Subantarctic Mode Water (SAMW) in the Pacific forms in two distinct pools in the south central and southeast Pacific, which subduct into the ocean interior and impact global storage of heat and ...carbon. Wintertime thickness of the central and eastern SAMW pools vary predominantly out of phase with each other, by up to ±150 m between years, resulting in an interannual thickness see‐saw. The thickness in the eastern (central) pool is found to be strongly positively (negatively) correlated with both the Southern Annular Mode (SAM) and El Niño–Southern Oscillation (ENSO). The relative phases of the SAM and ENSO set the SAMW thickness, with in phase reinforcing modes in 2005–2008 and 2012–2017 driving strong differences between the pools. Between 2008 and 2012 out of phase atmospheric modes result in less coherent SAMW patterns. SAMW thickness is dominated by local formation driven by SAM and ENSO modulated wind stress and turbulent heat fluxes.
Plain Language Summary
The Southern Ocean around Antarctica is a dominant pathway for moving heat and carbon from the atmosphere into the ocean interior, trapping it for hundreds of years. Most of this uptake is achieved through the formation of “mode waters”, homogeneous layers of water several hundred meters thick, by sinking and overturning as surface waters cool in winter. We find that two distinct pools of mode water in the South Pacific vary dramatically in winter thickness and volume from year to year. They vary in opposition to one another; when one is thicker than normal the other is thinner, with the pattern reversing after a year or so. We show that this “see‐saw” in thickness is strongest when the two main atmospheric patterns of climate variability over the Southern Ocean are reinforcing one another and weaken when they oppose one another. The combination of these patterns of atmospheric variability sets local mode water thickness via surface winds and ocean heat loss. The discovery of such strong dependence of mode water heat content on these atmospheric patterns is important for climate. Atmospheric variability is predicted to change into the future, potentially impacting heat uptake by mode waters and influencing global surface temperatures.
Key Points
South Pacific Subantarctic Mode Water (SAMW) layers display large (±150 m) changes in thickness from year to year
Two distinct pools of South Pacific SAMW exist and their thicknesses see‐saw out of phase with one another
The phase and magnitude of variability are set by changes in wind stress and heat flux driven by the main atmospheric modes of variability
The processes governing the seasonal evolution of the oceanic mixed layer temperature (MLT) and salinity (MLS) in the Red Sea (RS) are analyzed using the outputs of a high‐resolution (1/100°) ocean ...general circulation model for 2001 to 2015, forced by a high‐resolution (5 km) regional atmospheric reanalysis. We quantify the roles of atmospheric forcing and the advective, diffusive, and entrainment processes in the seasonal variability of mixed layer (ML) properties by analyzing the closed and complete potential temperature and salinity budgets integrated over the ML depth. The seasonal evolution of the ML density is predominantly driven by the MLT, which is dominated by the air–sea heat exchange. The seasonal evolution of MLS is predominantly driven by the advection of fresher waters from the Gulf of Aden, whereas atmospheric forcing governs its gradual increase along the basin. The spatial distribution of strong mesoscale circulation and semipermanent eddies is imprinted on all processes, whereas advective fluxes tend to follow meandering currents around the periphery of mesoscale eddies. Entrainment processes affect the ML density through the reemergence of heat and salt stored below the ML. Entrainment is especially important in the northern parts of the RS, where increased salinity preconditions the upper layers for ML deepening and denser water formation.
Plain Language Summary
The oceanic mixed layer (ML) mediates the exchanges of properties between the atmosphere and the interior of the sea and is of paramount importance for the ocean's physicochemical and biological characteristics. The seasonal and spatial variabilities of the ML characteristics are driven by the interplay of atmospheric forcing and oceanic processes, such as advection, diffusion, and entrainment. This study examines the relative importance of these processes in governing the seasonal evolution of the mixed layer temperature (MLT) and salinity (MLS), and hence the mixed layer density in the Red Sea (RS), using a high‐resolution (∼1 km) ocean general circulation model simulation for 2001 to 2015. These processes play important roles, which strongly vary spatially and seasonally, in a manner that can be different for MLT and MLS. The seasonal evolution of the ML density is largely driven by atmospheric forcing, dominated by the air–sea heat exchange, primarily opposed by advection via the upper‐layer circulation. In addition, ML properties are significantly influenced by the entrainment of waters below the ML, providing memory for the system because the heat and salt stored in previous periods can reemerge and influence the current properties.
