Agulhas Return Current Region Research Articles

Mesoscale Surface Wind‐SST Coupling in a High‐Resolution CESM Over the KE and ARC Regions

AbstractA strong positive correlation exists between mesoscale sea surface temperature (SST) and surface wind stress perturbations in the extratropical oceans, which is revealed by high‐resolution satellite but is difficult to be accurately simulated in low‐resolution coupled ocean‐atmosphere models. The extent to which the mesoscale coupling is captured in a high‐resolution Community Earth System Model (CESM‐HR) is assessed in this study, with a focus on two regions, the Kuroshio Extensions (KE) and Agulhas Return Current (ARC), respectively. Compared with satellite observations, it is found that the CESM‐HR can well depict the characteristics of the mesoscale air‐sea coupling, with its strength being comparable to that observed; but in the KE region, the simulated strength is only half of the observed during summer. Mechanisms for the atmospheric responses to mesoscale SST perturbations through the pressure adjustment (PA) and the downward momentum transport (DMT) are analyzed. The results highlight different mechanisms for controlling the atmospheric responses over the KE and ARC regions, which are regionally and seasonally dependent. In the ARC region, pronounced dipole patterns of sea surface pressure (SLP) and vertical velocity perturbation responses indicate that the DMT exerts a dominant effect in both winter and summer. In the KE region, monopole patterns show the main role played by the PA mechanism for the atmospheric adjustment in summer, while dipole patterns indicate the main role played by the DMT mechanism in winter. The weak coupling strength in summer over the KE is closely related to the inadequate parameterization of vertical heat mixing.

Seasonal and interannual variations of Chlorophyll-a concentrations in Agulhas return current region

The Agulhas Return Current (ARC) is one of the current systems in the Southern Indian Ocean (SIO) associated with high Chlorophyll-a Concentrations (CC). This work presents the seasonal and inter-annual variability of the CC during 2003–2017 in three different regions along the ARC pathway, which are selected based on the circulation pattern, bathymetry, and local dynamics. The parts are the retroflection region (R1), middle (R2), and end (R3) of the ARC. The analysis of the CC variability revealed the highest variability of CC in the R3 region, followed by R1, and the least variability in R2 for the entire study period. The CC is high during the austral summer in both R1 and R2 compared to the other seasons. However, at the end of the ARC region (R3), higher CC is observed during austral spring. The CC is high (low) during the negative (positive) phase of Southern Annular Mode (SAM) due to increased (decreased) westerlies. A comparative analysis was carried during the spring of 2004, 2008, and 2016, when the CC value was moderate, low, and high, respectively. It shows that the retroflection happened more eastward than its normal retroflection position during 2008. The dominance of anticyclonic eddies in the R3 reduced the CC during 2008. The study also identified the effect of the polar water in the variation of the chlorophyll.

Surface Stress and Atmospheric Boundary Layer Response to Mesoscale SST Structure in Coupled Simulations of the Northern California Current System

Abstract The influence of mesoscale sea surface temperature (SST) variations on wind stress and boundary layer winds is examined from coupled ocean–atmosphere numerical simulations and satellite observations of the northern California Current System. Model coupling coefficients relating the divergence and curl of wind stress and wind to downwind and crosswind SST gradients are generally smaller than observed values and vary by a factor of 2 depending on planetary boundary layer (PBL) scheme, with values larger for smoothed fields on the 0.25° observational grid than for unsmoothed fields on the 12-km model grid. Divergence coefficients are larger than curl coefficients on the 0.25° grid but not on the model grid, consistent with stronger scale dependence for the divergence response than for curl in a spatial cross-spectral analysis. Coupling coefficients for 10-m equivalent neutral stability winds are 30%–50% larger than those for 10-m wind, implying a correlated effect of surface-layer stability variations. Crosswind surface air temperature and SST gradients are more strongly coupled than downwind gradients, while the opposite is true for downwind and crosswind heat flux and SST gradients. Midlevel boundary layer wind coupling coefficients show a reversed response relative to the surface that is predicted by an analytical model; a predicted second reversal with height is not seen in the simulations. The relative values of coupling coefficients are consistent with previous results for the same PBL schemes in the Agulhas Return Current region, but their magnitudes are smaller, likely because of the effect of mean wind on perturbation heat fluxes.

