scholarly journals Understanding the Variations of the Length and the Seasonal Rainfall Anomalies of the Indian Summer Monsoon

2017 ◽  
Vol 30 (5) ◽  
pp. 1753-1763 ◽  
Author(s):  
Vasubandhu Misra ◽  
Amit Bhardwaj ◽  
Ryne Noska
Keyword(s):  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Large Scale ◽  
Seasonal Rainfall ◽  
Boreal Summer ◽  
Equatorial Pacific Ocean ◽  
Eastern Equatorial Pacific ◽  
Rainfall Anomalies ◽  
Static Energy ◽  
Eastern Equatorial Pacific Ocean

Abstract The canonical relationship between the length and the total seasonal rainfall anomalies of the Indian summer monsoon (ISM) is the association of the longer (shorter) season with wetter (drier) seasonal rainfall anomalies. This study shows that such canonical behavior is clearly associated with relatively strong ENSO SST anomalies in the eastern equatorial Pacific Ocean that appear in the boreal summer and fall seasons. The noncanonical relationship is caused by a longer (shorter) season associated with drier (wetter) ISM seasonal rainfall anomalies. A majority of these noncanonical seasons, with anomalously short season length but anomalously high seasonal mean rain, tend to occur under relatively weak La Niña forcing during the boreal summer season. Although the onset of such seasons occurs through canonical ENSO forcing of a large-scale meridional temperature gradient, the demise is dictated by the depletion of moist static energy from the underlying cooling of the upper ocean in the northern Indian Ocean. This is due to stronger meridional Ekman ocean heat transport forced by the stronger low-level atmospheric southwesterlies than those in the corresponding canonical wet ISM season.

scholarly journals Theoretical aspects of the onset of Indian summer monsoon from perturbed orography simulations in a GCM

2006 ◽  
Vol 24 (8) ◽  
pp. 2075-2089 ◽  
Author(s):  
A. Chakraborty ◽  
R. S. Nanjundiah ◽  
J. Srinivasan
Keyword(s):  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
General Circulation ◽  
Large Scale ◽  
Vertical Motion ◽  
Circulation Model ◽  
Threshold Value ◽  
The Earth ◽  
Static Energy ◽  
The Impact

Abstract. A theory is proposed to determine the onset of the Indian Summer Monsoon (ISM) in an Atmospheric General Circulation Model (AGCM). The onset of ISM is delayed substantially in the absence of global orography. The impact of orography over different parts of the Earth on the onset of ISM has also been investigated using five additional perturbed simulations. The large difference in the date of onset of ISM in these simulations has been explained by a new theory based on the Surface Moist Static Energy (SMSE) and vertical velocity at the mid-troposphere. It is found that onset occurs only after SMSE crosses a threshold value and the large-scale vertical motion in the middle troposphere becomes upward. This study shows that both dynamics and thermodynamics play profound roles in the onset of the monsoon.

scholarly journals Monitoring the Indian Summer Monsoon Evolution at the Granularity of the Indian Meteorological Sub-divisions using Remotely Sensed Rainfall Products

2019 ◽  
Vol 11 (9) ◽  
pp. 1080 ◽  
Author(s):  
Amit Bhardwaj ◽  
Vasubandhu Misra
Keyword(s):  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Spatial Scales ◽  
Tropical Rainfall Measuring Mission ◽  
Rain Gauge ◽  
Seasonal Rainfall ◽  
Onset Date ◽  
Rainfall Anomalies ◽  
Global Precipitation ◽  
Seasonal Length

We make use of satellite-based rainfall products from the Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) to objectively define local onset and demise of the Indian Summer Monsoon (ISM) at the spatial resolution of the meteorological subdivisions defined by the Indian Meteorological Department (IMD). These meteorological sub-divisions are the operational spatial scales for official forecasts issued by the IMD. Therefore, there is a direct practical utility to target these spatial scales for monitoring the evolution of the ISM. We find that the diagnosis of the climatological onset and demise dates and its variations from the TMPA product is quite similar to the rain gauge based analysis of the IMD, despite the differences in the duration of the two datasets. This study shows that the onset date variations of the ISM have a significant impact on the variations of the seasonal length and seasonal rainfall anomalies in many of the meteorological sub-divisions: for example, the early or later onset of the ISM is associated with longer and wetter or shorter and drier ISM seasons, respectively. It is shown that TMPA dataset (and therefore its follow up Global Precipitation Measurement (GPM) Integrated Multi-satellite Retrievals for GPM (IMERG)) could be usefully adopted for monitoring the onset of the ISM and therefore extend its use to anticipate the potential anomalies of the seasonal length and seasonal rainfall anomalies of the ISM in many of the Indian meteorological sub-divisions.

