Oceanic Influences on the Seasonal Cycle in Evaporation over the Indian Ocean
25
Citation
16
Reference
10
Related Paper
Citation Trend
Abstract:
The annual mean and seasonal cycle in latent heating over the Indian Ocean are investigated using a simple, analytical ocean model and a 3D, numerical, ocean model coupled to a prescribed atmosphere, which permits interaction through sea surface temperature (SST). The role of oceanic divergence in determining the seasonal cycle in evaporation rate is reexamined from the viewpoint that the amount of rainfall over India during the southwest monsoon is a function of the amount of water evaporated over the “monsoon streamtube” as well as orographically induced convective activity. Analysis of Comprehensive Ocean–Atmosphere Dataset (COADS) shows that nearly 90% of the water vapor available to precipitate over India during the southwest monsoon results from the annual mean evaporation field. The seasonal change in direction of airflow, which opens up a pathway from the southern Indian Ocean to the Arabian Sea, rather than the change in evaporation rate is key to explaining the climatological cycle, though the change in latent heating due to seasonal variations is similar to that needed to account for observed interannual-to-interdecadal variability in monsoon rainfall. The simple model shows that net oceanic heat advection is not required to sustain vigorous evaporation over the southern tropical Indian Ocean; its importance lies in ensuring that the maximum evaporation occurs during boreal summer. Also shown with the simple model is that evaporation over the Arabian Sea cannot increase sufficiently to make up for the loss of water vapor accumulated over the southern Indian Ocean should there be a change in circulation such that the Southern Ocean is no longer part of the monsoon streamtube. Analytical, periodic solutions of the linearized heat balance equation for the simple model are presented under the assumption that the residual of net surface heat flux minus rate of change of heat content (DIV) is considered to be an external periodic forcing independent of SST to first order. These solutions, expressed as functions of the amplitude and phase of DIV, lie in two regimes. The first regime is characterized by increases (decreases) in the amplitude of DIV resulting in an increase (decrease) in the amplitude of the solution. In contrast, in the second regime, the amplitude of the solution decreases (increases) as the amplitude of DIV increases (decreases). It is noteworthy that the regime boundaries for SST and latent heating do not necessarily coincide. For the present climate, as determined from COADS, the southern Indian Ocean’s annual harmonics of latent heating and SST lie in the second regime near the border, and so their tendencies are sensitive to the nature of the perturbation to the harmonic in DIV. The southern Indian Ocean’s semiannual harmonic of latent heating lies in the first regime, and so its tendency is robust to the nature of the perturbation to the harmonic in DIV; that of SST lies in the second regime near the border. Contrasting runs of the 3D numerical model, in which the Indonesian throughflow differs by less than 4 × 106 m3 s−1 in the annual mean and less than ±2 × 106 m3 s−1 in seasonal variability, provides new estimates for its potential role in the Indian Ocean heat balance. Net surface heat flux differences of over 20 W m−2 are found along the length and breadth of the southwest monsoon streamtube: particularly noteworthy regions are over the Somali jet and to the east of Madagascar. These changes can be explained in part by the changes in oceanic meridional transport generated by the throughflow as well as by its heat input. Spatial resolution and upper ocean physics are sufficient for the throughflow to retain its zonal jet character across the Indian Ocean and so inhibit meridional overturning. Significantly, its presence reduces the amount of heat imported into the Southern Ocean from the Arabian Sea during boreal summer, so making SSTs in the Arabian Sea higher.Keywords:
Water cycle
Abstract Satellite observations of the magnetic field induced by the general ocean circulation could provide new constraints on global oceanic water and heat transports. This opportunity is investigated in a model‐based twin experiment by assimilating synthetic satellite observations of the ocean‐induced magnetic field into a global ocean model. The general circulation of the world ocean is simulated over the period of 1 month. Idealized daily observations are generated from this simulation by calculating the ocean‐induced magnetic field at 450 km altitude and disturbing these global fields with error estimates. Utilizing an ensemble Kalman filter, the observations are assimilated into the same ocean model with a different initial state and different atmospheric forcing. Compared to a reference simulation without data assimilation, the corrected ocean‐induced magnetic field is improved throughout the whole simulation period and over large regions. The global RMS differences of the ocean‐induced magnetic field are reduced by up to 17%. Local improvements show values up to 54%. RMS differences of the depth‐integrated zonal and meridional ocean velocities are improved by up to 7% globally, and up to 50% locally. False corrections of the ocean model state are identified in the South Pacific Ocean and are linked to a deficient estimation of the ocean model error covariance matrices. Most Kalman filter induced changes in the ocean velocities extend from the sea surface down to the deep ocean. Allowing the Kalman filter to correct the wind stress forcing of the ocean model is essential for a successful assimilation.
