Ocean mixed layer processes in the Pacific Decadal Oscillation in coupled general circulation models (original) (raw)
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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.
Unforced decadal fluctuations in a coupled model of the atmosphere and ocean mixed layer
Journal of Geophysical Research, 1992
Global average temperature in a 100-year control run of a model used for greenhouse gas response simulations showed low-frequency natural variability comparable in magnitude to that observed over the last 100 years. The model variability was found to be barotropic in the atmosphere, and located in the tropical strip with largest values near the equator in the Pacific. The model variations were traced to complex, low-frequency interactions between the meridional sea surface temperature gradients in the eastern equatorial Pacific, clouds at both high and low levels, and features of the tropical atmospheric circulation. The variations in these and other model parameters appear to oscillate between two limiting climate states. The physical scenario accounting for the oscillations on decadal time scales is almost certainly not found in the real world on shorter time scales due to limited resolution and the omission of key physics (e.g., equatorial ocean dynamics) in the model. The real message is that models with dynamical limitations can still produce significant long-term variability. Only a thorough physical diagnosis of such simulations and comparisons with decadal-length data sets will allow one to decide if faith in the model results is, or is not, warranted. 112, 1524-1534, 1984.
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...
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•
Mixed layer depth variability and barrier layer formation over the North Pacific Ocean
Journal of Geophysical Research, 2000
Seasonal variability in the isothermal and isopycnal surface mixed layers of the North Pacific Ocean is examined using the Naval Research Laboratory Ocean Mixed Layer Depth (NMLD) Climatology. A comparison with observations from 11 ocean weather stations in the northeast Pacific Ocean is performed that validates the NMLD climatology in this region. The general features of the isothermal layer depth (ILD) and mixed layer depth (MLD) obtained from these mixed layers are explained with wind stress, surface net heat flux, and freshwater flux climatologies, given guidance from a mixed layer model. Departures from a surface-forced interpretation of turbulent mixing are found near the Kuroshio, where horizontal heat transport is important. The much deeper ILD in the northeast Pacific in winter and spring relative to the MLD reveals a 50 m "barrier layer" between the bottom of the MLD and the top of the thermocline. A detailed analysis shows this barrier layer extends over most of the North Pacific subpolar gyre. It forms when the seasonal thermocline is deepened in winter by surface cooling, such that salinity stratification due to evaporation minus precipitation less than zero (E-P • 0) becomes important in the formation of the MLD. A shallower halocline forms over the subpolar gyre than in other regions of the North Pacific because of precipitation dominating over evaporation in the annual mean. A mechanism for maintaining the shallow halocline is provided by upward vertical motion driven by positive wind stress curl in the presence of diapycnal mixing. Numerical models show this as part of a shallow meridional overturning cell. stratification play a role? 16,783 16,784 KARA ET AL.: N. PACIFIC MLD VARIABILITY AND BL FORMATION Studies of the equatorial Pacific have revealed that surface freshwater fluxes (i.e., heavy rainfall with the effect of horizontal advection) and strong wind bursts are responsible for the formation of the equatorial barrier layer [Ando and McPhaden, 1997; Vialard and Delecluse, 1998; Godfrey and Lindstrom, 1989]. As described by Anderson et al. [1996], this barrier layer formation occurs because strong wind bursts deepen the surface mixed layer to the top of the thermocline and precipitation and surface heating increase the surface buoyancy, forming a relatively warm and fresh thin surface mixed layer. Are similar or different processes responsible for the North Pacific barrier layer at midlatitudes? Direct surface ventilation of the upper layers of the North Pacific is known to be quite shallow because of the relatively low density of the surface waters in winter and the presence of saline deep waters [e.g., Yuan and Talley, 1992]. Tsuchiya [1982] showed that the shallow salinity minima of the North Pacific can be related to the subduction (possibly when ILD < MLD) of surface waters. The formation of the North Pacific barrier layer can be answered by understanding the reasons for the seasonal variability in the ILD and MLD. Henceforth we shall use LD to denote ILD and MLD whenever