Mesoscale Air-sea Interactions Research Articles

The role of eddy-wind interaction in the eddy kinetic energy budget of the Agulhas retroflection region

The Agulhas retroflection (AR) region possesses the highest eddy kinetic energy (EKE) level in the Indian Ocean. However, mechanisms regulating EKE of the AR remain uncertain. Here, by analyzing an eddy-resolving coupled model simulation with improved EKE representation, we show that the upper-ocean EKE of the AR is mainly generated through barotropic instability in its upstream and leakage zones and is by nonlocal transport in its downstream zone. The interaction between mesoscale eddies and local winds plays a key role in EKE dissipation. The lack of eddy-wind interaction results in flawed EKE budget in the leakage zone in ocean-alone models, leading to severe biases in EKE distribution with overestimation and over-strong penetration into the South Atlantic. Our results highlight the essence of mesoscale air-sea interaction in the dynamics of the AR, with implications for understanding the inter-basin transport of the Agulhas leakage.

Ocean Mesoscale and Frontal-Scale Ocean–Atmosphere Interactions and Influence on Large-Scale Climate: A Review

Abstract Two decades of high-resolution satellite observations and climate modeling studies have indicated strong ocean–atmosphere coupled feedback mediated by ocean mesoscale processes, including semipermanent and meandrous SST fronts, mesoscale eddies, and filaments. The air–sea exchanges in latent heat, sensible heat, momentum, and carbon dioxide associated with this so-called mesoscale air–sea interaction are robust near the major western boundary currents, Southern Ocean fronts, and equatorial and coastal upwelling zones, but they are also ubiquitous over the global oceans wherever ocean mesoscale processes are active. Current theories, informed by rapidly advancing observational and modeling capabilities, have established the importance of mesoscale and frontal-scale air–sea interaction processes for understanding large-scale ocean circulation, biogeochemistry, and weather and climate variability. However, numerous challenges remain to accurately diagnose, observe, and simulate mesoscale air–sea interaction to quantify its impacts on large-scale processes. This article provides a comprehensive review of key aspects pertinent to mesoscale air–sea interaction, synthesizes current understanding with remaining gaps and uncertainties, and provides recommendations on theoretical, observational, and modeling strategies for future air–sea interaction research. Significance Statement Recent high-resolution satellite observations and climate models have shown a significant impact of coupled ocean–atmosphere interactions mediated by small-scale (mesoscale) ocean processes, including ocean eddies and fronts, on Earth’s climate. Ocean mesoscale-induced spatial temperature and current variability modulate the air–sea exchanges in heat, momentum, and mass (e.g., gases such as water vapor and carbon dioxide), altering coupled boundary layer processes. Studies suggest that skillful simulations and predictions of ocean circulation, biogeochemistry, and weather events and climate variability depend on accurate representation of the eddy-mediated air–sea interaction. However, numerous challenges remain in accurately diagnosing, observing, and simulating mesoscale air–sea interaction to quantify its large-scale impacts. This article synthesizes the latest understanding of mesoscale air–sea interaction, identifies remaining gaps and uncertainties, and provides recommendations on strategies for future ocean–weather–climate research.

Sensitivity of ECMWF coupled forecasts to improved initialization of the ocean mesoscale

AbstractIt has been hypothesized that initial conditions derived from eddy‐resolving ocean models could benefit eddy‐permitting forecast systems at subseasonal lead times (2 weeks to 2 months) by providing improved ocean initial conditions and reduced sea‐surface temperature (SST) errors associated with a more realistic representation of mesoscale features and ocean fronts such as the Gulf Stream. Here, we test this hypothesis by running a series of coupled 32‐day reforecasts with ocean initial conditions derived from native resolution (i.e., eddy‐permitting) and higher resolution (i.e., eddy‐resolving) ocean reanalyses. Initial conditions derived from the eddy‐resolving reanalysis reproduce observed temperature and sea‐level variability associated with mesoscale ocean eddies more accurately. Furthermore, forecasts of ocean variables are significantly improved in the extratropics, particularly in regions of observed intense mesoscale variability. This result is relevant for the marine forecasting community and demonstrates the potential for improved probabilistic forecasts of ocean variables in affordable ensemble forecast systems. However, the impacts on the mean state and forecast skill in the atmosphere are more limited. Based on an evaluation of model biases as a function of lead time, we conclude that there is room for further improvements to the initialization of the Gulf Stream that could lead to more significant impacts on atmospheric skill. Furthermore, we demonstrate that our reforecasts underestimate the strength of mesoscale air–sea interaction in the vicinity of strong SST fronts by 20–40%. We speculate that the impacts of improved ocean eddy initialization on atmospheric forecast skill may be larger in coupled models with higher ocean and atmosphere resolutions that can simulate air–sea interaction in the midlatitudes more realistically.

