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Abstract Accurate representation of air‐sea interaction is crucial to numerical prediction of the ocean, weather, and climate. Sea surface temperature (SST) gradients and surface currents in the oceanic mesoscale regime are known to have significant influence on air‐sea fluxes of momentum. Studies based on high‐resolution numerical models and observations reveal that SST gradients and surface currents in the submesoscale regime are much stronger than those in the mesoscale. However, the feedback between the submesoscale processes and the air‐sea turbulent fluxes is not well understood. To quantitatively assess the responses between air‐sea flux of momentum and submesoscale processes, a non‐hydrostatic ocean model is implemented in this study. The inclusion of SST gradients and surface currents in air‐sea bulk fluxes are argued to be significant for modeling accurate wind stress in the submesoscale regime. Taking both into account, this study shows that the linear relationship between wind stress curl/divergence and crosswind/downwind SST gradients existing in the mesoscale regime is not obvious in the submesoscale. Instead, a linear relationship between wind stress curl/divergence and surface current curl/divergence is revealed in the submesoscale. Furthermore, the magnitude of wind stress curl introduced by submesoscale processes is much greater than that presented by mesoscale processes. Another key finding is that tracer subduction and potential vorticity distribution in the submesoscale is susceptible to submesoscale‐modified air‐sea turbulent momentum flux. This study serves as a starting point in investigating the feedbacks between atmospheric and oceanic submesoscale processes.
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On the Hyperbolicity of the Bulk Air–Sea Heat Flux Functions: Insights into the Efficiency of Air–Sea Moisture Disequilibrium for Tropical Cyclone Intensification
Abstract Sea-to-air heat fluxes are the energy source for tropical cyclone (TC) development and maintenance. In the bulk aerodynamic formulas, these fluxes are a function of surface wind speed U 10 and air–sea temperature and moisture disequilibrium (Δ T and Δ q , respectively). Although many studies have explained TC intensification through the mutual dependence between increasing U 10 and increasing sea-to-air heat fluxes, recent studies have found that TC intensification can occur through deep convective vortex structures that obtain their local buoyancy from sea-to-air moisture fluxes, even under conditions of relatively low wind. Herein, a new perspective on the bulk aerodynamic formulas is introduced to evaluate the relative contribution of wind-driven ( U 10 ) and thermodynamically driven (Δ T and Δ q ) ocean heat uptake. Previously unnoticed salient properties of these formulas, reported here, are as follows: 1) these functions are hyperbolic and 2) increasing Δ q is an efficient mechanism for enhancing the fluxes. This new perspective was used to investigate surface heat fluxes in six TCs during phases of steady-state intensity (SS), slow intensification (SI), and rapid intensification (RI). A capping of wind-driven heat uptake was found during periods of SS, SI, and RI. Compensation by larger values of Δ q > 5 g kg −1 at moderate values of U 10 led to intense inner-core moisture fluxes of greater than 600 W m −2 during RI. Peak values in Δ q preferentially occurred over oceanic regimes with higher sea surface temperature (SST) and upper-ocean heat content. Thus, increasing SST and Δ q is a very effective way to increase surface heat fluxes—this can easily be achieved as a TC moves over deeper warm oceanic regimes.
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- Award ID(s):
- 1941498
- PAR ID:
- 10299501
- Date Published:
- Journal Name:
- Monthly Weather Review
- Volume:
- 149
- Issue:
- 5
- ISSN:
- 0027-0644
- Page Range / eLocation ID:
- 1517 to 1534
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/MWR-D-20-0324.1
