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- Author or Editor: Geert Jan van Oldenborgh x
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Abstract
There has been intense debate about the causes of the 1997–98 El Niño. One side sees the obvious intense westerly wind events as the main cause for the exceptional heating in summer 1997, the other emphasizes slower oceanic processes. A quantitative analysis of all factors contributing to the onset of this El Niño is presented here. Specifically, the Niño-3 index in the Hamburg Ocean Primitive Equation Model OGCM at 1 June 1997 is decomposed into contributions from the fluxes and initial state at six months’ lead time. The initial-state thermal anomalies contribute about 40% compared with an average year, and the wind stress about 50%. Compared with the previous year, in which no El Niño developed, the main difference is in the zonal wind stress. This contribution is concentrated at the time and place of the strong westerly wind events in December 1996, and March and April 1997. As westerly wind events are difficult to predict, this limited the predictability of the onset of this El Niño.
Abstract
There has been intense debate about the causes of the 1997–98 El Niño. One side sees the obvious intense westerly wind events as the main cause for the exceptional heating in summer 1997, the other emphasizes slower oceanic processes. A quantitative analysis of all factors contributing to the onset of this El Niño is presented here. Specifically, the Niño-3 index in the Hamburg Ocean Primitive Equation Model OGCM at 1 June 1997 is decomposed into contributions from the fluxes and initial state at six months’ lead time. The initial-state thermal anomalies contribute about 40% compared with an average year, and the wind stress about 50%. Compared with the previous year, in which no El Niño developed, the main difference is in the zonal wind stress. This contribution is concentrated at the time and place of the strong westerly wind events in December 1996, and March and April 1997. As westerly wind events are difficult to predict, this limited the predictability of the onset of this El Niño.
Abstract
The nonlinearities that cause El Niño events to deviate more from the mean state than La Niña events are still not completely understood. This paper investigates the contribution of one candidate mechanism: ENSO nonlinearities originating from the atmosphere. The initially linear intermediate complexity model of the equatorial Pacific Ocean, in which all couplings were fitted to observations, describes the ENSO cycle reasonably well. In this linear model, extra terms are systematically introduced in the atmospheric component: the nonlinear response of mean wind stress to SST anomalies, the skewness of the driving noise term in the atmosphere, and the relation of this noise term to the background SST or the ENSO phase. The nonlinear response of mean wind stress to SST in the ENSO region is found to be the dominant term influencing the ENSO cycle. However, this influence is only visible when noise fields are used that are fitted to observed patterns of prescribed standard deviation and spatial decorrelation. Standard deviation and skewness of noise do have a dependence on the ENSO phase, but this has a relatively small influence on the ENSO cycle in this model. With these additional nonlinearities in the representation of the atmosphere, a step forward has been made toward building a realistic reduced complexity model for ENSO.
Abstract
The nonlinearities that cause El Niño events to deviate more from the mean state than La Niña events are still not completely understood. This paper investigates the contribution of one candidate mechanism: ENSO nonlinearities originating from the atmosphere. The initially linear intermediate complexity model of the equatorial Pacific Ocean, in which all couplings were fitted to observations, describes the ENSO cycle reasonably well. In this linear model, extra terms are systematically introduced in the atmospheric component: the nonlinear response of mean wind stress to SST anomalies, the skewness of the driving noise term in the atmosphere, and the relation of this noise term to the background SST or the ENSO phase. The nonlinear response of mean wind stress to SST in the ENSO region is found to be the dominant term influencing the ENSO cycle. However, this influence is only visible when noise fields are used that are fitted to observed patterns of prescribed standard deviation and spatial decorrelation. Standard deviation and skewness of noise do have a dependence on the ENSO phase, but this has a relatively small influence on the ENSO cycle in this model. With these additional nonlinearities in the representation of the atmosphere, a step forward has been made toward building a realistic reduced complexity model for ENSO.
Abstract
While sea surface temperature (SST) anomalies in the eastern equatorial Pacific are dominated by the thermocline feedback, in the central equatorial Pacific local wind effects, such as zonal advection, are important as well. El Niño–Southern Oscillation (ENSO) simulations with a linear model improve markedly if these effects are included as a local wind stress feedback on SST. An atmosphere model that reacts both to eastern and central Pacific SST anomalies is needed for producing a realistic ENSO cycle.
