scholarly journals Forced summer stationary waves: the opposing effects of direct radiative forcing and sea surface warming

2019 ◽  
Vol 53 (7-8) ◽  
pp. 4291-4309 ◽  
Author(s):  
Hugh S. Baker ◽  
Tim Woollings ◽  
Cheikh Mbengue ◽  
Myles R. Allen ◽  
Christopher H. O’Reilly ◽  
...  
Keyword(s):  
Radiative Forcing ◽  
Sea Surface ◽  
Stationary Waves ◽  
Surface Warming ◽  
Direct Radiative Forcing ◽  
Opposing Effects

scholarly journals Impact of Desert Dust Radiative Forcing on Sahel Precipitation: Relative Importance of Dust Compared to Sea Surface Temperature Variations, Vegetation Changes, and Greenhouse Gas Warming

2007 ◽  
Vol 20 (8) ◽  
pp. 1445-1467 ◽  
Author(s):  
Masaru Yoshioka ◽  
Natalie M. Mahowald ◽  
Andrew J. Conley ◽  
William D. Collins ◽  
David W. Fillmore ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Africa ◽  
Radiative Forcing ◽  
Sea Surface ◽  
Desert Dust ◽  
Direct Radiative Forcing ◽  
Sahel Region ◽  
Precipitation Reduction ◽  
Sahel Precipitation

Abstract The role of direct radiative forcing of desert dust aerosol in the change from wet to dry climate observed in the African Sahel region in the last half of the twentieth century is investigated using simulations with an atmospheric general circulation model. The model simulations are conducted either forced by the observed sea surface temperature (SST) or coupled with the interactive SST using the Slab Ocean Model (SOM). The simulation model uses dust that is less absorbing in the solar wavelengths and has larger particle sizes than other simulation studies. As a result, simulations show less shortwave absorption within the atmosphere and larger longwave radiative forcing by dust. Simulations using SOM show reduced precipitation over the intertropical convergence zone (ITCZ) including the Sahel region and increased precipitation south of the ITCZ when dust radiative forcing is included. In SST-forced simulations, on the other hand, significant precipitation changes are restricted to over North Africa. These changes are considered to be due to the cooling of global tropical oceans as well as the cooling of the troposphere over North Africa in response to dust radiative forcing. The model simulation of dust cannot capture the magnitude of the observed increase of desert dust when allowing dust to respond to changes in simulated climate, even including changes in vegetation, similar to previous studies. If the model is forced to capture observed changes in desert dust, the direct radiative forcing by the increase of North African dust can explain up to 30% of the observed precipitation reduction in the Sahel between wet and dry periods. A large part of this effect comes through atmospheric forcing of dust, and dust forcing on the Atlantic Ocean SST appears to have a smaller impact. The changes in the North and South Atlantic SSTs may account for up to 50% of the Sahel precipitation reduction. Vegetation loss in the Sahel region may explain about 10% of the observed drying, but this effect is statistically insignificant because of the small number of years in the simulation. Greenhouse gas warming seems to have an impact to increase Sahel precipitation that is opposite to the observed change. Although the estimated values of impacts are likely to be model dependent, analyses suggest the importance of direct radiative forcing of dust and feedbacks in modulating Sahel precipitation.

scholarly journals The Roles of Radiative Forcing, Sea Surface Temperatures, and Atmospheric and Land Initial Conditions in U.S. Summer Warming Episodes

2016 ◽  
Vol 29 (11) ◽  
pp. 4121-4135 ◽  
Author(s):  
Liwei Jia ◽  
Gabriel A. Vecchi ◽  
Xiaosong Yang ◽  
Richard G. Gudgel ◽  
Thomas L. Delworth ◽  
...  
Keyword(s):  
United States ◽  
Radiative Forcing ◽  
Heat Waves ◽  
Initial Conditions ◽  
The United States ◽  
Sea Surface ◽  
Temperature Anomalies ◽  
Summer Heat ◽  
Summer Warming ◽  
Direct Radiative Forcing

