CGD's Dr. Clara Deser

Abstract

A basin-wide interdecadal change in both the physical state and the ecology of the North 5 Pacific occurred near the end of 1976. Here we use a physical-ecosystem model to 6 examine whether changes in the physical environment associated with the 1976-77 7 transition influenced the lower trophic levels of the food web and if so by what means. 8 The physical component is an ocean general circulation model, while the biological 9 component contains ten compartments: two phytoplankton, two zooplankton, two detritus 10 pools, nitrate, ammonium, silicate and TCO2. The model is forced with observed 11 atmospheric fields during 1960-1999. During spring, there is a ~40% reduction in 12 plankton biomass in all four plankton groups during 1977-88 relative to 1970-76 in the 13 central Gulf of Alaska (GOA). The epoch difference in plankton appears to be controlled 14 by the mixed layer depth. Enhanced Ekman pumping after 1976 caused the halocline to 15 shoal, and thus the mixed layer depth did not penetrate as deep in the central GOA during 16 late winter. As a result, more phytoplankton remained in the euphotic zone and 17 phytoplankton biomass began to increase earlier in the year after the 1976 transition. 18 Zooplankton biomass also increased but then grazing pressure led to a strong decrease in 19 phytoplankton by April followed by a drop in zooplankton by May: essentially the mean 20 seasonal cycle of plankton biomass was shifted earlier in the year. As the seasonal cycle 21 progressed, the difference in plankton concentrations between epochs reversed sign 22 again, leading to slightly greater zooplankton biomass during summer in the later epoch.

Figure caption: The 1977-88 mean (contours) and ? (shading) for a) small phyto-plankton (P1) in March, b) large phytoplankton (diatoms, P2) in April, c) microzooplankton (Z1) in April and d) mesozooplankton (Z2) in May. Also shown are the values of P1, P2, Z1, Z2 for each calendar month for the periods e) 1970-76 and 1977-88, and f) ?, the difference between the two periods. The P and Z values (mmol N m ) presented here and the subsequent figures are from the top model level.

Support: NSF and NOAA

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Cassou, C., C. Deser, M. A. Alexander, 2007: Investigating the impact of reemerging sea surface temperature anomalies on the winter atmospheric circulation over the North Atlantic. J. Climate, 20, 3510-3526.

Abstract

Extratropical SSTs can be influenced by the "reemergence mechanism", whereby thermal anomalies in the deep winter mixed layer persist at depth through summer and are then re-entrained into the mixed layer in the following winter. The impact of reemergence in the North Atlantic Ocean upon the climate system is investigated using an atmospheric general circulation model coupled to a mixed layer ocean/thermodynamic sea ice model.

The dominant pattern of thermal anomalies below the mixed layer in summer in a 150-year control integration is associated with the North Atlantic sea surface temperature (SST) tripole forced by the North Atlantic Oscillation (NAO) in the previous winter as indicated by singular value decomposition (SVD). To isolate the reemerging signal, two additional 60-member ensemble experiments were conducted in which temperature anomalies below 40 m obtained from the SVD analysis are added to or subtracted from the control integration. The reemerging signal, given by the mean difference between the two 60-member ensembles, causes the SST anomaly tripole to recur, beginning in fall, amplifying through January and persisting through the following spring. The atmospheric response to these SST anomalies resembles the circulation that created them the previous winter but with reduced amplitude (10-20 m at 500 mb per °C), modestly enhancing the winter-to-winter persistence of the NAO. Changes in the transient eddies and their interaction with the mean flow contribute to the large-scale equivalent barotropic response throughout the troposphere. The latter can also be attributed to the change in occurrence of intrinsic weather regimes.