Circulation (fluid dynamics)
Assimilation (phonology)
Ocean dynamics
Cite
Citations (44)
Abstract. As the world ocean moves through the ambient geomagnetic core field, electric currents are generated in the entire ocean basin. These oceanic electric currents induce weak magnetic signals that are principally observable outside of the ocean and allow inferences about large-scale oceanic transports of water, heat, and salinity. The ocean-induced magnetic field is an integral quantity and, to first order, it is proportional to depth-integrated and conductivity-weighted ocean currents. However, the specific contribution of oceanic transports at different depths to the motional induction process remains unclear and is examined in this study. We show that large-scale motional induction due to the general ocean circulation is dominantly generated by ocean currents in the upper 2000 m of the ocean basin. In particular, our findings allow relating regional patterns of the oceanic magnetic field to corresponding oceanic transports at different depths. Ocean currents below 3000 m, in contrast, only contribute a small fraction to the ocean-induced magnetic signal strength with values up to 0.2 nT at sea surface and less than 0.1 nT at the Swarm satellite altitude. Thereby, potential satellite observations of ocean-circulation-induced magnetic signals are found to be likely insensitive to deep ocean currents. Furthermore, it is shown that annual temporal variations of the ocean-induced magnetic field in the region of the Antarctic Circumpolar Current contain information about sub-surface ocean currents below 1000 m with intra-annual periods. Specifically, ocean currents with sub-monthly periods dominate the annual temporal variability of the ocean-induced magnetic field. Keywords. Electromagnetics (numerical methods) – geomagnetism and paleomagnetism (geomagnetic induction) – history of geophysics (transport)
Oceanic basin
Ocean surface topography
Ocean dynamics
Cite
Citations (10)
How do ocean initial conditions impact historical and future climate projections in Earth system models? To answer this question, we use the 50-member Canadian Earth System Model (CanESM2) large ensemble, in which individual ensemble members are initialized using a strategic combination of different oceanic initial states and different atmospheric perturbations. We show that global ocean heat content anomalies associated with the different ocean initial states persist from initialization at year 1950 through the end of the simulations at year 2100. We also find that these anomalies most readily impact surface climate over the Southern Ocean. Ocean initial conditions affect Southern Ocean surface climate because persistent deep ocean temperature anomalies upwell along sloping isopycnal surfaces that delineate neighboring branches of the Upper and Lower Cells of the Global Meridional Overturning Circulation. As a result, up to a quarter of the ensemble variance in Southern Ocean turbulent heat fluxes, heat uptake, and surface temperature trends can be traced to variance in the ocean initial state. Such a discernible impact of varying ocean initial conditions on ensemble variance over the Southern Ocean is evident throughout the full 150 simulation years of the ensemble, even though upper ocean temperature anomalies due to varying ocean initial conditions rapidly dissipate over the first two decades of model integration over much of the rest of the globe.
Isopycnal
Ocean dynamics
Ocean surface topography
Sea-surface height
Initialization
Cite
Citations (0)
Abstract. Paleoreconstructions and modern observations provide us with anomalies of surface temperature over the past millennium. The history of deep ocean temperatures is much less well-known and was simulated in a recent study for the past 2000 years under forced surface temperature anomalies. In this study, we simulate the past 800 years with an illustrative forcing scenario in the Bern3D ocean model, which enables us to assess the role of changes in ocean circulation on deep ocean temperature. We quantify the effect of changing ocean circulation by comparing transient simulations (where the ocean dynamically adjusts to anomalies in surface temperature – hence density) to simulations with fixed ocean circulation. We decompose temperature, ocean heat content and meridional heat transport into the contributions from changing ocean circulation and changing sea surface temperature (SST). In the deep ocean, the contribution from changing ocean circulation is found to be as important as the changing SST signal itself. Firstly, the small changes in ocean circulation amplify the Little Ice Age signal around 3 km depth by at least a factor of two, depending on the basin. Secondly, they fasten the arrival of this atmospheric signal in the Pacific and Southern Ocean at all depths, whereas they delay the arrival in the Atlantic between about 2.5 and 3.5 km by two centuries. This delay is explained by an initial competition between the Little Ice Age cooling and a warming due to an increase in relatively warmer North Atlantic Deep Water at the cost of Antarctic Bottom Water. Under the consecutive AMOC slowdown, this shift in water masses is inverted and aging of the water causes a late additional cooling. Our results suggest that small changes in ocean circulation can have a large impact on the amplitude and timing of ocean temperature anomalies below 2 km depth.
