scholarly journals Wintertime weakening of low-cloud impacts on the subtropical high in the South Indian Ocean

2021 ◽  
pp. 1-40
Author(s):  
Ayumu Miyamoto ◽  
Hisashi Nakamura ◽  
Takafumi Miyasaka ◽  
Yu Kosaka
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
General Circulation ◽  
Deep Convection ◽  
Circulation Model ◽  
Sea Surface ◽  
Low Level ◽  
Mascarene High ◽  
South Indian ◽  
Low Cloud

AbstractTo elucidate the unique seasonality in the coupled system of the subtropical Mascarene high and low-level clouds, the present study compares wintertime cloud radiative impacts on the high with their summertime counterpart through coupled and atmospheric general circulation model simulations. A comparison of a fully coupled control simulation with another simulation in which the radiative effects of low-level clouds are artificially switched off demonstrates that the low-cloud effect on the formation of the Mascarene high is much weaker in winter. Background climatology plays an important role in this seasonality of the Mascarene high reinforcement. Relative to summer, the suppression of deep convection due to low-level clouds that acts to reinforce the high is much weaker in winter. This arises from 1) seasonally lower sea-surface temperature in concert with the smaller sea-surface temperature reduction due to the deeper ocean mixed layer and the weaker cloud radiative effect under weaker insolation, and 2) seasonally stronger subtropical subsidence associated with the Hadley circulation in winter. As verified through atmospheric dynamical model experiments, enhanced cloud-top radiative cooling by low-level clouds acts to reinforce the wintertime Mascarene high in comparable magnitude as in summer. The present study reveals that the self-sustaining feedback with low-level clouds alone is insufficient for replenishing the full strength of the wintertime Mascarene high. This implies that another internal feedback pathway and/or external driver must be operative in maintaining the wintertime high.

scholarly journals Radiative impacts of low-level clouds on the summertime subtropical high in the South Indian Ocean simulated in a coupled general circulation model

2021 ◽  
pp. 1-52
Author(s):  
Ayumu Miyamoto ◽  
Hisashi Nakamura ◽  
Takafumi Miyasaka ◽  
Yu Kosaka
Keyword(s):  
Indian Ocean ◽  
General Circulation ◽  
Circulation Model ◽  
South Indian Ocean ◽  
Coupled General Circulation Model ◽  
Low Level ◽  
Mascarene High ◽  
South Indian ◽  
Radiative Impacts ◽  
Low Cloud

AbstractOver the South Indian Ocean, the coupled system of the subtropical Mascarene high and lowlevel clouds exhibits marked seasonality. To investigate this seasonality, the present study assesses radiative impacts of low-level clouds on the summertime Mascarene high with a coupled general circulation model. Comparison between a fully coupled control simulation and a “no low-cloud simulation,” where the radiative effects of low-level clouds are artificially turned off, demonstrates that they act to reinforce the Mascarene high. Their impacts are so significant that the summertime Mascarene high almost disappears in the no low-cloud experiment, suggesting their essential role in the existence of the summertime Mascarene high. As the primary mechanism, lowered seasurface temperature by the cloud albedo effect suppresses deep convective precipitation, inducing a Matsuno-Gill type response that reinforces the high, as verified through an atmospheric dynamical model diagnosis. Associated reduction of high-top clouds, as well as increased low-level clouds, augments in-atmosphere radiative cooling, which further reinforces the high. The present study reveals that low-level clouds constitute a tight positive feedback system with the subtropical high via sea-surface temperature over the summertime South Indian Ocean.

scholarly journals Impact of high-resolution sea surface temperature representation on the forecast of small Mediterranean catchments' hydrological responses to heavy precipitation

2020 ◽  
Vol 24 (1) ◽  
pp. 269-291 ◽  
Author(s):  
Alfonso Senatore ◽  
Luca Furnari ◽  
Giuseppe Mendicino
Keyword(s):  
High Resolution ◽  
Boundary Conditions ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
General Circulation ◽  
Circulation Model ◽  
Sea Surface ◽  
Sources Of Uncertainty ◽  
Operational Forecasting ◽  
Forecasting System

