Axel Timmermann

Increased El Niño frequency in a climate model forced by future greenhouse warming

The El Niño/Southern Oscillation (ENSO) phenomenon is the strongest natural interannual climate fluctuation. ENSO originates in the tropical Pacific Ocean and has large effects on the ecology of the region, but it also influences the entire global climate system and affects the societies and economies of manycountries. ENSO can be understood as an irregular low-frequency oscillation between a warm (El Niño) and a cold (La Niña) state. The strong El Niños of 1982/1983 and 1997/1998, along with the more frequent occurrences of El Niños during the past few decades, raise the question of whether human-induced ‘greenhouse’ warming affects, or will affect, ENSO. Several global climate models have been applied to transient greenhouse-gas-induced warming simulations to address this question, but the results have been debated owing to the inability of the models to fully simulate ENSO (because of their coarse equatorial resolution). Here we present results from a global climate model with sufficient resolution in the tropics to adequately represent the narrow equatorial upwelling and low-frequency waves. When the model is forced by a realistic future scenario of increasing greenhouse-gas concentrations, more frequent El-Niño-like conditions and stronger cold events in the tropical Pacific Ocean result.

Northern Hemispheric Interdecadal Variability: A Coupled Air–Sea Mode

A coupled air‐sea mode in the Northern Hemisphere with a period of about 35 years is described. The mode was derived from a multicentury integration with a coupled ocean‐atmosphere general circulation model and involves interactions of the thermohaline circulation with the atmosphere in the North Atlantic and interactions between the ocean and the atmosphere in the North Pacific. The authors focus on the physics of the North Atlantic interdecadal variability. If, for instance, the North Atlantic thermohaline circulation is anomalously strong, the ocean is covered by positive sea surface temperature (SST) anomalies. The atmospheric response to these SST anomalies involves a strengthened North Atlantic Oscillation, which leads to anomalously weak evaporation and Ekman transport off Newfoundland and in the Greenland Sea, and the generation of negative sea surface salinity (SSS) anomalies. These SSS anomalies weaken the deep convection in the oceanic sinking regions and subsequently the strength of the thermohaline circulation. This leads to a reduced poleward heat transport and the formation of negative SST anomalies, which completes the phase reversal. The Atlantic and Pacific Oceans seem to be coupled via an atmospheric teleconnection pattern and the interdecadal Northern Hemispheric climate mode is interpreted as an inherently coupled air‐sea mode. Furthermore, the origin of the Northern Hemispheric warming observed recently is investigated. The observed temperatures are compared to a characteristic warming pattern derived from a greenhouse warming simulation with the authors’ coupled general circulation model and also with the Northern Hemispheric temperature pattern associated with the 35-yr climate mode. It is shown that the recent Northern Hemispheric warming projects well onto the temperature pattern of the interdecadal mode under consideration.

Strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation

The climate of the last glacial period was extremely variable, characterized by abrupt warming events in the Northern Hemisphere, accompanied by slower temperature changes in Antarctica and variations of global sea level. It is generally accepted that this millennial-scale climate variability was caused by abrupt changes in the ocean thermohaline circulation. Here we use a coupled ocean–atmosphere–sea ice model to show that freshwater discharge into the North Atlantic Ocean, in addition to a reduction of the thermohaline circulation, has a direct effect on Southern Ocean temperature. The related anomalous oceanic southward heat transport arises from a zonal density gradient in the subtropical North Atlantic caused by a fast wave-adjustment process. We present an extended and quantitative bipolar seesaw concept that explains the timing and amplitude of Greenland and Antarctic temperature changes, the slow changes in Antarctic temperature and its similarity to sea level, as well as a possible time lag of sea level with respect to Antarctic temperature during Marine Isotope Stage 3.

