| dc.description.abstract | In order to study the impact of global runoff, sensitivity experiment (NoRiv) was carried out
by shutting down the entire runoff into the ocean for a period of 200 years. The changes in
NoRiv, compared to the reference simulation (CR), shows the impact of global runoff (NoRiv
minus CR). The evolution of mean ocean SSS in the major ocean basins show that the upper–
ocean reached a quasi-stabilized stage in the first 100 years in the NoRiv. Globally, when there
is no runoff, the surface ocean turns saltier with the largest increase occurring off the mouths
of major river systems. High salinity patches of more than 2 psu are found near the mouths of
Amazon, Congo, Ganga–Brahmaputra, Yangtze, etc. The upper–ocean currents distribute the
positive salinity anomalies generated near the river mouth of NoRiv to far away regions and
open ocean. In accordance with our results using CCSM3.0 model, CESM1.0 also shows that
the impact of runoff into the ocean on SSS is significantly higher in the extratropical ocean than
in the tropics, though the tropical runoff is considerably higher (Vinayachandran, Jahfer, and
Nanjundiah, 2015). Arctic Ocean recorded the highest rise in salinity as the ratio of runoff to
the surface area of the basin is highest. The open ocean regions of Pacific exhibited the least
change in salinity due to the vastness of the basin. We further analyzed the significance of this
rise in SSS on the upper–ocean temperature, air–sea interaction, and rainfall.
A saltier ocean in the NoRiv deepens the mixed layer of the upper–ocean owing to the increase
in surface density. Though the mean MLD in the NoRiv is found to be deeper than
the CR, the SST response to this change in ML is complex. In the simulation without runoff,
the northern hemisphere oceans recorded a comparatively higher rise in surface temperature,
mostly in the Atlantic Ocean. The change in SST in the NoRiv is primarily due to the weakening
of upper–ocean stratification or due to changes in air–sea interaction in the coupled system.
The equatorial Ocean of the Pacific Ocean recorded a mean cooling (of 0.2oC). But, the magnitude
of the impact is relatively weaker in CESM1.0 as compared to CCSM3.0. The evolution of
SST anomaly in the equatorial Pacific is a consequence of complex air–sea interaction. The appearance
of cooler SST anomalies in the eastern equatorial Pacific of NoRiv generates a zonal
pressure gradient resulting in an enhancement in easterlies. This, in turn, enhances wind–
driven upwelling in the eastern equatorial Pacific leading to the formation of a La Niña–like
SST anomaly in the equatorial Pacific. This La Niña–like SST anomaly in the equatorial Pacific
has the potential to alter the Indian summer monsoon, one of the most dominant and regular
features in the present–day tropical climate. It is well established that the cooler phase of
equatorial Pacific, the La Niña, favors a higher rainfall over the Indian subcontinent. Existing
theories on the teleconnection between the Indian monsoon and ENSO states that the summer–
time changes in the equatorial Pacific SST results in convective anomalies in the Pacific Ocean
that can impact the rainfall over the Indian subcontinent (Shaman and Tziperman, 2007).
Chapter 6. Summary and Conclusions 151
A cooler Pacific, La Niña phase, favors negative convection anomalies in the tropical Pacific
which leads to atmospheric convergence over Indian landmass and thereby affects the
westward propagating jet termed as the North African–Asian jet (NAA; Shaman and Tziperman,
2007). The resultant negative vorticity anomalies over the Asian landmass during a La
Niña phase lead to overall warming over the region that reinforces the meridional temperature
gradient between equatorial Indian Ocean and landmass over Asia (Shaman and Tziperman,
2007). However, we find that the response of SSS, SST, and rainfall to the global runoff in the
recent version (CESM1.0) is weaker than the earlier one (CCSM3.0). A higher ocean–land temperature
gradient in the NoRiv is found to be favorable for increased intraseasonal activity over
the Indian region leading to a stronger summer monsoon (Jiang, Li, andWang, 2004). Thus the
improved monsoon in the absence of global runoff is thus a result of cooling in the equatorial
Pacific and an upper–level vorticity guided strengthening of the meridional temperature gradient.
The 0AMZ experiment is similar to the global runoff experiment (NoRiv) except that the runoff
into the ocean from the Amazon river is shut down. The last 100 years of the model simulation
shows that the Amazon runoff is a vital component of the large–scale, low–frequency oscillations
in the Atlantic Ocean. When there is no input from the Amazon river, saltier water from
the river mouth disseminates over the North Atlantic Ocean as far as the deepwater formation
sites in the extratropics. Consequently, the upper–ocean mixed layer over the deep–convection
zones in the Labrador and GIN Sea region deepen. AdeeperMLimplies strengthening of deepwater
formation rate and a reinforced Atlantic meridional overturning circulation (AMOC). On
a longer timescale, the atmospheric response to Amazon runoff is found to be consistent with
the well established global impacts during a positive phase of AMOC.
Chapter 6. Summary and Conclusions 152
A stronger AMOC brings cooler subsurface water to the surface in the southern tropical
Atlantic Ocean by enhancing the upwelling rate. This cooler upper–ocean water is carried to
the north of the equator by the cross–equatorial currents. Consequently, the upwelling branch
of Hadley Cell in the equatorial Atlantic Ocean becomes weaker. This weakening of Hadley
Cell has a negative feedback on the atmospheric meridional cells in the subtropics and extratropics.
The North Atlantic Oscillation, the strength of which depends on the strength of
winter–time (DJF) meridional circulation in the extratropical Atlantic Ocean, thus shifts to its
negative phase. Though the impact of Amazon runoff on rainfall during winter is relatively
weaker in the tropics, the landmass adjacent to the extratropical Atlantic Ocean shows significant
responses. We found that the runoff–induced changes in surface temperature, winds, and
rainfall over the United States and Europe are similar to the anomalies during a negative phase
of NAO. The winter storms over northern Europe turn weaker along with a reduction in local
rainfall. However, southern Europe and eastern Canada received higher rainfall in the absence
of Amazon runoff, whereas the rainfall over the eastern US decreased.
In the tropical Atlantic Ocean sector, the summer–time rainfall (JJA) response exhibits a
see–saw pattern with a higher rainfall over the northern half and lower rainfall to the south.
This spatial pattern of rainfall anomaly is a typical feature of feedback during a positive phase
of AMOC. During a stronger AMOC, the warm surface waters of the southern tropical Atlantic
is pushed to the northern tropics, and the cooler waters dominate over the south and along the
equator. The SST anomalies thus formed is strikingly similar to the surface ocean signature
during a positive Atlantic Multidecadal Variability (AMV), which is believed to be a subset
of AMOC. The changes in rainfall in the 0AMZ is closely tied to the spatial pattern of SST
anomaly such that a warmer northern equatorial Atlantic Ocean favors a higher rainfall over
the region and vice versa. Thus when there is no input from the Amazon river, the AMOC and
AMV shifts to a positive phase and thereby affect the surface temperature and rainfall over the
tropical Atlantic Ocean during boreal summer. | en_US |