Deep ocean water
Lead (geology)
Forcing (mathematics)
Circulation (fluid dynamics)
Oceanic basin
Cite
Citations (0)
Abstract In response to quadrupled CO 2 , the Southern Ocean primarily uptakes excess heat around 60°S, which is then redistributed by the northward ocean heat transport (OHT) and mostly stored in the ocean or released back to the atmosphere around 45°S. However, the relative roles of mean ocean circulation and ocean circulation change in the uptake and redistribution of heat in the Southern Ocean remain controversial. Here, a set of climate model experiments embedded with a novel partial coupling technique are used to separate the roles of mean ocean circulation (passive component) and ocean circulation change (active component). For the ocean heat uptake (OHU) response, the mean ocean circulation and ocean circulation change are of equal importance. The OHT response south of 50°S is mainly determined by mean ocean circulation, while the ocean circulation change generates an anomalous southward OHT north of 50°S. A heat budget analysis finds that the divergence of passive OHT acts to balance the passive surface heat gain to the south of ∼50°S, while the convergence of active OHT acts to balance the active surface heat loss to the north of ∼50°S. Intriguingly, all the increase in ocean heat storage (OHS) is attributable to the passive component, with the ocean circulation change playing almost no role. In the Southern Ocean, both the active and the passive ocean heat transports are overcompensated by the reverse atmospheric heat transport via the Bjerknes compensation.
Ocean dynamics
Cite
Citations (7)
The study of ocean circulation is vital to understanding how our climate works. The movement of the ocean is closely linked to the progression of atmospheric motion. Winds close to sea level add momentum to ocean surface currents. At the same time, heat that is stored and transported by the ocean warms the atmosphere above and alters air pressure distribution. Therefore, any attempt to model climate variation accurately must include reliable calculations of ocean circulation. Unlike movement of the atmosphere, movement of the ocean's waters takes place mostly near the surface. The major patterns of surface circulation form gigantic circular cells known as gyres. They are categorized according to their general location-equatorial, subtropical, subpolar, and polar-and may run across an entire ocean. The smaller-scale cell of ocean circulation is known' as an eddy. Eddies are much more common than gyres and much more difficult to track in computer simulations of ocean currents.
Eddy
Circulation (fluid dynamics)
Ocean dynamics
Cite
Citations (1)
Author(s): Tamsitt, Veronica | Advisor(s): Talley, Lynne D | Abstract: The circulation of the Southern Ocean is unique due to the lack of meridional boundaries at the latitudes of Drake Passage. Westerly winds drive the Antarctic Circumpolar Current (ACC), linking the major ocean basins and facilitating inter-basin exchange of properties. Additionally, the steeply tilted isopycnals in the Southern Ocean allow interaction between the deep ocean and the atmosphere, and as a result the Southern Ocean has an outsized contribution to the global uptake and redistribution of heat, carbon and nutrients. Complex topography and eddies make this circulation fundamentally three-dimensional, but many features and associated mechanisms of this three-dimensional circulation are not well understood.The objective of this thesis is to use the 1/6°, data-assimilating Southern Ocean State Estimate (SOSE), along with other high-resolution ocean models and available observations, to describe aspects of the three-dimensional structure of the upper cell of the Southern Ocean overturning circulation. First, we diagnose the upper ocean heat budget in the Southern Ocean (Chapter 2), and determine that a strong zonal asymmetry in the air-sea heat flux over the Southern Ocean is associated with large-scale meander the ACC mean path and associated asymmetry in geostrophic heat advection. Second, we use Lagrangian particle release experiments to show, for the first time, the full three-dimensional upwelling pathways of deep water from 30°S to the surface of the Southern Ocean (Chapter 3). We find that deep water moves south in narrow paths along the western and eastern boundaries of each ocean basin, then within the ACC upwelling is concentrated at hotspots associated with high eddy activity at major topographic features. Next, we quantify the water mass transformation along the upwelling pathways from Chapter 3, and find that although the upwelling in the ocean interior is largely along isopycnals, there is significant transformation just below the mixed layer and homogenization of deep water mass properties due to isopycnal mixing (Chapter 4). Finally, we highlight a newly identified poleward pathway of deep water along the eastern boundary of the Indian Ocean and describe the structure and variability of this pathway (Chapter 5).