Interannual Eddy Kinetic Energy Modulations in the Agulhas Return Current

AbstractInterannual variability in the mesoscale eddy field in the Agulhas Return Current (ARC) of 32–42°S and 15–35°E is investigated based on satellite altimeter observations and state estimate from the Estimating the Circulation and Climate of the Ocean, Phase II from 1993 to 2016. It is found that the interannual modulation of eddy kinetic energy in the ARC region is externally mediated by the wind stress forcing that generates the westward propagating sea surface height anomalies across the South Indian Ocean subtropical gyre. The wind‐forced sea surface height anomalies influence the upstream Agulhas Current volume transports. By modulating the intensity of barotropic instability of the ARC mean flow centered around the retroflection region, the Agulhas Current inflow variability leads to the downstream interannual eddy kinetic energy fluctuations in the ARC region.

Frontogenesis in the Agulhas Return Current Region Simulated by a High-Resolution CGCM

AbstractDetailed mechanisms for frontogenesis/frontolysis of the sea surface temperature (SST) front in the Agulhas Return Current (ARC) region are investigated using outputs from a high-resolution coupled general circulation model. The SST front is maintained throughout the year through an approximate balance between frontolysis by surface heat flux and frontogenesis by horizontal advection. Although a southward (northward) cross-isotherm flow on the northern (southern) side of the front is weaker than a strong eastward along-isotherm current in the frontal region, this cross-isotherm confluent flow advects warmer (cooler) temperature toward the SST front north (south) of the front and acts as the dominant frontogenesis mechanism. In addition, stronger (weaker) frontogenesis in austral summer (winter) is attributed to the stronger (weaker) cross-isotherm confluence, which may be linked to seasonal variations of the Agulhas Current, ARC, and Antarctic Circumpolar Current. On the other hand, the contribution from entrainment is relatively small, because frontolysis by larger (smaller) entrainment velocity on the northern (southern) side opposes frontogenesis by less (more) effective cooling associated with a thicker (thinner) mixed layer and smaller (larger) temperature difference between the mixed layer and entrained water in the northern (southern) region. To gain further insight into the time-mean cross-isotherm confluent flow in the frontal region, the vorticity balance is examined. It is shown that anticyclonic (cyclonic) vorticity advection north (south) of the front by the mean cross-isotherm confluence is in balance with the sum of cyclonic (anticyclonic) vorticity advection by the mean along-isotherm flow and cross-isotherm eddy–mean interaction.

An Investigation of the Stability Dependence of SST-Induced Vertical Mixing over the Ocean in the Operational Met Office Model

This study presents an assessment of the impact of a March 2006 change in the Met Office operational global numerical weather prediction model through the introduction of a nonlocal momentum mixing scheme. From comparisons with satellite observations of surface wind speed and sea surface temperature (SST), it is concluded that the new parameterization had a relatively minor impact on SST-induced changes in sea surface wind speed in the Met Office model in the September and October 2007 monthly averages over the Agulhas Return Current region considered here. The performance of the new parameterization of vertical mixing was evaluated near the surface layer and further through comparisons with results obtained using a wide range of sensitivity of mixing parameterization to stability in the Weather Research and Forecasting (WRF) Model, which is easily adapted to such sensitivity studies. While the new parameterization of vertical mixing improves the Met Office model response to SST in highly unstable (convective) conditions, it is concluded that significantly enhanced vertical mixing in the neutral to moderately unstable conditions (nondimensional stability [Formula: see text] between 0 and −2) typically found over the ocean is required in order for the model surface wind response to SST to match the satellite observations. Likewise, the reduced mixing in stable conditions in the new parameterization is also relatively small; for the range of the gradient Richardson number typically found over the ocean, the mixing was reduced by a maximum of only 10%, which is too small by more than an order of magnitude to be consistent with the satellite observations.

