Search Results

You are looking at 1 - 10 of 10 items for

  • Author or Editor: Esther C. Brady x
  • Refine by Access: All Content x
Clear All Modify Search
Esther C. Brady

Abstract

The interannual heat budget of the Pacific equatorial upwelling zone is studied using a primitive equation, a reduced gravity model of the upper Pacific equatorial ocean. The model is forced with monthly mean FSU winds from 1971 to 1990. A meridional overturning cell transports heat poleward with poleward flowing warm surface water compensated by colder, deeper equatorward flow. A horizontal cell transports heat equatorward as warmer equatorward flow, which enters the equatorial zone near the western boundary to feed the Equatorial Undercurrent, is compensated by colder poleward flow in the eastern basin. The heat transported by transient eddies is equatorward.

Heat transport anomalies from the 20-year mean balance the time rate of change of heat content on interannual and seasonal timescales. Anomalies of surface heat flux do not contribute significantly to the interannual heat budget.

Although each simulated ENSO event develops differently, similarities in the interannual heat budget emerge. Heat content increases due to an anomalous net equatorward heat transport during the initial stages when the easterlies are anomalously strong. This increase is associated first with an increase in the equatorward heat transport by the horizontal cell due to the increase of the zonal temperature gradient and then with a reduction of the poleward heat transported by meridional overturning as the easterlies slacken. Following the appearance of the maximum SST anomaly, anomalous net poleward heat transport compensates for the subsequent decrease in heat content. The anomalous poloward heat transport is associated first with a reduction of the equatorward heat transport by the horizontal cell as the Equatorial Undercurrent transport decreases, and later with an increase in the meridional overturning as the easterly wind stress returns to normal and the poleward surface flow is anomalously warm.

Northward heat transport occurs during the warm phase when there is an export of heat, southward heat transport occurs during the preconditioning phase when there is an import of heat into the equatorial region. This result indicates that a cross-equatorial exchange of warm water occurs on the interannual timescale as first suggested by the sea level analysis of Wyrtki and Wenzel.

Full access
Esther C. Brady and Peter R. Gent

Abstract

The seasonal heat transport mechanisms important in the Pacific equatorial upwelling zone are investigated using the primitive equation, reduced gravity model developed by Gent and Cane. Mechanisms of meridional heat transport are shown and discussed with respect to the heat budget of a box about the equator containing the upwelling. There is a horizontal cell in which warm water enters the upwelling box in the west in strong equatorward currents located near the, western boundary, which feed the eastward flowing undercurrent. To compensate, water leaves the section as a colder and weaker poleward thermocline flow in the eastern basin. The meridional-vertical cell comprises additional equatorward geostrophically balanced inflow in the upper thermocline, which is compensated by the warmer poleward outflow by Ekman divergence in the surface layer.

In the annual mean, the magnitude of the net heat exported by the meridional-vertical cell exceeds the net heat import due to the gyre exchange so that the net heat transport is poleward. This annual mean net heat export is compensated by the surface heat flux. The transient eddy heat transport is equatorward and much smaller. It is noted that in the winter seasons, boreal December–February and austral June–August, a large amount of heat is lost by a net excess of heat transport by meridional overturning. In the transition seasons, March–May and September–November, there is an equatorward heat transport anomaly in the upwelling box, either related to an excess of heat equatorward transport by gyre exchange in March–May or a reduction in the poleward heat transport by meridional overturning in September–November. March-May is the season during which the undercurrent has its maximum transport, and the strength of the gyre exchange is largest. During September–November, the season of strongest zonal wind when maximum overturning transport is expected, the poleward heat transport by meridional overturning is a minimum. This is partly because the temperature difference between the divergent surface water and convergent subsurface water is smallest in this season, which is the season of lowest SST in the cold tongue and the shallowest and warmest subsurface flow. Seasonally, the variations in the surface heat flux are much smaller than the variations in the heat transport. Thus, the seasonal heat content changes are compensated by the heat transport anomalies.