First, simulations are studied of a linear 1.5-layer reduced-gravity ocean model and a linear SST anomaly equation, forced by observed monthly wind stress. If only the thermocline feedback is present in the SST equation, SST can be simulated well in the eastern Pacific, but, contrary to observations, central Pacific SST is out of phase with the eastern Pacific. If a wind stress feedback is added in the SST equation, as a term proportional to the zonal wind stress, correlations between observed and simulated SST are above 0.8 in both the central and eastern Pacific, and the correlation between the Niño-3 (5°S–5°N, 90°–150°W) and Niño-4 (5°S–5°N, 150°W–160°E) indexes is close to the observed value of 0.75.
Next, a statistical atmosphere is added to the ocean module that is based on a regression of observed wind stress to the observed Niño-3 and Niño-4 indexes. The coupled system is driven by noise that is inferred from the residues of the fit and has a red component. The observed Niño-3–Niño-4 index correlation can be reproduced only with a wind stress feedback in the central Pacific. Also, the level of SST variability rises and the ENSO period increases to more realistic values.
The interplay between the local wind stress and the thermocline feedbacks, therefore, is an important factor in the structure of ENSO in the coupled linear model. In the eastern Pacific, the thermocline feedback dominates SST anomalies; in the central Pacific, the local wind stress feedback. Due to the local wind stress feedback, the ENSO wind stress response excites SST anomalies in the central Pacific, extending the ENSO SST anomaly pattern well into the central Pacific. In turn, these central Pacific SST anomalies give rise to wind stress anomalies that are situated more westward than the response to eastern Pacific SST anomalies. As a result, the ENSO amplitude is enhanced and the ENSO period increased. Also, central Pacific SST anomalies are not completely determined by eastern Pacific SST anomalies and they persist longer.
Abstract
While sea surface temperature (SST) anomalies in the eastern equatorial Pacific are dominated by the thermocline feedback, in the central equatorial Pacific local wind effects, such as zonal advection, are important as well. El Niño–Southern Oscillation (ENSO) simulations with a linear model improve markedly if these effects are included as a local wind stress feedback on SST. An atmosphere model that reacts both to eastern and central Pacific SST anomalies is needed for producing a realistic ENSO cycle.
First, simulations are studied of a linear 1.5-layer reduced-gravity ocean model and a linear SST anomaly equation, forced by observed monthly wind stress. If only the thermocline feedback is present in the SST equation, SST can be simulated well in the eastern Pacific, but, contrary to observations, central Pacific SST is out of phase with the eastern Pacific. If a wind stress feedback is added in the SST equation, as a term proportional to the zonal wind stress, correlations between observed and simulated SST are above 0.8 in both the central and eastern Pacific, and the correlation between the Niño-3 (5°S–5°N, 90°–150°W) and Niño-4 (5°S–5°N, 150°W–160°E) indexes is close to the observed value of 0.75.
Next, a statistical atmosphere is added to the ocean module that is based on a regression of observed wind stress to the observed Niño-3 and Niño-4 indexes. The coupled system is driven by noise that is inferred from the residues of the fit and has a red component. The observed Niño-3–Niño-4 index correlation can be reproduced only with a wind stress feedback in the central Pacific. Also, the level of SST variability rises and the ENSO period increases to more realistic values.
The interplay between the local wind stress and the thermocline feedbacks, therefore, is an important factor in the structure of ENSO in the coupled linear model. In the eastern Pacific, the thermocline feedback dominates SST anomalies; in the central Pacific, the local wind stress feedback. Due to the local wind stress feedback, the ENSO wind stress response excites SST anomalies in the central Pacific, extending the ENSO SST anomaly pattern well into the central Pacific. In turn, these central Pacific SST anomalies give rise to wind stress anomalies that are situated more westward than the response to eastern Pacific SST anomalies. As a result, the ENSO amplitude is enhanced and the ENSO period increased. Also, central Pacific SST anomalies are not completely determined by eastern Pacific SST anomalies and they persist longer.
Abstract
The pattern of global mean temperature (GMT) change is calculated by regressing local surface air temperature (SAT) to GMT for an ensemble of CMIP5 models and for observations over the last 132 years. Calculations are based on the historical period and climate change scenarios. As in the observations the warming pattern contains a warming hole over the subpolar North Atlantic. Using a bivariate regression of SAT to GMT and an index of the Atlantic meridional overturning circulation (AMOC), the warming pattern is decomposed in a radiatively forced part and an AMOC fingerprint. The North Atlantic warming hole is associated with a decline of the AMOC. The AMOC fingerprint resembles Atlantic multidecadal variability (AMV), but details of the pattern change when the AMOC decline increases, underscoring the nonlinearity in the response.