Frontolysis by surface heat flux in the Agulhas Return Current region with a focus on mixed layer processes: observation and a high-resolution CGCM

Detailed mechanisms for frontogenesis/frontolysis of the Agulhas Return Current (ARC) Front, defined as the maximum of the meridional sea surface temperature (SST) gradient at each longitude within the ARC region (40°–50°E, 55°–35°S), are investigated using observational datasets. Due to larger (smaller) latent heat release to the atmosphere on the northern (southern) side of the front, the meridional gradient of surface net heat flux (NHF) is found throughout the year. In austral summer, surface warming is weaker (stronger) on the northern (southern) side, and thus the NHF tends to relax the SST front. The weaker (stronger) surface warming, at the same time, leads to the deeper (shallower) mixed layer on the northern (southern) side. This enhances the frontolysis, because deeper (shallower) mixed layer is less (more) sensitive to surface warming. In austral winter, stronger (weaker) surface cooling on the northern (southern) side contributes to the frontolysis. However, deeper (shallower) mixed layer is induced by stronger (weaker) surface cooling on the northern (southern) side and suppresses the frontolysis, because the deeper (shallower) mixed layer is less (more) sensitive to surface cooling. Therefore, the frontolysis by the NHF becomes stronger (weaker) through the mixed layer processes in austral summer (winter). The cause of the meridional gradient of mixed layer depth is estimated using diagnostic entrainment velocity and the Monin–Obukhov depth. Furthermore, the above mechanisms obtained from the observation are confirmed using outputs from a high-resolution coupled general circulation model. Causes of model biases are also discussed.

Modeling the Atmospheric Boundary Layer Wind Response to Mesoscale Sea Surface Temperature Perturbations

Abstract The wind speed response to mesoscale SST variability is investigated over the Agulhas Return Current region of the Southern Ocean using the Weather Research and Forecasting (WRF) Model and the U.S. Navy Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) atmospheric model. The SST-induced wind response is assessed from eight simulations with different subgrid-scale vertical mixing parameterizations, validated using Quick Scatterometer (QuikSCAT) winds and satellite-based sea surface temperature (SST) observations on 0.25° grids. The satellite data produce a coupling coefficient of sU = 0.42 m s−1 °C−1 for wind to mesoscale SST perturbations. The eight model configurations produce coupling coefficients varying from 0.31 to 0.56 m s−1 °C−1. Most closely matching QuikSCAT are a WRF simulation with the Grenier–Bretherton–McCaa (GBM) boundary layer mixing scheme (sU = 0.40 m s−1 °C−1), and a COAMPS simulation with a form of Mellor–Yamada parameterization (sU = 0.38 m s−1 °C−1). Model rankings based on coupling coefficients for wind stress, or for curl and divergence of vector winds and wind stress, are similar to that based on sU. In all simulations, the atmospheric potential temperature response to local SST variations decreases gradually with height throughout the boundary layer (0–1.5 km). In contrast, the wind speed response to local SST perturbations decreases rapidly with height to near zero at 150–300 m. The simulated wind speed coupling coefficient is found to correlate well with the height-averaged turbulent eddy viscosity coefficient. The details of the vertical structure of the eddy viscosity depend on both the absolute magnitude of local SST perturbations, and the orientation of the surface wind to the SST gradient.

Role of mixed layer depth in surface frontogenesis: The Agulhas Return Current front

Air-sea interaction processes that modify the sea surface temperature (SST) front in the Agulhas Return Current region (between 40°E and 55°E) during austral summer and winter are examined using observational data and output from a high-resolution ocean general circulation model. While the air-sea heat flux frontal variations tend to relax the SST front, the frontolysis is amplified (damped) in summer (winter) when frontal variations in the mixed layer depth (MLD) are incorporated. The stronger (weaker) frontolysis associated with the MLD variations is due to the fact that the warming (cooling) by the surface heat flux is amplified south of the front where the MLD is shallower and is reduced north of the front where the MLD is deeper. This study is the first to show that the MLD variations play an important role in affecting the strength of the SST front.