Full access
Harry L. Bryden and Esther C. Brady

Abstract

To investigate the processes that maintain the large-scale, annual-average thermal structure of the equatorial Pacific, the three-dimensional ocean circulation for a large area is determined from a diagnostic model applied to repeated, meridional hydrographic sections along 150°W and 11O°W from 5°N to 5°S. Geostrophic balances are used to determine velocity profiles from 0 to 500 db across the boundaries of the region: zonal velocities across 150°W and 110°W at approximately 1° -lattitude intervals from 5°N to 5°S and meridional velocities across 5°N and 5°S averaged over the zonal distance between 150°W and 110°W. Poleward wind-driven flows across 5°N and 5°S based on climatological zonal wind stress are added to the geostrophic velocities in the mixed layers. To achieve overall mass conservation, the reference dynamic height field at 500 db is adjusted at four of the 21 stations by about 1 dyn cm. Horizontal nondivergence is used to determine meridional velocities between 0.75° and 5° latitudes. Three-dimensional nondivergence is used between 0.75°N and 0.75°S to determine a vertical profile of vertical velocity at the equator. The resulting model circulation, which is generally consistent with previous interpretations, is then analyzed to estimate the heat budget for the region and the zonal momentum balance at the equator.

The model circulation requires an annual-average hem gain from the atmosphere of 57 W m−2, which is consistent with existing estimates of air-sea heat exchange from bulk formula. The beat gain converts about 35 × 106 m3 s−1 of water flowing into the region with temperatures between 19 and 26°C into an equal amount of 27 to 28°C water flowing out of the region. Little of the heat gain warms the locally upwelled waters at the equator, however, rather, about half acts to increase the temperature of the westward flowing South Equatorial Current as it traverses the region and about half warms the poleward flow of water away from the equator. There is large upwelling at the equator extending down to 180 db with a maximum upward velocity of 2.9 × 10−3 cm s−1 and upward transport of 22 × 106 m3 2−1 across the 62.5 db surface. Because this upwelling occurs in conjunction with the eastward flow of the Equatorial Undercurrent which shallows to the east, the flow is predominantly along isotherms and the maximum cross-isotherm transport is only 7 × 106 m3 2−1 across the 23°C isotherm. Thus, the eastward and upward flow acts to decrease the surface water temperature to the east. In combination with the atmospheric warming of the poleward surface flow away from the equator, this eastward and upward flow along isotherms creates the Cold Tongue in the equatorial Pacific, which is characterized by minimum surface temperature at the equator in any meridional section and colder surface waters to the east.

For the zonal momentum balance at the equator, the vertically integrated zonal pressure gradient balances about 80 percent of the climatological westward wind stress. Eastward and vertical advection of zonal momentum each acts to balance about 20 percent of the wind stress. The sum of eastward and vertical advection indicates a deceleration of the eastward flow at all depths above 300 db. The inferred eastward stress profile suggests that eddy mixing of zonal momentum extends down to at least 200 m depth on the equator, well below the core of the Undercurrent.

Full access
Bette L. Otto-Bliesner and Esther C. Brady

Abstract

A 300-yr simulation with the NCAR Climate System Model (CSM), version 1, captured only ∼60% of the observed ENSO signal and exaggerated the interannual variability of SST in the western tropical Pacific. Here, a simulation with a new version of the CSM, which significantly improves the spatial and temporal patterns of tropical Pacific variability, is described. Maximum SST variability is shifted to the central and eastern Pacific. A better simulation of the equatorial Pacific thermocline structure results in Niño-3 and Niño-4 statistics comparable to the observed estimates for the last century. The evolution of SST and subsurface temperature anomalies is in excellent agreement with observed events. The majority of events evolve as a standing mode with weak SST anomalies occurring in the northern spring in the eastern tropical Pacific and maximum anomalies covering the eastern tropical Pacific Ocean to the date line by the following northern winter. At the same time, subsurface temperature anomalies spread eastward and upward along the tropical thermocline. The “delayed oscillator” and Wyrtki's “buildup” hypothesis are consistent with aspects of the CSM simulation. On the equator, a westerly wind stress anomaly in the central Pacific forces off-equatorial upwelling anomalies, which propagate westward, reaching the western boundary about one-half year later. This upwelling signal then propagates eastward along the equator, arriving 2 months before cooling in the eastern Pacific basin. The tropical Pacific atmospheric response to warm oceanic events also agrees with observational analyses with a negative Southern Oscillation pattern in sea level pressure, wind stress anomalies, and low-level convergence to the west of the maximum SST anomalies and enhanced deep convection and precipitation in the central and eastern tropical Pacific.