scholarly journals Dynamics on Seasonal Variability of EKE Associated with TIWs in the Eastern Equatorial Pacific Ocean

2019 ◽  
Vol 49 (6) ◽  
pp. 1503-1519 ◽  
Author(s):  
Minyang Wang ◽  
Yan Du ◽  
Bo Qiu ◽  
Shang-Ping Xie ◽  
Ming Feng
Keyword(s):  
Pacific Ocean ◽  
Surface Wind ◽  
Equatorial Pacific ◽  
Seasonal Migration ◽  
Sea Surface ◽  
Boreal Summer ◽  
Equatorial Pacific Ocean ◽  
Eastern Equatorial Pacific ◽  
Eastern Equatorial Pacific Ocean ◽  
Sst Fronts

AbstractEnergetic mesoscale eddies (vortices) associated with tropical instability waves (TIWs) exist in the eastern equatorial Pacific Ocean between 0° and 8°N. This study examines the seasonal variations in eddy kinetic energy (EKE) of TIWs using in situ and satellite observations and elucidates the underlying dynamical mechanisms. The results reveal that the cross-equatorial southerly winds are key to sustaining the high-level EKE (up to ~600 cm2 s−2) from boreal summer to winter in 0°–6°N and 155°–110°W. Because of the β effect and the surface wind divergence, the southerly winds generate anticyclonic wind curls north of the equator that intensify the sea surface temperature (SST) fronts and force the downwelling annual Rossby waves. The resultant sea surface height ridge induces strong horizontal current shears between 0° and 5°N. The intensified current shears and SST fronts generate EKE via barotropic and baroclinic instabilities, respectively. To the extent that the seasonal migration of a northward-displaced intertropical convergence zone intensifies the southerly winds north of, but not south of, the equator, our study suggests that the climatic asymmetry is important for the oceanic eddy generations in the eastern equatorial Pacific Ocean—a result with important implications for coupled climate simulation/prediction.

scholarly journals Galápagos upwelling driven by localized wind–front interactions

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Alexander Forryan ◽  
Alberto C. Naveira Garabato ◽  
Clément Vic ◽  
A. J. George Nurser ◽  
Alexander R. Hearn
Keyword(s):  
Ocean Circulation ◽  
Realistic Model ◽  
Surface Mixed Layer ◽  
Equatorial Pacific Ocean ◽  
World Heritage Sites ◽  
Theory Of Evolution ◽  
Heritage Sites ◽  
Galapagos Archipelago ◽  
Eastern Equatorial Pacific ◽  
Eastern Equatorial Pacific Ocean

AbstractThe Galápagos archipelago, rising from the eastern equatorial Pacific Ocean some 900 km off the South American mainland, hosts an iconic and globally significant biological hotspot. The islands are renowned for their unique wealth of endemic species, which inspired Charles Darwin’s theory of evolution and today underpins one of the largest UNESCO World Heritage Sites and Marine Reserves on Earth. The regional ecosystem is sustained by strongly seasonal oceanic upwelling events—upward surges of cool, nutrient-rich deep waters that fuel the growth of the phytoplankton upon which the entire ecosystem thrives. Yet despite its critical life-supporting role, the upwelling’s controlling factors remain undetermined. Here, we use a realistic model of the regional ocean circulation to show that the intensity of upwelling is governed by local northward winds, which generate vigorous submesoscale circulations at upper-ocean fronts to the west of the islands. These submesoscale flows drive upwelling of interior waters into the surface mixed layer. Our findings thus demonstrate that Galápagos upwelling is controlled by highly localized atmosphere–ocean interactions, and call for a focus on these processes in assessing and mitigating the regional ecosystem’s vulnerability to 21st-century climate change.

scholarly journals Large-scale extreme rainfall producing synoptic systems of the Indian summer monsoon

2020 ◽  
Author(s):  
Akshaya C Nikumbh ◽  
Arindam Chakraborty ◽  
G S Bhat ◽  
Dargan M. W. Frierson
Keyword(s):  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Large Scale ◽  
Extreme Rainfall

Dependence of Indian summer monsoon rainfall forecast skill of CFSv2 on initial conditions and the role of bias in SST boundary forcing

