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Gershunov, A, Roca R.  2004.  Coupling of latent heat flux and the greenhouse effect by large-scale tropical/subtropical dynamics diagnosed in a set of observations and model simulations. Climate Dynamics. 22:205-222.   10.1007/s00382-003-0376-7   AbstractWebsite

Coupled variability of the greenhouse effect (GH) and latent heat flux (LHF) over the tropical - subtropical oceans is described, summarized and compared in observations and a coupled ocean-atmosphere general circulation model (CGCM). Coupled seasonal and interannual modes account for much of the total variability in both GH and LHF. In both observations and model, seasonal coupled variability is locally 180degrees out-of-phase throughout the tropics. Moisture is brought into convergent/convective regions from remote source areas located partly in the opposite, non-convective hemisphere. On interannual time scales, the tropical Pacific GH in the ENSO region of largest interannual variance is 180degrees out of phase with local LHF in observations but in phase in the model. A local source of moisture is thus present in the model on interannual time scales while in observations, moisture is mostly advected from remote source regions. The latent cooling and radiative heating of the surface as manifested in the interplay of LHF and GH is an important determinant of the current climate. Moreover, the hydrodynamic processes involved in the GH-LHF interplay determine in large part the climate response to external perturbations mainly through influencing the water vapor feedback but also through their intimate connection to the hydrological cycle. The diagnostic process proposed here can be performed on other CGCMs. Similarly, it should be repeated using a number of observational latent heat flux datasets to account for the variability in the different satellite retrievals. A realistic CGCM could be used to further study these coupled dynamics in natural and anthropogenically altered climate conditions.

Gershunov, A, Michaelsen J.  1996.  Climatic-scale space-time variability of tropical precipitation. Journal of Geophysical Research-Atmospheres. 101:26297-26307.   10.1029/96jd01382   AbstractWebsite

More than 15 years of monthly microwave sounding unit rainfall data over the tropical oceans are analyzed to illustrate rainfall variability on various timescales and delineate its spatial patterns. The annual and semiannual components of the seasonal cycle are modeled with first and second annual harmonics at every 2.5 degrees x 2.5 degrees grid square. Regions of highest rainfall variability tend to be characterized by a powerful annual cycle. The semiannual cycle is generally a trivial component of the seasonal cycle, except in some regions where either the mean climatological precipitation is low or where the total seasonal cycle is weak. An interesting exception, in this respect, is a band of the southeastern tropical Pacific extending immediately to the south of the eastern equatorial Pacific cold tongue. Regions of highest climatological mean rainfall are characterized by weak seasonality but strong nonseasonal variability. After seasonality is described and removed from the data, nonseasonal variability is considered via principal component analysis in the time domain. The two dominant modes together describe precipitation variability associated with the El Nino-Southern Oscillation: they outline the evolution of warm- and cold-event precipitation anomalies and contrast the intense 1982-1983 warm event with the moderate events of 1986-1987 and 1992-1993. These two modes display oscillations with predominantly quasi-biennial and similar to 5-year periods. Another coherent mode summarizes intraseasonal variability which, although inadequately resolved by the monthly average rainfall data, displays typical signs of the 40- to 50-day oscillation. All coherent modes, despite having much of their energy concentrated around rather different frequencies, show signs of interaction.

White, WB, Gershunov A, Annis J.  2008.  Climatic influences on Midwest drought during the twentieth century. Journal of Climate. 21:517-531.   10.1175/2007jcli1465.1   AbstractWebsite

