An Assessment of the Southern Ocean Mixed Layer Heat Budget (original) (raw)
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Journal Of Geophysical Research: Oceans, 2018
Oceanic and atmospheric processes were investigated in order to explore the causes of seasonal and interannual variability of sea surface temperatures (SST) in the western tropical Atlantic (WTA; 208S-208N, 158W-608W). A mixed-layer (ML) heat budget was performed by using Argo profiles and supplementary data sets based on satellite and atmospheric products during the period 2007-2012. The WTA is divided into four boxes which represent the main temporal and spatial heterogeneities of this region. An analysis of error of each term pointed out that the mean net surface heat fluxes are systematically underestimated by 20 W m 22. A correction of this term provides realistic estimates of the vertical mixing which was obtained as residual term. In agreement with previous studies, the results show that surface flux is the most important process that governs the seasonal cycle of the heat content. Changes in shortwave radiation and latent heat fluxes dictate the oceanic response to the meridional migration of the ITCZ. Along the equator, surface fluxes modulate the annual cycle of ML temperature, but are strongly balanced by horizontal advection. The entrainment term proves a small contribution to the cooling of the ML. On an interannual time scale, the strong positive (negative) SST anomalies observed in 2010 (2012) were generated during the previous winter in both years, mainly north of 108N, during which the wind anomalies were at the origin of intense heat loss anomalies. Horizontal advection may contributes to the maintaining of these SST anomalies in the equatorial zone and south Atlantic.
The diurnal mixed layer and upper ocean heat budget in the western equatorial Pacific
Journal of Geophysical Research, 1995
This paper presents the results of an experiment in the western equatorial Pacific centered on the equator at 165øE which was designed to study the changes to the structure of the upper ocean on timescales of a few days and spatial scales of tens of kilometers. The results show that the response of the upper ocean to atmospheric forcing is very sensitive to the vertical structure of both the temperature and salinity. The diurnal response of the near-surface temperature to daytime heating and nighttime cooling was found to have an amplitude of a few tenths of a degree Celsius. This compares with a horizontal variation of temperature on scales of a few tens of kilometers of a similar magnitude. Even away from the very fresh surface layers typical of the area, salinity is found to play an important role in limiting the depth of nighttime mixing. In this case a subsurface salinity maximum restricts the depth to around 40 m. The nighttime convection is severely limited by either a small change in the surface forcing or the horizontal advection of slightly cooler waters from the east; we are unable to determine which is the dominant mechanism in the present case. The reduced mixing leads to an increase of the diurnal variation of sea surface temperature to over 1øC. The estimated net surface heat flux from the atmosphere to the ocean was found to be not significantly differeni from zero at 10 W m -e in agreement with recent measurements. The net surface heat flux during the period of the heat budget experiment, which took place on the equator, was substantially higher at 65 W m -e. Changes of in situ temperature are found to be dominated by advection. The vertical velocity is estimated to be of order 10 m d -• and to be caused by advection along east-west sloping density surfaces. Changes to the temperature structure of the upper ocean induced by motions with a timescale of a few days (possibly planetary waves) are found to be significantly greater than longer-term wind-induced upwelling or advection. SSTs of the warm pool of around 0.5øC have been observed to occur prior to ENSO events [Hanawa el al., Paper number 94JC03228. 0148-0227/95/94 J C-03228505.00 1988]. On a shorter timescale the structure of the upper ocean changes dramatically during strong westerly wind events [Lukas and Lindslrom, 1991; McPhaden el al, 1992] and such events can trigger the production of an equatorial Kelvin wave that traverses the width of the Pacific [McPhaden el al., 1988]. Small changes in the SST of the warm pool (again of around 0.5øC) are also known to affect the global atmospheric circulation [e.g., Hoskins and Karoly, 198i; Palmer and Mansfield, 1984, 1986]. As pointed out by Godfrey and Lindstrom [1989], a temperature change of 0.5øC of a mixed iayer 50 m deep in 3 months requires a net heating of around 10 W m -2, putting a strong constraint on the accuracy of observed heat fluxes. The problem in obtaining reliable estimates of the net heat flux over large time and space scales has been highlighted by recent measurements. Indirect estimates of the net surface heat flux into the ocean by Godfrey and Lindstrom [1989] suggest that it is below 20 W m -2, which is in accord with the suggestion of Priestley [1966] and Newell [1986] that the net flux is 6865 ,•1•
Quantifying the Role of Ocean Dynamics in Ocean Mixed Layer Temperature Variability
Journal of Climate, 2021
Understanding the role of the ocean in climate variability requires first understanding the role of ocean dynamics in the ocean mixed layer and thus sea surface temperature variability. However, key aspects of the spatially and temporally varying contributions of ocean dynamics to such variability remain unclear. Here, the authors quantify the contributions of ocean dynamical processes to mixed layer temperature variability on monthly to multiannual time scales across the globe. To do so, they use two complementary but distinct methods: 1) a method in which ocean heat transport is estimated directly from a state-of-the-art ocean state estimate spanning 1992–2015 and 2) a method in which it is estimated indirectly from observations between 1980–2017 and the energy budget of the mixed layer. The results extend previous studies by providing quantitative estimates of the role of ocean dynamics in mixed layer temperature variability throughout the globe, across a range of time scales, in...
