Relationship between surface fields over Indian ocean and monsoon rainfall over homogeneous zones of India

The monthly mean atmospheric fields and surface parameters of NCEP/NCAR reanalysis for the period 1948-1998 have been studied to examine the characteristics of monsoon circulation features, sea surface temperature (SST), sea level pressure, surface wind stress and latent heat flux over the Indian Ocean and nearby seas during deficient, normal and excess rain years. The entire period of study has been classified into deficient, normal and excess rain years for all India as well as for each of the five homogeneous zones separately based on the observed seasonal mean rainfall. On the basis of the mean characteristics of the surface fields, the oceanic region covering the Indian Ocean and adjacent seas has been divided into four regional sectors. Using various statistical means the relation between the surface fields over the four regional sectors and the monsoon rainfall over five homogeneous zones of Indian landmass has been examined. Attempt have been made to identify some surface parameters which can be used as predictors for seasonal mean monsoon rainfall over the entire India and also over some homogeneous zones.

The potential influence of falling ice radiative effects on Central-Pacific El Niño variability under progressive global warming

The impacts of falling ice (snow) radiative effects (FIREs) on simulated surface wind stress and sea surface temperature (SST) in Central Pacific El Niño (CP-El Niño) under a progressive warming climate are examined. Using controlled simulations with the CESM1 model, it is shown that the exclusion of FIREs (No snow: NOS) generates persistent westerly anomalies in surface wind stress relative to that with FIREs (Snow on: SON). These anomalies subsequently lead to a weakening of the easterly trade winds associated with warmer SST anomalies in modeled life cycle. Results over three separated 40-year intervals (P1: 21-60 years; P2: 61-100 years; P3: 101-140 years) are compared with Coupled Model Intercomparison Project phase 5 (CMIP5) models without FIREs. Both NOS configuration and CMIP5 models simulate longer life cycles of CP-El Niño events with weakening easterlies and warmer SST anomalies on the equator, persistently propagating eastward from the mature to dissipating phases. Compared to NOS, SON, on the other hand, produces a shorter CP-El Niño life cycle together with stronger easterlies and colder SSTs over the eastern to central equatorial Pacific. The magnitudes of the simulated westerlies and warm SST anomalies tend to diminish without eastward shifting following the peak of the CP-El Niño activity. There are substantial differences in CP-El Niño characteristics from P1 to P3 between NOS and SON. During P1, both SON and NOS show patterns which are consistent with their present-day counterparts. In P2 and P3, SON exhibits a prolonged CP-El Niño life cycle, while NOS develops a double-peak El Niño evolution at the mature and decaying phases. Regarding El Niño diversity and the projections, the CMIP5 models have not reached a consensus. The inclusion of the FIREs would increase the confidence in simulating El Niño future behavior.

Effects of spatial scale modification on the responses of surface wind stress to the thermal front in the northern South China Sea

The responses of surface wind stress to the mesoscale sea surface temperature (SST) anomalies associated with the SST front in the northern South China Sea (NSCS) are studied using satellite observations and reanalysis data. Both satellite and reanalysis data explicitly show the linear relationships between the spatial-high-pass filtered wind stress perturbation derivatives and the underlying SST gradient field. However, the noise in the linear relationships is much smaller in the reanalysis data than in the satellite observations. This result is rarely reported in other frontal areas.The wavelet analysis shows that the satellite scatterometer observed numerous high wavenumber perturbations within 100 km in the NSCS, but these perturbations were absent in the reanalysis data. The linear relationship between the perturbation SST gradient and derivative wind stress fields is not significant at this scale, which enhances the noise in the linear relationship. The spatial bandpass-filtered perturbation between 100 km and 300 km can give reasonable estimates of the coupling coefficients between the wind stress divergence and downwind SST gradient (αd) and between the wind stress curl and crosswind SST gradient (αc) in the NSCS, with values of 1.33 × 10−2 N m−2 per °C and 0.95 × 10−2 N m−2 per °C, respectively.

