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Sumant Nigam and Chul Chung

Abstract

The structure of surface-wind anomalies associated with ENSO variability is extracted from ComprehensiveOcean–Atmosphere Dataset observations and European Centre for Medium-Range Forecasts (ECMWF) and National Centers for Environmental Prediction (NCEP) reanalyses, along with estimates of uncertainty. The targets are used to evaluate ENSO surface winds produced by the National Center for Atmospheric Research’s atmospheric GCM known as the Community Climate Model, version 3 (CCM3), when integrated in the climate-simulation mode. Simulated anomalies have stronger easterlies in the off-equatorial Tropics and stronger equatorward flow in the Pacific than any of the observational estimates do. CCM3’s wind departures are found to be large when compared with the difference of the reanalysis anomalies and should thus be considered to be errors.

In a companion paper, the authors make a compelling case for the presence of robust errors in CCM3’s ENSO heating distribution, based on comparisons with the residually diagnosed heating anomalies from ECMWF and NCEP reanalyses.

The linkage between specific features of CCM3’s surface-wind and heating errors is investigated using a steady, linear, global, primitive equation model (18 vertical σ levels, 30 zonal waves, and latitude spacing of 2.5°). Diagnostic modeling indicates that stronger equatorward flow in the Pacific results largely from excessive diabatic cooling in the off-equatorial Tropics, a key heating error linked to a more meridional redistribution of ENSO heating in CCM3. The “bottom-heavy” structure of CCM3’s equatorial heating anomalies, on the other hand, is implicated in the generation of zonal-wind errors in the central and eastern tropical Pacific.

In the diagnostic simulation of CCM3’s ENSO variability, the longwave heating anomalies, with peak values near 850 mb, contribute as much to surface zonal winds as do all other heating components together—a novel finding, needing corroboration.

This study, along with the companion paper, illustrates the dynamical diagnosis strategy—of circulation and forcing intercomparisons with observed counterparts, followed by diagnostic modeling—for analyzing errors in the GCM’s simulation of climate variability.

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Chul Chung and Sumant Nigam

Abstract

The Asian summer monsoon heating anomalies are parameterized in terms of the concurrent ENSO SST anomalies and used as additional forcing in the Cane–Zebiak (CZ) Pacific ocean–atmosphere anomaly model. The Asian heating parameterization is developed from the rotated principal component analysis of combined interannual variability of the tropical Pacific SSTs, residually diagnosed tropical diabatic heating at 400 mb (from ECMWF’s analyses), and the 1000-mb tropical winds during the 1979–97 summer months of June, July, and August.

Analysis of the 95 000-yr-long model integrations conducted with and without the interactive Asian sector heating anomalies reveals that their influence on the Pacific surface winds leads to increased ENSO occurrence—an extra ENSO event every 20 yr or so. An examination of the ENSO distribution w.r.t. the peak SST anomaly in the eastern equatorial Pacific shows increased El Niño occurrence in the 2.2–3.6 K range (and −1.0 to −1.6 K range in case of cold events) along with a modest reduction in the 0.6–1.2 K range, that is, a population shift due to the strengthening of weak El Niños in the monsoon run. The interaction of ENSO-related Asian summer monsoon heating with the CZ model’s ocean–atmosphere also results in a wider period distribution of ENSO variability, but with the El Niño peak phase remaining seasonally locked with the northern winter months.

The above modeling results confirm the positive feedback between Asian summer monsoon and ENSO suggested by previous empirical and diagnostic modeling studies; the feedback is generated primarily by the diabatic heating changes in the Asian Tropics.

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Chul Eddy Chung and V. Ramanathan

Abstract

Aerosols are regionally concentrated and are subject to large temporal variations, even on interannual timescales. In this study, the focus is on the observed large interannual variability of the South Asian (SA) haze, estimating the corresponding variations in its radiative forcing, and using a general circulation model to study their impacts on global climate variability. The SA haze is a widespread haze, covering most of South Asia and the northern Indian Ocean during December–April. The southernmost extent of the haze varies year to year from about 10°S to about 5°N. In order to understand the impact of this interannual variation in the haze forcing, two numerical studies were conducted with two extreme locations of the forcing: 1) extended haze forcing (EHF) and 2) shrunk haze forcing (SHF). The former has the forcing extending to 10°S, while the latter is confined to regions north of the equator.

