. Kulakowski, 2013: “Drought induces spruce beetle (Dendroctonus rufipennis) outbreaks across northwestern Colorado.” Ecology, doi: 10.1890/13-0230.1.

This study examines influences of climate variability on spruce beetle (Dendroctonus rufipennis) outbreak across NW Colorado during the CE 1650-2011 period. Periods of broad-scale outbreak reconstructed using documentary records and tree-rings were dated to 1843 to 1860, 1882-1889, 1931-1957, and 2004-2010. Periods of outbreak were compared with seasonal temperature, precipitation, vapor pressure deficit (VPD), the Palmer Drought Severity Index (PDSI), and indices of ocean-atmosphere oscillation that include the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO). Classification trees showed that outbreaks can be predicted most successfully from above average annual AMO values and above average summer VPD values, indicators of drought across Colorado. Notably, we find that spruce beetle outbreaks appear to be predicted best by interannual to multidecadal variability in drought, not by temperature alone. This finding may imply that spruce beetle outbreaks are triggered by decreases in host tree defenses, which are hypothesized to occur with drought stress. Given the persistence of the AMO, the shift to a positive AMO phase in the late 1990s is likely to promote continued spruce beetle disturbance.

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Simulated changes in Northwest U.S. climate in response to Amazon deforestation

Medvigy, D., R.L. Walko, M.J. Otte, and R. Avissar, 2013: “Simulated changes in Northwest U.S. climate in response to Amazon deforestation.” Journal of Climate, v. 26, pp. 9115-9136, doi: 10.1175/JCLI-D-12-00775.1.

Numerical models have long predicted that the deforestation of the Amazon would lead to large regional changes in precipitation and temperature, but the extratropical effects of deforestation have been a matter of controversy. This paper investigates the simulated impacts of deforestation on the northwest United States December–February climate. Integrations are carried out using the Ocean–Land–Atmosphere Model (OLAM), here run as a variable-resolution atmospheric GCM, configured with three alternative horizontal grid meshes: 1) 25-km characteristic length scale (CLS) over the United States, 50-km CLS over the Andes and Amazon, and 200-km CLS in the far-field; 2) 50-km CLS over the United States, 50-km CLS over the Andes and Amazon, and 200-km CLS in the far-field; and 3) 200-km CLS globally. In the high-resolution simulations, deforestation causes a redistribution of precipitation within the Amazon, accompanied by vorticity and thermal anomalies. These anomalies set up Rossby waves that propagate into the extratropics and impact western North America. Ultimately, Amazon deforestation results in 10%–20% precipitation reductions for the coastal northwest United States and the Sierra Nevada. Snowpack in the Sierra Nevada experiences declines of up to 50%. However, in the coarse-resolution simulations, this mechanism is not resolved and precipitation is not reduced in the northwest United States. These results highlight the need for adequate model resolution in modeling the impacts of Amazon deforestation. It is concluded that the deforestation of the Amazon can act as a driver of regional climate change in the extratropics, including areas of the western United States that are agriculturally important.

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Identification of extreme precipitation threat across midlatitude regions based on short-wave circulations

Wang, S.-Y., R.E. Davies, and R.R. Gillies, 2013: “Identification of extreme precipitation threat across midlatitude regions based on short-wave circulations.” Journal of Geophysical Research: Atmospheres, v. 118, pp. 11,059-11,074, doi: 10.1002/jgrd.50841.

The most severe thunderstorms, producing extreme precipitation, occur over subtropical and midlatitude regions. Atmospheric conditions conducive to organized, intense thunderstorms commonly involve the coupling of a low-level jet (LLJ) with a synoptic short wave. The midlatitude synoptic activity is frequently modulated by the circumglobal teleconnection (CGT), in which meridional gradients of the jet stream act as a guide for short Rossby waves. Previous research has linked extreme precipitation events with either the CGT or the LLJ but has not linked the two circulation features together. In this study, a circulation-based index was developed by combining (a) the degree of the CGT and LLJ coupling, (b) the extent to which this CGT-LLJ coupling connects to regional precipitation and (c) the spatial correspondence with the CGT (short wave) trending pattern over the recent 32 years (1979–2010). Four modern-era global reanalyses, in conjunction with four gridded precipitation data sets, were utilized to minimize spurious trends. The results are suggestive of a link between the CGT/LLJ trends and several recent extreme precipitation events, including those leading to the 2008 Midwest flood in U.S., the 2011 tornado outbreaks in southeastern U.S., the 2010 Queensland flood in northeastern Australia, and to the opposite side the 2012 central U.S. drought. Moreover, an analysis of three Coupled Model Intercomparison Project Phase 5 models from the historical experiments points to the role of greenhouse gases in forming the CGT trends during the warm season.