Abstract This study investigates the roles of radiative forcing, sea surface temperatures (SSTs), and atmospheric and land initial conditions in the summer warming episodes of the United States. The summer warming episodes are defined as the significantly above-normal (1983–2012) June–August 2-m temperature anomalies and are referred to as heat waves in this study. Two contrasting cases, the summers of 2006 and 2012, are explored in detail to illustrate the distinct roles of SSTs, direct radiative forcing, and atmospheric and land initial conditions in driving U.S. summer heat waves. For 2012, simulations with the GFDL atmospheric general circulation model reveal that SSTs play a critical role. Further sensitivity experiments reveal the contributions of uniform global SST warming, SSTs in individual ocean basins, and direct radiative forcing to the geographic distribution and magnitudes of warm temperature anomalies. In contrast, for 2006, the atmospheric and land initial conditions are the key drivers. The atmospheric (land) initial conditions play a major (minor) role in the central and northwestern (eastern) United States. Because of changes in radiative forcing, the probability of areal-averaged summer temperature anomalies over the United States exceeding the observed 2012 anomaly increases with time over the early twenty-first century. La Niña (El Niño) events tend to increase (reduce) the occurrence rate of heat waves. The temperatures over the central United States are mostly influenced by El Niño/La Niña, with the central tropical Pacific playing a more important role than the eastern tropical Pacific. Thus, atmospheric and land initial conditions, SSTs, and radiative forcing are all important drivers of and sources of predictability for U.S. summer heat waves.

scholarly journals Time-evolving sea-surface warming patterns modulate the climate change response of subtropical precipitation over land

2020 ◽  
Vol 117 (9) ◽  
pp. 4539-4545 ◽  
Author(s):  
Giuseppe Zappa ◽  
Paulo Ceppi ◽  
Theodore G. Shepherd
Keyword(s):  
Time Evolution ◽  
Radiative Forcing ◽  
Climate Model ◽  
Ghg Emissions ◽  
Sea Surface ◽  
Surface Warming ◽  
Climate Change Response ◽  
Sst Warming ◽  
Rapid Adjustment ◽  
Change Response

Greenhouse gas (GHG) emissions affect precipitation worldwide. The response is commonly described by two timescales linked to different processes: a rapid adjustment to radiative forcing, followed by a slower response to surface warming. However, additional timescales exist in the surface-warming response, tied to the time evolution of the sea-surface-temperature (SST) response. Here, we show that in climate model projections, the rapid adjustment and surface mean warming are insufficient to explain the time evolution of the hydro-climate response in three key Mediterranean-like areas—namely, California, Chile, and the Mediterranean. The time evolution of those responses critically depends on distinct shifts in the regional atmospheric circulation associated with the existence of distinct fast and slow SST warming patterns. As a result, Mediterranean and Chilean drying are in quasiequilibrium with GHG concentrations, meaning that the drying will not continue after GHG concentrations are stabilized, whereas California wetting will largely emerge only after GHG concentrations are stabilized. The rapid adjustment contributes to a reduction in precipitation, but has a limited impact on the balance between precipitation and evaporation. In these Mediterranean-like regions, future hydro-climate–related impacts will be substantially modulated by the time evolution of the pattern of SST warming that is realized in the real world.

Time-evolving sea-surface warming patterns modulate the climate change response of precipitation in Mediterranean-like regions

2021 ◽  
Author(s):  
Giuseppe Zappa ◽  
Paulo Ceppi ◽  
Theodore Shepherd
Keyword(s):  
Climate Change ◽  
Time Evolution ◽  
Water Availability ◽  
Radiative Forcing ◽  
Climate Models ◽  
Climate Model ◽  
Sea Surface ◽  
Surface Warming ◽  
The Mediterranean ◽  
Change Response

<p>Regions with a Mediterranean-like climate, apart for California, are projected to receive less rainfall due to climate change, thus posing serious implications for future water availability for societal and agricultural needs. At a first order, it is often assumed that water availability is proportional to global mean warming. Yet, the mechanisms controlling the precipitation response in Mediterranean climates remain only partly understood, as shown by the substantial uncertainty that still characterises the climate model projections. Here, by analysing projections from the CMIP5 climate models, we show that the linear scaling with warming does not apply in three key Mediterranean-like regions, namely Chile, California and the Mediterranean proper. In particular, despite long-term warming, the models show that the projected precipitation reduction in Chile and the Mediterranean halts as soon as anthropogenic forcing is stabilised, while the precipitation increase in California accelerates. By examining the response to an abrupt quadrupling of CO2, we demonstrate that such non-linearity in the time-evolution of precipitation cannot be solely explained by the well-known rapid adjustment to radiative forcing, but it is instead due to distinct fast and slow patterns of atmospheric circulation change, that are themselves forced by the time-evolution in the spatial patterns of sea-surface temperature warming. In particular, while the fast warming is favourable to force a poleward shift of the mid-latitudes jets, hence drying the Mediterranean and Chile, the slow warming, including an el nino-like pattern in the tropical Pacific, inhibits such shifts and precipitation changes, while favouring the wetting of California. The results show that stabilising GHG concentrations will have an immediate benefit to the hydro-climate of these Mediterranean-like regions, while pointing to constraining uncertainty in the patterns of surface warming as an important step to increase confidence in the future projections.<span> </span></p>