Climate Dynamics, 2009
We analyze the processes responsible for the generation and evolution of sea-surface temperature anomalies observed in the Southern Ocean during a decade based on a 2D diagnostic mixed-layer model in which geostrophic advection is prescribed from altimetry. Anomalous air-sea heat flux is the dominant term of the heat budget over most of the domain, while anomalous Ekman heat fluxes account for 20-40% of the variance in the latitude band 40°-60°S. In the ACC pathway, lateral fluxes of heat associated with anomalous geostrophic currents are a major contributor, dominating downstream of several topographic features, reflecting the influence of eddies and frontal migrations. A significant fraction of the variability of large-scale SST anomalies is correlated with either ENSO or the SAM, each mode contributing roughly equally. The relation between the heat budget terms and these climate modes is investigated, showing in particular that anomalous Ekman and air-sea heat fluxes have a cooperating effect (with regional exceptions), hence the large SST response associated with each mode. It is further shown that ENSO-or SAM-locked anomalous geostrophic currents generate substantial heat fluxes in all three basins with magnitude comparable with that of atmospheric forcings for ENSO, and smaller for the SAM except for limited areas. ENSO-locked forcings generate SST anomalies along the ACC pathway, and advection by mean flows is found to be a non-negligible contribution to the heat budget, exhibiting a wavenumber two zonal structure, characteristic of the Antarctic Circumpolar Wave. By contrast SAM-related forcings are predominantly zonally uniform along the ACC, hence smaller zonal SST gradients and a lesser role of mean advection, except in the SouthWest Atlantic. While modeled SST anomalies are significantly correlated with observations over most of the Southern Ocean, the analysis of the data-model discrepancies suggests that vertical ocean physics may play a significant role in the nonseasonal heat budget, especially in some key regions for mode water formation.
Geophysical Research Letters, 2018
The Ocean Observatories Initiative air‐sea flux mooring deployed at 54.08°S, 89.67°W, in the southeast Pacific sector of the Southern Ocean, is the farthest south long‐term open ocean flux mooring ever deployed. Mooring observations (February 2015 to August 2017) provide the first in situ quantification of annual net air‐sea heat exchange from one of the prime Subantarctic Mode Water formation regions. Episodic turbulent heat loss events (reaching a daily mean net flux of −294 W/m2) generally occur when northeastward winds bring relatively cold, dry air to the mooring location, leading to large air‐sea temperature and humidity differences. Wintertime heat loss events promote deep mixed layer formation that lead to Subantarctic Mode Water formation. However, these processes have strong interannual variability; a higher frequency of 2 σ and 3 σ turbulent heat loss events in winter 2015 led to deep mixed layers (>300 m), which were nonexistent in winter 2016.
Seasonal mixed layer heat budget of the tropical Atlantic Ocean
Journal of Geophysical Research, 2003
This paper addresses the atmospheric and oceanic causes of the seasonal cycle of sea surface temperature (SST) in the tropical Atlantic based on direct observations. Data sets include up to four years (September 1997 -February 2002 of measurements from moored buoys of the Pilot Research Array in the Tropical Atlantic (PIRATA), near-surface drifting buoys, and a blended satellite-in situ SST product. We analyze the mixed layer heat balance at eight PIRATA mooring locations and find that the seasonal cycles of latent heat loss and absorbed shortwave radiation are responsible for seasonal SST variability in the northwest basin (8 -15ºN along 38ºW). Along the equator (10ºW -35ºW) contributions from latent heat loss are diminished, while horizontal temperature advection and vertical entrainment contribute significantly. Zonal temperature advection is especially important during boreal summer near the western edge of the cold tongue, while horizontal eddy temperature advection, which most likely results from tropical instability waves, opposes temperature advection by the mean flow. The dominant balance in the southeast (6 -10ºS along 10ºW) is similar to that in the northwest, with both latent heat loss and absorbed solar radiation playing important roles.