Figure caption: REM response in NDJFM for (a) SLP, (b) T850, (d) Z500, and (e) U200. The mean U200 elimatology given by CTL is superimposed (m s^-1, thick solid line) and contours start at 30 m s^-1, every 10 m s^-1. Contour intervals are 0.3 hPa for SLP, 0.1 °C for T850, 2 m for Z500 and 0.4 m s^-1 for U200 anomalies. Shaded areas exceed the 95% significance level based on Student's t statistic. (c) REM sea ice response for JFM (grey shading for decrease, dotting for increase fraction) on which REM surface wind response is superimposed.

Support: in part by NOAA under Grant NA06GP0394 and by CNRS and by the European Community via the sixth framework ENSEMBLES project under Contract GOCE-CT-2003-505539.

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Deser, C., and A. S. Phillips, 2006: Simulation of the 1976/1977 climate transition over the North Pacific: sensitivity to tropical forcing. Journal of Climate, 19, 170-6180.

Abstract

This study examines the contribution of tropical Sea Surface Temperature (SST) forcing to the 1976/1977 climate transition of the winter atmospheric circulation over the North Pacific using a combined observational and modeling approach. The National Center for Atmospheric Research (NCAR) Community Atmospheric Model Version 3 (CAM3) simulates approximately 75% of the observed 4 hPa deepening of the wintertime Aleutian Low from 1950-1976 to 1977-2000 when forced with the observed evolution of tropical SSTs in a 10-member ensemble average. This response is driven by precipitation increases over the western half of the equatorial Pacific Ocean. In contrast, the NCAR Community Climate Model Version 3 (CCM3), the predecessor to CAM3, simulates no significant change in the strength of the Aleutian Low when forced with the same tropical SSTs in a 12-member ensemble average. The lack of response in CCM3 is traced to an erroneously large precipitation increase over the tropical Indian Ocean whose dynamical impact is to weaken the Aleutian Low; this, when combined with the response to rainfall increases over the western and central equatorial Pacific, results in near-zero net change in the strength of the Aleutian Low. The observed distribution of tropical precipitation anomalies associated with the 1976/1977 transition, estimated from a combination of direct measurements at land stations and indirect information from surface marine cloudiness and wind divergence fields, supports the models' simulated rainfall increases over the western half of the Pacific but not the magnitude of CCM3's rainfall increase over the Indian Ocean.

Figure caption: Epoch differences of winder (December - February) sea level pressure (SLP; left) and 500 hPa geopotential height (Z500; right), obtained by subtracting the period 1950-1976 from the period 1977-2000, from observations and three atmospheric general circulation models forced with observed time-varying tropical sea surface temperatures (referred to as "TOGA" simulations): CAM3 at T85 horizontal resolution, CAM3 at T42 horizontal resolution, and CCM3 at T42 horizontal resolution. The contour interval for SLP (Z500) is 1 hPa (10 m), the zero contours are omitted, negative contours are dashed, and the shading indicates epoch difference values that are statistically significant at the 95% level based on a Student's t-test.

Support: in part by a grant from NOAA's Office of Global Programs.

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Deser, C., R.A. Tomas, and S. Peng, 2007: The transient atmospheric circulation response to North Atlantic SST and sea ice anomalies. J. Climate, 20, 4751-4767.

Abstract

The objective of this study is to investigate the transient evolution of the wintertime atmospheric circulation response to imposed patterns of SST and sea ice extent anomalies in the North Atlantic sector using a large ensemble of experiments with the NCAR Community Climate Model Version 3 (CCM3). The initial adjustment of the atmospheric circulation is characterized by an out-of-phase relationship between geopotential height anomalies in the lower and upper troposphere localized to the vicinity of the forcing. This initial baroclinic response reaches maximum amplitude in ~ 5 - 10 days, and persists for 2 \u2013 3 weeks. Diagnostic results with a linear primitive equation model indicate that this initial response is forced by diabatic heating anomalies in the lower troposphere associated with surface heat flux anomalies generated by the imposed thermal forcing. Following the initial baroclinic stage of adjustment, the response becomes progressively more barotropic and increases in both spatial extent and magnitude. The equilibrium stage of adjustment is reached in 2 \u2013 2.5 months, and is characterized by an equivalent barotropic structure that resembles the hemispheric NAO/NAM pattern, the model\u2019s leading internal mode of circulation variability over the northern hemisphere. The maximum amplitude of the equilibrium response is approximately 2-3 times larger than that of the initial response. The equilibrium response is maintained primarily by non-linear transient eddy fluxes of vorticity (and, to a lesser extent, heat), with diabatic heating making a limited contribution in the vicinity of the forcing.