Effect of drag coefficient parameterizations on air-sea coupled simulations: a case study for typhoons Haima and Nida in 2016

Reasonable parameterization of air-sea momentum flux is important for the accuracy of ocean and atmosphere simulations, and in the numerical model, the parameterization of the air-sea momentum flux becomes a problem of parameterization of the sea surface wind stress drag coefficient (Cd). In this study, five kinds of typical Cd parameterization methods were assessed in the simulation of two typhoon cases, one of which was a super typhoon and another was a common severe typhoon, based on an atmosphere-wave-ocean coupled model. Based on the two case study, it was found that the typhoon path and minimum sea level pressure were not very sensitive to Cd parameterizations, though the spatial distribution of Cd and its variation with wind speed were all very different across the parameterization methods. However, Cd has a significant effect on the wind speed, and at high wind speed, the simulated maximum wind speed compared better with the observation in the experiment which adopted the Cd calculation method considering the effects of sea spray. Also, Cd plays an important role in the feedback processes between atmosphere and ocean during the typhoon process, through its effect on the air-sea heat and momentum flux, SST, ocean mixed layer depth, ocean currents etc. The results of this study answered the question of how the Cd affects the atmosphere and ocean during the typhoon process, and to what extent they are affected, which can help to explain or even further improve the simulation results.

Horizontal Gravity in Ocean Ekman transport

Three-dimensional gravity vector field g (= igλ+jgφ+kgz) in geodesy has been greatly simplified to a uniform vertical vector (-g0k) in oceanography with (λ, φ, z) the (longitude, latitude, height), (i, j, k) the corresponding unit vectors, and g0 = 9.81 m/s2. Recent studies by the author show such simplification incorrect. The horizontal gravity is important in ocean dynamics. Along the same path, the horizontal gravity is included into the classical Ekman layer dynamics with constant eddy viscosity and depth-dependent-only density ρ(z) represented by an e-folding near-inertial buoyancy frequency. A new Ekman spiral and in turn a new formula for the Ekman transport are obtained. With the horizontal gravity data from the global static gravity model EIGEN-6C4 and the surface wind stress data from the Comprehensive Ocean-Atmosphere Data Set (COADS), the Ekman transport due to the horizontal gravity is crucial and cannot be neglected.

High-Frequency Sea-Level Variations driven by Self-Attraction and Loading from Atmospheric Surface Pressure at the Continents

<p>Surface mass anomalies on Earth modify the external gravity field via both Newtonian attraction and elastic deformation of the underlying crust. Time-variable mass transport divergence leading to quickly changing surface mass distributions induces additional horizontal pressure gradients that feed back into the dynamics of the transport process. In view of the present-day accuracy of geodetic observations, this feedback is well known to be important for global ocean tide modelling (Ray, 1998). The same feedback, however, is also affecting the barotropic response of the global oceans to surface wind stress and atmospheric pressure loading. It is typically termed as "Self Attraction and Loading" and can be seen as one contribution to sea-level variability induced by "Gravity, Rotation, and Deformation (GRD)" as defined by Gregory et al. (2019).</p><p>In this presentation, we will specifically discuss the contribution to sea-level variability induced by surface pressure variations over the continents, which are by now often ignored in numerical ocean modelling. Induced ocean bottom pressure signals are specifically prominent at the shortest periods between hours and days, and frequently exceed 1 hPa in coastal regions. The signals are found to be relevant for the satellite gravimetry missions GRACE and GRACE-FO, and the process will be therefore included in the next release of the AOD1B non-tidal de-aliasing product.  </p>

Overturning response to a surface wind stress doubling in an eddying and a non-eddying ocean

In this paper, the overturning responses to wind stress changes of an eddying and a non-eddying ocean are compared. Differences are found in the deep overturning cell in the low-latitude North Atlantic with substantial implications for the deep western boundary current (DWBC). In an ocean-only twin experiment with one eddying and one non-eddying configuration of the MPI ocean model, two different forcings are being applied: the standard NCEP forcing and the NCEP forcing with 2x surface wind stress. The response to the wind stress doubling in the Atlantic meridional overturning circulation is similar in the eddying and the non-eddying configuration, showing an increase by about 4 Sv (~25%, 1 Sv = 106 m3s−1). In contrast, the DWBC responds with a speedup in the non-eddying and a slowdown in the eddying configuration. This paper demonstrates that the DWBC slowdown in the eddying configuration is largely balanced by eddy vorticity fluxes. Because those fluxes are not resolved and also not captured by an eddy parameterization in the non-eddying configuration, such a DWBC slowdown is likely not to occur in non-eddying ocean models which therefore might not capture the whole range of overturning responses. Furthermore, evidence is provided that the balancing effect of the eddies is not a passive reaction to a remotely triggered DWBC slowdown. Instead, deep eddies which are sourced from the upper ocean provide an excess input of relative vorticity which then actively forces the DWBC mean flow to slow down.

