Rain‐Induced Stratification of the Equatorial Indian Ocean and Its Potential Feedback to the Atmosphere

Abstract

Surface freshening through precipitation can act to stably stratify the upper ocean, forming a rain layer (RL). RLs inhibit subsurface vertical mixing, isolating deeper ocean layers from the atmosphere. This process has been studied using observations and idealized simulations. The present ocean modeling study builds upon this body of work by incorporating spatially resolved and realistic atmospheric forcing. Fine‐scale observations of the upper ocean collected during the Dynamics of the Madden‐Julian Oscillation field campaign are used to verify the General Ocean Turbulence Model (GOTM). Spatiotemporal characteristics of equatorial Indian Ocean RLs are then investigated by forcing a 2D array of GOTM columns with realistic and well‐resolved output from an existing regional atmospheric simulation. RL influence on the ocean‐atmosphere system is evaluated through analysis of RL‐induced modification to surface fluxes and sea surface temperature (SST). This analysis demonstrates that RLs cool the ocean surface on time scales longer than the associated precipitation event. A second simulation with identical atmospheric forcing to that in the first, but with rainfall set to zero, is performed to investigate the role of rain temperature and salinity stratification in maintaining cold SST anomalies within RLs. Approximately one third, or 0.1°C, of the SST reduction within RLs can be attributed to rain effects, while the remainder is attributed to changes in atmospheric temperature and humidity. The prolonged RL‐induced SST anomalies enhance SST gradients that have been shown to favor the initiation of atmospheric convection. These findings encourage continued research of RL feedbacks to the atmosphere.

A Cold Lid on a Warm Ocean: Indian Ocean Surface Rain Layers and Their Feedbacks to the Atmosphere

“…Their findings are consistent with in situ observations of RLs analyzed by Thompson et al (2019), and corroborate the results of previous model experiments (Pei et al, 2018) that RLs reduce local SST through the surface input of cold rain and sustain and enhance SST reductions through a stable salinity stratification that confines wind-driven evaporative cooling to the near-surface RL. Shackelford et al (2022) also demonstrated the role of RLs in enhancing small-scale SST gradients that induce pressure perturbations in the atmospheric boundary layer and potentially excite atmospheric convection (Back & Bretherton, 2009;Li & Carbone, 2012;Lindzen & Nigam, 1987). Additionally, Pei et al (2018) demonstrated that RLs may produce a slight subsurface ocean heating effect below the RL base.…”

“…However, the idealized, 1D nature of these ocean simulations provide limited information on RL behavior under realistic surface forcing and on RL feedbacks to the atmosphere. Shackelford et al (2022) studied RL formation under realistic atmospheric conditions by forcing a 2D array of 1D ocean column models using output from a convection-permitting simulation of the November 2011 DYNAMO event. Their findings are consistent with in situ observations of RLs analyzed by Thompson et al (2019), and corroborate the results of previous model experiments (Pei et al, 2018) that RLs reduce local SST through the surface input of cold rain and sustain and enhance SST reductions through a stable salinity stratification that confines wind-driven evaporative cooling to the near-surface RL.…”

“…During the MJO disturbed phase, these signatures increase in magnitude and frequency, reflecting the increasing presence of RLs as the MJO transitions from a convectively suppressed to convectively active state. Since RLs inhibit vertical mixing of surface waters with those of the deeper ocean (Reverdin et al., 2012; Shackelford et al., 2022; Thompson et al., 2019), cooling by surface evaporation in RL remains trapped near the ocean surface. Meanwhile, the reduced vertical mixing below the RL combined with the additional ∼1 W m −2 shortwave radiation that penetrates below the RL base amplify subsurface warming to a depth of about 40 m relative to the no‐RL simulation.…”

“…While DWLs and RLs can occur during all MJO phases, and can be present simultaneously, DWLs are most frequent during the MJO suppressed phase and RLs are most frequent during the MJO disturbed phase (Shackelford et al., 2022; Thompson et al., 2019). During the MJO active phase, the stabilizing buoyancy flux from surface heat and freshwater inputs is typically insufficient to withstand mixing by strong surface winds (Moum et al., 2014; Shackelford et al., 2022; Thompson et al., 2019). Thus, the ocean frequently becomes well‐mixed to the thermocline with a diurnally uniform SST during this period.…”

“…Shackelford et al. (2022) studied RL formation under realistic atmospheric conditions by forcing a 2D array of 1D ocean column models using output from a convection‐permitting simulation of the November 2011 DYNAMO event. Their findings are consistent with in situ observations of RLs analyzed by Thompson et al.…”