Full access
Wei Liu, Zhengyu Liu, and Esther C. Brady

Abstract

This paper is concerned with the question: why do coupled general circulation models (CGCM) seem to be biased toward a monostable Atlantic meridional overturning circulation (AMOC)? In particular, the authors investigate whether the monostable behavior of the CGCMs is caused by a bias of model surface climatology. First observational literature is reviewed, and it is suggested that the AMOC is likely to be bistable in the real world in the past and present. Then the stability of the AMOC in the NCAR Community Climate System Model, version 3 (CCSM3) is studied by comparing the present-day control simulation (without flux adjustment) with a sensitivity experiment with flux adjustment. It is found that the monostable AMOC in the control simulation is altered to a bistable AMOC in the flux-adjustment experiment because a reduction of the surface salinity biases in the tropical and northern North Atlantic leads to a reduction of the bias of freshwater transport in the Atlantic. In particular, the tropical bias associated with the double ITCZ reduces salinity in the upper South Atlantic Ocean and, in turn, the AMOC freshwater export, which tends to overstabilize the AMOC and therefore biases the AMOC from bistable toward monostable state. This conclusion is consistent with a further analysis of the stability indicator of two groups of IPCC Fourth Assessment Report (AR4) CGCMs: one without and the other with flux adjustment. Because the tropical bias is a common feature among all CGCMs without flux adjustment, the authors propose that the surface climate bias, notably the tropical bias in the Atlantic, may contribute significantly to the monostability of AMOC behavior in current CGCMs.

Full access
Bette L. Otto-Bliesner, Esther C. Brady, Gabriel Clauzet, Robert Tomas, Samuel Levis, and Zav Kothavala

Abstract

The climate sensitivity of the Community Climate System Model version 3 (CCSM3) is studied for two past climate forcings, the Last Glacial Maximum (LGM) and the mid-Holocene. The LGM, approximately 21 000 yr ago, is a glacial period with large changes in the greenhouse gases, sea level, and ice sheets. The mid-Holocene, approximately 6000 yr ago, occurred during the current interglacial with primary changes in the seasonal solar irradiance.

The LGM CCSM3 simulation has a global cooling of 4.5°C compared to preindustrial (PI) conditions with amplification of this cooling at high latitudes and over the continental ice sheets present at LGM. Tropical sea surface temperature (SST) cools by 1.7°C and tropical land temperature cools by 2.6°C on average. Simulations with the CCSM3 slab ocean model suggest that about half of the global cooling is explained by the reduced LGM concentration of atmospheric CO2 (∼50% of present-day concentrations). There is an increase in the Antarctic Circumpolar Current and Antarctic Bottom Water formation, and with increased ocean stratification, somewhat weaker and much shallower North Atlantic Deep Water. The mid-Holocene CCSM3 simulation has a global, annual cooling of less than 0.1°C compared to the PI simulation. Much larger and significant changes occur regionally and seasonally, including a more intense northern African summer monsoon, reduced Arctic sea ice in all months, and weaker ENSO variability.

Full access
Esther C. Brady, Bette L. Otto-Bliesner, Jennifer E. Kay, and Nan Rosenbloom

Abstract

Results are presented from the Community Climate System Model, version 4 (CCSM4), simulation of the Last Glacial Maximum (LGM) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) at the standard 1° resolution, the same resolution as the majority of the CCSM4 CMIP5 long-term simulations for the historical and future projection scenarios. The forcings and boundary conditions for this simulation follow the protocols of the Paleoclimate Modeling Intercomparison Project, version 3 (PMIP3). Two additional CCSM4 CO2 sensitivity simulations, in which the concentrations are abruptly changed at the start of the simulation to the low 185 ppm LGM concentrations (LGMCO2) and to a quadrupling of the preindustrial concentration (4×CO2), are also analyzed. For the full LGM simulation, the estimated equilibrium cooling of the global mean annual surface temperature is 5.5°C with an estimated radiative forcing of −6.2 W m−2. The radiative forcing includes the effects of the reduced LGM greenhouse gases, ice sheets, continental distribution with sea level lowered by approximately 120 m from the present, and orbital parameters, but not changes to atmospheric aerosols or vegetation biogeography. The LGM simulation has an equilibrium climate sensitivity (ECS) of 3.1(±0.3)°C, comparable to the CCSM4 4×CO2 result. The LGMCO2 simulation shows a greater ECS of 4.2°C. Other responses found at the LGM in CCSM4 include a global precipitation rate decrease at a rate of ~2% °C−1, similar to climate change simulations in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4); a strengthening of the Atlantic meridional overturning circulation (AMOC) with a shoaling of North Atlantic Deep Water and a filling of the deep basin up to sill depth with Antarctic Bottom Water; and an enhanced seasonal cycle accompanied by reduced ENSO variability in the eastern Pacific Ocean’s SSTs.