The Dustbowl Era drought in the 1930s was the principal Midwest drought of the twentieth century, occurring primarily in late spring-summer [April-August (AMJJA)] when > 70% of annual rainfall normally occurred. Another major Midwest drought occurred in the 1950s but primarily in fall-early winter [September-December (SOND)] when normal rainfall was similar to 1/2 as much. Optimized canonical correlation analysis (CCA) is applied to forecast AMJJA and SOND Midwest rainfall variability in cross-validated fashion from antecedent DJF and JJA sea surface temperature (SST) variability in the surrounding oceans. These CCA models simulate (i. e., hindcast, not forecast) the Dustbowl Era drought of the 1930s and four of seven secondary AMJJA droughts (>= 3-yr duration) during the twentieth century, and the principal Midwest drought of the 1950s and one of three secondary SOND droughts. Diagnosing the model canonical correlations finds the superposition of tropical Pacific cool phases of the quasi-decadal oscillation (QDO) and interdecadal oscillation (IDO) responsible for secondary droughts in AMJJA when ENSO was weak and finds the eastern equatorial Pacific cool phase of the ENSO responsible for secondary droughts during SOND when ENSO was strong. These explain why secondary droughts in AMJJA occurred more often (nearly every decade) and were of longer duration than secondary droughts in SOND when decadal drought tendencies were usually interrupted by ENSO. These diagnostics also find the AMJJA Dustbowl Era drought in the 1930s and the principal SOND drought in the 1950s driven primarily by different phases (i. e., in quadrature) of the pentadecadal signal in the Pacific decadal oscillation (PDO).

Macias, D, Landry MR, Gershunov A, Miller AJ, Franks PJS.  2012.  Climatic control of upwelling variability along the western North American coast. Plos One. 7   10.1371/journal.pone.0030436   AbstractWebsite

The high biological production of the California Current System (CCS) results from the seasonal development of equatorward alongshore winds that drive coastal upwelling. While several climatic fluctuation patterns influence the dynamics and biological productivity of the CCS, including the El Nino-Southern Oscillation (ENSO), the Pacific Decadal Oscillation index (PDO) and the North Pacific Gyre Oscillation (NPGO), the mechanisms of interaction between climatic oscillations and the CCS upwelling dynamics have remained obscure. Here, we use Singular Spectral Analysis (SSA) to reveal, for the first time, low-frequency concordance between the time series of climatic indices and upwelling intensity along the coast of western North America. Based on energy distributions in annual, semiannual and low-frequency signals, we can divide the coast into three distinct regions. While the annual upwelling signal dominates the energy spectrum elsewhere, low-frequency variability is maximal in the regions south of 33 degrees N. Non-structured variability associated with storms and turbulent mixing is enhanced at northerly locations. We found that the low-frequency signal is significantly correlated with different climatic indices such as PDO, NPGO and ENSO with the correlation patterns being latitude-dependent. We also analyzed the correlations between this upwelling variability and sea surface temperature (SST) and sea level pressure (SLP) throughout the North Pacific to visualize and interpret the large-scale teleconnection dynamics in the atmosphere that drive the low-frequency coastal winds. These results provide new insights into the underlying mechanisms connecting climatic patterns with upwelling dynamics, which could enhance our prediction and forecast capabilities of the effects of future oceanographic and climatic variability in the CCS.

Guzman-Morales, J, Gershunov A.  2019.  Climate change suppresses Santa Ana winds of Southern California and sharpens their seasonality. Geophysical Research Letters. 46:2772-2780.   10.1029/2018gl080261   AbstractWebsite

We downscale Santa Ana winds (SAWs) from eight global climate models (GCMs) and validate key aspects of their climatology over the historical period. We then assess SAW evolution and behavior through the 21st century, paying special attention to changes in their extreme occurrences. All GCMs project decreases in SAW activity, starting in the early 21st century, which are commensurate with decreases in the southwestward pressure gradient force that drives these winds. The trend is most pronounced in the early and late SAW season: fall and spring. It is mainly determined by changes in the frequency of SAW events, less so by changes in their intensity. The peak of the SAW season (November-December-January) is least affected by anthropogenic climate change in GCM projections. Plain Language Summary Dry and gusty Santa Ana winds (SAWs) drive the most catastrophic wildfires in Southern California. Their sensitivity to the changing climate has been a matter of uncertainty and debate. We have assessed the response of SAW activity to global warming and describe these results in detail here. The overall decrease in SAW activity robustly projected by downscaled global climate models is strongest in the early and late seasons-fall and spring. SAWs are expected to decrease least at the peak of their season approximately December. Importantly, decreased SAW activity in the future climate is driven mainly by decreased frequency rather than the peak intensity of these winds. These results, together with what we know from recent literature about how precipitation is projected to change in this region, suggest a later wildfire season in the future.

