Search Results

You are looking at 61 - 70 of 102 items for

  • Author or Editor: Clara Deser x
  • Refine by Access: All Content x
Clear All Modify Search
Yuko M. Okumura
,
Masamichi Ohba
,
Clara Deser
, and
Hiroaki Ueda

Abstract

El Niño and La Niña exhibit significant asymmetry not only in their spatial structure but also in their duration. Most El Niños terminate rapidly after maturing near the end of the calendar year, whereas many La Niñas persist into the following year and often reintensify in boreal winter. Through atmospheric general circulation model experiments, it is shown that the nonlinear response of atmospheric deep convection to the polarity of equatorial Pacific sea surface temperature anomalies causes an asymmetric evolution of surface wind anomalies over the far western Pacific around the mature phase of El Niño and La Niña. Because of the eastward displacement of precipitation anomalies in the equatorial Pacific during El Niño compared to La Niña, surface winds in the western Pacific are more affected by SST forcing outside the equatorial Pacific, which acts to terminate the Pacific event.

Full access
Michael A. Alexander
,
Robert Tomas
,
Clara Deser
, and
David M. Lawrence

Abstract

Two atmospheric general circulation model experiments are conducted with specified terrestrial snow conditions representative of 1980–99 and 2080–99. The snow states are obtained from twentieth-century and twenty-first-century coupled climate model integrations under increasing greenhouse gas concentrations. Sea surface temperatures, sea ice, and greenhouse gas concentrations are set to 1980–99 values in both atmospheric model experiments to isolate the effect of the snow changes. The reduction in snow cover in the twenty-first century relative to the twentieth century increases the solar radiation absorbed by the surface, and it enhances the upward longwave radiation and latent and sensible fluxes that warm the overlying atmosphere. The maximum twenty-first-century minus twentieth-century surface air temperature (SAT) differences are relatively small (<3°C) compared with those due to Arctic sea ice changes (∼10°C). However, they are continental in scale and are largest in fall and spring, when they make a significant contribution to the overall warming over Eurasia and North America in the twenty-first century. The circulation response to the snow changes, while of modest amplitude, involves multiple components, including a local low-level trough, remote Rossby wave trains, an annular pattern that is strongest in the stratosphere, and a hemispheric increase in geopotential height.

Full access
Michael S. Timlin
,
Michael A. Alexander
, and
Clara Deser

Abstract

The reemergence mechanism, whereby temperature anomalies extending over the deep winter mixed layer are stored beneath the surface in summer and are reentrained into the mixed layer when it deepens again in the following autumn and winter, is studied in the North Atlantic using approximately 40 years of surface and subsurface data. Reemergence is found to be robust in the Sargasso Sea and the northeast Atlantic, regions where (i) the mixed layer is much deeper in winter than in summer, (ii) currents are relatively weak, and (iii) temperature anomalies are coherent over broad areas. The two leading empirical orthogonal functions of North Atlantic SST anomalies also exhibit strong reemergence signatures. A novel application of empirical orthogonal function analysis to temperature anomalies in the time–depth plane, which also incorporates information from all grid points, is shown to be an efficient and useful approach for detecting reemergence without dependence on specific spatial patterns or prior selection of regions.

Full access
Christophe Cassou
,
Clara Deser
,
Laurent Terray
,
James W. Hurrell
, and
Marie Drévillon

Abstract

The origin of the so-called summer North Atlantic “Horseshoe” (HS) sea surface temperature (SST) mode of variability, which is statistically linked to the next winter's North Atlantic Oscillation (NAO), is investigated from data and experiments with the CCM3 atmospheric general circulation model (AGCM). Lagged observational analyses reveal a linkage between HS and anomalous rainfall in the vicinity of the Atlantic intertropical convergence zone. Prescribing the observed anomalous convection in the model generates forced atmospheric Rossby waves that propagate into the North Atlantic sector. The accompanying perturbations in the surface turbulent and radiative fluxes are consistent with forcing the SST anomalies associated with HS. It is suggested that HS can therefore be interpreted as the remote footprint of tropical atmospheric changes.

