Indo-Pacific Ocean Response to Intraseasonal Forcing

ABSTRACT

The objectives of our research are to examine the Tropical Indo-Pacific Ocean response to intraseasonal forcing and determine how this response modulates the seasonal and interannual variability of the Indo-Pacific Ocean, as well as its basic state. To address the above objectives, we are employing the NASA Laboratory for Hydrospheric Processes version of the Gent & Cane ocean general circulation model (GC-OGCM) along with observed high-frequency forcing fields to investigate the role intraseasonal forcing plays in the circulation and thermodynamic structure of the Indian Ocean. Our version of the GC-OGCM explicitly accounts for mixed-layer processes and upper ocean hydrology, contains a representation of the Indonesian Throughflow, and includes some aspects of the feedback from SST onto the surface fluxes via an atmospheric advective mixed-layer model. Composite forcing of the intraseasonal oscillation (ISO) has been constructed based on satellite-derived and reanalysis values of wind, rainfall, and surface shortwave forcing. Given that the Tropical ISO has a strong seasonal dependence, with an eastward-propagating ISO mode favored in boreal winter and a northeast-propagating mode favored in boreal summer, composite forcing was constructed for each of these two seasons/modes.

To examine the influence of ISO forcing, we compare a simulation that employs only climatological forcing (control) to cases that include a sequence of composite ISO events during their respective season. Using this general framework, we plan to examine a number of issues, such as the relative roles of the different ISO forcing components on the response, the nature of the recovery mechanisms of the ocean to one or more ISO events, the rectification of the intraseasonal time scale to longer time scales and/or the mean state, ISO modulation of the Indonesian throughflow, etc. In addition, recent evidence from theoretical and GCM models suggests that that coupled intraseasonal variations in SST play an important role in determining the characteristics of the model representation of the ISO - a chronically difficult phenomena for GCMs to simulate. As these studies greatly simplified the ocean feedback (e.g., fixed-depth slab mixed-layer), we will examine how the more complete ocean response from our GCM compares to these simplified treatments in order to assess the realism of the ocean feedbacks employed in such studies.

Preliminary results to date indicate that the low-frequency rectification of the intraseasonal signal involves a warming of the near-equatorial western Pacific Ocean of a few tenths of a degree. Important to the development of this low-frequency warming is the inclusion of the shortwave and rainfall forcing components, and a proper representation of the negative evaporation anomalies in light of many model's uses of a low-wind threshold. When these forcing components are not properly represented/included, the ISO forcing results in a low-frequency cooling rather than warming.