Tug of war on summertime circulation between radiative forcing and sea surface warming

2015 ◽  
Vol 8 (7) ◽  
pp. 560-566 ◽  
Author(s):  
T. A. Shaw ◽  
A. Voigt
Keyword(s):  
Radiative Forcing ◽  
Sea Surface ◽  
Surface Warming

scholarly journals An Assessment of Direct Radiative Forcing, Radiative Adjustments, and Radiative Feedbacks in Coupled Ocean–Atmosphere Models*

2015 ◽  
Vol 28 (10) ◽  
pp. 4152-4170 ◽  
Author(s):  
Eui-Seok Chung ◽  
Brian J. Soden
Keyword(s):  
Surface Temperature ◽  
Radiative Forcing ◽  
Climate Models ◽  
Substantial Part ◽  
Regional Patterns ◽  
Surface Warming ◽  
Ocean Atmosphere ◽  
Direct Radiative Forcing ◽  
Radiative Feedbacks ◽  
Global Mean

Abstract In this study, radiative kernels are used to separate direct radiative forcing from radiative adjustments to that forcing to quantify the magnitude and intermodel spread of tropospheric and stratospheric adjustments in coupled ocean–atmosphere climate models. Radiative feedbacks are also quantified and separated from radiative forcing by assuming that feedbacks are a linear response to changes in global-mean surface temperature. The direct radiative forcing due to a quadrupling of CO2 is found to have an intermodel spread of ~3 W m−2. In contrast to previous studies, relatively small estimates of cloud adjustments are obtained, which are both positive and negative. This discrepancy is at least partially attributable to small, but nonnegligible, global-mean surface warming in fixed sea surface temperature experiments, which aliases a surface-driven feedback response into estimates of the adjustments. This study suggests that correcting for the bias induced from this global-mean surface warming offers a more accurate estimate of tropospheric adjustments. It is shown that the regional patterns in the tropospheric adjustments tend to oppose the radiative feedback. This compensation is closely tied to spatial inhomogeneities in the initial rate of surface warming, suggesting that a substantial part of the spatial variation in the estimated tropospheric adjustment is an artifact of the linear regression methodology. Even when assuming that the global-mean estimates of the tropospheric adjustments are valid, neglecting them introduces little uncertainty in estimates of the total forcing, feedback, or effective climate sensitivity relative to the intermodel spread in these values.

scholarly journals Effects of sea surface winds on marine aerosols characteristics and impacts on longwave radiative forcing over the Arabian Sea

2008 ◽  
Vol 8 (4) ◽  
pp. 15855-15899 ◽  
Author(s):  
Vijayakumar S. Nair ◽  
S. Suresh Babu ◽  
S. K. Satheesh ◽  
K. Krishna Moorthy
Keyword(s):  
Wind Speed ◽  
Radiative Forcing ◽  
Arabian Sea ◽  
Near Infrared ◽  
Surface Wind ◽  
Sea Surface ◽  
Surface Winds ◽  
Near Surface ◽  
Sea Surface Winds ◽  
Direct Radiative Forcing

Abstract. Collocated measurements of spectral aerosol optical depths (AODs), total and BC mass concentrations, and number size distributions of near surface aerosols, along with sea surface winds, made onboard a scientific cruise over southeastern Arabian Sea, are used to delineate the effects of changes in the wind speed on aerosol properties and its implication on the shortwave and longwave radiative forcing. The results indicated that an increase in the sea-surface wind speed from calm to moderate (<1 to 8 m s−1) values results in a selective increase of the particle concentrations in the size range 0.5 to 5 μm, leading to significant changes in the size distribution, increase in the mass concentration, decrease in the BC mass fraction, a remarkable increase in AODs in the near infrared and a flattening of the AOD spectrum. The consequent increase in the longwave direct radiative forcing almost entirely offsets the corresponding increase in the short wave direct radiative forcing (or even overcompensates) at the top of the atmosphere; while the surface forcing is offset by about 50%.

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