The impact of topography and eddy parameterization on the simulated Southern Ocean circulation response to changes in surface wind stress

It remains uncertain how the Southern Ocean circulation responds to changes in surface wind stress, and whether coarse resolution simulations, where meso-scale eddy fluxes are parameterized, can adequately capture the response. We address this problem using two idealized model setups mimicking the Southern Ocean: a flat bottom channel, and a channel with moderately complex topography. Under each topographic configuration and varying wind stress, we compare several coarse resolution simulations, configured with different eddy parameterizations, against an eddy-resolving simulation. We find that: (1) without topography, sensitivity of the Antarctic Circumpolar Current (ACC) to wind stress is overestimated by coarse resolution simulations, due to an underestimate of the sensitivity of the eddy diffusivity; (2) in the presence of topography, stationary eddies dominate over transient eddies in counteracting the direct response of the ACC and overturning circulation to wind stress changes; (3) coarse resolution simulations with parameterized eddies capture this counteracting effect reasonably well, largely due to their ability to resolve stationary eddies. Our results highlight the importance of topography in modulating the response of the Southern Ocean circulation to changes in surface wind stress. The interaction between meso-scale eddies and stationary meanders induced by topography requires more attention in future development and testing of eddy parameterizations.

The thermocline biases in the tropical North Pacific and their attributions

The tropical thermocline plays an important role in regulating equatorial sea surface temperature (SST); at present, it is still poorly simulated in the state-of-the-art climate models. In this paper, thermocline biases in the tropical North Pacific are investigated using the newly released CMIP6 historical simulations. It is found that CMIP6 models tend to produce an overly shallow thermocline in the northwestern tropics, accompanied by a deep thermocline in the northeastern tropics. A pronounced thermocline strength bias arises in the tropical northeastern Pacific, demonstrating a dipole structure with a sign change at about 8° N. These thermocline biases are accompanied with biases in the simulations of oceanic circulations, including a too weak North Equatorial Counter Current (NECC), a reduction in water exchanges between the subtropics and the equatorial regions, and an eastward extension of the equatorward interior water transport. The causes of these thermocline biases are further analyzed. The thermocline bias is primarily caused by the model deficiency in simulating the surface wind stress curl, which can be further attributed to the longstanding double-ITCZ bias in the tropical North Pacific. Besides, thermocline strength bias can be partly attributed to the poor prescription of oceanic background diffusivity. By constraining the diffusivity to match observations, the thermocline strength in the tropical northeastern Pacific is greatly increased.

Instability Processes in Simulated Finite-Width Ocean Fronts

A simple, isolated front is modeled using a turbulence resolving, large-eddy simulation (LES) to examine the generation of instabilities and inertial oscillations by surface fluxes. Both surface cooling and surface wind stress are considered. Coherent roll instabilities with 200–300-m horizontal scale form rapidly within the front after the onset of surface forcing. With weak surface cooling and no wind, the roll axis aligns with the front, yielding results that are equivalent to previous constant gradient symmetric instability cases. After ~1 day, the symmetric modes transform into baroclinic mixed modes with an off-axis orientation. Traditional baroclinic instability develops by day 2 and thereafter dominates the overall circulation. Addition of destabilizing wind forcing produces a similar behavior, but with off-axis symmetric-Ekman shear modes at the onset of instability. In all cases, imbalance of the geostrophic shear by vertical mixing leads to an inertial oscillation in the frontal currents. Analysis of the energy budget indicates an exchange between kinetic energy linked to the inertial currents and potential energy associated with restratification as the front oscillates in response to the vertically sheared inertial current. Inertial kinetic energy decreases from enhanced mixed layer turbulence dissipation and vertical propagation of inertial wave energy into the pycnocline.