2021 ◽  
Author(s):  
Stella Jes Varghese ◽  
Kavirajan Rajendran ◽  
Sajani Surendran ◽  
Arindam Chakraborty
Keyword(s):  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Lead Time ◽  
Monsoon Rainfall ◽  
Indian Summer Monsoon Rainfall ◽  
Initial Conditions ◽  
Summer Monsoon Rainfall ◽  
Forecast Skill ◽  
Boreal Summer ◽  
Central Pacific

<p>Indian summer monsoon seasonal reforecasts by CFSv2, initiated from January (4-month lead time, L4) through May (0-month lead time, L0) initial conditions (ICs), are analysed to investigate causes for the highest Indian summer monsoon rainfall (ISMR) forecast skill of CFSv2 with February (3-month lead time, L3) ICs. Although theory suggests forecast skill should degrade with increase in lead-time, CFSv2 shows highest skill with L3, due to its forecasting of ISMR excess of 1983 which other ICs failed to forecast. In contrast to observation, in CFSv2, ISMR extremes are largely decided by sea surface temperature (SST) variation over central Pacific (NINO3.4) associated with El Niño-Southern Oscillation (ENSO), where ISMR excess (deficit) is associated with La Niña (El Niño) or cooling (warming) over NINO3.4. In 1983, CFSv2 with L3 ICs forecasted strong La Niña during summer, which resulted in 1983 ISMR excess. In contrast, in observation, near normal SSTs prevailed over NINO3.4 and ISMR excess was due to variation of convection over equatorial Indian Ocean, which CFSv2 fails to capture with all ICs. CFSv2 reforecasts with late-April/early-May ICs are found to have highest deterministic ISMR forecast skill, if 1983 is excluded and Indian monsoon seasonal biases are also reduced. During the transitional ENSO in Boreal summer of 1983, faster and intense cooling of NINO3.4 SSTs in L3, could be due to larger dynamical drift with longer lead time of forecasting, compared to L0. Boreal summer ENSO forecast skill is also found to be lowest for L3 which gradually decreases from June to September. Rainfall occurrence with strong cold bias over NINO3.4, is because of the existence of stronger ocean-atmosphere coupling in CFSv2, but with a shift of the SST-rainfall relationship pattern to slightly colder SSTs than the observed. Our analysis suggests the need for a systematic approach to minimize bias in SST boundary forcing in CFSv2, to achieve improved ISMR forecasts.</p>

scholarly journals The ENSO teleconnections to the Indian summer monsoon climate through the Last Millennium as simulated by the PMIP3

2018 ◽  
Author(s):  
Charan Teja Tejavath ◽  
Karumuri Ashok ◽  
Supriyo Chakraborty ◽  
Rengaswamy Ramesh
Keyword(s):  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Little Ice Age ◽  
Walker Circulation ◽  
Summer Monsoon Rainfall ◽  
Ice Age ◽  
Boreal Summer ◽  
Last Millennium ◽  
Enso Teleconnections ◽  
The Mean

Abstract. Using seven model simulations from the PMIP3, we study the mean summer (June–September) climate and its variability in India during the Last Millennium (LM; CE 850–1849) with emphasis on the Medieval Warm Period (MWP) and Little Ice Age (LIA), after validation of the simulated current day climate and trends. We find that the above (below) LM-mean summer global temperatures during the MWP (LIA) are associated with relatively higher (lower) number of concurrent El Niños as compared to La Niñas. The models simulate higher (lower) Indian summer monsoon rainfall (ISMR) during the MWP (LIA). This is notwithstanding a strong simulated negative correlation between the timeseries of NINO3.4 index and that of the area-averaged ISMR, Interestingly, the percentage of strong El Niños (La Niñas) causing negative (positive) ISMR anomalies is higher in the LIA (MWP), a non-linearity that apparently is important for causing higher ISMR in the MWP. Distribution of simulated boreal summer velocity potential at 850 hPa during MWP in models, in general, shows a zone of anomalous convergence in the central tropical Pacific flanked by two zones of divergence, suggesting a westward shift in the Walker circulation as compared to the simulations for LM as well as and a majority of historical simulations, and current day observed signal. The anomalous divergence centre in the west also extends into the equatorial eastern Indian Ocean, resulting in an anomalous convergence zone over India and therefore excess rainfall during the MWP as compared to the LM; the results are qualitative, given the inter-model spread.

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