State Estimates for the Tropical Pacific: A Reanalysis for Evaluating the Model, Observations, and Mass, Heat, and Salt Fluxes

PIs: Bruce Cornuelle, Ariane Verdy (Scripps Institution of Oceanography, University of California San Diego)

Description

We have produced a series of overlapping Four-dimensional variational (4D-Var) state estimates for the tropical Pacific covering 2010-2019 at a resolution of 1/3˚, with some examples at 1/6˚. Each estimate is a free forward model run that has had initial conditions, forcing, and other controls adjusted so that it is consistent with observations and can provide diagnoses of mass, heat, and salt fluxes. The model domain includes the entire tropical Pacific, and the TAO mooring observations were withheld from the state estimate to be used for cross-validation to check the performance of the state estimate and highlight the unique information coming from the TAO data.  The estimates will be used to help design process studies, to assess regions of good and bad model skill to guide model improvement, and for observing system design.

Funded by NOAA Climate Variability Program.

Model domain with bathymetry. Dashed white lines bound the region in which observations were assimilated. TAO moorings are shown as orange and blue markers, with the blue ones indicating the moorings that were used for the plots in Figure 2. The mooring data were not assimilated but are used for independent comparisons.

Figure 1. Model domain with bathymetry. Dashed white lines bound the region in which observations were assimilated. TAO moorings are shown as orange and blue markers, with the blue ones indicating the moorings that were used for the plots in Figure 2. The mooring data were not assimilated but are used for independent comparisons.

Accomplishments

The dynamically consistent, property-conserving ocean state estimates are available on the website (see link in Data section). We have used these results to diagnose heat and salt fluxes in the equatorial region during an El Niño-La Niña cycle. We see changes to stratification, the Equatorial undercurrent, and upwelling structure during this period. Freshwater and heat budgets for the upper 300 m of the tropical Pacific between 5˚S and 5˚N show how the region cooled and freshened during the El Niño.

Vertical mixing across 300 m is negligible, meaning property changes are balanced by horizontal and vertical advective divergence and surface forcing. The overturning circulation across the northern boundary of the analysis domain collapses during the El Niño. However, the geostrophic input of heat reduces by a larger amount than the Ekman removal of heat, and the region cools. While these advective processes across the north wall act to make the region saltier, the eastward propagation of the western Pacific warm-fresh pool and enhanced precipitation result in a freshening of the region during the El Niño.

Lessons Learned

1. The state estimation procedure worked well even for the strong El Niño-La Niña cycle meaning that nonlinearity in the adjoint iteration was not a problem.

2. Doubling the resolution to 1/6˚ showed changes in many details of the circulation, so increasing the model resolution is a priority, particularly for flow interactions with topography.

3. The state estimate reproduces the tropical instability waves and the Kelvin and Rossby waves propagating along the equator. Comparing these waves to the observations is a measure of the skill of the atmospheric reanalysis used to force the model.

Publications

Verdy, A., M.R. Mazloff, B.D. Cornuelle, A.C. Subramanian. ENSO influence on heat and freshwater budgets in a tropical Pacific Ocean state estimate, In Preparation.

Data

Model output is saved in NetCDF format.
The output is available at http://www.ecco.ucsd.edu/tropac.html.

(a) Niño 3.4 index anomalies over the period 2010-2019 and (b-f) daily averaged temperatures at 100 m from the state estimate (black) and TAO observations (blue) at five moorings on the equator with longitudes labeled in each panel. The five mooring locations are highlighted in Figure 1. Orange and gray shading indicate periods when the Niño 3.4 anomaly is above 0.4 or below -0.4 for at least 6 months, respectively.

Figure 2. (a) Niño 3.4 index anomalies over the period 2010-2019 and (b-f) daily averaged temperatures at 100 m from the state estimate (black) and TAO observations (blue) at five moorings on the equator with longitudes labeled in each panel. The five mooring locations are highlighted in Figure 1. Orange and gray shading indicate periods when the Niño 3.4 anomaly is above 0.4 or below -0.4 for at least 6 months, respectively.

Temperature along the equator from the state estimate showing the November mean. Gray contours show isopycnals; arrows show the zonal and vertical components of velocity. The white contour shows the 20˚C isotherm.
Temperature along the equator from the state estimate showing November 2015. Gray contours show isopycnals; arrows show the zonal and vertical components of velocity. The white contour shows the 20˚C isotherm.
Temperature along the equator from the state estimate during the El Niño, and November 2017. Gray contours show isopycnals; arrows show the zonal and vertical components of velocity. The white contour shows the 20˚C isotherm.

Figure 3. (a-c) Temperature along the equator from the state estimate showing the November mean (a), November 2015 (b), during the El Niño, and November 2017 (c). Gray contours show isopycnals; arrows show the zonal and vertical components of velocity. The white contour shows the 20˚C isotherm.

(a) Niño 3.4 index anomalies over the period 2014-2018, (b) depth of the sigma-25 isopycnal, (c) average temperature in the top 300 m, and (d) average salinity in the top 300 m in the region 5˚S to 5˚N, east of 140˚E. Daily averages in the state estimate (black) are compared to the monthly Argo mapped product (blue) by Roemmich and Gilson (2009). Orange and gray shading indicate periods when the Niño 3.4 anomaly is above 0.4 or below -0.4 for at least 6 months, respectively.

Figure 4. (a) Niño 3.4 index anomalies over the period 2014-2018, (b) depth of the sigma-25 isopycnal, (c) average temperature in the top 300 m, and (d) average salinity in the top 300 m in the region 5˚S to 5˚N, east of 140˚E. Daily averages in the state estimate (black) are compared to the monthly Argo mapped product (blue) by Roemmich and Gilson (2009). Orange and gray shading indicate periods when the Niño 3.4 anomaly is above 0.4 or below -0.4 for at least 6 months, respectively.