Enhanced Warming over the Global Subtropical Western Boundary Currents

Subtropical western boundary currents are warm, fast-flowing currents that form on the western side of ocean basins. They carry warm tropical water to the mid-latitudes and vent large amounts of heat and moisture to the atmosphere along their paths, affecting atmospheric jet streams and mid-latitude storms, as well as ocean carbon uptake1, 2, 3, 4. The possibility that these highly energetic currents might change under greenhouse-gas forcing has raised significant concerns5, 6, 7, but detecting such changes is challenging owing to limited observations. Here, using reconstructed sea surface temperature datasets and century-long ocean and atmosphere reanalysis products, we find that the post-1900 surface ocean warming rate over the path of these currents is two to three times faster than the global mean surface ocean warming rate. The accelerated warming is associated with a synchronous poleward shift and/or intensification of global subtropical western boundary currents in conjunction with a systematic change in winds over both hemispheres. This enhanced warming may reduce the ability of the oceans to absorb anthropogenic carbon dioxide over these regions. However, uncertainties in detection and attribution of these warming trends remain, pointing to a need for a long-term monitoring network of the global western boundary currents and their extensions.

The influence of a weakening of the Atlantic meridional overturning circulation on ENSO

The influences of a substantial weakening of the Atlantic meridional overturning circulation (AMOC) on the tropical Pacific climate mean state, the annual cycle, and ENSO variability are studied using five different coupled general circulation models (CGCMs). In the CGCMs, a substantial weakening of the AMOC is induced by adding freshwater flux forcing in the northern North Atlantic. In response, the well-known surface temperature dipole in the low-latitude Atlantic is established, which reorganizes the large-scale tropical atmospheric circulation by increasing the northeasterly trade winds. This leads to a southward shift of the intertropical convergence zone (ITCZ) in the tropical Atlantic and also the eastern tropical Pacific. Because of evaporative fluxes, mixing, and changes in Ekman divergence, a meridional temperature anomaly is generated in the northeastern tropical Pacific, which leads to the development of a meridionally symmetric thermal background state. In four out of five CGCMs this leads to a substantial weakening of the annual cycle in the eastern equatorial Pacific and a subsequent intensification of ENSO variability due to nonlinear interactions. In one of the CGCM simulations, an ENSO intensification occurs as a result of a zonal mean thermocline shoaling. Analysis suggests that the atmospheric circulation changes forced by tropical Atlantic SSTs can easily influence the large-scale atmospheric circulation and hence tropical eastern Pacific climate. Furthermore, it is concluded that the existence of the present-day tropical Pacific cold tongue complex and the annual cycle in the eastern equatorial Pacific are partly controlled by the strength of the AMOC. The results may have important implications for the interpretation of global multidecadal variability and paleo-proxy data.

Deepwater Formation in the North Pacific During the Last Glacial Termination

Switching Basins Most of the densest, deepest water at the bottom of the oceans comes from two regions, the North Atlantic and the circum-Antarctic. Have other regions been able to produce significant quantities of deep water in the past? For decades, researchers have looked, with limited success, for evidence of deepwater formation in the North Pacific since the time of the Last Glacial Maximum, about 23,000 years ago. Okazaki et al. (p. 200) combine published observational evidence from the North Pacific with model simulations to suggest that deep water did form in the North Pacific during the early part of the Last Glacial Termination, between about 17,500 and 15,000 years ago. The switch between deep-water formation in the North Atlantic and the North Pacific is likely to have had an important effect on heat transport and climate. The Atlantic was not the only ocean in the Northern Hemisphere in which deep water formed during the last deglaciation. Between ~17,500 and 15,000 years ago, the Atlantic meridional overturning circulation weakened substantially in response to meltwater discharges from disintegrating Northern Hemispheric glacial ice sheets. The global effects of this reorganization of poleward heat flow in the North Atlantic extended to Antarctica and the North Pacific. Here we present evidence from North Pacific paleo surface proxy data, a compilation of marine radiocarbon age ventilation records, and global climate model simulations to suggest that during the early stages of the Last Glacial Termination, deep water extending to a depth of ~2500 to 3000 meters was formed in the North Pacific. A switch of deepwater formation between the North Atlantic and the North Pacific played a key role in regulating poleward oceanic heat transport during the Last Glacial Termination.

