Dynamics of sea-surface temperature anomalies in the Southern Ocean diagnosed from a 2D mixed-layer model (original) (raw)
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Climate Dynamics, 2022
The South Atlantic Convergence Zone (SACZ) is an atmospheric system occurring in austral summer on the South America continent and sometimes extending over the adjacent South Atlantic. It is characterized by a persistent and very large, northwest-southeast-oriented, cloud band. Its presence over the ocean causes sea surface cooling that some past studies indicated as being produced by a decrease of incoming solar heat flux induced by the extensive cloud cover. Here we investigate ocean-atmosphere interaction processes in the Southwestern Atlantic Ocean (SWA) during SACZ oceanic episodes, as well as the resulting modulations occurring in the oceanic mixed layer and their possible feedbacks on the marine atmospheric boundary layer. Our main interests and novel results are on verifying how the oceanic SACZ acts on dynamic and thermodynamic mechanisms and contributes to the sea surface thermal balance in that region. In our oceanic SACZ episodes simulations we confirm an ocean surface cooling. Model results indicate that surface atmospheric circulation and the presence of an extensive cloud cover band over the SWA promote sea surface cooling via a combined effect of dynamic and thermodynamic mechanisms, which are of the same order of magnitude. The sea surface temperature (SST) decreases in regions underneath oceanic SACZ positions, near Southeast Brazilian coast, in the South Brazil Bight (SBB) and offshore. This cooling is the result of a complex combination of factors caused by the decrease of solar shortwave radiation reaching the sea surface and the reduction of horizontal heat advection in the Brazil Current (BC) region. The weakened southward BC and adjacent offshore region heat advection seems to be associated with the surface atmospheric circulation caused by oceanic SACZ episodes, which rotate the surface wind and strengthen cyclonic oceanic mesoscale eddy. Another singular feature found in this study is the presence of an atmospheric cyclonic vortex Southwest of the SACZ (CVSS), both at the surface and aloft at 850 hPa near 24°S and 45°W. The CVSS induces an SST decrease southwestward from the SACZ position by inducing divergent Ekman transport and consequent offshore upwelling. This shows that the dynamical effects of atmospheric surface circulation associated with the oceanic SACZ are not restricted only to the region underneath the cloud band, but that they extend southwestward where the CVSS presence supports the oceanic SACZ convective activity and concomitantly modifies the ocean dynamics. Therefore, the changes produced in the oceanic dynamics by these SACZ events may be important to many areas of scientific and applied climate research. For example, episodes of oceanic SACZ may influence the pathways of pollutants as well as fish larvae dispersion in the region.
Role of the upper ocean structure in the response of ENSO-like SST variability to global warming
2010
The response of El Niño and Southern Oscillation (ENSO)-like variability to global warming varies comparatively between the two different climate system models, i.e., the Meteorological Research Institute (MRI) and Geophysical Fluid Dynamics Laboratory (GFDL) Coupled General Circulation Models (CGCMs). Here, we examine the role of the simulated upper ocean temperature structure in the different sensitivities of the simulated ENSO variability in the models based on the different level of CO 2 concentrations. In the MRI model, the sea surface temperature (SST) undergoes a rather drastic modification, namely a tendency toward a permanent El Niño-like state. This is associated with an enhanced stratification which results in greater ENSO amplitude for the MRI model. On the other hand, the ENSO simulated by GFDL model is hardly modified although the mean temperature in the near surface layer increases. In order to understand the associated mechanisms we carry out a vertical mode decomposition of the mean equatorial stratification and a simplified heat balance analysis using an intermediate tropical Pacific model tuned from the CGCM outputs. It is found that in the MRI model the increased stratification is associated with an enhancement of the zonal advective feedback and the non-linear advection. In the GFDL model, on the other hand, the thermocline variability and associated anomalous vertical advection are reduced in the eastern equatorial Pacific under global warming, which erodes the thermocline feedback and explains why the ENSO amplitude is reduced in a warmer climate in this model. It is suggested that change in stratification associated with global warming impacts the equatorial wave dynamics in a way that enhances the second baroclinic mode over the gravest one, which leads to the change in feedback processes in the CGCMs. Our results illustrate that the upper ocean vertical structure simulated in the CGCMs is a key parameter of the sensitivity of ENSO-like SST variability to global warming.
