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Title: Bay of Bengal salinity stratification and Indian summer monsoon intraseasonal oscillation: 1. Intraseasonal variability and causes: SALINITY STRATIFICATION AND MISO
Award ID(s):
1658132 1658218
Author(s) / Creator(s):
; ; ; ; ;
Date Published:
Journal Name:
Journal of Geophysical Research: Oceans
Page Range / eLocation ID:
4291 to 4311
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Abstract

    The freshwater input from rain to the surface ocean is a key component of the global water cycle. Frequent rainfall in the inter‐tropical convergence zone creates regions of strong surface stratification and low salinity, which vary seasonally. We evaluate how variations in rain type and preexisting upper ocean stratification influence the timing and duration of the salinity response to rainfall using the General Ocean Turbulence Model. A series of model simulations was run by prescribing three typical background stratification conditions and idealized rain and wind forcing that was consistent with observed convective, stratiform, and mixed convective and stratiform rainfall. Background stratification was assessed using underway CTD observations and rain forcing was identified from mooring observations collected in the eastern tropical Pacific during the second Salinity Processes in the Upper Ocean Regional Study. Model results show that strong stratification, whether preexisting or from convective rainfall, inhibits downward mixing of freshwater and allows near‐surface salinity anomalies to persist following rain. In contrast, when stratiform rain precedes convective rain, salinity anomalies are quickly mixed downward and longer lasting deeper in the mixed layer. This implies that accurately quantifying the salinity structure following rain should consider preexisting stratification and the type of rainfall. Furthermore, patterns of rainfall and stratification likely affect the bias between salinity observations at the surface and deeper in the mixed layer. Because satellite rain data do not correctly represent the small scales of rain forcing, the small‐scale surface salinity response to rain cannot be predicted from satellite data.

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  2. Abstract

    Modifications for navigation since the late 1800s have increased channel depth (H) in the lower Hudson River estuary by 10–30%, and at the mouth the depth has more than doubled. Observations along the lower estuary show that both salinity and stratification have increased over the past century. Model results comparing predredging bathymetry from the 1860s with modern conditions indicate an increase in the salinity intrusion of about 30%, which is roughly consistent with theH5/3scaling expected from theory for salt flux dominated by steady exchange. While modifications including a recent deepening project have been concentrated near the mouth, the changes increase salinity and threaten drinking water supplies more than 100 km landward. The deepening has not changed the responses to river discharge (Qr) of the salinity intrusion (~Qr−1/3) or mean stratification (Qr2/3). Surprisingly, the increase in salinity intrusion with channel deepening results in almost no change in the estuarine circulation. This contrasts sharply with local scaling based on local dynamics of anH2dependence, but it is consistent with a steady state salt balance that allows scaling of the estuarine circulation based on external forcing factors and is independent of depth. In contrast, the observed and modeled increases in stratification are opposite of expectations from the steady state balance, which could be due to reduction in mixing with loss of shallow subtidal regions. Overall, the mean shift in estuarine parameter space due to channel deepening has been modest compared with the monthly‐to‐seasonal variability due to tides and river discharge.

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