Strong El Niño events and nonlinear dynamical heating

[1] We present evidence showing that the nonlinear dynamic heating (NDH) in the tropical Pacific ocean heat budget is essential in the generation of intense El Nino events as well as the observed asymmetry between El Nino (warm) and La Nina (cold) events. The increase in NDH associated with the enhanced El Nino activity had an influence on the recent tropical Pacific warming trend and it might provide a positive feedback mechanism for climate change in the tropical Pacific.

Wind Effects on Past and Future Regional Sea Level Trends in the Southern Indo-Pacific*

Abstract Global sea level rise due to the thermal expansion of the warming oceans and freshwater input from melting glaciers and ice sheets is threatening to inundate low-lying islands and coastlines worldwide. At present the global mean sea level rises at 3.1 ± 0.7 mm yr−1 with an accelerating tendency. However, the magnitude of recent decadal sea level trends varies greatly spatially, attaining values of up to 10 mm yr−1 in some areas of the western tropical Pacific. Identifying the causes of recent regional sea level trends and understanding the patterns of future projected sea level change is of crucial importance. Using a wind-forced simplified dynamical ocean model, the study shows that the regional features of recent decadal and multidecadal sea level trends in the tropical Indo-Pacific can be attributed to changes in the prevailing wind regimes. Furthermore, it is demonstrated that within an ensemble of 10 state-of-the-art coupled general circulation models, forced by increasing atmospheric CO2 co...

ENSO Suppression due to Weakening of the North Atlantic Thermohaline Circulation

Changes of the North Atlantic thermohaline circulation (THC) excite wave patterns that readjust the thermocline globally. This paper examines the impact of a freshwater-induced THC shutdown on the depth of the Pacific thermocline and its subsequent modification of the El Nino–Southern Oscillation (ENSO) variability using an intermediate-complexity global coupled atmosphere–ocean–sea ice model and an intermediate ENSO model, respectively. It is shown by performing a numerical eigenanalysis and transient simulations that a THC shutdown in the North Atlantic goes along with reduced ENSO variability because of a deepening of the zonal mean tropical Pacific thermocline. A transient simulation also exhibits abrupt changes of ENSO behavior, depending on the rate of THC change. The global oceanic wave adjustment mechanism is shown to play a key role also on multidecadal time scales. Simulated multidecadal global sea surface temperature (SST) patterns show a large degree of similarity with previous climate reconstructions, suggesting that the observed pan-oceanic variability on these time scales is brought about by oceanic waves and by atmospheric teleconnections.

Eastern tropical Pacific hydrologic changes during the past 27,000 years from D/H ratios in alkenones

[1] The tropical Pacific plays a central role in the climate system by providing large diabatic heating that drives the global atmospheric circulation. Quantifying the role of the tropics in late Pleistocene climate change has been hampered by the paucity of paleoclimate records from this region and the lack of realistic transient climate model simulations covering this period. Here we present records of hydrogen isotope ratios (dD) of alkenones from the Panama Basin off the Colombian coast that document hydrologic changes in equatorial South America and the eastern tropical Pacific over the past 27,000 years (a) and the past 3 centuries in detail. Comparison of alkenone dD values with instrumental records of precipitation over the past � 100 a suggests that dD can be used as a hydrologic proxy. On long timescales our records indicate reduced rainfall during the last glacial period that can be explained by a southward shift of the mean position of the Intertropical Convergence Zone and an associated reduction of Pacific moisture transport into Colombia. Precipitation increases at � 17 ka in concert with sea surface temperature (SST) cooling in the North Atlantic and the eastern tropical Pacific. A regional coupled model, forced by negative SST anomalies in the Caribbean, simulates an intensification of northeasterly trade winds across Central America, increased evaporative cooling, and a band of increased rainfall in the northeastern tropical Pacific. These results are consistent with the alkenone SST and dD reconstructions that suggest increasing precipitation and SST cooling at the time of Heinrich event 1.