The Role of Eddies in the Southern Ocean Temperature Response to the Southern Annular Mode
The role of eddies in modulating the Southern Ocean response to the southern annular mode (SAM) is examined, using an ocean model run at multiple resolutions from coarse to eddy resolving. The high-resolution versions of the model show an increase in eddy kinetic energy that peaks 2-3 yr after a positive anomaly in the SAM index. Previous work has shown that the instantaneous temperature response to the SAM is characterized by predominant cooling south of 458S and warming to the north. At all resolutions the model captures this temperature response. This response is also evident in the coarse-resolution implementation of the model with no eddy mixing parameterization, showing that eddies do not play an important role in the instantaneous response. On the longer time scales, an intensification of the mesoscale eddy field occurs, which causes enhanced poleward heat flux and drives warming south of the oceanic Polar Front. This warming is of greater magnitude and occurs for a longer period than the initial cooling response. The results demonstrate that this warming is surface intensified and strongest in the mixed layer. Non-eddy-resolving models are unable to capture the delayed eddy-driven temperature response to the SAM. The authors therefore question the ability of coarse-resolution models, such as those commonly used in climate simulations, to accurately represent the full impacts of the SAM on the Southern Ocean.
Ocean Dynamics, Thermocline Adjustment, and Regulation of Tropical SST
Journal of Climate, 1997
The role of tropical Pacific ocean dynamics in regulating the ocean response to thermodynamic forcing is investigated using an ocean general circulation model (GCM) coupled to a model of the atmospheric mixed layer. It is found that the basin mean sea surface temperature (SST) change is less in the presence of varying ocean heat transport than would be the case if the forcing was everywhere balanced by an equivalent change in the surface heat flux. This occurs because the thermal forcing in the eastern equatorial Pacific is partially compensated by an increase in heat flux divergence associated with the equatorial upwelling. This constitutes a validation of a previously identified ''ocean dynamical thermostat.'' A simple two-box model of subtropical-equatorial interaction shows that the SST regulation mechanism crucially depends on spatial variation in the sensitivity of the surface fluxes to SST perturbations. In the GCM, this sensitivity increases with latitude, largely a result of the wind speed dependence of the latent heat flux, so that a uniform forcing can be balanced by a smaller SST change in the subtropics than in equatorial latitudes. The tropical ocean circulation moves heat to where the ocean more readily loses it to the atmosphere. Water that subducts in subtropical latitudes and returns to the equatorial thermocline therefore has a smaller temperature perturbation than the surface equatorial waters. The thermocline temperature adjusts on timescales of decades to the imposed forcing, but the adjustment is insufficient to cancel the thermostat mechanism. The results imply that an increase in the downward heat flux at the ocean surface, as happens with increasing concentrations of greenhouse gases, should be accompanied by a stronger equatorial SST gradient. This contradicts the results of coupled atmosphere-ocean GCMs. Various explanations are offered. None are conclusive, but the possibility that the discrepancy lies in the low resolution of the ocean GCMs typically used in the study of climate change is discussed.
A global ocean model (ORCA2) forced with 50 yr of NCEP-NCAR reanalysis winds and heat fluxes has been used to investigate the evolution and forcing of interannual dipolelike sea surface temperature (SST) variability in the South Indian and South Atlantic Oceans. Although such patterns may also exist at times in only one of these basins and not the other, only events where there are coherent signals in both basins during the austral summer have been chosen for study in this paper. A positive (negative) event occurs when there is a significant warm (cool) SST anomaly evident in the southwest of both the South Indian and South Atlantic Oceans and a cool (warm) anomaly in the eastern subtropics.
Interannual SST Variability in the Southern Subtropical and Extra- tropical Ocean
2006
The interannual variability of the sea surface temperature (SST) in the southern oceans shows an unrecognized ubiquitous feature. In contrast to their northern counterparts, active SST fluctuations occur in the open oceans in all three major basins, which are unattached to the coastal processes. Using historical SST observations for 1950-2000, it is shown that these extra-tropical/subtropical SST anomalies have a tilted southwest-northeast dipole pattern in both the Atlantic and Indian Oceans and, to a certain extent, in the central and eastern Pacific. SST fluctuations in all basins show similar seasonal enhancement in austral summer. A long-term simulation of a coupled ocean-atmosphere general circulation model reproduces some of these major features realistically, especially in the South Atlantic. A composite analysis of the objectively selected major events in the South Atlantic from the observations and the simulation shows that the anomalous SST pattern is initiated by mid-latitude atmospheric fluctuations. Through a coupled air-sea feedback, the center of the subtropical branch of the SST anomalies can shift towards the tropics in the next season.