Lavers, DA, Ralph FM, Waliser DE, Gershunov A, Dettinger MD.  2015.  Climate change intensification of horizontal water vapor transport in CMIP5. Geophysical Research Letters. 42:5617-5625.   10.1002/2015gl064672   AbstractWebsite

Global warming of the Earth's atmosphere is hypothesized to lead to an intensification of the global water cycle. To determine associated hydrological changes, most previous research has used precipitation. This study, however, investigates projected changes to global atmospheric water vapor transport (integrated vapor transport (IVT)), the key link between water source and sink regions. Using 22 global circulation models from the Climate Model Intercomparison Project Phase 5, we evaluate, globally, the mean, standard deviation, and the 95th percentiles of IVT from the historical simulations (1979-2005) and two emissions scenarios (2073-2099). Considering the more extreme emissions, multimodel mean IVT increases by 30-40% in the North Pacific and North Atlantic storm tracks and in the equatorial Pacific Ocean trade winds. An acceleration of the high-latitude IVT is also shown. Analysis of low-altitude moisture and winds suggests that these changes are mainly due to higher atmospheric water vapor content.

Semenza, JC, Caplan JS, Buescher G, Das T, Brinks MV, Gershunov A.  2012.  Climate change and microbiological water quality at California beaches. Ecohealth. 9:293-297.   10.1007/s10393-012-0779-1   AbstractWebsite

Daily microbiological water quality and precipitation data spanning 6 years were collected from monitoring stations at southern California beaches. Daily precipitation projected for the twenty-first century was derived from downscaled CNRM CM3 global climate model. A time series model of Enterococcus concentrations that was driven by precipitation, matched the general trend of empirical water quality data; there was a positive association between precipitation and microbiological water contamination (P < 0.001). Future projections of precipitation result in a decrease in predicted Enterococcus levels through the majority of the twenty-first century. Nevertheless, variability of storminess due to climate change calls for innovative adaptation and surveillance strategies.

Rodo, X, Pascual M, Doblas-Reyes FJ, Gershunov A, Stone DA, Giorgi F, Hudson PJ, Kinter J, Rodriguez-Arias MA, Stenseth NC, Alonso D, Garcia-Serrano J, Dobson AP.  2013.  Climate change and infectious diseases: Can we meet the needs for better prediction? Climatic Change. 118:625-640.   10.1007/s10584-013-0744-1   AbstractWebsite

The next generation of climate-driven, disease prediction models will most likely require a mechanistically based, dynamical framework that parameterizes key processes at a variety of locations. Over the next two decades, consensus climate predictions make it possible to produce forecasts for a number of important infectious diseases that are largely independent of the uncertainty of longer-term emissions scenarios. In particular, the role of climate in the modulation of seasonal disease transmission needs to be unravelled from the complex dynamics resulting from the interaction of transmission with herd immunity and intervention measures that depend upon previous burdens of infection. Progress is also needed to solve the mismatch between climate projections and disease projections at the scale of public health interventions. In the time horizon of seasons to years, early warning systems should benefit from current developments on multi-model ensemble climate prediction systems, particularly in areas where high skill levels of climate models coincide with regions where large epidemics take place. A better understanding of the role of climate extremes on infectious diseases is urgently needed.

Westerling, AL, Gershunov A, Brown TJ, Cayan DR, Dettinger MD.  2003.  Climate and wildfire in the western United States. Bulletin of the American Meteorological Society. 84:595-+.   10.1175/bams-84-5-595   AbstractWebsite

A 21-yr gridded monthly fire-starts and acres-burned dataset from U.S. Forest Service, Bureau of Land Management, National Park Service, and Bureau of Indian Affairs fire reports recreates the seasonality and interannual variability of wildfire in the western United States. Despite pervasive human influence in western fire regimes, it is striking how strongly these data reveal a fire season responding to variations in climate. Correlating anomalous wildfire frequency and extent with the Palmer Drought Severity Index illustrates the importance of prior and accumulated precipitation anomalies for future wildfire season severity. This link to antecedent seasons' moisture conditions varies widely with differences in predominant fuel type. Furthermore, these data demonstrate that the relationship between wildfire season severity and observed moisture anomalies from antecedent seasons is strong enough to forecast fire season severity at lead times of one season to a year in advance.

