Bay of Bengal Intraseasonal Oscillations and the 2018 Monsoon Onset

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  • 1 College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA
  • | 2 University of Massachusetts, Dartmouth, MA, USA
  • | 3 Indian Institute of Science, Bengaluru, India
  • | 4 University of Notre Dame, Notre Dame, IN, USA
  • | 5 Scripps Institution of Oceanography, University of California, San Diego, CA, USA
  • | 6 Woods Hole Oceanographic Institution, Woods Hole, MA, USA
  • | 7 Indian Institute of Tropical Meteorology, Pune, India
  • | 8 College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA
  • | 9 U.S. Naval Research Laboratory, Monterey, CA, USA
  • | 10 U.S. Naval Research Laboratory, Stennis, MS, USA
  • | 11 NOAA/Pacific Marine Environmental Laboratory, Seattle, WA, USA
  • | 12 Woods Hole Oceanographic Institution, Woods Hole, MA, USA
  • | 13 University of Colorado, Boulder, CO, USA
  • | 14 National Institute of Ocean Technology, Chennai, India
  • | 15 Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA
  • | 16 University of Notre Dame, Notre Dame, IN, USA
  • | 17 University of Massachusetts, Dartmouth, MA, USA
  • | 18 Woods Hole Oceanographic Institution, Woods Hole, MA, USA
  • | 19 Indian Institute of Science, Bengaluru, India
  • | 20 Scripps Institution of Oceanography, University of California, San Diego, CA, USA
  • | 21 Naval Information Warfare Center Pacific, San Diego, CA, USA
  • | 22 The Ocean University of Sri Lanka, Crow Island, Colombo 15, Sri Lanka
  • | 23 Indian Institute of Technology Madras, Chennai, India
  • | 24 India Meteorological Department, New Delhi, India
  • | 25 Indian National Centre for Ocean Information Services, Hyderabad, India
  • | 26 Indian Institute of Tropical Meteorology, Pune, India
  • | 27 Space Applications Centre, Indian Space Research Organisation, Ahmedabad, India
  • | 28 Applied Physics Laboratory, University of Washington, Seattle, WA, USA
  • | 29 College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA
  • | 30 Scripps Institution of Oceanography, University of California, San Diego, CA, USA
  • | 31 US Air Force, 53rd Weather Reconnaissance Squadron, Biloxi, MS, USA
  • | 32 DEVCOM - Army Research Lab, Adelphi, MD, USA
  • | 33 NorthWest Research Associates, Redmond, WA, USA
  • | 34 University of Colorado, Boulder, CO, USA
  • | 35 University of South Carolina, Columbia, SC, USA
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Abstract

In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air-sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the US, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air-sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ~20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10−12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ~20 to 50 m), cooling SST (by ~ 1°C), and warming/drying of the lower to mid-troposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air-sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.

Current affiliation: National Center for Atmospheric Research, Boulder, CO, USA

Current affiliation: Institute of Marine Science, Burapha University, Chonburi, Thailand

Corresponding author: Emily Shroyer, emily.shroyer@oregonstate.edu

Abstract

In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air-sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the US, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air-sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ~20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10−12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ~20 to 50 m), cooling SST (by ~ 1°C), and warming/drying of the lower to mid-troposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air-sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.

Current affiliation: National Center for Atmospheric Research, Boulder, CO, USA

Current affiliation: Institute of Marine Science, Burapha University, Chonburi, Thailand

Corresponding author: Emily Shroyer, emily.shroyer@oregonstate.edu
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