RESEARCH POSTER PRESENTATION DESIGN © 2015

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Fig. 3: Hovmöller diagram for late summer SMOS salinity (PSU) along 90°E for latitudes 12-20°N, with meridional Ekman transport overlaid in green.

The Monsoon Intraseasonal Oscillations (MISO) is strongly coupled to upper ocean variability in the Bay of Bengal (BoB). This study analyzes high-resolution moored observations from the northern BoB (18°N, 90°E) in comparison to three RAMA moorings to the south (8, 12, and 15°N, at 90°E). Examination of moored mixed layer salinity and satellite sea surface salinity (SSS) fields suggests that this is due to episodic intrusions of river discharge to the northern BoB driven by strong southward Ekman drift. Our results indicate the critical importance of river water intrusions to improved prediction of upper ocean variability in the BoB in relation to MISO and ISM rainfall.

Summary

• Explain the observed fluctuations in sea surface temperature (SST) and sea surface salinity (SSS) from the mooring data by computing mixed layer heat and salt budgets. Satellite data was used to supplement missing measurements from the RAMA dataset.

Objectives

Past studies suggest that net heat flux largely drives changes in sea surface temperature (SST) in the BoB during the Indian Summer Monsoon (ISM). However, our mixed layer heat budget analysis shows that intraseasonal temperature variability deviated greatly from the simple 1-D balance prediction in late summer.

Results

Analysis of interannual SSS variability using satellite data suggests that freshwater plumes tend to arrive to the northern bay in late summer, but the timing and extent of individual intrusion events depends highly on synoptic wind conditions as well as the presence of freshwater at the head of the Bay.

Conclusions

Freshwater plumes and associated salinity fronts caused shoaling of the mixed layer to less than 1 m at times. The presence of this shallow mixed layer led to amplified SST response to MISO-driven surface heat flux in the northern Bay.

Southward advection of freshwater from river discharge plays an important role in the heat and salt budgets in the northern Bay of Bengal in late summer. However, the exact timing and extent of individual freshwater intrusions depends highly on synoptic wind conditions and the associated Ekman transport. Our results indicate that freshwater plumes have a large role in determining upper ocean variability and air-sea interaction in the northern part of the Bay. This is significant because better understanding air-sea coupling in the Bay of Bengal is necessary to increase monsoon predictability.

Channing Prend1, Hyodae Seo2, Robert Weller2, J. Tom Farrar2 1. Columbia University, 2. Woods Hole Oceanographic Institution

Impact of freshwater plumes on intraseasonal upper ocean variability from moored observations in the Bay of Bengal

Fig. 1: BoB snapshots of SMOS salinity (PSU) data every 6 days for a 3 week period in August 2015 showing the arrival of a freshwater plume to the northern Bay.

Analysis of satellite SST data and the Price-Weller-Pinkel (PWP) 1-D ocean model further reveals that accounting for entrainment significantly improved the temperature balance in late summer via barrier layer dynamics, while advection was of secondary importance due to the overall weak SST gradients

• Examine how upper ocean variability varies spatially in the BoB by comparing the moorings and satellite data. In particular we will focus on the impact of the episodic intrusions of river discharge, which caused extreme shoaling of the mixed layer – particularly in the northern Bay.

Fig 2: Late summer time series of 10-day lowpass filtered observed SST tendency, 1-D balance predicted SST tendency, SST tendency calculated from PWP output, and PWP tendency with advection

Fig. 4: Hovmöller diagram of Aquarius and SMOS SSS for 8/2011-1/2016 along 90°E for latitudes 12-21°N with meridional Ekman transport overlaid in green.

Fig. 5: JJAS time series of 10-day lowpass filtered SST for WHOI and RAMA moorings

Acknowledgments This project was made possible by the WHOI Summer Student Fellowship, with funding through the NSF-REU program as well as donations and grants to the institution. The surface mooring was deployed by the Upper Ocean Processes Group of the Woods Hole Oceanographic Institution (WHOI) with support from the U.S. Office of Naval Research, (Grant N00014-13-1-0453). HS is supported by the ONR Young Investigator Program (N00014-15-1-2588);