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Vasubandhu Misra and Amit Bhardwaj

Abstract

In this study we examine the thermodynamically and dynamically forced hydroclimatic changes in the four representative seasons over Peninsular Florida (PF) from an unprecedented pair of high-resolution regional coupled ocean–atmosphere model simulations conducted at 10-km grid spacing for both the atmospheric and the oceanic components forced by one of the global climate models that participated in CMIP5. The model simulation verifies reasonably well with the observations and captures the distinct seasonal cycle of the region. The projected change in the freshwater flux in the mid-twenty-first century (2041–60) relative to the late twentieth century (1986–2005) shows a precipitation deficit in the summer over PF, which is statistically significant. This projected change in freshwater flux over PF is enabled by the strengthening of the anticyclonic North Atlantic subtropical high circulation with corresponding changes in divergence of moisture, advection of moisture from changes in the winds and in the change in humidity gradient, and from the change in moisture flux convergence by the transient eddies. These changes suggest that a future warm climate could witness a drier summer over PF at the expense of a wetter West Florida Shelf. The analysis conducted in this study reveals that the changes in atmospheric circulation have a significant impact on the hydroclimate, far more than that implied by the Clausius–Clapeyron equation from changes in temperature.

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Vasubandhu Misra and Amit Bhardwaj

Abstract

This study introduces an objective definition for onset and demise of the northeast Indian monsoon (NEM). The definition is based on the land surface temperature analysis over the Indian subcontinent. It is diagnosed from the inflection points in the daily anomaly cumulative curve of the area-averaged surface temperature over the provinces of Andhra Pradesh, Rayalseema, and Tamil Nadu located in the southeastern part of India. Per this definition, the climatological onset and demise dates of the NEM season are 6 November and 13 March, respectively. The composite evolution of the seasonal cycle of 850-hPa winds, surface wind stress, surface ocean currents, and upper-ocean heat content suggest a seasonal shift around the time of the diagnosed onset and demise dates of the NEM season. The interannual variations indicate onset date variations have a larger impact than demise date variations on the seasonal length, seasonal anomalies of rainfall, and surface temperature of the NEM. Furthermore, it is shown that warm El Niño–Southern Oscillation (ENSO) episodes are associated with excess seasonal rainfall, warm seasonal land surface temperature anomalies, and reduced lengths of the NEM season. Likewise, cold ENSO episodes are likely to be related to seasonal deficit rainfall anomalies, cold land surface temperature anomalies, and increased lengths of the NEM season.

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Vasubandhu Misra, Amit Bhardwaj, and Ryne Noska

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.

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Vasubandhu Misra, Akhilesh Mishra, and Amit Bhardwaj

Abstract

This paper describes a novel simulation of active and break spells of the Indian summer monsoon (ISM) using a relatively high-resolution regional coupled ocean–atmosphere climate model (RCM) run at 10-km grid spacing. Similar to what is seen in observations, the RCM-simulated active (break) spells are characterized by stronger (weaker) rainfall over central India and anomalous low-level atmospheric flow that enhances (weakens) the climatological flow pattern. Highlights of this study include the improved spatiotemporal structure, propagation characteristics, and amplitude of the intraseasonal variations of the ISM rainfall in the RCM simulation as compared with some of the more recent simulations conducted with global models at coarser spatial resolutions. This study’s RCM simulation also displays associated variations in the upper ocean, with active (break) spells of the ISM coinciding with colder (warmer) sea surface temperatures (SSTs) in both the Arabian Sea and the Bay of Bengal. These SST anomalies are mainly sustained by corresponding net heat flux anomalies on the ocean surface. The active (break) spells are further associated with shoaling (deepening) of the mixed layer depth, which is critical for the SST response to heat flux. All of these simulated features of intraseasonal variations of the ISM have been seen in earlier observational studies, which further confirms the fidelity of the model simulation and the importance of coupled air–sea interactions and upper-ocean stratification.

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John Uehling, Vasubandhu Misra, Amit Bhardwaj, and Nirupam Karmakar

Abstract

In this study, we introduce a localized definition of the onset and retreat of the northern Australian rainy season that is solely based on gridded rainfall analysis. Our analysis shows that the local onset/retreat of the rainy season has considerable spatial heterogeneity. Onset is earlier and the length of the rainy season is longer to the west of the Gulf of Carpentaria than to its east. Furthermore, we also find the local onset/retreat is influenced by the wet and dry spells of the 30–60-day intraseasonal oscillation. Much of the retreat of the rainy season occurs in the dry phases of the intraseasonal oscillation. However, intriguingly, a majority of the local onset of the rainy season occurs during dry phases of the intraseasonal oscillation. The ENSO teleconnection with the variable-length northern Australian rainy season also exhibits spatial heterogeneity and significant differences from rainfall anomalies using the fixed-length boreal winter season. The onset, the retreat, the length, and the seasonal rainfall anomalies of the rainy season display a stronger correlation with the ENSO SST anomalies for the region east of 140°E relative to its west. The strong covariability of the local onset date with the corresponding seasonal length and seasonal rainfall anomalies over northern Australia offers the advantage of monitoring the onset of the northern Australian rainy season to provide an outlook for the forthcoming season. The proposed local definition of onset/retreat of the northern Australian rainy season is simple, objective, and unambiguous and is ideally suited for real-time monitoring of the evolution of the season.

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Amit Bhardwaj, Vasubandhu Misra, Ben Kirtman, Tirusew Asefa, Carolina Maran, Kevin Morris, Ed Carter, Christopher Martinez, and Daniel Roberts

Abstract

We present here the analysis of 20 years of high-resolution experimental winter seasonal climate reforecasts for Florida (CLIFF). These winter seasonal reforecasts were dynamically downscaled by a regional atmospheric model at 10-km grid spacing from a global model run at T62 spectral resolution (~210-km grid spacing at the equator) forced with sea surface temperatures (SST) obtained from one of the global models in the North American Multimodel Ensemble (NMME). CLIFF was designed in consultation with water managers (in utilities and public water supply) in Florida targeting its five water management districts, including two smaller watersheds of two specific stakeholders in central Florida that manage the public water supply. This enterprise was undertaken in an attempt to meet the climate forecast needs of water management in Florida. CLIFF has 30 ensemble members per season generated by changes to the physics and the lateral boundary conditions of the regional atmospheric model. Both deterministic and probabilistic skill measures of the seasonal precipitation at the zero-month lead for November–December–January (NDJ) and one-month lead for December–January–February (DJF) show that CLIFF has higher seasonal prediction skill than persistence. The results of the seasonal prediction skill of land surface temperature are more sobering than precipitation, although, in many instances, it is still better than the persistence skill.

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