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Robust twenty-first-century projections of El Niño and related precipitation variability

Power, S., F. Delage, C. Chung, G. Kociuba, and K. Keay, 2013: “Robust twenty-first-century projections of El Niño and related precipitation variability.” Nature, doi: 10.1038/nature12580.

The El Niño–Southern Oscillation (ENSO) drives substantial variability in rainfall, severe weather, agricultural production, ecosystems and disease in many parts of the world. Given that further human-forced changes in the Earth’s climate system seem inevitable, the possibility exists that the character of ENSO and its impacts might change over the coming century. Although this issue has been investigated many times during the past 20 years, there is very little consensus on future changes in ENSO, apart from an expectation that ENSO will continue to be a dominant source of year-to-year variability. Here we show that there are in fact robust projected changes in the spatial patterns of year-to-year ENSO-driven variability in both surface temperature and precipitation. These changes are evident in the two most recent generations of climate models, using four different scenarios for CO2 and other radiatively active gases. By the mid- to late twenty-first century, the projections include an intensification of both El-Niño-driven drying in the western Pacific Ocean and rainfall increases in the central and eastern equatorial Pacific. Experiments with an Atmospheric General Circulation Model reveal that robust projected changes in precipitation anomalies during El Niño years are primarily determined by a nonlinear response to surface global warming. Uncertain projected changes in the amplitude of ENSO-driven surface temperature variability have only a secondary role. Projected changes in key characteristics of ENSO are consequently much clearer than previously realized.

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Reexamining El Niño and cholera in Peru: A climate affairs approach

Ramírez, I.J., S.C. Grady, and M.H. Glantz, 2013: “Reexamining El Niño and cholera in Peru: A climate affairs approach.” Weather, Climate, and Society, v. 5, pp. 148–161, doi: 10.1175/WCAS-D-12-00032.1.

In the 1990s Peru experienced the first cholera epidemic after almost a century. The source of emergence was initially attributed to a cargo ship, but later there was evidence of an El Niño association. It was hypothesized that marine ecosystem changes associated with El Niño led to the propagation of V. cholerae along the coast of Peru, which in turn initiated the onset of the epidemic in 1991. Earlier studies supported this explanation by demonstrating a relationship between elevated temperatures and increased cholera incidence in Peru; however, other aspects of El Niño–Southern Oscillation (ENSO) and their potential impacts on cholera were not investigated. Therefore, this study examines the relationship between El Niño and cholera in Peru from a holistic view of the ENSO cycle. A “climate affairs” approach is employed as a conceptual framework to incorporate ENSO’s multidimensional nature and to generate new hypotheses about the ENSO and cholera association in Peru. The findings reveal that ENSO may have been linked to the cholera epidemic through multiple pathways, including rainfall extremes, La Niña, and social vulnerability, with impacts depending on the geography of teleconnections within Peru. When the definition of an ENSO event is examined, cholera appears to have emerged either during ENSO neutral or La Niña conditions. Furthermore, the analysis herein suggests that the impact of El Niño arrived much later, possibly resulting in heightened transmission in the austral summer of 1992. In conclusion, a modified hypothesis with these new insights on cholera emergence and transmission in Peru is presented.

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Changes in extreme temperature and precipitation in the Arab region: Long-term trends and variability related to ENSO and NAO

Donat, M.G., T.C. Peterson, M. Brunet, A.D. King, M. Almazroui, R.K. Kolli, D. Boucherf, A.Y. Al-Mulla, A.Y. Nour, A.A. Aly, T.A.A. Nada, M.M. Semawi, H.A.A. Dashti, T.G. Salhab, K.I. El Fadli, M.K. Muftah, S.D. Eida, W. Badi, F. Driouech, K.E. Rhaz, M.J.Y. Abubaker, A.S. Ghulam, A.S. Erayah, M.B. Mansour, W.O. Alabdouli, J.S.A. Dhanhani, and M.N. Al Shekaili, 2013: “Changes in extreme temperature and precipitation in the Arab region: Long-term trends and variability related to ENSO and NAO.” International Journal of Climatology, doi: 10.1002/joc.3707.