Estimation of eddy heat transport in the global ocean from Argo data

The Argo data are used to calculate eddy (turbulence) heat transport(EHT) in the global ocean and analyze its horizontal distribution and vertical structure. We calculate the EHT by averaging all the v0, T0profiles within each 2 � �2 � bin. The velocity and temperature anomalies are obtained by removing their climatological values from the Argo “instantaneous” values respectively. Through the Student’s t-test and an error evaluation, we obtained a total of 87% Argo bins with significant depth-integrated EHTs (D-EHTs). The results reveal a positive-and-negative alternating D-EHT pattern along the western boundary currents (WBC) and Antarctic Circumpolar Current (ACC). The zonally-integrated D-EHT (ZI-EHT) of the global ocean reaches 0.12 PW in the northern WBC band and –0.38 PW in the ACC band respectively. The strong ZI-EHT across the ACC in the global ocean is mainly caused by the southern Indian Ocean. The ZI-EHT in the above two bands accounts for a large portion of the total oceanic heat transport, which may play an important role in regulating the climate. The analysis of vertical structures of the EHT along the 35 � N and 45 � S section reveals that the oscillating EHT pattern can reach deep in the northern WBC regions and the Agulhas Return Current (ARC) region. It also shows that the strong EHT could reach 600 m in the WBC regions and 1 000 m in the ARC region, with the maximum mainly located between 100 and 400 m depth. The results would provide useful information for improving the parameterization scheme in models.

High-Resolution Satellite Measurements of the Atmospheric Boundary Layer Response to SST Variations along the Agulhas Return Current

Abstract The marine atmospheric boundary layer (MABL) response to sea surface temperature (SST) perturbations with wavelengths shorter than 30° longitude by 10° latitude along the Agulhas Return Current (ARC) is described from the first year of SST and cloud liquid water (CLW) measurements from the Advanced Microwave Scanning Radiometer (AMSR) on the Earth Observing System (EOS) Aqua satellite and surface wind stress measurements from the QuikSCAT scatterometer. AMSR measurements of SST at a resolution of 58 km considerably improves upon a previous analysis that used the Reynolds SST analyses, which underestimate the short-scale SST gradient magnitude over the ARC region by more than a factor of 5. The AMSR SST data thus provide the first quantitatively accurate depiction of the SST-induced MABL response along the ARC. Warm (cold) SST perturbations produce positive (negative) wind stress magnitude perturbations, leading to short-scale perturbations in the wind stress curl and divergence fields that are linearly related to the crosswind and downwind components of the SST gradient, respectively. The magnitudes of the curl and divergence responses vary seasonally and spatially with a response nearly twice as strong during the winter than during the summer along a zonal band between 40° and 50°S. These seasonal variations closely correspond to seasonal and spatial variability of large-scale MABL stability and surface sensible heat flux estimated from NCEP reanalysis fields. SST-induced deepening of the MABL over warm water is evident in AMSR measurements of CLW. Typical annual mean differences in cloud thickness between cold and warm SST perturbations are estimated to be about 300 m.

Reply [to [Comment on] “Comment on “New global drifter data set available’” by Ohann Lutjeharms and Isabelle Ansorge

The Eos article referred to in the letter by Lutjeharms et al. discusses only the time mean drifter velocity data in the Agulhas Return Current region. Because of Eos space limitations, we could not discuss the large amount of research devoted to synoptic meanders and eddies, which are also seen by the individual drifter tracks in our data set while passing through the Agulhas Return Current system.More than 10 years of drifter data were used in the time mean velocity plot in our Figure 2. Underlying the synoptic variations were seven semi‐permanent meanders of the Agulhas Return Current. In our global drifter data set, no other western boundary current had such an extensive, steady or stable meander pattern. We thought that this indeed was a “discovery” and are pleased that Lutjeharms et al. agree.