Influence of oceanic mesoscale eddy on the atmospheric boundary layer based on an idealized model

Often, numerical models rely on parameterization schemes to represent currently unresolved or insufficiently understood phenomena.The influence of ocean mesoscale eddies on the overlying atmosphere is such a phenomenon. Using the Weather Research and Forecasting (WRF) model, the Regional Ocean Modeling System (ROMS) and the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modelling system, this study investigates the influence of idealized oceanic mesoscale eddies on the atmospheric boundary layer (ABL). The idealized model is horizontally homogeneous and set on an f-plane at 35°N. The cyclonic (anticyclonic) eddy causes sea level pressure (SLP) to increase (decrease) and reduces (increases) latent and sensible heat fluxes, surface winds, marine ABL, and atmospheric precipitable water in the coupled and uncoupled cases. Moreover, a momentum budget analysis of the model output shows that the pressure gradient term is the primary contributor to atmospheric momentum balance. Additionally, it should be noted that the SLP adjustment mechanism is important when advection is weak. Comparing the uncoupled and coupled WRF cases, the eddy's effect on atmospheric parameters is more intense in the uncoupled than in the coupled case, which can increase precipitation by 50% and wind speed and heat flux by 20–30%. These results may be potentially important for future parameterizations of mesoscale air-sea interaction processes in numerical model development.

Weak Mesoscale Variability in the Optimum Interpolation Sea Surface Temperature (OISST)-AVHRR-Only Version 2 Data before 2007

Mesoscale sea surface temperature (SST) variability triggers mesoscale air–sea interactions and is linked to ocean subsurface mesoscale dynamics. The National Oceanic and Atmospheric Administration (NOAA) daily Optimum Interpolation SST (OISST) products, based on various satellite and in situ SST data, are widely utilized in the investigation of multi-scale SST variabilities and reconstruction of subsurface and deep-ocean fields. The quality of OISST datasets is subjected to temporal inhomogeneity due to alterations in the merged data. Yet, whether this issue can significantly affect mesoscale SST variability is unknown. The analysis of this study detects an abrupt enhancement of mesoscale SST variability after 2007 in the OISST-AVHRR-only version 2 and version 2.1 datasets (hereafter OI.v2-AVHRR-only and OI.v2.1-AVHRR-only). The contrast is most stark in the subtropical western boundary current (WBC) regions, where the average mesoscale SST variance during 2007–2018 is twofold larger than that during 1993–2006. Further comparisons with other satellite SST datasets (TMI, AMSR-E, and WindSAT) suggest that the OISST-AVHRR-only datasets have severely underestimated mesoscale SST variability before 2007. An evaluation of related documents of the OISST data indicates that this bias is mainly caused by the change of satellite AVHRR instrument in 2007. There are no corresponding changes detected in the associated fields, such as the number and activity of mesoscale eddies or the background SST gradient in these regions, confirming that the underestimation of mesoscale SST variability before 2007 is an artifact. Another OISST product, OI.v2-AVHRR-AMSR, shows a similar abrupt enhancement of mesoscale SST variability in June 2002, when the AMSR-E instrument was incorporated. This issue leaves potential influences on scientific research that utilize the OISST datasets. The composite SST anomalies of mesoscale eddies based on the OI.v2-AVHRR-only data are underestimated by up to 37% before 2007 in the subtropical WBC regions. The underestimation of mesoscale variability also affects the total (full-scale) SST variability, particularly in winter. Other SST data products based on the OISST datasets were also influenced; we identify suspicious changes in J-OFURO3 and CFSR datasets; the reconstructed three-dimensional ocean products using OISST data as input may also be inevitably affected. This study reminds caution in the usage of the OISST and relevant data products in the investigation of mesoscale processes.