The warming hole is situated south of deep convection sites, indicating that it involves an adjustment of the gyre circulation, although it should be noted that some models feature deep convection in the middle of the subpolar gyre. The warming hole is already prominent in historical runs, where the response of the AMOC to GMT is weak, which suggests that it is involved in an ocean adjustment that precedes the AMOC decline. In the more strongly forced scenario runs, the warming hole over the subpolar gyre becomes weaker, while cooling over the Nordic seas increases, consistent with previous findings that deep convection in the Labrador and Irminger Seas is more vulnerable to changes in external forcing than convection in the Nordic seas, which only reacts after a threshold is passed.
Abstract
The pattern of global mean temperature (GMT) change is calculated by regressing local surface air temperature (SAT) to GMT for an ensemble of CMIP5 models and for observations over the last 132 years. Calculations are based on the historical period and climate change scenarios. As in the observations the warming pattern contains a warming hole over the subpolar North Atlantic. Using a bivariate regression of SAT to GMT and an index of the Atlantic meridional overturning circulation (AMOC), the warming pattern is decomposed in a radiatively forced part and an AMOC fingerprint. The North Atlantic warming hole is associated with a decline of the AMOC. The AMOC fingerprint resembles Atlantic multidecadal variability (AMV), but details of the pattern change when the AMOC decline increases, underscoring the nonlinearity in the response.
The warming hole is situated south of deep convection sites, indicating that it involves an adjustment of the gyre circulation, although it should be noted that some models feature deep convection in the middle of the subpolar gyre. The warming hole is already prominent in historical runs, where the response of the AMOC to GMT is weak, which suggests that it is involved in an ocean adjustment that precedes the AMOC decline. In the more strongly forced scenario runs, the warming hole over the subpolar gyre becomes weaker, while cooling over the Nordic seas increases, consistent with previous findings that deep convection in the Labrador and Irminger Seas is more vulnerable to changes in external forcing than convection in the Nordic seas, which only reacts after a threshold is passed.
Abstract
The time dependence of the local relation between sea surface temperature (SST) and thermocline depth in the central and eastern equatorial Pacific Ocean is analyzed for the period 1990–99, using subsurface temperature measurements from the Tropical Atmosphere–Ocean Array/Triangle Trans-Ocean Buoy Network (TAO/TRITON) buoy array. Thermocline depth anomalies lead SST anomalies in time, with a longitude-dependent delay ranging from 2 weeks in the eastern Pacific to 1 year in the central Pacific. The lagged correlation between thermocline depth and SST is strong, ranging from r > 0.9 in the east to r ≈ 0.6 at 170°W. Time-lagged correlations between thermocline depth and subsurface temperature anomalies indicate vertical advection of temperature anomalies from the thermocline to the surface in the eastern Pacific. The measurements are compared with the results of forced OGCM and linear model experiments. Using model results, it is shown that the delay between thermocline depth and SST is caused mainly by upwelling and mixing between 140° and 90°W. Between 170°E and 140°W the delay has a different explanation: thermocline depth anomalies travel to the eastern Pacific, where upwelling creates SST anomalies that in turn cause anomalous wind in the central Pacific. SST is then influenced by these wind anomalies.
Abstract
The time dependence of the local relation between sea surface temperature (SST) and thermocline depth in the central and eastern equatorial Pacific Ocean is analyzed for the period 1990–99, using subsurface temperature measurements from the Tropical Atmosphere–Ocean Array/Triangle Trans-Ocean Buoy Network (TAO/TRITON) buoy array. Thermocline depth anomalies lead SST anomalies in time, with a longitude-dependent delay ranging from 2 weeks in the eastern Pacific to 1 year in the central Pacific. The lagged correlation between thermocline depth and SST is strong, ranging from r > 0.9 in the east to r ≈ 0.6 at 170°W. Time-lagged correlations between thermocline depth and subsurface temperature anomalies indicate vertical advection of temperature anomalies from the thermocline to the surface in the eastern Pacific. The measurements are compared with the results of forced OGCM and linear model experiments. Using model results, it is shown that the delay between thermocline depth and SST is caused mainly by upwelling and mixing between 140° and 90°W. Between 170°E and 140°W the delay has a different explanation: thermocline depth anomalies travel to the eastern Pacific, where upwelling creates SST anomalies that in turn cause anomalous wind in the central Pacific. SST is then influenced by these wind anomalies.
