Dynamics of Regional Heat Convergence and Deep-Ocean Warming in the subtropical South Pacific and Indian Oceans
- (University Of Miami, Key Biscayne)
Global mean sea level is steadily increasing, but the spatial pattern of sea level change is remarkably uneven, mainly due to the spatial redistribution of heat by the ocean and atmospheric dynamics. The combination of satellite and in situ observations provides a more complex view on the horizontal and vertical heat redistribution in the ocean, and quantification of the dynamical processes involved. The difference between total and mass-related sea level fields, observed by the NASA satellite altimetry and GRACE missions, respectively, gives the full-depth steric sea level variability, which can be used as a proxy for heat content changes. While over the decade 2005-2014 the global mean steric sea level rose at a rate of ~1.3 mm/yr, two regions of enhanced warming stood out. The largest increase of steric sea level and heat content occurred in the subtropical South Indian and Pacific Oceans, which made these two regions the Earth’s major heat accumulators. Nevertheless, they substantially differed from each other. In the Indian Ocean, warming was mostly limited to the upper 1000 m, while the subtropical South Pacific also experienced significant warming in the 1000-2000 m depth interval. Preliminary analysis of satellite altimetry, GRACE, and Argo observations suggests that in the subtropical South Pacific, warming could extend even deeper, below 2000 m. Both the subtropical South Indian and Pacific warm pools started to loose heat in early 2014. During this time, the Pacific Ocean and the atmosphere aloft started to exhibit features suggesting the impending onset of an El Nino event. By early 2016, this El Nino had already expressed itself as one of the most powerful ENSO events, approximately equal in strength to the previous record-holding 1997-98 El Nino. If the cooling of the South Pacific warm pool that began in 2014 continues, heat accumulated at depth can be fullyreleased to the surrounding ocean and the atmosphere aloft leading to far-reaching effects on the global weather and climate.
The goal of the proposed research is to monitor and analyze the horizontal and vertical heat redistribution in the ocean using available satellite and in situ observing systems and ocean model simulations. Specifically, we intend to answer the following science questions: (i) Does the observed accumulation of heat in the South Pacific warm pool extend below 2000 m depth? (ii) What physical processes and mechanisms are responsible for the observed regional accumulation of heat? (iii) Why is heat accumulation in the subtropical South Indian Ocean limited to the upper 1000 m, but extends deeper in the subtropical South Pacific? (iv) How is the 2015-2016 El Nino event affecting the subtropical South Pacific and South Indian Oceans? Does it signify the onset of a long-term cooling trend, or is it a transient negative anomaly?
To address these questions, we will carry out a comprehensive analysis of satellite altimetry, GRACE, Argo, and repeat hydrography measurements, thoroughly accounting for data errors and paying particular attention to systematic uncertainties. Biases in regional ocean mass trends likely present in GRACE data will be quantified and reduced. Physical processes responsible for the regional accumulation of heat will be investigated using available observations and ocean model experiments. To monitor the time evolution of the full-depth heat content in the southwest Pacific, in particular as a response to 2015-2016 El Nino, and to obtain unique time series of meridional transports, with a particular focus on the Antarctic Bottom Water transport, we propose to deploy two Pressure Inverted Echo Sounders along P06 section.
By studying the regional sea level variability in relation to heat convergence, the proposed research will directly address the OSTST goal to demonstrate the Earth science and applications arising from analyses of ocean surface topography data.