FYI
Ph. D. Thesis Defense: CAOS: TODAY: 11:00 AM, Impact of river runoff into the ocean on climate in a coupled model
Title: " Impact of river runoff into the ocean on climate in a coupled model"
Candidate: Mr. Jahfer Sharif K. K.
Date: 03 March 2020 (Tuesday)
Time: 11:00 AM
Venue: CAOS Seminar Hall
Tea/Coffee at 10:45 AM
ALL ARE WELCOME
Rivers of the world discharge about 40 x 103 km3 of freshwater into the oceans, yet the impact of runoff on climate is not well known. Using a coupled model, the response of oceans and climate to river discharge is investigated in this thesis. Model experiments were carried out for a period of 200 years by switching off or doubling the river discharge into the ocean. In one such experiment, the runoff into the ocean was intercepted globally. Model studies show that the largest changes in SST were found to be away from the river mouth where the SSS anomalies are high. While the northern Atlantic and Pacific Oceans exhibited warmer SST without the runoff, the equatorial and southern tropical oceans become cooler. The cooling in the equatorial Pacific Ocean resembles the La Niña phase of El Niño Southern Oscillation (ENSO) and consequently the Indian summer monsoon rainfall (ISMR) enhanced. Based on existing theories, we find that the equatorial Pacific affects the ISMR through upper tropospheric meridional temperature via the westward propagating jet termed as the North African–Asian Jet (NAA). The resultant negative vorticity anomalies over the Asian landmass during a La Niña phase leads to overall warming over the region that reinforces the meridional temperature gradient between equatorial Indian Ocean and landmass over Asia, strengthening the ISMR.
The objective of the second part of this study is to investigate the role of Amazon runoff on climate. In this section, we examine the climatic response to fluctuations in the Amazon river runoff into the equatorial Atlantic Ocean. We find that the Amazon river runoff has a major impact on the Atlantic Ocean, Europe, and North America. In the absence of Amazon runoff, the Atlantic meridional overturning circulation (AMOC) strengthens and the Atlantic Ocean turns warmer in the northern hemisphere, and cooler water spreads in the equatorial and South Atlantic Ocean. In the boreal winter, an enhanced AMOC weakens the atmospheric ascending motion and thereby the Hadley cell strength over the equatorial Atlantic Ocean. Consequently, the meridional cells in the mid-latitude and extratropics also weaken. The surface ocean and atmospheric anomalies in the absence of Amazon runoff resemble spatial patterns of anomalies during a negative phase of North Atlantic Oscillation (NAO). The boreal wintertime northern Europe and the eastern United States turn cooler and drier in the absence of Amazon runoff and southern Europe and eastern Canada experience warmer and wetter winters. During boreal summer, there are significant changes in rainfall in the tropical Atlantic sector. When Amazon runoff is absent, the AMOC enhances, and large–scale warming appears in the North Atlantic Ocean (positive AMV). This positive phase of AMV favors a more northerly position of intertropical convergence zone (ITCZ) during boreal summer. Changes in rainfall patterns affect the local freshwater budget in the Atlantic Ocean and the rainfall over northwest Africa.
In order to study the impact of runoff into the Bay of Bengal, coupled model experiments were carried out with and without runoff into the Bay for a period of 200 years. When the runoff into the Bay is intercepted, the surface salinity in the upper ocean increases and the mixed layer becomes deeper. On short–timescale (1–10 years), the response of the climate system to the change in runoff is contrasting in summer and winter. During winter, the western equatorial Indian Ocean turns warmer in the first ten years whereas, during summer, the largest cooling is found in the eastern half. Significant changes in rainfall over India is observed during June and September.
Results from the coupled model experiments show how climatically significant is the freshwater input from the rivers. Modification of runoff reaching the ocean can have a significant impact on climate at a wide range of spatial and temporal scales. The impact of runoff on the climatic oscillations like ENSO, monsoon, NAO, AMO, AMOC, etc., shows that the impacts are not just confined to local oceanic regions where the runoff enters. Further, detailed studies on the changes in the freshwater usage and its impacts need to be done to better understand the climatic roles of the freshwater components of the earth system.
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Dr. P. N. Vinayachandran
Professor & J C Bose National Fellow
Centre for Atmospheric and Oceanic Sciences | Indian Institute of Science | Bangalore 560 012 | Tel: +91 80 2293 3065
http://caos.iisc.ac.inCo-chair, Ocean Predict Science Team
http://www.godae-oceanview.org
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