A workshop was held in Casablanca, Morocco, in March 2012, to enhance knowledge of climate extremes and their changes in the Arab region. This workshop initiated intensive data compilation activities of daily observational weather station data from the Arab region. After conducting careful control processes to ensure the quality and homogeneity of the data, climate indices for extreme temperatures and precipitation were calculated.

This study examines the temporal changes in climate extremes in the Arab region with regard to long-term trends and natural variability related to ENSO and NAO. We find consistent warming trends since the middle of the 20th Century across the region. This is evident in the increased frequencies of warm days and warm nights, higher extreme temperature values, fewer cold days and cold nights and shorter cold spell durations. The warming trends seem to be particularly strong since the early 1970s. Changes in precipitation are generally less consistent and characterised by a higher spatial and temporal variability; the trends are generally less significant. However, in the western part of the Arab region, there is a tendency towards wetter conditions. In contrast, in the eastern part, there are more drying trends, although, these are of low significance.

We also find some relationships between climate extremes in the Arab region and certain prominent modes of variability, in particular El Niño-Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO). The relationships of the climate extremes with NAO are stronger, in general, than those with ENSO, and are particularly strong in the western part of the Arab region (closer to the Atlantic Ocean). The relationships with ENSO are found to be more significant towards the eastern part of the area of study.

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Interannual variability of Indian winter monsoon over the Western Himalayas

Dimri, A.P., 2013: “Interannual variability of Indian winter monsoon over the Western Himalayas.” Global and Planetary Change, v. 106, pp. 39-50, doi: 10.1016/j.gloplacha.2013.03.002.

The Indian subcontinent is surrounded by mighty Himalayas in the north. It is characterized by heterogeneous topography and variable landuse from northwest to northeast. Apart from these, due to seasonal changes western, central and eastern Himalayas are having different precipitation patterns. In the present study Indian winter (December, January, February — DJF) monsoon (IWM) precipitation over the Western Himalayas (WH) is analyzed. During IWM, the WH receives almost one third of annual precipitation due to eastward moving cyclonic storms, western disturbances (WDs). Wet and dry precipitation years’ composite analysis shows anomalous cyclonic flow over and across the northern India with higher clouding associated with precipitation over the Himalayan region during wet year. Significant southward shift of 200 hPa subtropical westerly jet (SWJ) with stationary wave pattern over south Asian region is seen during wet years. Over equatorial Pacific increased response of attenuated Walker circulation during El Niño situations is associated with higher precipitation wet years. Also, strengthening of Hadley circulation response within 30°S to 30°N provides symmetrical upper tropospheric meridional transport from Southern Hemisphere to Northern Hemisphere during wet years. Significant precursor dependency on evolution of sea surface temperature warming over equatorial eastern Pacific and cooling over western equatorial Pacific is seen.

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Improved simulation of Indian summer monsoon in latest NCEP climate forecast system free run

Saha, S.K., S. Pokhrel, H.S. Chaudhari, A. Dhakate, S. Shewale, C.T. Sabeerali, K. Salunke, A. Hazra, S, Mahapatra, and A.S. Rao, 2013: “Improved simulation of Indian summer monsoon in latest NCEP climate forecast system free run.” International Journal of Climatology, doi: 10.1002/joc.3791.

Simulation of Indian summer monsoon features by latest coupled model of National Centers for Environmental Prediction (NCEPs) Climate Forecast System version 2 (CFSv2) is attempted in its long run. Improvements in the simulation of Indian summer monsoon as compared with previous version (CFSv1) is accessed and areas which still require considerable refinements are introduced. It is found that spatial pattern of seasonal mean rainfall and wind circulations are more realistic in CFSv2 as compared with CFSv1. Variance and northward propagation of intraseasonal oscillation (ISO), which also contribute to the seasonal mean rainfall are remarkably improved. However, the central Indian dry bias still persists and amplified. Pervasive cold bias in surface (2 m air temperature) as well as in the whole troposphere is further increased in CFSv2. These cold biases may be partly attributed to the lack of model’s ability to realistically simulate the ratio of convective and stratiform rainfall. Sea-surface temperature (SST) over the Indian Ocean is underestimated in CFSv2. However, CFSv1 shows east–west dipole structure in the bias. The teleconnection of El Nino Southern Oscillation (ENSO) and Indian summer monsoon rainfall (ISMR) in terms of Niño3 SST and monsoon rainfall correlation is more realistic in the latest version of the model. Overall, there are substantial improvements in CFSv2 as compared with CFSv1, but it has to evolve further to realistically simulate the mean and variability of ISMR.

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