Full-Depth Seasonal Sea Level Budgets

Gregory Johnson - (NOAA)

Co-Investigator(s):

Paige Lavin (University of Maryland, College Park )

Abstract:

Sea level budget (SLB) analyses provide critical validation of sea surface height (SSH) measurement system accuracies, including the net, steric, and barystatic components. We consider the SLB “closed” if altimeter-based SSH matches the sum of the independently observed steric (from Core and Deep Argo floats) and barystatic (from GRACE/GRACE-FO) sea level components. Closure not only provides confidence in use of these observations, but can be used to demonstrate whether altimeters are meeting mission requirements for accuracy. Successful mitigation of and adaptation to sea level rise (SLR) requires knowledge of which components are driving that change. Through the use of climate models, that insight can be extended to help predict future SLR. On the global scale, the SLB closes within the uncertainties, while at regional scales closure is basin-dependent and typically only occurs at longer timescales (e.g., Leuliette and Willis, 2011). This project aims to close the SLB on seasonal timescales in areas where the full-depth steric term is currently being measured at much higher temporal resolution than ever before by Deep Argo floats.

Many recent attempts to close the SLB use the Core Argo float network data to obtain steric sea level (SSL) since 2005. While these observations are revolutionary in measuring SSL near-globally at high temporal resolution, there are still gaps – notably in marginal seas and at depths below 2000 dbar. Since 2014, regional pilot arrays of Deep Argo floats have been deployed in a few deep basins. These floats sample from the surface to 4000 or 6000 dbar depending on the model. We will focus on the multiyear float arrays in the Southwest Pacific, South Australian, and Brazil basins. We will combine the in-situ profiling float data with satellite observations of SSH and gravity to construct seasonal SLBs. We will use Core Argo floats in these regions to augment our SSL calculations in the upper 2000 dbar. To assess the barystatic sea level component, we will use the monthly, JPL-RL06M mass concentration (mascon) fields calculated from GRACE/GRACE-FO measurements. For total sea level we will use data from the Radar Altimeter Database System (RADS) and the upcoming Sentinel-6A mission. Since these three basins differ greatly in terms of their bottom water dynamics, mass addition trends, and distance from bottom water formation regions, the seasonal variations and dominant terms of their SLBs are likely to differ greatly. The impact of the addition of the Deep Argo steric term on the closure of the SLB in the Southwest Pacific basin is of specific interest since closing the SLB there has been challenging (Purkey et al, 2014; Royston et al, 2020).

There is particular value in understanding the seasonal variability in the components of the SLB in the Southwest Pacific Basin as it is located just off of the coast of American Samoa. Islands in this region are already experiencing significant flooding from storms which will be exacerbated under future warming scenarios. When considering coastal flooding risk, the gradual rate of global SLR may seem insignificant compared to the magnitude of existing water level fluctuations due to tides, waves, and storm surge. However, in regions where the magnitude of this existing water-level variability is small, such as in the tropical Pacific, the small amount of SLR (e.g., 5–10 cm) expected to occur by 2050 will more than double the frequency of extreme water-level events (Vitousek et al, 2017). Therefore, providing detailed insight into the magnitude of the full-depth seasonal variability of the SLB in the Southwest Pacific Basin, and the primary driver(s) of this signal, will help coastal inundation modelers more accurately assess the present and future coastal flooding risk for these islands. The work done will also validate whether Sentinel-6A achieves its drift error requirement of <5 mm/year for the regional SLR trend in each of the basins mentioned above.

Supported by NASA