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Gerald A. Meehl

Abstract

Recent advances in computer power and climate modeling capability have provided the opportunity for several modeling groups to undertake extended integrations with global coupled ocean-atmosphere climate models that allow the study of coupled processes thought to be important in producing Southern Oscillation and El Niño phenomena. Results are shown here from such a coupled model, developed at the National Center for Atmospheric Research (NCAR), that consists of a global, spectral (R15) atmospheric general circulation model (GCM) coupled to a global, 5° latitude-longitude, four-layer, ocean GCM. In spite of limitations of the coarse model grid, Southern Oscillation-type interannual variability of the ocean-atmosphere system is inherent in the coupled model. One of the mysteries of the Southern Oscillation cycle is how the system makes the transition from cold to warm phase and back again in the tropical Pacific in northern spring. Evidence is shown from the NCAR coupled model that a modulation of the mean seasonal cycle in the eastern Pacific drives the initiation and decay of warm and cold episodes in that region. The mechanism of this forcing in the model involves coupled seasonal anomalies of sea-level pressure (SLP), sea-surface temperature, surface wind stress, ocean upwelling, and convection-precipitation. These coupled anomalies form as a consequence of land-sea contrast in the eastern Pacific, in association with the evolution of seasonally low SLP over South America during northern winter and its movement with the seasonal cycle of solar forcing northwestward during northern spring. The anomalies become established farther west in the tropical Pacific as the year progresses and are associated with global patterns that resemble, in some ways, the phenomena observed with warm and cold events–the extremes of the Southern Oscillation. Similar sets of coupled processes exist in the observed long-term mean seasonal cycle, and the interannual events in the eastern tropical Pacific are manifested as a modulation of the mean seasonal cycle in the observations analogous to the coupled model. A reduction of coupling strength in the model (by reducing the strength of the wind-stress forcing from the atmosphere) eliminates both seasonal dependence and interannual anomaly signals Turning off the seasonal cycle of solar forcing in the model changes the nature and regular evolution of the warm and cold events. Since the model fails to simulate any of the observed phenomena in the western Pacific, it is likely that only one of several possible sets of mechanisms involved with the observed El Niño-Southern Oscillation is simulated in the present global coupled model.

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Gerald A. Meehl

Abstract

A seasonal global ocean-current data set (OCDS) digitized on a 5° grid from long-term mean shipdrift-derived currents from pilot charts is presented and described. Annual zonal means of v-component currents show subtropical convergence zones which move closest to the equator during the respective winters in each hemisphere. Net annual v-component surface flow at the equator is northward. Zonally averaged u-component currents have greatest seasonal variance in the tropics with strongest westward currents in the winter hemisphere. An ensemble of ocean currents measured by buoys and current meters compares favorably with OCDS data in spite of widely varying time and space scales. The OCDS currents and directly measured currents are about twice as large as computed geostrophic currents. An analysis of equatorial Pacific currents suggests that dynamic topography and sea-level changes indicative of the geostrophic flow component cannot be relied on solely to infer absolute strength of surface currents which include a strong Ekman component. Comparison of OCDS v-component currents and meridional transports predicted by Ekman theory shows agreement in the sign of transports in the midlatitudes and tropics in both hemispheres. Ekman depths required to scale OCDS v-component currents to computed Ekman transports are reasonable at most latitudes with layer depths deepening closer to the equator.

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Gerald A. Meehl

Abstract

In the hierarchy of simple ocean formulations available for coupling to atmospheric GCMs, a scheme whereby ocean surface-layer depths vary geographically and seasonally is deemed better than a fixed depth layer at all locations and seasons, but still is less sophisticated than dynamic ocean models. Yet such simple ocean formulations are useful for basic sensitivity studies. Here, a calculation of varying surface layer depths is done by first performing an ocean heat storage calculation using gridded, long-term mean mixed-layer depths and sea surface temperatures with a parameterized temperature structure beneath the mixed layer derived from weather ship data. Heat storage values in the midlatitudes are larger in the Atlantic than in the Pacific, which is in qualitative agreement with the weather ship data. Variants of the basic calculation show that neither mixed layer data nor SST data alone are sufficient to compute heat storage adequately. Using the results from the parameterized heat storage calculations, effective ocean surface-layer depths are computed. These are found to be deeper in the Atlantic than in the Pacific, with a strong semiannual monsoon signal apparent in the Indian Ocean. Since these calculations exclude the effects of vertical and horizontal motion further analysis as to the viability of these calculations can be done with the specified depths coupled to an atmospheric GCM.