Full access
Bette L. Otto-Bliesner, Robert Tomas, Esther C. Brady, Caspar Ammann, Zav Kothavala, and Gabriel Clauzet

Abstract

Preindustrial (PI) simulations of the Community Climate System Model version 3 (CCSM3) at two resolutions, a moderate and a low resolution, are described and compared to the standard controls for present-day (PD) simulations. Because of computational efficiency, the moderate- and low-resolution versions of CCSM3 may be appropriate for climate change studies requiring simulations of the order of hundreds to thousands of years. The PI simulations provide the basis for comparison for proxy records that represent average late Holocene conditions.

When forced with PI trace gases, aerosols, and solar irradiance estimates, both resolutions have a global cooling of 1.2°–1.3°C, increased sea ice in both hemispheres, and less precipitation near the equator and at midlatitudes as compared to simulations using PD forcing. The response to PI forcings differs in the two resolutions for North Atlantic meridional overturning circulation (MOC), the Antarctic Circumpolar Current (ACC), and ENSO. The moderate-resolution CCSM3 has enhanced ACC, North Atlantic MOC, and tropical Pacific ENSO variability for PI forcings as compared to PD. The low-resolution CCSM3 with more extensive sea ice and colder climate at high northern latitudes in the PD simulation shows less sensitivity of the North Atlantic MOC to PI forcing. ENSO variability and the strength of the ACC do not increase with PI forcing in the low-resolution CCSM3.

Full access
Aixue Hu, Bette L. Otto-Bliesner, Gerald A. Meehl, Weiqing Han, Carrie Morrill, Esther C. Brady, and Bruce Briegleb

Abstract

Responses of the thermohaline circulation (THC) to freshwater forcing (hosing) in the subpolar North Atlantic Ocean under present-day and the last glacial maximum (LGM) conditions are investigated using the National Center for Atmospheric Research Community Climate System Model versions 2 and 3. Three sets of simulations are analyzed, with each set including a control run and a freshwater hosing run. The first two sets are under present-day conditions with an open and closed Bering Strait. The third one is under LGM conditions, which has a closed Bering Strait. Results show that the THC nearly collapses in all three hosing runs when the freshwater forcing is turned on. The full recovery of the THC, however, is at least a century earlier in the open Bering Strait run than the closed Bering Strait and LGM runs. This is because the excessive freshwater is diverged almost equally toward north and south from the subpolar North Atlantic when the Bering Strait is open. A significant portion of the freshwater flowing northward into the Arctic exits into the North Pacific via a reversed Bering Strait Throughflow, which accelerates the THC recovery. When the Bering Strait is closed, this Arctic to Pacific transport is absent and freshwater can only be removed through the southern end of the North Atlantic. Together with the surface freshwater excess due to precipitation, evaporation, river runoff, and melting ice in the closed Bering Strait experiments after the hosing, the removal of the excessive freshwater takes longer, and this slows the recovery of the THC. Although the background conditions are quite different between the present-day closed Bering Strait run and the LGM run, the THC responds to the freshwater forcing added in the North Atlantic in a very similar manner.

Full access
Bette L. Otto-Bliesner, Esther C. Brady, John Fasullo, Alexandra Jahn, Laura Landrum, Samantha Stevenson, Nan Rosenbloom, Andrew Mai, and Gary Strand

Abstract

The climate of the past millennium provides a baseline for understanding the background of natural climate variability upon which current anthropogenic changes are superimposed. As this period also contains high data density from proxy sources (e.g., ice cores, stalagmites, corals, tree rings, and sediments), it provides a unique opportunity for understanding both global and regional-scale climate responses to natural forcing. Toward that end, an ensemble of simulations with the Community Earth System Model (CESM) for the period 850–2005 (the CESM Last Millennium Ensemble, or CESM-LME) is now available to the community. This ensemble includes simulations forced with the transient evolution of solar intensity, volcanic emissions, greenhouse gases, aerosols, land-use conditions, and orbital parameters, both together and individually. The CESM-LME thus allows for evaluation of the relative contributions of external forcing and internal variability to changes evident in the paleoclimate data record, as well as providing a longer-term perspective for understanding events in the modern instrumental period. It also constitutes a dynamically consistent framework within which to diagnose mechanisms of regional variability. Results demonstrate an important influence of internal variability on regional responses of the climate system during the past millennium. All the forcings, particularly large volcanic eruptions, are found to be regionally influential during the preindustrial period, while anthropogenic greenhouse gas and aerosol changes dominate the forced variability of the mid- to late twentieth century.

Full access