Support: NSF Grant ATM-0233568 and NCAR's Water Cycle Program.

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Hurrell, J. W., and C. Deser, 2007: North Atlantic climate variability: the role of the North Atlantic Oscillation. Journal of Marine Systems, in press.

Summary

Marine ecosystems are undergoing rapid change at local and global scales. To understand these changes, including the relative roles of natural variability and anthropogenic effects, and to predict the future state of marine ecosystems requires quantitative understanding of the physics, biogeochemistry and ecology of oceanic systems at mechanistic levels. Central to this understanding is the role played by dominant patterns or "modes" of atmospheric and oceanic variability, which orchestrate coherent variations in climate over large regions with profound impacts on ecosystems. We review the spatial structure of extratropical climate variability over the Northern Hemisphere and, specifically, focus on modes of climate variability over the extratropical North Atlantic.

A leading pattern of weather and climate variability over the Northern Hemisphere is the North Atlantic Oscillation (NAO). The NAO refers to a redistribution of atmospheric mass between the Arctic and the subtropical Atlantic, and swings from one phase to another produce large changes in surface air temperature, winds, storminess and precipitation over the Atlantic as well as the adjacent continents. The NAO also affects the ocean through changes in heat content, gyre circulations, mixed layer depth (Fig. 1), salinity, high latitude deep water formation and sea ice cover. Thus, indices of the NAO have become widely used to document and understand how this mode of variability alters the structure and functioning of marine ecosystems.

There is no unique way, however, to define the NAO. Several approaches are discussed, including both linear (e.g., principal component analysis) and nonlinear (e.g., cluster analysis) techniques. The former, which have been most widely used, assume preferred atmospheric circulation states come in pairs, in which anomalies of opposite polarity have the same spatial structure. In contrast, nonlinear techniques search for recurrent patterns of a specific amplitude and sign. They reveal, for instance, spatial asymmetries between different phases of the NAO that are likely important for ecological studies.

It also follows that there is no universally accepted index to describe the temporal evolution of the NAO. Several of the most common measures are presented and compared. All reveal that there is no preferred time scale of variability for the NAO: large changes occur from one winter to the next and from one decade to the next. There is also a large amount of within-season variability in the patterns of atmospheric circulation of the North Atlantic, so that most winters cannot be characterized solely by a canonical NAO structure. A better understanding of how the NAO responds to external forcing, including sea surface temperature changes in the tropics, stratospheric influences, and increasing greenhouse gas concentrations, is crucial to the current debate on climate variability and change.

Figure caption: Difference in mean winter (December-March) ocean mixed layer depth (m) between years when an index of the NAO exceeds one standard deviation over 1955-2003. The contour increment is 6 m and positive (negative) differences are given by the solid (dashed) contours.

Support: CLIVAR Program of the NOAA Office of Global Programs. The National Center for Atmospheric Research is sponsored by the National Science Foundation.

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Jochum, M., C. Deser, A. Phillips, 2007: Tropical atmospheric variability forced by oceanic internal variability. J. Climate, 20, 765-771.

Abstract

Atmospheric general circulation model experiments are conducted to quantify the contribution of internal oceanic variability in the form of tropical instability waves (TIWs) to interannual wind and rainfall variability in the tropical Pacific. It is found that in the tropical Pacific, along the equator, and near 25°N and 25°S, TIWs force a significant increase in wind and rainfall variability from interseasonal to interannual time scales. Because of the stochastic nature of TIWs, this means that climate models that do not take them into account will underestimate the strength and number of extreme events and may overestimate forecast capability.

