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Mesoscale coupled ocean-atmosphere feedbacks in boundary current systems

  • Author(s): Putrasahan, Dian Ariyani
  • et al.
Abstract

The focus of this dissertation is on studying ocean- atmosphere (OA) interactions in the Humboldt Current System (HCS) and Kuroshio Extension (KE) region using satellite observations and the Scripps Coupled Ocean- Atmosphere Regional (SCOAR) model. Within SCOAR, a new technique is introduced by implementing an interactive 2-D spatial smoother within the SST-flux coupler to remove the mesoscale SST field felt by the atmosphere. This procedure allows large-scale SST coupling to be preserved while extinguishing the mesoscale eddy impacts on the atmospheric boundary layer (ABL). This technique provides insights to spatial-scale dependence of OA coupling, and the impact of mesoscale features on both the ABL and the surface ocean. For the HCS, the use of downscaled forcing from SCOAR, as compared to NCEP Reanalysis 2, proves to be more appropriate in quantifying wind-driven upwelling indices along the coast of Peru and Chile. The difference in their wind stress distribution has significant impact on the wind-driven upwelling processes and total upwelling transport along the coast. Although upwelling induced by coastal Ekman transport dominates the wind-driven upwelling along coastal areas, Ekman pumping can account for 30% of the wind-driven upwelling in several coastal locations. Control SCOAR shows significant SST-wind stress coupling during fall and winter, while Smoothed SCOAR shows insignificant coupling throughout, indicating the important role of ocean mesoscale eddies on air-sea coupling in HCS. The SST-wind stress coupling however, did not produce any rectified response on the ocean eddies. Coupling between SST, wind speed and latent heat flux is insignificant on large-scale coupling and full coupling mode. On the other hand, coupling between these three variables are significant on the mesoscale for most of the model run, which suggests that mesoscale SST affects latent heat through direct flux anomalies as well as indirectly through stability changes on the overlying atmosphere, which affects surface wind speeds and thus latent heat flux. In the KE region, differences in the strength of coupling between the Control and Smoothed SCOAR runs indicate how the spatial scale of SST fronts affects the OA coupling via two distinct mechanisms, the vertical mixing mechanism (VMM) and the pressure adjustment mechanism (PAM). Intuitively, one might expect that the VMM would be most active on the ocean mesoscale and less significant on the large scale. Instead, the model revealed that the VMM, expressed through the coupling between downwind SST gradient and wind stress divergence, acts strongly on both the large scale and mesoscale. In contrast, coupling between crosswind SST gradients and wind stress curl is seen on the mesoscale, but extinguished over large-scale SST gradients, revealing the vital role of ocean mesoscale. For PAM, one might expect the large-scale coupling to be dominant in establishing the PAM. Instead, model results suggest that in PAM, the coupling between the Laplacian of sea level pressure and surface wind convergence are active on both the mesoscale and the large scale, though the coupling strength nearly doubles with the inclusion of ocean mesoscale. Ocean mesoscale imprints are also seen on precipitation anomalies, for which their differences are more aligned with the differences in SST gradients and surface wind convergence rather than SST anomalies

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