Ocean dynamics
Oceanic basin
Boundary current
Eddy
Ocean surface topography
Physical oceanography
Antarctic Bottom Water
Cite
Citations (0)
Abstract How do ocean initial states impact historical and future climate projections in Earth system models? To answer this question, we use the 50-member Canadian Earth System Model (CanESM2) large ensemble, in which individual ensemble members are initialized using a combination of different oceanic initial states and atmospheric microperturbations. We show that global ocean heat content anomalies associated with the different ocean initial states, particularly differences in deep ocean heat content due to ocean drift, persist from initialization at year 1950 through the end of the simulations at year 2100. We also find that these anomalies most readily impact surface climate over the Southern Ocean. Differences in ocean initial states affect Southern Ocean surface climate because persistent deep ocean temperature anomalies upwell along sloping isopycnal surfaces that delineate neighboring branches of the upper and lower cells of the global meridional overturning circulation. As a result, up to a quarter of the ensemble variance in Southern Ocean turbulent heat fluxes, heat uptake, and surface temperature trends can be traced to variance in the ocean initial state, notably deep ocean temperature differences of order 0.1 K due to model drift. Such a discernible impact of varying ocean initial conditions on ensemble variance over the Southern Ocean is evident throughout the full 150 simulation years of the ensemble, even though upper ocean temperature anomalies due to varying ocean initial conditions rapidly dissipate over the first two decades of model integration over much of the rest of the globe.
Isopycnal
Ocean dynamics
Initialization
Deep ocean water
Mixed layer
Cite
Citations (9)
Abstract. Paleoreconstructions and modern observations provide us with anomalies of surface temperature over the past millennium. The history of deep ocean temperatures is much less well-known and was simulated in a recent study for the past 2000 years under forced surface temperature anomalies and fixed ocean circulation. In this study, we simulate the past 800 years with an illustrative forcing scenario in the Bern3D ocean model, which enables us to assess the impact of changes in ocean circulation on deep ocean temperature. We quantify the effect of changing ocean circulation by comparing transient simulations (where the ocean dynamically adjusts to anomalies in surface temperature – hence density) to simulations with fixed ocean circulation. We decompose temperature, ocean heat content and meridional heat transport into the contributions from changing ocean circulation and changing sea surface temperature (SST). In the deep ocean, the contribution from changing ocean circulation is found to be as important as the changing SST signal itself. Firstly, the small changes in ocean circulation amplify the Little Ice Age signal at around 3 km depth by at least a factor of 2, depending on the basin. Secondly, they fasten the arrival of this atmospheric signal in the Pacific and Southern Ocean at all depths, whereas they delay the arrival in the Atlantic between about 2.5 and 3.5 km by two centuries. This delay is explained by an initial competition between the Little Ice Age cooling and a warming due to an increase in relatively warmer North Atlantic Deep Water at the cost of Antarctic Bottom Water. Under the consecutive Atlantic meridional overturning circulation (AMOC) slowdown, this shift in water masses is inverted and ageing of the water causes a late additional cooling. Our results suggest that small changes in ocean circulation can have a large impact on the amplitude and timing of ocean temperature anomalies below 2 km depth.
Lead (geology)
Deep ocean water
Forcing (mathematics)
Circulation (fluid dynamics)
Oceanic basin
Cite
Citations (3)
This book is written for college juniors and seniors and new graduate students in meteorology, ocean engineering, and oceanography. It begins with a brief overview of what is known about the ocean. This is followed by a description of the ocean basins, for the shape of the seas influences the physical processes in the water. Next, students will study the external forces, wind and heat, acting on the ocean, and the ocean's response. It also includes the equations describing dynamic response of the ocean. For example, the equations of motion, the influence of earth's rotation, and viscosity. Finally, students consider some particular examples: the deep circulation, the equatorial ocean and El NiEœno, and the circulation of particular areas of the ocean. Contents: 1) A Voyage of Discovery. 2) The Historical Setting. 3) The Physical Setting. 4) Atmospheric Influences. 5) The Oceanic Heat Budget. 6) Temperature, Salinity and Density. 7) The Equations of Motion. 8) Equations of Motion with Viscosity. 9) Response of the Upper Ocean to Winds. 10) Geostrophic Currents. 11) Wind Driven Ocean Circulation. 12) Vorticity in the Ocean. 13) Deep Circulation in the Ocean. 14) Equatorial Processes. 15) Numerical Models. 16) Ocean Waves. 17) Coastal Processes and Tides.
Physical oceanography
Ocean surface topography
Ocean dynamics
Oceanic basin
Circulation (fluid dynamics)
Cite
Citations (683)