The tropical pacific plays an important role in the global climate system: its interannual sea surface temperature (SST) variability, the EL NI o southern oscillation, affects weather and climate…

This is a $259,308 grant awarded by the National Aeronautics and Space Administration (NASA) to the University Corporation for Atmospheric Research (UCAR) to conduct research on heat absorption and water mass transformation in the tropical Pacific Ocean. The grant will fund a study that evaluates upwelling and heat budget processes in the tropical Pacific using satellite data and ocean state estimates from the Estimating the Circulation and Climate of the Ocean (ECCO) model. The research aims to understand how heat enters the tropical Pacific, the mechanisms that maintain the cold tongue, and the role of ocean mixing in driving sea surface temperature variability. This work supports NASA's Earth science research objectives. The grant does not have a set-aside designation.

The tropical pacific plays an important role in the global climate system: its interannual sea surface temperature (SST) variability, the EL NI o southern oscillation, affects weather and climate all over the globe. The eastern tropical pacific features an area of relatively cool SST, the cold tongue, which absorbs large amounts of heat, and which is subsequently RE-distributed throughout the global ocean. In this study we evaluate pathways by which heat enters the tropical pacific, and mechanisms that maintain cold ssts and lead to SST variability. We evaluate upwelling in the eastern tropical pacific from satellite surface measurements and two estimating the circulation and climate of the ocean (ecco) based ocean state estimates (oses) using a number of analysis techniques. Ocean state estimates aim to minimize the model-observation misfit by adjusting chosen control variables, such as surface (momentum) fluxes. They provide the user with full heat budget output to perform process studies on a realistic representation of the ocean state. Here, we compare a regional OSE with . Degree resolution to a global OSE with 1-degree resolution. The oses also differ in their handling of the error minimization, the global state estimate (ECCOV4) allows for diffusive coefficients to adjust and adjusts wind stress, while the regional OSE (tropical pacific ocean state estimation, tpose) adjusts the wind speed. Satellite data present an independent set of measurements against which to compare the state estimates. First, we compare the representation of vertical motion between the oses, and surface layer divergence (SLD), its variability, and uncertainty derived from satellite data to OSE SLD. This analysis allows US to answer the question whether oses robustly represent vertical motion in the tropical pacific. Then, we apply two different techniques to calculate motion crossing isotherms, that is, water being transformed from one temperature class to another. The first is a kinematic approach, where we evaluate the continuity equation in temperature space. This approach is usually taken when estimating upwelling from observations. The second is based on thermodynamics, which evaluates the heat budget. The thermodynamic method is especially powerful, because in addition to informing US on when and where water is being transformed, IT also allows US to determine how the transformation happens. We can distinguish between vertical and horizontal mixing, solar penetration, and mesoscale (eddy) effects, thus allowing US to understand how heat is absorbed into the ocean. Time series of this analysis in conjunction with wind stress from satellite data allows US to investigate how water mass transformation modulates the tropical pacific under weak and strong wind regimes, as well as what role IT plays in longer term modes of climate variability, such as enso. Where the isotherms are approximately horizontal, i.e., in the thermocline, we can partition the total upwelling into cross-isothermal and along-isothermal flow. The cross-isothermal flow again is related to ocean mixing. We attempt to find relationships between total vertical flow and cross-isothermal flow, and between cross-isothermal flow and ocean mixing. From these relationships IT will be possible to estimate ocean mixing from large scale quantities, for example by using surface layer divergence to estimate total vertical flow, then apply a relationship to find the fraction of that total flow that crosses isotherms, and then find the fraction of that flow that is explained by mixing.