the latter can be commonly referred to in the given context. A basin-scale study has been done on the seasonal changes in surface LD for the Pacific Ocean [Bathen, 1972], using an ILD definition applied to a monthly mean climatology constructed from observations. In that study the depth of mixing is defined using a temperature definition with a prescribed temperature gradient of 0.02 øC m -• because at the time the salinity observations needed were much less common for large regions of the world's oceans. With the present availability of higher-resolution global ocean climatologies for temperature [Levitus et al., 1994], salinity [Levitus and Boyer, 1994], wind stress, heat flux, and freshwater flux [e.g., da Silva et al., 1994], not only is investigating seasonal variability of isothermal and isopycnal mixed layers possible, but so is interpreting them in terms of surface-forced turbulent mixing [Price et al., 1986; Gordon and Corry, 1991], which can provide insight into the physical processes that are responsible for the formation of the winter barrier layer in the North Pacific. Given the known sensitivity of the LD to the criteria used to define them [Kava et al., this issue], be it a property gradient definition or a property change definition, appropriate care must be taken to apply an optimal definition. The major circulation systems in the North Pacific [e.g., Talley, 1993; Hurlbutt et al., 1996; $hriver and Hurlbutt, 1997] are found to influence the seasonal variability in LD in certain regions of the basin [Qiu and Joyce, 1995]. The surface circulation in the Pacific consists of the cyclonic subpolar gyve in the north, the anticyclonic North Pacific subtropical gyve, and the north Equatorial Counter current near the equator [e.g., Tal-ley, 1993]. Roden [1979] explained that a correlation exists between the positions of the North Pacific Current and the MLD variations. Monterey and Levitus [1997] noted a few small regions in the North Pacific where ILD is shallower than MLD, occurring inside the subtropical gyres during March and April. Differential radiative heating can sharpen or weaken existing temperature fronts in the North Pacific because radiative heat fluxes are effective in changing the temperature of the upper layer and in altering the hydrostatic stability [e.g., Roden, 1980; Dinniman and Rienecker, 1999]. Horizontal heat transport by advection and diffusion can alter local heat balances in the North Pacific [Gent, 1991; Qiu and Kelly, 1993], which in turn, diminish or enhance the KARA ET AL.: N. PACIFIC MLD VARIABILITY AND BL FORMATION 16,785 16,788 KARA ET AL.: N. PACIFIC MLD VARIABILITY AND BL FORMATION At midlatitudes, mean depths of more than 250 m occur for hoe(T) in the 40ø-55øN latitude band, while hoe(ert) reaches maximum depths of 175-225 m in the western mid-Pacific east of Japan. The deep winter structure
ICES Journal of Marine Science, 2011
Jang, C. J., Park, J., Park, T., and Yoo, S. 2011. Response of the ocean mixed layer depth to global warming and its impact on primary production: a case for the North Pacific Ocean. – ICES Journal of Marine Science, 68: 996–1007. This study investigates changes in the mixed layer depth (MLD) in the North Pacific Ocean in response to global warming and their impact on primary production by comparing outputs from 11 models of the coupled model intercomparison projects phase 3. The MLD in the 21st century decreases in most regions of the North Pacific, whereas the spatial pattern of the MLD is nearly unchanged. The overall shoaling results in part from intensified upper-ocean stratification caused by both surface warming and freshening. A significant MLD decrease (>30 m) is found in the Kuroshio extension (KE), which is predominantly driven by reduced surface cooling, caused by weakening of wind. Associated with the mixed layer shoaling in the KE, the primary production component r...
Journal of Climate, 2010
The mixed layer heat budget in the tropical Pacific is diagnosed using pentad (5 day) averaged outputs from the Global Ocean Data Assimilation System (GODAS), which is operational at the National Centers for Environmental Prediction (NCEP). The GODAS is currently used by the NCEP Climate Prediction Center (CPC) to monitor and to understand El Niño and La Niña in near real time. The purpose of this study is to assess the feasibility of using an operational ocean data assimilation system to understand SST variability. The climatological mean and seasonal cycle of mixed layer heat budgets derived from GODAS agree reasonably well with previous observational and model-based estimates. However, significant differences and biases were noticed. Large biases were found in GODAS zonal and meridional currents, which contributed to biases in the annual cycle of zonal and meridional advective heat fluxes. The warming due to tropical instability waves in boreal fall is severely underestimated owi...