The Global Sink of Available Potential Energy by Mesoscale Air-Sea Interaction.

The thermal component of oceanic eddy available potential energy (EPE) generation due to air‐sea interaction is proportional to the product of anomalous sea surface temperature (SST) and net air‐sea heat flux (SHF). In this study we assess EPE generation and its timescale and space‐scale dependence from observations and a high‐resolution coupled climate model. A dichotomy exists in the literature with respect to the sign of this term, that is, whether it is a source or a sink of EPE. We resolve this dichotomy by partitioning the SST and net heat flux into climatological mean, climatological seasonal cycle, and remaining transient contributions, thereby separating the mesoscale eddy variability from the forced seasonal cycle. In this decomposition the mesoscale air‐sea SST‐SHF feedbacks act as a 0.1 TW global sink of EPE. In regions of the ocean with a large seasonal cycle, for example, midlatitudes of the Northern Hemisphere, the EPE generation by the forced seasonal cycle exceeds the mesoscale variability sink, such that the global generation by seasonal plus eddy variability acts as a 0.8 TW source. EPE destruction is largest in the midlatitude western boundary currents due to mesoscale air‐sea interaction and in the tropical Pacific where SST variability is due mainly to the El Niño–Southern Oscillation. The EPE sink in western boundary currents is spatially aligned with SST gradients and offset to the poleward side of currents, while the mean and seasonal generation are aligned with the warm core of the current. By successively smoothing the data in space and time we find that half of the EPE sink is confined to timescales less than annual and length scales less than 2°, within the oceanic mesoscale band.

A study of mesoscale air–sea interaction in the Southern Ocean with a regional coupled model

Coupling between the atmosphere and ocean is scale-dependent. For example, in the mid-latitudes and at oceanic mesoscales (spatial scales between 10 and hundreds of kilometers), the air–sea interactions are driven by the oceanic variability, and the atmosphere responds to the changes in the sea surface temperature anomalies (SSTA), which are created by fast oceanic advection. This study explores these interactions, using a regional high-resolution atmosphere–ocean coupled model with a realistic atmospheric component and a semi-idealized oceanic model of a zonal flow. The atmospheric component consists of two nested domains: the inner domain fully coupled with the ocean model, and the outer domain one-way coupled with the observed SST. Two 2-year simulations are discussed here: one in which the oceanic isopycnals are steep and ocean currents are strong (“Strong Currents” or SC) and another with less steep isopycnals and weaker currents (“Weak Currents” or WC). Simulated mesoscale variability occurs on a wide range of spatial scales, and we distinguish large-mesoscale (hundreds of kilometers and shorter) and small-mesoscale (tens of kilometers and shorter) anomalies. The model is most applicable to the mid-latitude Southern Ocean far from any boundaries.Relationships between atmospheric variables and SSTA are studied using temporal correlations and coupling coefficients, and for both large-mesoscale and small-mesoscale anomalies. Significant positive correlations between large-mesoscale anomalies are found for following pairs of variables: equivalent neutral stability (ENS) 10-meter winds and SSTA, wind stress and SSTA, and wind stress divergence/curl and SSTA downwind/crosswind gradients. The temporal correlations are smaller for the small-mesoscale anomalies. The correlation coefficients are also higher for the model SC region, whereas the corresponding coupling coefficients are higher in the model WC region. Among all pairs, the coupling coefficients for the ENS winds and SSTA are the most consistent in time and various environmental conditions. Coupling coefficients for the wind stress and SSTA show a nearly linear dependence on the ENS wind speed. As a result, the reported variability in these coupling coefficients indicates a complex, nonlinear relationship between the wind and SST anomalies. Our numerical analysis indicates strong presence of the Vertical Mixing Mechanism involving the downward momentum mixing on both small- and large-mesoscales. In contrast, the active presence of the Pressure Adjustment Mechanism in the Marine Boundary Layer could not be confirmed on the spatial and time scales considered in this study.