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Gerald A. Meehl

Abstract

The present generation of global coupled ocean–atmosphere GCMs contains considerable systematic errors both in terms of net surface heat flux and simulated SSTs. Here, a global coupled GCM is used to illustrate how systematic errors in the separate coupled model components (atmosphere and ocean) contribute to the simulations of net surface heat flux and SST when the components are coupled together. Features of the coupled model simulation are a combination of errors in the component models and errors introduced due to the dynamic interaction, both local and nonlocal, between atmosphere and ocean. Various regions and latitudinal zones are examined to determine the processes that produce the net surface heat fluxes and SSTs in the coupled simulation. In the coupled model, a good simulation of net surface heat flux does not always produce a correspondingly accurate simulation of SST. Alterations of surface winds and/or ocean currents can introduce SST errors and consequent compensating surface fluxes that have apparent agreement with observed estimates (e.g., near 60°N in the North Atlantic). Additionally, an SST error that occurs due to a combination of surface flux errors from atmosphere and ocean components in the coupled simulation, as well as an alteration of the ocean surface currents, can produce a better agreement of the net surface fluxes in the coupled model with observed estimates in spite of the large SST errors (e.g., near 50°N in the Atlantic and Pacific). Conversely, a good simulation of SST in the coupled model can be associated with surface heat flux errors also due to dynamic adjustments in the atmosphere and ocean in the coupled simulation (e.g., near 20°N and 20°S). A high-quality coupled model simulation does not necessarily require a precise reproduction of observed net surface heat fluxes, even though accurate observed surface fluxes are necessary to calibrate model parameterizations in the components and to provide an index of model performance. Rather, improved coupled model simulations must rely on improvement of the entire thermodynamic and dynamic simulations (and verification of state variables) in the components separately and when coupled.

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Gerald A. Meehl

Abstract

Two ocean formulations, one a simple, 50-m slab ocean and another a coarse-resolution global ocean general circulation model (GCM), are coupled to a global atmospheric GCM. To determine what part of the simulation errors is introduced by the atmospheric model and what part arises from limitations of the ocean formulation, results are compared to observations as well as to integrations involving the ocean GCM run with observed atmospheric forcing and the atmospheric GCM run with specified observed sea surface temperatures (SSTs). The tropical Indian and Pacific regions are studied because of the associations involving the dynamically coupled ocean-atmosphere system in those regions related to large-scale tropics and global interannual variability. Analysis of the net surface heat flux leads to the conclusion that limitations in the ocean formulations contribute more to errors in the coupled climate simulations than inherent deficiencies in the atmospheric model. In spite of the limitations of the ocean formulations in simulating SST, the atmospheric model simulates most major features associated with the low-level wind fields in the tropical Indian and Pacific regions with differences consistent with the SSTs supplied by the ocean models. The implication is that increased quality of the ocean simulation will result in substantial improvements of the coupled climate model simulations even without upgrades to the atmospheric model. The point is raised that a coupled model with surface fluxes computed interactively produces a more internally consistent climate simulation than could be expected from the same atmospheric model forced with observed SSTs, even if the SSTs computed by the coupled model do not exactly match the observed values. Surface fluxes, then, are not “absolute” values and must be interpreted as compensatory products of the limitations (and strengths) of the respective media in simulating SST patterns. Thus, even the present generation of imperfect coupled models can provide insight into some of the processes, mechanisms, and sensitivities of the coupled climate system that no other research tool can.

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Gerald A. Meehl

Abstract

Variations in the upper-ocean beat content are part of a mechanism to explain biennial signals in the tropical Indian and Pacific ocean regions. The mechanism involves modulations of the annual cycle of convection and processes of the dynamically coupled ocean and atmosphere system in the tropics. A critical element of that mechanism is persistent sea surface temperature anomalies on the time scale of one seasonal cycle. Analyses of composite vertical temperature profiles from hydrographic station data for various near-equatorial areas in the Indian and Pacific oceans show that variations in the ocean heat content depend on the depth of the thermocline in the warm-pool region (both eastern Indian and western Pacific) and temperatures in the upper-ocean mixed layer away from the warm pool (western Indian and eastern Pacific). The variations in thermocline depth in the Pacific are similar to those for El Niño-Southern Oscillation (ENSO) events and are present in the biennial mode as well. Apparently, similar sets of mechanisms operate on both ENSO and biennial time scales. These results suggest that changes in upper-ocean heat content contribute to the persistence of sea surface temperature anomalies important to the biennial mechanism, that both the Indian and Pacific oceans are actively involved in ENSO, and that ENSO could be an amplification of the biennial cycle.

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Gerald A. Meehl

Major conclusions and recommendations regarding the status of global coupled general circulation models are presented here from a workshop convened by the World Climate Research Programme Steering Group on Global Coupled Modelling that was held from 10 to 12 October 1994 at the Scripps Institution of Oceanography, La Jolla, California. The purpose of the workshop was to assess the current state of the art of global coupled modeling on the decadal and longer timescales in terms of methodology and results to identify the major issues and problems facing this activity and to discuss possible alternatives for making progress in light of these problems. This workshop brought together representatives from nearly every group in the world actively involved in formulating and running such models. After presentations by workshop participants, four working groups identified key issues involving 1) initialization and model spinup, 2) strategies and techniques for coupling of model components, 3) flux correction/adjustment, and 4) secular drift and systematic errors. The participants concluded that improved communication between those engaged in this activity will be important to enhance further progress. Consequently, the World Climate Research Programme intends to continue the support of internationally coordinated activities in global coupled modeling.