The ARPEGE AGCM is then used to test if the persistence of HS SST anomalies from summer to late fall can feed back to the atmosphere and have an impact on the next winter's North Atlantic variability. Observed HS SST patterns are imposed in the model from August to November. They generate a weak but coherent early winter response projecting onto the NAO and therefore reproduce the observed HS–NAO relationship obtained from lagged statistics. Changes in the simulated upper-level jet are associated with the anomalous HS meridional SST gradient and interact with synoptic eddy activity from October onward. The strength and position of the transients as a function of seasons are hypothesized to be of central importance to explain the nature, timing, and sign of the model response.

In summary, the present study emphasizes the importance of summer oceanic and atmospheric conditions in both the Tropics and extratropics, and their persistence into early winter for explaining part of the NAO's low-frequency variability.

Full access
Clara Deser
,
Adam S. Phillips
, and
James W. Hurrell

Abstract

This study examines the tropical linkages to interdecadal climate fluctuations over the North Pacific during boreal winter through a comprehensive and physically based analysis of a wide variety of observational datasets spanning the twentieth century. Simple difference maps between epochs of high sea level pressure over the North Pacific (1900–24 and 1947–76) and epochs of low pressure (1925–46 and 1977–97) are presented for numerous climate variables throughout the tropical Indo-Pacific region, including rainfall, cloudiness, sea surface temperature (SST), and sea level pressure. The results support the notion that the Tropics play a key role in North Pacific interdecadal climate variability. In particular, SST anomalies in the tropical Indian Ocean and southeast Pacific Ocean, rainfall and cloudiness anomalies in the vicinity of the South Pacific convergence zone, stratus clouds in the eastern tropical Pacific, and sea level pressure differences between the tropical southeast Pacific and Indian Oceans all exhibit prominent interdecadal fluctuations that are coherent with those in sea level pressure over the North Pacific. The spatial patterns of the interdecadal tropical climate anomalies are compared with those associated with ENSO, a predominantly interannual phenomenon; in general, the two are similar with some differences in relative spatial emphasis. Finally, a published 194-yr coral record in the western tropical Indian Ocean is shown to compare favorably with the twentieth-century instrumental records, indicating the potential for extending knowledge of tropical interdecadal climate variability to earlier time periods.

Full access
Clara Deser
,
Michael A. Alexander
, and
Michael S. Timlin

Abstract

An extension of the simple stochastic climate model of Frankignoul and Hasselman that includes the effects of seasonal variations in upper-ocean mixed layer depth upon the persistence of winter sea surface temperature (SST) anomalies is proposed. Seasonal variations in mixed layer depth allow for the “reemergence mechanism,” whereby thermal anomalies stored in the deep winter mixed layer persist at depth through summer and become partially reentrained into the mixed layer during the following winter. In this way, SST anomalies can recur from winter to winter without persisting through the intervening summer. Reformulating the simple stochastic climate model in terms of an effective ocean thermal capacity given by the depth of the winter mixed layer, thereby implicitly taking into account reemergence, is shown to provide a favorable fit to the observed winter-to-winter SST autocorrelations in the North Atlantic and Pacific, and represents a considerable improvement over the original model. The extended model also compares favorably with results from an entraining bulk ocean mixed layer model coupled to an atmospheric general circulation model. The authors propose that the extended model be adopted as the new “null hypothesis” for interannual SST variability in middle and high latitudes.

Full access
Christopher E. Lietzke
,
Clara Deser
, and
Thomas H. Vonder Haar

Abstract

For about a month near the boreal vernal equinox, the eastern Pacific intertropical convergence zone (ITCZ) is observed to form two troughs quasi-symmetrically situated about the equator near 5°–7° latitude during years when an equatorial sea surface cold tongue is present (e.g., La Niña years). The three-dimensional structure and temporal evolution of the eastern Pacific double ITCZ is documented using weekly cloud liquid and ice water fields and relative humidity profiles retrieved from Special Sensor Microwave/Temperature-2 measurements. The depth of convection in the southern branch of the double ITCZ, as determined by the coincident presence of cloud liquid and ice as well as by upward motion inferred from the relative humidity field, is observed to be sensitive to both the underlying SST and subsidence from the northern branch. The equatorial sea surface cold tongue appears to be the determining factor regulating the formation of a double ITCZ in the eastern Pacific. Areas of deep convection within the double ITCZ are accompanied by surface wind convergence maxima. However, the coincident maxima in deep convection and surface convergence are located several degrees of latitude equatorward of the highest sea surface temperatures.