Figure caption: Regression of SST, precipitation, and wind stress anomalies onto the SST anomaly averaged between 3°S-3°N and 145°-139°W. Only the statistically significant regressions are shown [correlation coefficient of at least 0.15 for a 95% confidence interval; the contour interval for precipitation is 0.1 mm day-1 (color) and for SST is 0.05°C (black)]. Thus, an SST anomaly of 0.25° (approximately the TIW-induced rms of SST; see Fig. 6) in this area will at 8°N, 150°W reduce the rainfall rate by 0.3 mm day-1 and reduce the easterlies at 1°S, 160°W by 0.004 N m-2. Regressions for SST anomalies farther west and east are smaller because toward the west TIWs are weaker and toward the east the mean SST is colder (and convection is less sensitive to SST changes).

Support: NSF and NOAA.

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Kwon, Y.-O., and C. Deser, 2007: North Pacific decadal variability in the Community Climate System Model Version 2. Journal of Climate, 20, 2416-2433.

Abstract

North Pacific decadal oceanic and atmospheric variability is examined from a 650-year control integration of Community Climate System Model version 2. The dominant pattern of winter sea surface temperature (SST) variability is similar to the observed "Pacific Decadal Oscillation", with maximum amplitude along the Kuroshio Current Extension. SST anomalies in this region exhibit significant spectral peaks at approximately 16 years and 40 years. Lateral geostrophic heat flux divergence, caused by a meridional shift of the Kuroshio Current Extension forced by basin scale wind stress curl anomalies 3-5 years earlier, is responsible for the decadal SST variability; local surface heat flux and Ekman heat flux divergence act as a damping and positive feedback, respectively. A simple linear Rossby wave model is invoked to explicitly demonstrate the link between the wind stress curl forcing and decadal variability in the Kuroshio Current Extension. The Rossby wave model not only successfully reproduces the two decadal spectral peaks, but also illustrates that only the low-frequency (> 10-year period) portion of the approximately white-noise wind stress curl forcing is relevant. This model also demonstrates that the weak and insignificant decadal spectral peaks in the wind stress curl forcing are necessary for producing the corresponding strong and significant oceanic peaks in the Kuroshio Current Extension. The wind stress curl response to decadal SST anomalies in the Kuroshio Current Extension is similar in structure but opposite in sign and somewhat weaker than the wind stress curl forcing pattern. These results suggest that the simulated North Pacific decadal variability owes its existence to two-way ocean-atmosphere coupling.

Figure caption: Lag-regressions of DJFM (left) SST and (right) 500 m temperature on the SST Kuroshio Extention Index. Lags are noted in the center column, with positive (negative) lag indicating the index time series is leading (lagging) the temperature field. Positive (negative) regression coefficients are contoured with red (blue) lines. Zero contours are plotted with black lines. Contour interval is 0.3°C °C^-1, and gray shadings indicate regression values significant at 99%. All variables are low-pass filtered to retain periods longer than 10 years.

Support: NOAA's Office of Global Programs.

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Park, S., M. A. Alexander, and Clara Deser, 2006: The impact of cloud radiative feedback, remote ENSO forcing, and entrainment on the persistence of North Pacific sea surface temperature anomalies. Journal of Climate; 19, 6243-6261.

