Abstract
Abstract. The predictability of the sea surface height expression of baroclinic tides is examined with 96 h forecasts produced by the AMSEAS operational forecast model during 2013–2014. The phase-locked tide, both barotropic and baroclinic, is identified by harmonic analysis of the 2-year record and found to agree well with observations from tide gauges and satellite altimetry within the Caribbean Sea. The non-phase-locked baroclinic tide, which is created by time-variable mesoscale stratification and currents, may be identified from residual sea level anomalies (SLAs) near the tidal frequencies. The predictability of the non-phase-locked tide is assessed by measuring the difference between a forecast – centered at T+36, T+60, or T+84 h – and the model's later verifying analysis for the same time. Within the Caribbean Sea, where a baroclinic tidal sea level range of ±5 cm is typical, the forecast error for the non-phase-locked tidal SLA is correlated with the forecast error for the subtidal (mesoscale) SLA. Root mean square values of the former range from 0.5 to 2 cm, while the latter ranges from 1 to 6 cm, for a typical 84 h forecast. The spatial and temporal variability of the forecast error is related to the dynamical origins of the non-phase-locked tide and is briefly surveyed within the model.
Highlights
Sea level fluctuations of several centimeters associated with the astronomically forced baroclinic tide are nearly ubiquitous throughout the ocean (Ray and Mitchum, 1996; Zhao et al, 2016)
This study has examined the predictability of non-phaselocked baroclinic tides using 4 d ocean forecast products from the AMSEAS system
It was motivated by the desire to understand our present capability for predicting baroclinic tidal sea level anomalies (SLAs), which is expected to be a key limitation for measuring mesoscale and submesoscale processes with the forthcoming Surface Water & Ocean Topography (SWOT) wide-swath satellite altimeter (Callies and Wu, 2019)
Summary
Sea level fluctuations of several centimeters associated with the astronomically forced baroclinic tide are nearly ubiquitous throughout the ocean (Ray and Mitchum, 1996; Zhao et al, 2016). It is only possible to separate the baroclinic tide from the barotropic tide in altimetry data because of the large separation in spatial scales between these classes of waves (Zaron, 2019) There is another component of sea level variability associated with the tidal frequencies that represents non-phaselocked baroclinic tides, which are created by temporal modulations of the propagation medium (Munk and Cartwright, 1966; Rainville and Pinkel, 2006; Colosi and Munk, 2006; Zilberman et al, 2011; Ray and Zaron, 2011). Because modulations of the propagation medium – caused by mesoscale eddies and other processes – are, in part, represented within operational ocean forecasting systems, it ought to be possible to predict some component of the non-phase-locked tide with such a forecasting system
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