Kyu-Myong Kim

The 2010 Pakistan Flood and Russian Heat Wave: Teleconnection of Hydrometeorological Extremes

AbstractIn this paper, preliminary results are presented showing that the two record-setting extreme events during 2010 summer (i.e., the Russian heat wave–wildfires and Pakistan flood) were physically connected. It is found that the Russian heat wave was associated with the development of an extraordinarily strong and prolonged extratropical atmospheric blocking event in association with the excitation of a large-scale atmospheric Rossby wave train spanning western Russia, Kazakhstan, and the northwestern China–Tibetan Plateau region. The southward penetration of upper-level vorticity perturbations in the leading trough of the Rossby wave was instrumental in triggering anomalously heavy rain events over northern Pakistan and vicinity in mid- to late July. Also shown are evidences that the Russian heat wave was amplified by a positive feedback through changes in surface energy fluxes between the atmospheric blocking pattern and an underlying extensive land region with below-normal soil moisture. The Pakis...

The North Pacific as a Regulator of Summertime Climate over Eurasia and North America

Abstract The role of the North Pacific as a regulator of boreal summer climate over Eurasia and North America is investigated using observational data. Two summertime interannual climate modes associated with sea surface temperature (SST) variability in the North Pacific are identified. The first mode shows an elongated zone of warm (cold) SST anomalies in the central North Pacific along 40°N, with temporal variability significantly correlated with El Nino during the preceding spring, but its subsequent evolution is quite different from El Nino. The second mode exhibits a seesaw SST variation between the northern and southern North Pacific and is independent of El Nino. Both modes are linked to coherent SST anomalies over the North Atlantic, suggesting the presence of an “atmospheric bridge” linking the two extratropical oceans. Using the principal component of the most dominant mode as the North Pacific index (NPI), composite analyses show that the positive (negative) phase of NPI features a warm (cold) ...

Fingerprinting the impacts of aerosols on long-term trends of the Indian summer monsoon regional rainfall

In this paper, we present corroborative observational evidences from satellites, in-situ observations, and re-analysis data showing possible impacts of absorbing aerosols (black carbon and dust) on subseasonal and regional summer monsoon rainfall over India. We find that increased absorbing aerosols in the Indo-Gangetic Plain in recent decades may have lead to long-term warming of the upper troposphere over northern India and the Tibetan Plateau, enhanced rainfall in northern India and the Himalayas foothill regions in the early part (may-June) of the monsoon season, followed by diminished rainfall over central and southern India in the latter part (July-August) of the monsoon season. These signals which are consistent with current theories of atmospheric heating and solar dimming by aerosol and induced cloudiness in modulating the Indian monsoon, would have been masked by conventional method of using al-India rainfall averaged over the entire monsoon season.

Dynamics of monsoon‐induced biennial variability in ENSO

The mechanism of the quasi-biennial tendency in ENSO-monsoon coupled system is investigated using an intermediate coupled model. The monsoon wind forcing is prescribed as a function of SST anomalies based on the relationship between zonal wind anomalies over the western Pacific to sea level change in the equatorial eastern Pacific. The key mechanism of quasi-biennial tendency in El Nino evolution is found to be in the strong coupling of ENSO to monsoon wind forcing over the western Pacific. Strong boreal summer monsoon wind forcing, which lags the maximum SST anomaly in the equatorial eastern Pacific approximately 6 months, tends to generate Kelvin waves of the opposite sign to anomalies in the eastern Pacific and initiates the turnabout in the eastern Pacific. Boreal winter monsoon forcing, which has zero lag with maximum SST in the equatorial eastern Pacific, tends to damp the ENSO oscillations.

How nature foiled the 2006 hurricane forecasts

The 2006 hurricane season proved again that predicting Mother Nature is a very precarious undertaking. At the beginning of the season, all signs indicated that it would be more active than average: Sea surface temperature (SST) was above normal, vertical wind shear was low, and sea level pressure was reduced over the tropical Atlantic. Many forecasters believed that these features foreshadowed a continuation of the trend of nine preceding years of above-normal hurricane seasons. Given the recent warming tendency in the Atlantic and the prevailing favorable preseason conditions, there was no wonder that even by August 2006, forecasters were still calling for an above-normal frequency of tropical storms and hurricanes.

Atmospheric Teleconnection over Eurasia Induced by Aerosol Radiative Forcing during Boreal Spring

Abstract The direct effects of aerosols on global and regional climate during boreal spring are investigated based on numerical simulations with the NASA Global Modeling and Assimilation Office finite-volume general circulation model (fvGCM) with Microphyics of Clouds with the Relaxed–Arakawa Schubert Scheme (McRAS), using aerosol forcing functions derived from the Goddard Ozone Chemistry Aerosol Radiation and Transport model (GOCART). The authors find that anomalous atmospheric heat sources induced by absorbing aerosols (dust and black carbon) excite a planetary-scale teleconnection pattern in sea level pressure, temperature, and geopotential height spanning North Africa through Eurasia to the North Pacific. Surface cooling due to direct effects of aerosols is found in the vicinity and downstream of the aerosol source regions, that is, South Asia, East Asia, and northern and western Africa. Significant atmospheric heating is found in regions with large loading of dust (over northern Africa and the Middle...