Comparison of bulk sea surface and mixed layer temperatures
Journal of Geophysical Research, 2008
1] Mixed layer temperature (MLT) and bulk sea surface temperature (SST) are frequently used interchangeably or assumed to be proportional in climate studies. This study examines historical analyses of bulk SST and MLT from contemporaneous ocean profile observations during 1960-2007, looking for systematic differences between these variables. The results show that globally and time-averaged MLT is cooler than SST by approximately 0.1°C. MLT is cooler than SST in upwelling zones where abundant net surface warming is compensated for by cooling across the base of the mixed layer. In the upwelling zone of the equatorial eastern Pacific this negative MLT-SST difference varies out of phase with seasonal SST, reaching a negative extreme of À0.8°C in boreal spring when SST is warm, solar radiation is high, and winds are weak. In contrast, on interannual timescales MLT-SST varies in phase with SST with small differences during El Niños as a result of low solar heating and enhanced rainfall and with large differences, approaching À0.8°C, during La Niñas. On shorter diurnal timescales, during El Niños, MLT-SST differences associated with temperature inversions occur in response to nocturnal cooling in the presence of near-surface freshening. Near-surface freshening produces persistent shallow (a few meters depth) warm layers in the northwestern Pacific during boreal summer when solar heating is strong. In contrast, shallow cool layers occur in the Gulf Stream area of the northwest Atlantic in boreal winter when fresh surface layers caused by lateral exchange are cooled by abundant turbulent heat loss. The different impacts of shallow barrier layers on near-surface temperature gradients in these different dynamical regimes are explored with a one-dimensional mixed layer model. Citation: Grodsky, S. A., J. A. Carton, and H. Liu (2008), Comparison of bulk sea surface and mixed layer temperatures,
Journal of Climate, 2007
A global Ocean General Circulation Model (OGCM) is used to investigate the mixed layer heat budget of the Northern Indian Ocean (NIO). The model is validated against observations and shows a fairly good agreement with mixed layer depth data in the NIO. The NIO has been separated into three sub-basins: the western Arabian Sea (AS), the eastern AS, and the Bay of Bengal (BoB). This study reveals strong differences between the western and eastern AS heat budget, while the latter basin has similarities with the BoB. Interesting new results on seasonal timescales are shown. Penetration of solar heat flux needs to be taken into account for two reasons. First, an average of 28 W m ¤ is lost beneath the mixed layer over the year. Second, the penetration of solar heat flux tends to reduce the effect of solar heat flux on the SST seasonal cycle in the AS because seasons of strongest flux are also seasons of thin mixed layer. This enhances the control of SST seasonal variability by latent heat flux. Impact of salinity on SST variability is demonstrated. Salinity stratification plays a clear role in maintaining a high winter SST in the BoB and eastern AS while not in the western AS. The presence of fresh water near the surface allows to store heat below the surface layer that can later be recovered by entrainment warming during winter cooling (with a winter contribution of +2.1 ¡ C in the BoB). On interannual timescale, the eastern AS and BoB are strongly controlled by the winds through the latent heat flux anomalies. In the western AS, vertical processes and also horizontal advection contribute significantly to SST interannual variability and the wind is not the only factor controlling the heat flux forcing.
Journal of Geophysical Research, 2000
A 50-year coupled atmosphere-ocean model integration is used to study sea surface temperature (SST) and mixed layer depth (h), and the processes which influence them. The model consists of an atmospheric general circulation model coupled to an ocean mixed layer model in ice-free regions. The midlatitude SST variability is simulated fairly well, although the maximum variance is underestimated and located farther south than observed. The model is clearly deficient in the vicinity of the Gulf Stream and in the eastern tropical Pacific where advective processes are important. The model generally reproduces the observed structure of the mean h in both March and September but underestimates it in the North Atlantic during winter.