Abstract
The changes in model ENSO behavior due to an increase in greenhouse gases, according to the Intergovernmental Panel on Climate Change (IPCC) Business-As-Usual scenario, are investigated using a 62-member ensemble 140-yr simulation (1940–2080) with the National Center for Atmospheric Research Community Climate System Model (CCSM; version 1.4). Although the global mean surface temperature increases by about 1.2 K over the period 2000–80, there are no significant changes in the ENSO period, amplitude, and spatial patterns. To explain this behavior, an analysis of the simulation results is combined with results from intermediate complexity coupled ocean–atmosphere models. It is shown that this version of the CCSM is incapable of simulating a correct meridional extension of the equatorial wind stress response to equatorial SST anomalies. The wind response pattern is too narrow and its strength is insensitive to background SST. This leads to a more stable Pacific climate system, a shorter ENSO period, and a reduced sensitivity of ENSO to global warming.
Abstract
The changes in model ENSO behavior due to an increase in greenhouse gases, according to the Intergovernmental Panel on Climate Change (IPCC) Business-As-Usual scenario, are investigated using a 62-member ensemble 140-yr simulation (1940–2080) with the National Center for Atmospheric Research Community Climate System Model (CCSM; version 1.4). Although the global mean surface temperature increases by about 1.2 K over the period 2000–80, there are no significant changes in the ENSO period, amplitude, and spatial patterns. To explain this behavior, an analysis of the simulation results is combined with results from intermediate complexity coupled ocean–atmosphere models. It is shown that this version of the CCSM is incapable of simulating a correct meridional extension of the equatorial wind stress response to equatorial SST anomalies. The wind response pattern is too narrow and its strength is insensitive to background SST. This leads to a more stable Pacific climate system, a shorter ENSO period, and a reduced sensitivity of ENSO to global warming.
Abstract
Probable changes in mean and extreme precipitation in East Africa are estimated from general circulation models (GCMs) prepared for the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4). Bayesian statistics are used to derive the relative weights assigned to each member in the multimodel ensemble. There is substantial evidence in support of a positive shift of the whole rainfall distribution in East Africa during the wet seasons. The models give indications for an increase in mean precipitation rates and intensity of high rainfall events but for less severe droughts. Upward precipitation trends are projected from early this (twenty first) century. As in the observations, a statistically significant link between sea surface temperature gradients in the tropical Indian Ocean and short rains (October–December) in East Africa is simulated in the GCMs. Furthermore, most models project a differential warming of the Indian Ocean during boreal autumn. This is favorable for an increase in the probability of positive Indian Ocean zonal mode events, which have been associated with anomalously strong short rains in East Africa. On top of the general increase in rainfall in the tropics due to thermodynamic effects, a change in the structure of the Eastern Hemisphere Walker circulation is consistent with an increase in East Africa precipitation relative to other regions within the same latitudinal belt. A notable feature of this change is a weakening of the climatological subsidence over eastern Kenya. East Africa is shown to be a region in which a coherent projection of future precipitation change can be made, supported by physical arguments. Although the rate of change is still uncertain, almost all results point to a wetter climate with more intense wet seasons and less severe droughts.
Abstract
Probable changes in mean and extreme precipitation in East Africa are estimated from general circulation models (GCMs) prepared for the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4). Bayesian statistics are used to derive the relative weights assigned to each member in the multimodel ensemble. There is substantial evidence in support of a positive shift of the whole rainfall distribution in East Africa during the wet seasons. The models give indications for an increase in mean precipitation rates and intensity of high rainfall events but for less severe droughts. Upward precipitation trends are projected from early this (twenty first) century. As in the observations, a statistically significant link between sea surface temperature gradients in the tropical Indian Ocean and short rains (October–December) in East Africa is simulated in the GCMs. Furthermore, most models project a differential warming of the Indian Ocean during boreal autumn. This is favorable for an increase in the probability of positive Indian Ocean zonal mode events, which have been associated with anomalously strong short rains in East Africa. On top of the general increase in rainfall in the tropics due to thermodynamic effects, a change in the structure of the Eastern Hemisphere Walker circulation is consistent with an increase in East Africa precipitation relative to other regions within the same latitudinal belt. A notable feature of this change is a weakening of the climatological subsidence over eastern Kenya. East Africa is shown to be a region in which a coherent projection of future precipitation change can be made, supported by physical arguments. Although the rate of change is still uncertain, almost all results point to a wetter climate with more intense wet seasons and less severe droughts.