Mesoscale Air–Sea Interaction and Its Role in Eddy Energy Dissipation in the Kuroshio Extension

ABSTRACT Using the high-resolution Community Earth System Model (CESM) output, this study investigates air–sea interaction and its role in eddy energy dissipation in the Kuroshio Extension (KE) region. Based on an eddy energetics analysis, it is found that the baroclinic pathway associated with temperature variability is the main eddy energy source in this region. Both the air–sea heat flux and wind stress act as eddy killers that remove energy from oceanic eddies. Heat exchange between atmosphere and oceanic eddies dominates the dissipation of eddy temperature variance within the surface layer and accounts for 36% of the total dissipation in the upper 350-m layer. Compared to the heat exchange, the role of wind power in damping the eddy kinetic energy (EKE) is relatively small. Only 18% of EKE dissipation in the upper 350 m is attributed to eddy wind power. Misrepresentation of the damping role of mesoscale ocean–atmosphere interaction can result in an incorrect vertical structure of eddy energy dissipation, leading to an erroneous representation of vertical mixing in the interior ocean.

Mesoscale air-sea interaction during the passage of a typhoon over the Yellow and East China Seas

In order to best capture the mutual forcing and feedback process to improve the accuracy of modeling of the mesoscale process, we developed the two-way coupling model MCM (Mesoscale Coupled Model) based on two independent models, the atmospheric model MM5v3 and the ocean model ECOM-si, using a flux coupler designed with the concurrent process communication technique. We selected Typhoon Winnie (9711) passing over the Yellow and East China Seas (YECS) to test the performance of the coupling model, and analyzed the quantitative impacts on atmospheric and oceanic variations. The results show that the simulated air-sea heat and momentum exchanges strongly affect circulation in both the atmosphere and the ocean. When mesoscale air-sea interaction is considered, the results for the atmosphere and ocean improved considerably. In terms of atmosphere, the air-sea interaction updates the distribution of elements (e.g., wind, heat flux, relative humidity and temperature) in the vertical and horizontal fields. The forecast result of typhoon intensity becomes more realistic, with decreases in central pressure error of 3.4 hPa at 30 hr and 9 hPa at 60 hr. A strong wind drives the cyclonic ocean current in the upper layer and induces horizontal transport and vertical mixing. All of these processes directly influence the Sea Surface Temperature (SST), which decreases over a large area, particularly near the typhoon center, and can reach 5°C. Finally, we discuss a possible negative feedback mechanism.

A Model of Mesoscale Air-Sea Interaction in a Sea Breeze-Coastal Upwelling Regime

Abstract A time-dependent, two-dimensional numerical model is constructed by coupling a four-layer atmosphere to a two-layer ocean through fluxes of heat and momentum. Idealized experiments are performed to investigate the oceanic response to sea breeze forcing, changes induced in the sea breeze by coastal upwelling, and air-sea feedback during periods of active coastal upwelling. This problem is motivated by the fact that the time scale of the coastal upwelling response is short compared to most oceanic response times and is comparable to the sea breeze time scale. When forced with a longshore wind stress, the model ocean reproduces several features commonly observed in coastal upwelling regimes, including an equatorward surface jet, a poleward undercurrent, and a region of low sea surface temperatures near the coast. For the cases considered here, the sea breeze contributes significantly to the mean longshore wind stress and, consequently, plays a role in driving the coastal upwelling circulation. It al...

Mesoscale Variability of Sea Surface Temperatures

Abstract Results of an investigation of the nature of the mesoscale variability of sea surface temperature (SST) inthe upwelling zone off the central coast of Oregon are presented. A knowledge of SST mesoscale variability isimportant toward understanding the mesoscale air-sea interaction process in an upwelling zone. Almost-daily sea surface temperature data gathered by remote sensing techniques provided the basis for this investi-gation. These data were gathered over a period of 60 days during the COHO Project in summer 1973. In order to study the influence of wind forcing on the mesoscale SST field, wind data were gathered from ananemometer located at Newport, Ore. Some important results of this investigation are: 1) the daily SST fields respond rapidly to wind forcing;2) the three-week mean SST's tend to follow the large-scale bathymetry; and 3) identification of SST eddiesfrom day to day on daily perturbation (from the three-week means) maps is made difficult because of theexistence of strong horizont...