Full access
Terrence M. Joyce
,
Clara Deser
, and
Michael A. Spall

Abstract

The Bermuda station “S” time series has been used to define the variability of subtropical mode water (STMW) from 1954 to 1995. This record, which shows decadal variability at a nominal period of about 12–14 yr, has been used as a baseline for seeking correlation with large-scale atmospheric forcing and with decadal north–south excursions of the Gulf Stream position defined by the subsurface temperature at 200-m depth. A common time period of 1954–89 inclusive, defined by the data sources, shows a high degree of correlation among the STMW potential vorticity (PV), Gulf Stream position, and large-scale atmospheric forcing (buoyancy flux, SST, and sea level pressure). Two pentads with anomalously small and large STMW PV were further studied and composites were made to define a revised North Atlantic Oscillation (NAO) index associated with the decadal forcing. During years of low PV at Bermuda, the NAO index is low, the Gulf Stream is in a southerly position, and the zero wind stress curl latitude is shifted south as are the composite extratropical winter storm tracks, in comparison to the period of high PV at Bermuda. Because the NAO, Gulf Stream separation latitude, and STMW PV variations are in phase with maximum annually averaged correlation at zero year time lag, the authors hypothesize that all must be either coupled with one another or with some other phenomenon that determines the covariability. A mechanism is proposed that could link all of the above together. It relies on the fact that during periods of high STMW PV, associated with a northerly Gulf Stream and a high NAO, one finds enhanced production of mode water in the subpolar gyre and Labrador Sea. Export of the enhanced Labrador Sea Water (LSW) component into the North Atlantic via the Deep Western Boundary Current can influence the separation point of the Gulf Stream in the upper ocean once the signal propagates from the source region to the crossover point with the Gulf Stream. If the SST signal produced by the 100-km shift of the Gulf Stream along a substantial (1000 km) length of its path as it leaves the coast can influence the NAO, a negative feedback oscillation may develop with a timescale proportional to the time delay between the change of phase of the air–sea forcing in the Labrador Basin and the LSW transient at the crossover point. Both a simple mechanistic model as well as a three-layer numerical model are used to examine this feedback, which could produce decadal oscillations given a moderately strong coupling.

Full access
Clara Deser
,
Michael A. Alexander
, and
Michael S. Timlin
Full access
Sarah M. Kang
,
Clara Deser
, and
Lorenzo M. Polvani

Abstract

The uncertainty arising from internal climate variability in climate change projections of the Hadley circulation (HC) is presently unknown. In this paper it is quantified by analyzing a 40-member ensemble of integrations of the Community Climate System Model, version 3 (CCSM3), under the Special Report on Emissions Scenarios (SRES) A1B scenario over the period 2000–60. An additional set of 100-yr-long time-slice integrations with the atmospheric component of the same model [Community Atmosphere Model, version 3.0 (CAM3)] is also analyzed.

Focusing on simple metrics of the HC—its strength, width, and height—three key results emerge from the analysis of the CCSM3 ensemble. First, the projected weakening of the HC is almost entirely confined to the Northern Hemisphere, and is stronger in winter than in summer. Second, the projected widening of the HC occurs only in the winter season but in both hemispheres. Third, the projected rise of the tropical tropopause occurs in both hemispheres and in all seasons and is, by far, the most robust of the three metrics.

This paper shows further that uncertainty in future trends of the HC width is largely controlled by extratropical variability, while those of HC strength and height are associated primarily with tropical dynamics. Comparison of the CCSM3 and CAM3 integrations reveals that ocean–atmosphere coupling is the dominant source of uncertainty in future trends of HC strength and height and of the tropical mean meridional circulation in general. Finally, uncertainty in future trends of the hydrological cycle is largely captured by the uncertainty in future trends of the mean meridional circulation.

Full access