Each of the two haze forcing simulations was implemented into a 3D global climate model (NCAR CCM3) with a prescribed SST seasonal cycle to estimate the sensitivity of the model climate to the aerosol forcing area. In both simulations, the haze forcing was prescribed only during the dry season between November and April. Over India where the forcing is centered, the simulated climate changes are very similar between EHF and SHF. In remote regions, however, the responses differ remarkably. Focusing on the remote effects of the haze, it is shown that some of the recent observed boreal-wintertime changes of the southwest Asian monsoon, El Niño–Southern Oscillation (ENSO), and the Arctic Oscillation (AO) could be explained by the SA haze forcing and its fluctuation.

First, both simulations reveal the wintertime drought over southwest Asia, with the EHF generating far more severe drought. Second, the EHF experiment simulates a poleward shift of the Northern Hemisphere (NH) zonal-mean zonal momentum during the winter season, while the SHF effect rather moves the NH extratropical zonal momentum only slightly equatorward. Thus, the interannual fluctuations in the extension of the haze forcing area can explain the recently documented increased variability of the AO.

Third, the EHF significantly suppresses the convection in the western equatorial Pacific during the boreal wintertime, and the SHF leads to much less suppression. Since the western Pacific convection suppression would weaken the trade winds over the Pacific and induce warm anomalies in the eastern basin, it is proposed that the SA haze may be partially responsible for the observed El Niño–like warming during the recent decades. When the convection suppression in the EHF experiment is imposed in the Cane–Zebiak Pacific ocean–atmosphere model, the coupled model actually simulates a warm bias similar to the observed El Niño trends of the recent decades. These findings have to be verified with a fully coupled ocean–atmosphere climate model.

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Chul Eddy Chung and V. Ramanathan

Abstract

Sea surface temperatures (SSTs) in the equatorial Indian Ocean have warmed by about 0.6–0.8 K since the 1950s, accompanied by very little warming or even a slight cooling trend over the northern Indian Ocean (NIO). It is reported that this differential trend has resulted in a substantial weakening of the meridional SST gradient from the equatorial region to the South Asian coast during summer, to the extent that the gradient has nearly vanished recently. Based on simulations with the Community Climate Model Version 3 (CCM3), it is shown that the summertime weakening in the SST gradient weakens the monsoon circulation, resulting in less monsoon rainfall over India and excess rainfall in sub-Saharan Africa. The observed trend in SST is decomposed into a hypothetical uniform warming and a reduction in the meridional gradient. The uniform warming of the tropical Indian Ocean in the authors’ simulations increases the Indian summer monsoon rainfall by 1–2 mm day−1, which is opposed by a larger drying tendency due to the weakening of the SST gradient. The net effect is to decrease the Indian monsoon rainfall, while preventing the sub-Saharan region from becoming too dry. Published coupled ocean–atmosphere model simulations are used to describe the competing effects of the anthropogenic radiative forcing due to greenhouse gases and the anthropogenic South Asian aerosols on the observed SST gradient and the monsoon rainfall.

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Kwang-Y. Kim and Chul Chung

Abstract

Extraction of the accurate annual cycle in the tropical Pacific sea surface temperature field is addressed as a demonstration of the utility of a new technique called the cyclostationary empirical orthogonal function (CSEOF) analysis. The strength of the annual cycle has fluctuated roughly by 15% in the past 30 yr (1970–99), and this fluctuation includes swings every 4–6 yr. Accurate extraction of the detailed structure and temporal modulation of the annual cycle was owing to the application of CSEOF analysis, the concept of which has been introduced and compared to the conventional EOF analysis.