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Gerald A. Meehl

Abstract

A mechanism is described that involves the south Asian monsoon as an active part of the tropospheric biennial oscillation (TBO) described in previous studies. This mechanism depends on coupled land–atmosphere–ocean interactions in the Indian sector, large-scale atmospheric east–west circulations in the Tropics, convective heating anomalies over Africa and the Pacific, and tropical–midlatitude interactions in the Northern Hemisphere. A key element for the monsoon role in the TBO is land–sea or meridional tropospheric temperature contrast, with area-averaged surface temperature anomalies over south Asia that are able to persist on a 1-yr timescale without the heat storage characteristics that contribute to this memory mechanism in the ocean. Results from a global coupled general circulation model show that soil moisture anomalies contribute to land-surface temperature anomalies (through latent heat flux anomalies) for only one season after the summer monsoon. A global atmospheric GCM in perpetual January mode is run with observed SSTs with specified convective heating anomalies to demonstrate that convective heating anomalies elsewhere in the Tropics associated with the coupled ocean–atmosphere biennial mechanism can contribute to altering seasonal midlatitude circulation. These changes in the midlatitude longwave pattern, forced by a combination of tropical convective heating anomalies over East Africa, Southeast Asia, and the western Pacific (in association with SST anomalies), are then able to maintain temperature anomalies over south Asia via advection through winter and spring to set up the land–sea meridional tropospheric temperature contrast for the subsequent monsoon. The role of the Indian Ocean, then, is to provide a moisture source and a low-amplitude coupled response component for meridional temperature contrast to help drive the south Asian monsoon. The role of the Pacific is to produce shifts in regionally coupled convection–SST anomalies. These regions are tied together and mutually interact via the large-scale east–west circulation in the atmosphere and contribute to altering midlatitude circulations as well. The coupled model results, and experiments with an atmospheric GCM that includes specified convective heating anomalies, suggest that the influence of south Asian snow cover in the monsoon is not a driving force by itself, but is symptomatic of the larger-scale shift in the midlatitude longwave pattern associated with tropical SST and convective heating anomalies.

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Gerald A. Meehl

Abstract

The basic concept of land-sea temperature contrast and the strength of the Asian summer monsoon is investigated here by comparing the relative contributions of external conditions (involving surface albedo) and internal feedbacks (involving soil moisture) in a number of atmospheric general circulation model (GCM) mean climate simulations and in a GCM sensitivity experiment. All models are run with the same long-term mean sea surface temperatures so that only land-surface conditions affect the land-sea temperature contrast. There is a surprising consistency among the various models such that stronger summer monsoons (defined as high area-averaged precipitation over south Asia) are associated with greater land-sea temperature contrast (i.e., higher land temperatures), lower sea level pressure over land, less snow cover, and greater soil moisture. In a sensitivity study with land albedos uniformly raised from 0. 13 to 0.20 in one of the models, the winter-spring-summer sequence over southern Asia shows that there is a high sensitivity to the specified land albedos. Lower land albedos are associated with warmer land temperatures, greater land-sea temperature contrast, and a stronger Asian summer monsoon. There is also a positive feedback between soil moisture and precipitation (increased soil moisture provides a surface moisture source for further precipitation).

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Gerald A. Meehl

Abstract

The cause of the semiannual oscillation (SAO) at middle and high southern latitudes, proposed by van Loon, is reexamined using observations and general circulation model (GCM) simulations. The model results and the more recent observed data [sea surface temperature (SST), ocean heat storage, temperature profiles in the upper ocean, and atmosphere transient eddy momentum and heat fluxes] support van Loon's original hypothesis that the mechanism involves the different annual cycles of temperature between the Antarctic polar continent and the surrounding midlatitude southern oceans. A strong semiannual oscillation is noted in the observed atmospheric transient eddy momentum flux in southern midlatitudes corresponding with the two times of year when the circumpolar trough around Antarctica is most intense. The products of the dynamical coupling of ocean and atmosphere–the annual cycle of SST near 50°S and associated ocean beat storage–are important to the amplitude and phase of the SAO in the atmosphere. GCM simulations are analyzed to provide insights into the consequences of changing elements of the ocean forcing near 50°S. The GCM simulation with the specified annual cycle of SSTs has the correct phase of the SAO but reduced amplitude. The model with a simple mixed-layer ocean (shallow fixed depth with no dynamics) produces an altered annual cycle of SSTs and ocean heat storage at 50°S and a similarly altered SAO as a consequence. These model results, along with the observed upper-ocean temperature profiles and heat storage values, suggest that changes in the annual cycle of SST and ocean heat storage near 50°S could lead to a modulation of the observed SAO and affect its role in the El Niño–Southern Oscillation and the Indian monsoon.

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