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In recent years, studies have put forth various theories on the role of oceanic equatorial Rossby waves (OERW) in the subseasonal‐to‐seasonal (S2S) predictability of the Indian Ocean (IO). While much of the scientific literature uses data from in‐situ, satellite, and/or reanalysis datasets, this study focuses on reforecast fields from the European Centre for Medium‐Range Weather Forecasting's (ECMWF) S2S dataset. Evaluation of the model's predictive skill in representing OERWs and the associated variations in subsurface‐to‐surface interaction and air–sea coupling are discussed. This work provides a unique methodology to calculate and evaluate the predictability of OERWs from model forecast data. Our results indicate that the model forecasts OERWs with high skill (anomaly correlation > 0.8 out to 40 days), indicating they are a key source of oceanic subseasonal predictability at extended lead times. Analysis of the wavenumber–frequency spectra for the IO indicates a strong reduction in power throughout the model forecast time period in the oceanic equatorial Kelvin wave (OEKW) regime and modest reduction of the OERW power. Both Kelvin and Rossby waves are modulated by the subseasonal zonal wind stress anomalies and the reduction of power is impacted by biases in winds at longer forecast leads. The erroneous weakening of the OEKWs contributes to the weakening of the reflected OERWs. Previous studies have documented that ocean heat content (OHC), particularly associated with downwelling OERWs, is important to maintaining and amplifying subseasonal precipitation in the IO. The reduced OERW power results in weaker advection of enhanced OHC anomalies by the OERWs, which has numerous implications for air–sea and subsurface‐to‐surface coupling, as discussed. The atmospheric response to the waning westward transport of OHC anomalies in the western IO by OERWs is associated with a weakening of intraseasonal precipitation anomalies associated with the intraseasonal oscillation. [ABSTRACT FROM AUTHOR]

Existing theories of the Madden-Julian oscillation neglect the feedback between the modification of sea surface temperature by the convection and development of a convective cluster itself. The authors show that the convection-generated SST gradient plays an important role in cluster propagation and development. The relative importance of radiative and evaporative fluxes in SST regulation is also discussed. Various Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment and Central Equatorial Pacific Experiment observation platforms are used to estimate the effects of equatorial convection on SST changes during March 1993. The data include drifting buoys and TAO-buoy array measurements, combined with the Navy Operational Global Atmospheric Prediction System analyzed surface wind fields and Geostationary Meteorological Satellite cloud-top temperatures. It is shown that during the equatorial convection episode SST is decreasing under and to the west of the convective heat source due to the large wind velocities and solar flux reduction. To the east of the source, in the convergence region of a Kelvin wave, low wind speeds and high insolation cause the SST to increase. The data are used to formulate an empirical relationship between wind speed and the 24-h SST change on the equator. Although formulated in terms of wind speed, this relationship implicitly includes radiative effects. This equation is then used in a global circulation model to examine the effect of SST feedback on the behavior of equatorial convection. A series of experiments is performed using an R15 general circulation model of the 'aquaplanet' with a zonally symmetric SST distribution. In the case with fixed SSTs, equatorial wind fluctuations have the character of waves propagating around the globe with a phase speed of about 20 m [s.sup.-1]. When the effect of SST modification is included, the fluctuations slow down and become more organized. In addition, a 40-60-day peak appears in the spectral analysis of equatorial precipitation.

Upper-ocean heat content and heat fluxes of 10–60-day intraseasonal oscillations (ISOs) were examined using high-resolution currents and hydrographic fields measured at five deep-water moorings in the central Bay of Bengal (BoB) and satellite observations as part of an international effort examining the role of the ocean on monsoon intraseasonal oscillations (MISOs) in the BoB. Currents, temperature, and salinity were sampled over the upper 600–1200 m from July 2018 to June 2019. The 10–60-day velocity ISOs of magnitudes 20–30 cm s−1 were observed in the upper 200 m, and temperature ISOs as large as 3°C were observed in the thermocline near 100 m. The wavelet cospectral analysis reveals multiple periods of ISOs carrying heat southward. The meridional heat-flux divergence associated with the 10–60-day band was strongest in the central BoB at depths between 40 and 100 m, where the averaged flux divergence over the observational period is as large as 10−7 °C s−1. The vertically integrated heat-flux divergence in the upper 200 m is about 20–30 W m−2, which is comparable to the annual-average net surface heat flux in the northern BoB. Correlations between the heat content over the 26°C isotherm and the outgoing longwave radiation indicate that the atmospheric forcing typically leads changes of the oceanic heat content, but in some instances, during fall–winter months, oceanic heat content leads the atmospheric convection. Our analyses suggest that ISOs play an important role in the upper-ocean heat balance by transporting heat southward, while aiding the air–sea coupling at ISO time scales. [ABSTRACT FROM AUTHOR]

The numerical study presented here focuses on baroclinic processes that contribute to tropical cyclone (TC) propagation. A three-dimensional, semispectral, primitive equation model of baroclinic vortex was developed to study TC motion. In a tilted vortex, interaction between upper- and lower-level vorticity anomalies leads to vortex propagation relative to the steering flow. On a [Beta] plane, with no environmental flow, the vortex is tilted toward the south and the interaction between the layers reduces the westward movement of the vortex. The vortex tilting can also occur due to the vertical shear in the environmental wind. On an f plane, the interaction between the layers causes the northward movement of the vortex in westerly linear shear, and southward movement in easterly linear shear, with a meridional velocity of about 1 m [s.sup.-1]. This velocity increases with increasing vortex intensity and vertical motion.

The paper examines the role of the development of outflow-layer instabilities on the motion of tropical cyclones. The influence of barotropic instability is examined by comparing the time changes in the storm tracks with the frequencies of free, unstable barotropic modes. For intense vortices barotropic instability is shown to contribute to the slow (periods of a few days) trochoidal motion of a cyclone. The development of instability depends on the horizontal distribution and frequency of environmental forcing. The strongest response occurs when the frequency of the forcing matches the frequency of an unstable mode.