Bay of Bengal Intraseasonal Oscillations and the 2018 Monsoon Onset

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.

Multi‐Scale Temporal Variability of Turbulent Mixing During a Monsoon Intra‐Seasonal Oscillation in the Bay of Bengal: An LES Study

“…The shoaling of the ML or the formation of RL is rather short-lived (Figure 5). This result suggests that the sustained shallow MLD and the formation of BL during the active phase as observed in the BoB (Shroyer et al, 2021) are more likely to be the result of lateral advection of riverine water than of the surface forcing like the precipitation. Furthermore, the simulations with the TIL highlight that, in addition to SST, SSS, and MLD, OHC (i.e., heat content below the mixed layer) is also a control on the ocean response to MISO forcing.…”

“…The initial profiles of temperature, salinity and stratification ( N 2 ) for the two types of BL are shown in Figure 2. The profiles in the BL‐TI‐LT case are motivated by the observations of subsurface temperature inversion collected by a fastCTD during the 2018 MISO‐BOB experiment (Shroyer et al., 2021). From these profiles, the no‐thermal‐inversion temperature profile is created by replacing the region of the temperature inversion only with linearly decreasing temperature.…”

“…Shroyer et al. (2021) indicated a reduction of 300 MJ m −2 over the MISO event which suggests the significant contribution of lateral advection of cold water via 3‐D processes. It should be noted that the presence of the TIL, which affects the initial values of the OHC, does not influence the ΔOHC which reflects the effect of Q net assuming that there is no feedback between the TIL and the surface flux.…”

“…Multiple LES are performed using realistic surface fluxes and initial conditions in the present study. In Section 2, we present the numerical method behind the LES and discuss the use of the MISO surface fluxes and background conditions which were collected during the summer 2018 MISO‐BOB field experiment (Shroyer et al., 2021). Questions 1, 2 and 3 are addressed in Sections 3, 4, and 5, respectively.…”

Ocean Convection

“…In addition to the prevalent night-time convection, day-time convection may also occur due to changes in atmospheric forcing, such as the movement of cold air, evaporative cooling, prolonged cloudy days, and so on. During the active phase of the summer monsoon, many parts of the Bay of Bengal experience cloud cover for several days, which further leads to Q net being negative even during the day-time [48]. The evaporative cooling over the ocean during the dry season or winter period has the potential to drive stronger convection and a deeper mixed layer [49].…”