Key Points
Heat fluxes dominate the development of the mixed layers (MLs) in the Red Sea, whereas salinity preconditions denser water formation
Advective fluxes dominate the restratification processes in spring, primarily through their effect on salinity
Entrainment plays an important role in the evolution of ML density throughout the Red Sea
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.
Plain Language Summary
We studied how the air and ocean exchange heat in the Southern Ocean (the ocean that surrounds Antarctica). Harsh weather conditions in the Southern Ocean make it hard to directly observe; therefore, not much is known about how the air and ocean interact in this region. We used data from a mooring (instrument‐laden buoy on the ocean surface anchored to the ocean floor) installed by Ocean Observatories Initiative in February 2015 off the west coast of Chile. The data from this mooring are important because they are the farthest south instrument ever deployed for multiple years that can study how the air and ocean interact. The mooring data shows that storms bring cold, dry air, and high winds (blowing to the northeast), causing the ocean to rapidly lose heat. This heat loss makes the surface ocean more dense than the water below, forcing the deeper water to mix with the surface water. This results in a thick layer of relatively dense water that is important for absorbing and storing carbon dioxide in the deep ocean. In our results, 2015 and 2017 had significantly more winter storms that caused the ocean to lose heat than in 2016.
Key Points
The southernmost long‐term open ocean mooring yields the first multiyear air‐sea flux results south of 50 degrees south
Episodic turbulent heat loss events occur year‐round and are driven primarily by cold, dry northeastward winds
Winter 2015 had more intense heat loss events, deeper mixed layers, and greater Subantarctic Mode Water formation than 2016
A strong decrease of volume and density of North Pacific Subtropical Mode Water (NPSTMW) in 1999 was analyzed in a regional high‐resolution (0.1°) numerical ocean circulation model simulation. Both ...shoaling of the bottom and deepening of the top of the NPSTMW layer contributed equally to volume decrease. They were locally governed by different physical processes, but both seem to be associated with basin‐wide changes in wind. A westward propagating negative thermocline depth anomaly that developed in the Central Pacific when the Pacific Decadal Oscillation index changed from a positive to a negative phase in 1998 caused shoaling of the bottom of the NPSTMW layer. Deepening of the top of the NPSTMW layer was due to an increase in the near surface stratification, caused by an increase in wind‐driven lateral heat transport convergence by the Kuroshio Extension jet starting in 1997. Both processes increased the potential vorticity in the NPSTMW region, decreasing the volume of water in the NPSTMW density range that satisfied the low potential vorticity constraint that is part of the definition of “mode water.” The strong near‐surface density decrease provided preconditioning for preferential surface formation of a lighter variety of NPSTMW, further decreasing its density. It also resulted in decrease of the outcrop window in the NPSTMW density range, strongly reducing its formation rate in 1998 and 1999 despite strong surface heat loss.
Plain Language Summary
We analyze physical processes that, in 1999, governed a strong decrease in volume and density of a very large water mass that forms each year on the equatorward side of the North Pacific Kuroshio Extension jet—one of the regions of greatest ocean heat loss to the atmosphere—by wintertime surface ocean heat loss. Because of their large volume, water masses like the one analyzed here (called the North Pacific Subtropical Mode Water, NPSTMW) are major subsurface oceanic reservoirs of heat, carbon, and other properties, strongly affecting both the amount of heat stored in upper ocean (in approximately top 500 m) as well as the heat exchange between the ocean and the atmosphere. We show that the strong NPSTMW density decrease played a crucial role in the strong NPSTMW volume decrease in 1999, and they were both ultimately caused by North Pacific basin‐wide changes in wind.
Key Points
Strong NPSTMW density decrease played a crucial role in the strong North Pacific Subtropical Mode Water (NPSTMW) volume decrease in 1999
Deepening of the top and shoaling of the bottom of the NPSTMW layer contributed equally to 1999 NPSTMW volume decrease
Although locally governed by different physical processes, both seem to be associated with basin‐wide changes in wind
Abstract
Eddy modulation of the air–sea interaction and convection that occurs in the process of mode water formation is analyzed in simulations of a baroclinically unstable wind- and buoyancy-driven ...jet. The watermass transformation analysis of Walin is used to estimate the formation rate of mode water and to characterize the role of eddies in that process. It is found that diabatic eddy heat flux divergences in the mixed layer are comparable in magnitude, but of opposite sign, to the surface air–sea heat flux and largely cancel the direct effect of buoyancy loss to the atmosphere. The calculations suggest that mode water formation estimates based on climatological air–sea heat flux data and outcrops, which do not fully resolve ocean eddies, may neglect a large opposing term in the heat budget and are thus likely to significantly overestimate true formation rates. In Walin’s watermass transformation framework, this manifests itself as a sensitivity of formation rate estimates to the averaging period over which the outcrops and air–sea fluxes are subjected. The key processes are described in terms of a transformed Eulerian-mean formalism in which eddy-induced mean flow tends to cancel the Eulerian-mean flow, resulting in weaker residual mean flow, subduction, and mode water formation rates.