Abstract

The influence of cloud radiative feedback, remote ENSO heat flux forcing, and oceanic entrainment on persisting North Pacific sea surface temperature (SST) anomalies is investigated using a stochastically-forced ocean mixed layer model. The stochastic heat flux is estimated from an atmospheric general circulation model, the seasonally-varying radiative feedback parameter and remote ENSO forcing are obtained from observations, and entrainment is derived from the observed mean seasonal cycle of ocean mixed layer depth. Persistence is examined via SST autocorrelations in the western, central and subtropical eastern North Pacific and for the leading pattern of variability across the basin. The contribution of clouds, ENSO, and entrainment to SST persistence is evaluated by comparing simulations with and without each term. The SST autocorrelation structure in the model closely resembles nature: the pattern correlation between the two is 0.87-0.9 in the three regions and for the basin-wide analyses, and 0.35- 0.66 after subtracting an exponential function representing the background damping due to air-sea heat fluxes. Positive radiative feedback enhances SST autocorrelations (~0.1-0.3) from late spring to summer in the central and western Pacific and from late summer through fall in the subtropical eastern Pacific. The influence of the remote ENSO forcing on SST autocorrelation varies with season and location with a maximum impact on the correlation magnitude of 0.2-0.3. The winter-to- winter recurrence of higher autocorrelations is caused by entrainment, which generally suppresses SST variability but returns thermal anomalies sequestered beneath the mixed layer in summer back to the surface in the following fall/winter. This reemergence mechanism enhances SST autocorrelation by ~0.3 at lags of 9-12 months from the previous winter in the western and central Pacific but only slightly enhances autocorrelation (~0.1) in the subtropical eastern Pacific.

The impact of clouds, ENSO and entrainment on the autocorrelation structure of the basinwide SST anomaly pattern is similar to that in the western region. ENSO's impact on the basinwide North Pacific SST autocorrelation in an atmospheric general circulation model coupled to an ocean mixed layer model with observed SSTs specified in the tropical Pacific is very similar to the results from the stochastic model developed here.

Figure caption: Ensemble mean autocorrelation (r) of high-pass ?ltered (< 12.5 years) monthly SST anomalies in CWP as a function of calendar month and lag from (a) observations obtained from the average of 4 different data sets and (b) 50-member ensemble of the complete stochastic model. The contour/shading interval is 0.1 and the zero contour is indicated by a red line. Lagged auto-correlations out to 24 months for the reference months of (c) April and (d) September, where vertical bars indicate one standard deviation of the autocorrelations at each lag.

Support: NOAA Office of Global Programs CLIVAR Project.

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Ueyama, Rei and Clara Deser, 2007: A climatology of diurnal and semidiurnal surface wind variations over the tropical Pacific ocean based on the tropical atmosphere ocean moored buoy array. Journal of Climate, in press.

Abstract

Hourly measurements from 51 moored buoys in the Tropical Atmosphere Ocean array (9°N-8°S, 165°E-95°W) during 1993-2004 are used to document the climatological seasonal and annual mean patterns of diurnal and semidiurnal near-surface wind variability over the tropical Pacific Ocean. In all seasons, the amplitude of the semidiurnal harmonic is approximately twice as large as the diurnal harmonic for the zonal wind component, while the diurnal harmonic is at least three times as large as the semidiurnal harmonic for the meridional wind component, averaged across the buoy array. Except for the eastern equatorial Pacific, the semidiurnal zonal wind harmonic exhibits uniform amplitude (~ 0.14 m s-1) and phase (maximum westerly wind anomalies ~ 0325/1525 local time) across the basin in all seasons. This pattern is well explained by atmospheric thermal tidal theory. The semidiurnal zonal wind signal is diminished over the cold surface waters of the eastern equatorial Pacific where it is associated with enhanced boundary layer stability. Diurnal meridional wind variations tend to be out-of-phase north and south of the equator (maximum southerly wind anomalies ~ 0700 LT at 5°N and ~ 1900 LT at 5°S), while a noon southerly wind anomaly maximum is observed on the equator in the eastern Pacific particularly during the cold season (Jun-Nov). The diurnal meridional wind variations result in enhanced divergence along the equator and convergence along the southern border of the intertropical convergence zone ~ 0700 LT (opposite conditions ~ 1900 LT); the amplitude of the divergence diurnal cycle is ~ 5 x 10-7 s-1. The diurnal meridional wind variations are largely consistent with the diurnal pressure gradient force.

Support: NSF.