Inversion for the heat anomaly transport from the SST time series in the Northwestern Pacific
Journal of Geophysical Research Atmospheres
We describe a heat anomaly transport in the upper ocean mixed layer in the Kuroshio extension region and the subtropical gyre of the northwest Pacific. Emphasis is on behavior in the cool season (December-March) during the Asian Winter Monsoon. The heat anomaly transport is estimated by applying an inversion technique to the stochastic partial differential equation for the heat anomaly balance of advection, diffusion, stabilizing feedback, and atmospheric forcing. The inversion consists of (1) derivation of statistical parametric model from the heat anomaly balance equation; (2) fitting the derived statistical model to the sea surface temperature (SST) anomaly covariances; and (3) calculation of the heat anomaly net advection velocity, horizontal diffusion coefficient, feedback factor and atmospheric forcing correlation from the parameters of the evaluated statistical model. The inversion was applied to the Comprehensive Ocean-Atmosphere Data Set Compressed Marine Reports SST dam, averaged at 1 ø latitude x 2' longitude boxes on a 10-day mean basis from 1965 to 1990. The estimates of the net advection velocity are consistent in magnitude and direction with the general circulation in the surface layer of the Northwest Pacific in winter. SST anomalies are transported to the west at -0.15 m s -• in the northern part of the North Equatorial Current. Between 21 ø and 29øN in the recirculating region, SST anomalies propagate westward with the mean velocity less than 0.1 m s '•. South and east of Honshu the observed pattern of the SST anomaly transport agrees broadly with the circulations of the Kuroshio current and its extension and the Oyashio current. South of Honshu, the eastward transport is about 200-300 km wide; its absolute velocity is up to 0.2 m s -•. One branch of the transport separates from the coast near the large meander path of the Kuroshio current and follows the east-southeast direction. The second separation from the coast occurs south of Hokkaido. Over the analysis domain the estimates of the diffusion coefficient are in the range of 3x 103 to 6x 103 m 2 s -•. The higher values of the diffusion coefficient confirm the enhancement of the mesoscale eddy processes near the subtropical convergence zone. The analysis supports Hasselmann's (1976) theory in which generation of midlatitude SST anomalies lasting the dominant timescale of atmospheric processes is primarily attributed to the short period stochastic weather forcing. However, the analysis indicates that the inertia of SST anomalies to their "memory" of earlier winds can not be neglected in the vicinity of the western boundary and in the tropics. 1. Introduction Since the late 1950s, sea surface temperature (SST) anomalies have been regarded as one of the key elements of climate variations [Bjerknes, 1959; Namias, 1959]. During the past three decades many publications have described the generation and evolution of SST anomalies (c.f. review of Frankignoul [1985]). They considered a heat budget of the top layer of the ocean, atmospheric forcing of the sea, feedbacks, and multiple timescale interactions in the coupled oceanatmosphere system. The concept of the uniform mixed layer [Kraus and Turner, 1967, Niiler and Kraus, 1977] played a major role in the formulation of a model for the upper sea heat anomaly balance. Statistical studies of the global SST Copyright 1995 by the American Geophysical Union. Paper number 94JC03041. 0148-0227/95/94JC-03041 $05.00 anomaly variability in terms of empirical orthogonal functions were originated by Davis [1976]. Numerical simulations of SST anomalies with ocean general circulation models were initiated by Haney et al. [1978]. At the beginning of the 1980s it was generally accepted that the SST anomaly behavior differs between the tropics and midlatitudes. The tropical SST anomalies were suggested to be generated by the large-scale ocean-atmosphere feedback processes [see Philander, 1990]. White et al. [1985] and Pazan et al. [1986] examined heat content redistribution in the tropical western Pacific during E1 Nifio-Southern Oscillation events. The heat content redistribution was shown to be associated with wind-driven baroclinic Rossby and Kelvin wave activity. Recent numerical experiments indicate that there are at least two classes of ocean-atmosphere modes in tropics. In the first class of modes, SST and surface wind variations can be in phase, but other oceanic parameters, for example, thermocline depth variations, have a phase lag that represents the inertia of the ocean and its "memory" of earlier 4845 4846 OSTROVSKII AND PITERBARG: HEAT ANOMALY TRANSPORT IN NW PACIFIC winds [Philander et al., 1992]. In numerical models that capture this class of modes the ocean response to the wind is of the "delayed oscillator" type, and the simulated Southern Oscillation can be made irregular by introducing highfrequency modes such as atmospheric "weather" forcing. The second class of ocean-atmosphere modes in the tropics is characterized by phase differences between SST and surface wind fluctuations [Lau et al., 1992].
On the persistence of cold-season SST anomalies associated with the annular modes
2011
In this study, a simple stochastic climate model is used to examine the impact of the ocean mixed layer depth, surface turbulent energy fluxes, and Ekman currents on the persistence of cold-season extratropical sea surface temperature (SST) anomalies associated with variability in the annular modes of atmospheric circulation in both hemispheres. Observational analysis reveals that during the cold season, SST anomalies associated with the southern annular mode (SST SAM) persist considerably longer than those associated with the northern annular mode (SST NAM). Using the simple model, it is shown that the persistence of the cold-season SST SAM is consistent with the simple stochastic climate paradigm in which the atmospheric forcing is approximated as white noise, and the persistence of SST anomalies can be largely determined by the thermal inertia of the ocean mixed layer. In the North Atlantic, however, the simple climate model overestimates the persistence of the cold-season SST NAM. It is thought that this overestimate occurs because the NAM-related heat flux forcing cannot be described purely as white noise but must also include a feedback from the underlying SST anomalies.