Abstract. Operational meteo-hydrological forecasting chains are affected by many sources of uncertainty. In coastal areas characterized by complex topography, with several medium-to-small size catchments, quantitative precipitation forecast becomes even more challenging due to the interaction of intense air–sea exchanges with coastal orography. For such areas, which are quite common in the Mediterranean Basin, improved representation of sea surface temperature (SST) space–time patterns can be particularly important. The paper focuses on the relative impact of different resolutions of SST representation on regional operational forecasting chains (up to river discharge estimates) over coastal Mediterranean catchments, with respect to two other fundamental options while setting up the system, i.e. the choice of the forcing general circulation model (GCM) and the possible use of a three-dimensional variational assimilation (3D-Var) scheme. Two different kinds of severe hydro-meteorological events that affected the Calabria region (southern Italy) in 2015 are analysed using the WRF-Hydro atmosphere–hydrology modelling system in its uncoupled version. Both of the events are modelled using the 0.25∘ resolution global forecasting system (GFS) and the 16 km resolution integrated forecasting system (IFS) initial and lateral atmospheric boundary conditions, which are from the European Centre for Medium-Range Weather Forecasts (ECMWF), applying the WRF mesoscale model for the dynamical downscaling. For the IFS-driven forecasts, the effects of the 3D-Var scheme are also analysed. Finally, native initial and lower boundary SST data are replaced with data from the Medspiration project by Institut Français de Recherche pour L'Exploitation de la Mer (IFREMER)/Centre European Remote Sensing d'Archivage et de Traitement (CERSAT), which have a 24 h time resolution and a 2.2 km spatial resolution. Precipitation estimates are compared with both ground-based and radar data, as well as discharge estimates with stream gauging stations' data. Overall, the experiments highlight that the added value of high-resolution SST representation can be hidden by other more relevant sources of uncertainty, especially the choice of the general circulation model providing the boundary conditions. Nevertheless, in most cases, high-resolution SST fields show a non-negligible impact on the simulation of the atmospheric boundary layer processes, modifying flow dynamics and/or the amount of precipitated water; thus, this emphasizes the fact that uncertainty in SST representation should be duly taken into account in operational forecasting in coastal areas.

scholarly journals An objective tropical Atlantic sea surface temperature gradient index for studies of south Amazon dry-season climate variability and change

2008 ◽  
Vol 363 (1498) ◽  
pp. 1761-1766 ◽  
Author(s):  
Peter Good ◽  
Jason A Lowe ◽  
Mat Collins ◽  
Wilfran Moufouma-Okia
Keyword(s):  
Climate Change ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
General Circulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Dry Season ◽  
Sea Surface ◽  
Tropical Atlantic ◽  
Sst Gradient

Future changes in meridional sea surface temperature (SST) gradients in the tropical Atlantic could influence Amazon dry-season precipitation by shifting the patterns of moisture convergence and vertical motion. Unlike for the El Niño-Southern Oscillation, there are no standard indices for quantifying this gradient. Here we describe a method for identifying the SST gradient that is most closely associated with June–August precipitation over the south Amazon. We use an ensemble of atmospheric general circulation model (AGCM) integrations forced by observed SST from 1949 to 2005. A large number of tropical Atlantic SST gradient indices are generated randomly and temporal correlations are examined between these indices and June–August precipitation averaged over the Amazon Basin south of the equator. The indices correlating most strongly with June–August southern Amazon precipitation form a cluster of near-meridional orientation centred near the equator. The location of the southern component of the gradient is particularly well defined in a region off the Brazilian tropical coast, consistent with known physical mechanisms. The chosen index appears to capture much of the Atlantic SST influence on simulated southern Amazon dry-season precipitation, and is significantly correlated with observed southern Amazon precipitation. We examine the index in 36 different coupled atmosphere–ocean model projections of climate change under a simple compound 1% increase in CO 2 . Within the large spread of responses, we find a relationship between the projected trend in the index and the Amazon dry-season precipitation trends. Furthermore, the magnitude of the trend relationship is consistent with the inter-annual variability relationship found in the AGCM simulations. This suggests that the index would be of use in quantifying uncertainties in climate change in the region.

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