The seasonal and spatial evolution of mixed layers (MLs) in the Red Sea (RS) is analyzed for the 2001-2015 period using the results of a high resolution (~1km horizontal, 50 vertical layers) ocean ...circulation model forced by a novel regional high resolution (5 km) atmospheric reanalysis dataset. The simulation reproduces the main features of the near-surface stratification, as described by the available observations. The seasonal evolution of the modeled mixed layer depths (MLDs) in the RS is predominantly driven by atmospheric buoyancy forcing, especially its heat flux component. Everywhere in the basin the model MLs are deepest in January and February. The deepest MLDs develop in the northern parts of the Gulf of Aqaba and in the western parts of the north RS. The MLDs gradually shoal towards the south, reflecting the meridional gradient of wintertime surface buoyancy loss. In spring and summer, the surface ocean heat gain increases the stratification and the MLs are becoming shallow everywhere in the basin. During this season wind may have a significant local impact on the MLD. Particularly important are strong winds channeled by topography, such as in the vicinity of the Strait of Bab-Al-Mandeb and the straits connecting the two gulfs in the north, and lateral jets blowing through mountain gaps, such as the Tokar jet in the central RS. The MLD distribution further suggests influence by the general and mesoscale circulation. The complex patterns of air-sea buoyancy flux, wind forcing, and the thermohaline and mesoscale circulation, are all strongly imprinted on the MLD distribution.
Abstract
Satellite observations and idealized numerical studies reveal intensification of long-period (on the order of one cycle per year) waves in the western part of ocean basins. The authors ...explore the idea that the intensification is associated with the spatial growth of purely time periodic, but baroclinically unstable, motions. The framework is a simple idealized 2½-layer model in which only the upper layer is directly forced by the wind, a setting similar to the shadow zone of the Luyten–Pedlosky–Stommel (LPS) model. The upper two layers participate in the wave motion, which is driven by a large-scale wind stress fluctuating with the annual period, representing the seasonal cycle. Although possibly unstable solutions exist everywhere in the subtropical gyre on account of the nonzero meridional background flow, they are not seen in the eastern part of the basin in satellite observations nor are they excited there by model gyre-scale annual-period winds. Instead, energy injected into the model ocean at a fixed frequency and with zonal and meridional wavenumbers, such that the resulting flow perturbation is locally stable, refracts westward as it propagates through the spatially varying background flow without change of frequency and reaches distant regions where the spatial wavenumber becomes complex so that spatial growth occurs. This process results in spatially growing solutions of annual or near-annual frequency only in the southwestern part of the model subtropical gyre, thus explaining why the intensification is preferentially manifested in the southwestern subtropical gyre in published numerical model results. The paper concludes with a discussion of relevant satellite and in situ observations.
Abstract
The purpose of this paper is to understand how long planetary waves evolve when propagating in a subtropical gyre. The steady flow of a wind-driven vertically sheared model subtropical gyre ...is perturbed by Ekman pumping that is localized within a region of finite lateral extent and oscillates periodically at about the annual frequency after sudden initiation. Both the background flow and the infinitesimal perturbations are solutions of a 2½-layer model. The region of forcing is located in the eastern part of the gyre where the steady flow is confined to the uppermost layer (shadow zone). The lateral scales of the forcing and of the response are supposed to be small enough with respect to the overall gyre scale that the background flow may be idealized as horizontally uniform, yet large enough (greater than the baroclinic Rossby radii) that the long-wave approximation may be made. The latter approximation limits the length of time over which the solutions remain valid. The solutions consist of (i) a forced response oscillating at the forcing frequency in which both stable (real) and zonally growing (complex) meridional wavenumbers are excited plus (ii) a localized transient structure that grows as it propagates away from the region of forcing. Application of the method of stationary phase provides analytical solutions that permit clear separation of the directly forced part of the solution and the transient as well as estimation of the temporal growth rate of the transient, which proves to be convectively unstable. The solutions presented here are relevant to understanding the instability of periodic (including annual period) perturbations of oceanic subtropical gyres on scales larger than the baroclinic Rossby radii of deformation.