Guirguis, K, Gershunov A, Clemesha RES, Shulgina T, Subramanian AC, Ralph FM.  2018.  Circulation drivers of atmospheric rivers at the North American West Coast. Geophysical Research Letters. 45:12576-12584.   10.1029/2018gl079249   AbstractWebsite

Atmospheric rivers (ARs) are mechanisms of strong moisture transport capable of bringing heavy precipitation to the West Coast of North America, which drives water resources and can lead to large-scale flooding. Understanding links between climate variability and landfalling ARs is critical for improving forecasts on timescales needed for water resource management. We examined 69years of landfalling ARs along western North America using reanalysis and a long-term AR catalog to identify circulation drivers of AR landfalls. This analysis reveals that AR activity along the West Coast is largely associated with a handful of influential modes of atmospheric variability. Interaction between these modes creates favorable or unfavorable atmospheric states for landfalling ARs at different locations, effectively steering moisture plumes up and down the coast from Mexico to British Columbia. Seasonal persistence of certain modes helps explain interannual variability of landfalling ARs, including recent California drought years and the wet winter of 2016/2017. Plain Language Summary Understanding links between large-scale climate variability and landfalling ARs is important for improving subseasonal-to-seasonal (S2S) predictability of water resources in the western United States. We have analyzed a seven-decade-long catalog of ARs impacting western North America to quantify synoptic influence on AR activity. Our results identify dominant circulation patterns associated with landfalling ARs and show how seasonal variation in the prevalence of certain circulation features modulates the frequency of AR landfalls at different latitudes in a given year. AR variability played an important role in the recent California drought as well as the wet winter of 2016/2017, and we show how this variability was associated with the relative frequency of favorable versus unfavorable atmospheric states. Our findings also reveal that the bulk of AR landfalls along the West Coast is associated with only a handful of influential circulation features, which has implications for S2S predictability.

Clemesha, RES, Guirguis K, Gershunov A, Small IJ, Tardy A.  2018.  California heat waves: their spatial evolution, variation, and coastal modulation by low clouds. Climate Dynamics. 50:4285-4301.   10.1007/s00382-017-3875-7   AbstractWebsite

We examine the spatial and temporal evolution of heat waves through California and consider one of the key modulating factors of summertime coastal climate-coastal low cloudiness (CLC). Heat waves are defined relative to daytime maximum temperature (T-max) anomalies after removing local seasonality and capture unseasonably warm events during May-September. California is home to several diverse climate regions and characteristics of extreme heat events are also variable throughout these regions. Heat wave events tend to be shorter, but more anomalously intense along the coast. Heat waves typically impact both coastal and inland regions, although there is more propensity towards coastally trapped events. Most heat waves with a strong impact across regions start at the coast, proceed inland, and weaken at the coast before letting up inland. Typically, the beginning of coastal heat waves are associated with a loss of CLC, followed by a strong rebound of CLC starting close to the peak in heat wave intensity. The degree to which an inland heat wave is expressed at the coast is associated with the presence of these low clouds. Inland heat waves that have very little expression at the coast tend to have CLC present and an elevated inversion base height compared with other heat waves.

Gershunov, A, Guirguis K.  2012.  California heat waves in the present and future. Geophysical Research Letters. 39   10.1029/2012gl052979   AbstractWebsite

Current and projected heat waves are examined over California and its sub-regions in observations and downscaled global climate model (GCM) simulations. California heat wave activity falls into two distinct types: (1) typically dry daytime heat waves and (2) humid nighttime-accentuated events (Type I and Type II, respectively). The four GCMs considered project Type II heat waves to intensify more with climate change than the historically characteristic Type I events, although both types are projected to increase. This trend is already clearly observed and simulated to various degrees over all sub-regions of California. Part of the intensification in heat wave activity is due directly to mean warming. However, when one considers non-stationarity in daily temperature variance, desert heat waves are expected to become progressively and relatively less intense while coastal heat waves are projected to intensify even relative to the background warming. This result generally holds for both types of heat waves across models. Given the high coastal population density and low acclimatization to heat, especially humid heat, this trend bodes ill for coastal communities, jeopardizing public health and stressing energy resources. Citation: Gershunov, A., and K. Guirguis (2012), California heat waves in the present and future, Geophys. Res. Lett., 39, L18710, doi:10.1029/2012GL052979.