Robust Hadley Circulation changes and increasing global dryness due to CO2 warming from CMIP5 model projections

Significance In spite of increasing research efforts, global warming signals of the Hadley Circulation (HC) and its dynamical linkages to water cycle changes remain largely unknown. Here, from model projections, we find robust signals of both strengthening and weakening components of the HC induced by CO2 warming. These changes in the HC drive a pattern of global dryness featuring widespread reduction of tropospheric humidity and increased frequency of dry months, particularly over subtropical and tropical land regions. We also find that global warming signal in increased dryness is the most detectable among numerous water-cycle quantities examined. Our results provide a scientific basis for inferring that greenhouse warming is likely to contribute to the observed prolonged worldwide droughts in recent decades. In this paper, we investigate changes in the Hadley Circulation (HC) and their connections to increased global dryness (suppressed rainfall and reduced tropospheric relative humidity) under CO2 warming from Coupled Model Intercomparison Project Phase 5 (CMIP5) model projections. We find a strengthening of the HC manifested in a “deep-tropics squeeze” (DTS), i.e., a deepening and narrowing of the convective zone, enhanced ascent, increased high clouds, suppressed low clouds, and a rise of the level of maximum meridional mass outflow in the upper troposphere (200−100 hPa) of the deep tropics. The DTS induces atmospheric moisture divergence and reduces tropospheric relative humidity in the tropics and subtropics, in conjunction with a widening of the subsiding branches of the HC, resulting in increased frequency of dry events in preferred geographic locations worldwide. Among various water-cycle parameters examined, global dryness is found to have the highest signal-to-noise ratio. Our results provide a physical basis for inferring that greenhouse warming is likely to contribute to the observed prolonged droughts worldwide in recent decades.

A multimodel study of the twentieth-century simulations of Sahel drought from the 1970s to 1990s

[1] In this paper, we evaluate the performance of 19 coupled general circulation models (CGCMs) in twentieth-century simulations of the Sahel during the 1970s to 1990s. Correlation, regression, and cluster analyses are applied to observations and model outputs including Sahel monthly precipitation, evaporation, soil moisture, and sea surface temperature (SST). We find that only eight CGCMs (hit models) produce a reasonable Sahel drought signal, while seven CGCMs (miss models) produce excessive rainfall over the Sahel during the observed drought period. Even the model with the highest prediction skill of the Sahel drought could only predict the increasing trend of severe drought events but not the beginning and duration of the events. From analyses of the statistical characteristics of the hit and miss models, we conclude that a good simulation of the Sahel drought requires (1) a strong coupling between Sahel rainfall and Indian Ocean SST, with warm (cold) SST identified with Sahel drought (flood), (2) a significant coupling between Sahel rainfall and the Atlantic Ocean SST, with a warm equatorial Atlantic and cold extratropical North Atlantic coexisting with Sahel drought, and vice versa, and (3) a robust land surface feedback with strong sensitivity of precipitation and land evaporation to soil moisture. These three characteristics constitute sufficient conditions for a good simulation of Sahel drought in CGCMs.

Amplification of ENSO effects on Indian summer monsoon by absorbing aerosols

In this study, we present observational evidence, based on satellite aerosol measurements and MERRA reanalysis data for the period 1979–2011, indicating that absorbing aerosols can have strong influence on seasonal-to-interannual variability of the Indian summer monsoon rainfall, including amplification of ENSO effects. We find a significant correlation between ENSO (El Nino Southern Oscillation) and aerosol loading in April–May, with La Nina (El Nino) conditions favoring increased (decreased) aerosol accumulation over northern India, with maximum aerosol optical depth over the Arabian Sea and Northwestern India, indicative of strong concentration of dust aerosols transported from West Asia and Middle East deserts. Composite analyses based on a normalized aerosol index (NAI) show that high concentration of aerosol over northern India in April–May is associated with increased moisture transport, enhanced dynamically induced warming of the upper troposphere over the Tibetan Plateau, and enhanced rainfall over northern India and the Himalayan foothills during May–June, followed by a subsequent suppressed monsoon rainfall over all India, consistent with the elevated heat pump (EHP) hypothesis (Lau et al. in Clim Dyn 26:855–864, 2006. doi:10.1007/s00382-006-0114-z). Further analyses from sub-sampling of ENSO years, with normal (<1-σ), and abnormal (>1-σ) NAI over northern India respectively show that the EHP may lead to an amplification of the Indian summer monsoon response to ENSO forcing, particularly with respect to the increased rainfall over the Himalayan foothills, and the warming of the upper troposphere over the Tibetan Plateau. Our results suggest that absorbing aerosol, particular desert dusts can strongly modulate ENSO influence, and possibly play important roles as a feedback agent in climate change in Asian monsoon regions.

Impact of snow darkening via dust, black carbon, and organic carbon on boreal spring climate in the Earth system

Dust, black carbon (BC), and organic carbon (OC) aerosols, when deposited onto snow, are known to reduce the albedo of the snow (i.e., snow darkening effect (SDE)). Here using the NASA Goddard Earth Observing System Model, Version 5 (GEOS-5) with aerosol tracers and a state-of-the-art snow darkening module (GOddard SnoW Impurity Module: GOSWIM) for the land surface, we examine the role of SDE on climate in the boreal spring snowmelt season. SDE is found to produce significant surface warming (over 15 W m−2) over broad areas in midlatitudes, with dust being the most important contributor to the warming in central Asia and the western Himalayas and with BC having larger impact in the Europe, eastern Himalayas, East Asia, and North America. The contribution of OC to the warming is generally low but still significant mainly over southeastern Siberia, northeastern East Asia, and western Canada (~19% of the total solar visible absorption by these snow impurities). The simulations suggest that SDE strengthens the boreal spring water cycle in East Asia through water recycling and moisture advection from the ocean and contributes to the maintenance of dry conditions in parts of a region spanning Europe to central Asia, partially through feedback on the models background climatology. Overall, our study suggests that the existence of SDE in the Earth system associated with dust, BC, and OC contributes significantly to enhanced surface warming over continents in northern hemisphere midlatitudes during boreal spring, raising the surface skin temperature by approximately 3–6 K near the snowline.