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Sumant Nigam, Chul Chung, and Eric DeWeaver

Abstract

Diabatic heating associated with El Niño–Southern Oscillation (ENSO) variability is residually diagnosed from the European Centre for Medium-Range Forecasts (ECMWF) and National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) atmospheric reanalysis datasets during the overlapping 1979–93 period. Quantitative characterization of the horizontal and vertical structure of ENSO heating anomalies, including estimates of uncertainty, provides observationally constrained validation targets for GCM physical parameterizations.

The diagnosed ENSO heating anomalies have similar horizontal structure, but the vertically averaged ECMWF heating is stronger and in better agreement with the Xie–Arkin precipitation anomalies, particularly with respect to precipitation reduction over the western tropical Pacific. Comparison of heating vertical structures in the central equatorial Pacific shows ECMWF heating to be considerably stronger in the lower troposphere, where it exhibits a local maximum.

The ENSO covariant tropospheric temperature in the two reanalyses was also examined along the equator and found to have an intriguing vertical structure, with sizeable amplitude in the lower and upper troposphere and vanishing amplitude in between. The largest temperature anomalies in the lower troposphere are at the surface, and the ECMWF one is about 50% stronger.

The three-dimensional heating anomalies diagnosed from the reanalyses are used to evaluate the ENSO heating distribution produced by NCAR’s Community Climate Model, version 3 (CCM3) atmospheric GCM, when integrated in a climate simulation mode. At least, in context of ENSO variability, the differences in ECMWF and NCEP heating anomalies are small in comparison with CCM3’s heating departures from either of these anomalies, allowing characterization of the CCM3’s ENSO heating structure: horizontally, as a more meridional redistribution (“Hadley-like”), and vertically, as a substantially “bottom-heavy” profile, relative to the reanalyses anomalies.

In a companion paper, deficiencies in the simulated ENSO surface winds are related to specific features of the CCM3’s heating error, from diagnostic modeling.

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Chul Eddy Chung, V. Ramanathan, and Jeffrey T. Kiehl

Abstract

The effects of the south Asian haze on the regional climate are assessed using the National Center for Atmospheric Research Community Climate Model version 3 (CCM3) at the T42/L18 resolution. This haze, as documented during the Indian Ocean Experiment (INDOEX) campaign (1995–2000), consists mainly of anthropogenic aerosols, and spans over most of south Asia and the north Indian Ocean. It reduces the net solar flux at the surface by as much as 20–40 W m−2 on a monthly mean basis and heats the lowest 3-km atmosphere by as much as 0.4–0.8 K day−1, which enhances the solar heating of this layer by 50%–100%. This widespread haze layer is a seasonal phenomenon limited to the dry period between November and May.

The imposed haze radiative forcing leads to several large and statistically significant climate changes during the dry monsoon season, which include cooling of the land surface, and warming of the atmosphere. These temperature change features lead to the stabilization of the boundary layer that results in a reduction of evaporation and sensible heat flux from the land. The dynamical response to the aerosol forcing is surprisingly large. The aerosol forcing weakens the north–south temperature gradient in the lower level, which results in an enhancement of the area mean low-level convergence and a northward shift of the ITCZ. The increase in low-level convergence leads to increased convective rainfall and latent heat release, which in turn leads to a further increase in low-level convergence. This positive feedback between the low-level convergence and deep convective heating increases the average precipitation over the haze area by as much as 20%. The ocean surface undergoes a suppression of evaporation. Because of this decreased evaporation accompanied by the increase in the haze-area precipitation, the precipitation over the rest of the Tropics decreases, with a large fraction of this decrease concentrated over the Indonesian and the western Pacific warm pool region. The prescribed dry monsoon haze effect affects the summertime wet monsoon too, but a detailed analysis has to await the availability of year-round aerosol data.

The major inference from this study is that the effects of absorbing aerosols on the regional climate can be quite large. The simulated surface temperature response was very sensitive to the ratio (R) of the surface aerosol forcing to the atmospheric forcing. The R itself varies from −1.5 in clear skies to about −0.5 in overcast skies over ocean, and available experimental data are not sufficient to constrain its value more narrowly.

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