Abstract
The authors investigate the dynamics of zonal jets in a semihemisphere zonally reentrant ocean model. The forcings imposed in the model are an idealized atmospheric wind stress and ...relaxation to a latitudinal temperature profile held constant in time. While there are striking similarities to the observed atmospheric annular modes, where the leading mode of variability is associated with the primary zonal jet’s meridional undulation, secondary (weaker) jets emerge and systematically migrate equatorward.
The model output suggests the following mechanism for the equatorward migration: while the eddy momentum fluxes sustain the jets, the eddy heat fluxes have a poleward bias causing an anomalous residual circulation with poleward (equatorward) flow on the poleward (equatorward) flanks. By conservation of mass, there must be a rising residual flow at the jet. From the thermodynamics equation, the greatest cooling occurs at the jet core, thus creating a tendency to reduce the baroclinicity on the poleward flank, while enhancing it on the equatorward flank. Consequently, the baroclinic zone shifts, perpetuating the jet migration.
A salient feature of sea level records from the Adriatic Sea is the frequent occurrence of energetic seiches of period about 21 h. Once excited by a sudden wind event, such seiches often persist for ...days. They lose energy either to friction within the Adriatic, or by radiation through Otranto Strait into the Mediterranean.
The free decay time of the dominant (lowest mode) seiche was determined from envelopes of handpassed sea level residuals from three locations (Bakar, Split and Dubrovnik) along the Croatian coast during twelve seiche episodes between 1963 and 1986 by taking into consideration only time intervals when the envelopes decreased exponentially in time, when the modelled effects of along-basin winds were smaller than the error of estimation of decay time from the envelopes and when across-basin winds were small. The free decay time thus obtained was 3.2±0.5 d. This value is consonant with the observed width of the spectral peak.
The decay caused by both bottom friction and radiation was included in a one dimensional variable cross section shallow water model of the Adriatic. Bottom friction is parameterized by the coefficient
k appearing in the linearized bottom stress term
ρ
0u
(where u is the along-basin velocity and
ρ
0 the fluid density). The coefficient k is constrained by values obtained from linearization of the quadratic bottom stress law using estimates of near bottom currents associated with the seiche, with wind driven currents, with tides and with wind waves. Radiation is parameterized by the coefficient
f appearing in the open strait boundary condition
ζ =
auh/
c (where ζ is sea level,
h is depth and
c is phase speed). This parameterization of radiation provides results comparable to allowing the Adriatic to radiate into an unbounded half plane ocean. Repeated runs of the model delineate the dependence of model free seiche decay time on
k and
a, and these plus the estimates of
k allow estimation of
a.
The principle conclusions of this work are as follows.
1.
(1) Exponential decay of seiche amplitude with time does not necessarily guarantee that the observed decay is free of wind influence.
2.
(2) Winds blowing across the Adriatic may be of comparable importance to winds blowing along the Adriatic in influencing apparent decay of seiches; across-basin winds are probably coupled to the longitudinal seiche on account of the strong along-basin variability of across-basin winds forced by Croatian coastal orography.
3.
(3) The free decay time of the 21.2 h Adriatic seiche is 3.2±0.5 d.
4.
(4) A one dimensional shallow water model of the seiche damped by bottom stress represented by Godin's (1988) approximation to the quadratic bottom friction law
ρ
0
C
D
u|
u| using the commonly accepted drag coefficient
C
D = 0.0015 and quantitative estimates of bottom currents associated with wind driven currents, tides and wind waves, as well as with the seiche itself with no radiation gives a damping time of 9.46 d; radiation sufficient to give the observed damping time must then account for 66% of the energy loss per period. But independent estimates of bottom friction for Adriatic wind driven currents and inertial oscillations, as well as comparisons between quadratic law bottom stress and directly measured bottom stress, all suggest that the quadratic law with
C
D=0.0015 substantially underestimates the bottom stress. Based on these studies, a more appropriate value of the drag coefficient is at least
C
D=0. In this case, bottom friction with no radiation leads to a damping time of 4.73 d, radiation sufficient to give the observed damping time then accounts for 32% of the energy loss per period.
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