Mihai Dima

A Hemispheric Mechanism for the Atlantic Multidecadal Oscillation

Abstract The physical processes associated with the ∼70-yr period climate mode, known as the Atlantic multidecadal oscillation (AMO), are examined. Based on analyses of observational data, a deterministic mechanism relying on atmosphere–ocean–sea ice interactions is proposed for the AMO. Variations in the thermohaline circulation are reflected as uniform sea surface temperature anomalies in the North Atlantic. These anomalies are associated with a hemispheric wavenumber-1 sea level pressure (SLP) structure in the atmosphere that is amplified through atmosphere–ocean interactions in the North Pacific. The SLP pattern and its associated wind field affect the sea ice export through Fram Strait, the freshwater balance in the northern North Atlantic, and consequently the strength of the large-scale ocean circulation. It generates sea surface temperature anomalies with opposite signs in the North Atlantic and completes a negative feedback. The authors find that the time scale of the cycle is associated with the...

Subtropical coral reveals abrupt early-twentieth-century freshening in the western North Pacific Ocean

Instrumental climate observations provide robust records of global land and ocean temperatures during the twentieth century. Unlike for temperature, continuous salinity observations in the surface ocean are scarce prior to 1970, and the magnitude of salinity changes during the twentieth century is largely unknown. Surface ocean salinity is a major component in climate dynamics, as it influences ocean circulation and water mass formation. Here we present an annually resolved reconstruction of salinity variations in the surface waters of the western subtropical North Pacific Ocean since 1873, based on bimonthly records of δ18O, Sr/Ca, and U/Ca in a coral from the Ogasawara Islands. The reconstruction indicates that an abrupt regime shift toward fresher surface ocean conditions occurred between 1905 and 1910. Observational atmospheric data suggest that the abrupt freshening was associated with a weakening of the winds that drive the Kuroshio Current system and the associated subtropical gyre circulation. We note that the abrupt early-twentieth-century freshening in the western subtropical North Pacific precedes abrupt climate change in the northern North Atlantic by a few years. The potential for abrupt regime shifts in surface ocean salinity should be considered in climate predictions for the coming decades.

Impacts of the North Atlantic Oscillation and the El Niño–Southern Oscillation on Danube river flow variability

The impact of the North Atlantic Oscillation (NAO) and El Nino-Southern Oscillation (ENSO) on the Danube river streamflow variability is investigated for the period 1840 to 1998. A composite analysis reveals that positive streamflow anomalies are related to a large scale atmospheric circulation pattern that contains elements of the positive phase of the Pacific North American (PNA) pattern and negative phase of the NAO. The corresponding sea surface temperature (SST) pattern shows positive anomalies over most of the tropical region. Opposite atmospheric circulation and SST patterns are associated to negative streamflow anomalies. Significant decadal variations of the NAO and ENSO impact on the Danube streamflow are detected for the observational period. A lag-correlation analysis reveals that winter SST from tropical Pacific and some regions from the North Atlantic are significantly correlated with the streamflow variations from spring and summer suggesting a possible predictive skill of the Danube streamflow anomalies in these seasons using winter SST as a predictor.

Distinct Modes of Internal Variability in the Global Meridional Overturning Circulation Associated with the Southern Hemisphere Westerly Winds

The internal variability of the global meridional overturning circulation (GMOC) in long-term integration of the earth system model Community Earth System Models (COSMOS) is examined in this study. Two distinct modes of the GMOC, which are closely linked to the Southern Hemisphere westerly winds (SWW) anomalies,are found to vary on multidecadaland centennial time scales.The dominantmode is characterized by Southern Ocean dynamics: strengthening and poleward shift of the SWW associated with a positive phase of the southern annular mode yield Ekman-induced northward mass transport, including a zonally asymmetric response in the Southern Ocean sea surface temperature and a cooling in the tropical Pacific Ocean due to large-scale upwelling. The second mode projects mainly onto the Atlantic meridional overturning circulation (AMOC). It is driven by a combination of SWW variation and buoyancy forcing. Based on the relationship between the two modes together with the wind perturbation experiments, the authors emphasize that the full AMOC response to the SWW change takes several centuries in their model. The sea surface temperature in Northern Hemisphere high latitudes is significantly affected in this mode, showing a largescale warming. Their results from a mid-Holocene experiment imply that both modes are independent from the climate background conditions in the Holocene. Finally, the authors argue that the natural modes of GMOC are important to understand trends in ocean circulation, with consequences for heat and carbon budgets for past, present, and future climate.

Evidence for Two Distinct Modes of Large-Scale Ocean Circulation Changes over the Last Century

Through its nonlinear dynamics and involvement in past abrupt climate shifts the thermohaline circulation (THC) represents a key element for the understanding of rapid climate changes. The expected THC weakening under global warming is characterized by large uncertainties, and it is therefore of significant importance to identify ocean circulation changes over the last century. By applying various statistical techniques on two global sea surface temperature datasets two THC-related modes are separated. The first one involves relatively slow adjustment of the whole conveyor belt circulation and has an interhemispherically symmetric pattern. The second mode is associated with the relatively fast adjustment of the North Atlantic overturning cell and has the seesaw structure. Based on the separation of these two patterns the authors show that the global conveyor has been weakeningsince the late 1930sand that the North Atlanticoverturningcell suffered an abrupt shift around 1970. The distinction between the two modes provides also a new frame for interpreting past abrupt climate changes.

Intensification and poleward shift of subtropical western boundary currents in a warming climate

A significant increase in sea surface temperature (SST) is observed over the midlatitude western boundary currents (WBCs) during the past century. However, the mechanism for this phenomenon remains poorly understood due to limited observations. In the present paper, several coupled parameters (i.e., sea surface temperature (SST), ocean surface heat fluxes, ocean water velocity, ocean surface winds and sea level pressure (SLP)) are analyzed to identify the dynamic changes of the WBCs. Three types of independent data sets are used, including reanalysis products, satellite-blended observations. and climate model outputs from the fifth phase of the Climate Model Intercomparison Project (CMIP5). Based on these broad ranges of data, we find that the WBCs (except the Gulf Stream) are intensifying and shifting toward the poles as long-term effects of global warming. An intensification and poleward shift of near-surface ocean winds, attributed to positive annular mode-like trends, are proposed to be the forcing of such dynamic changes. In contrast to the other WBCs, the Gulf Stream is expected to be weaker under global warming, which is most likely related to a weakening of the Atlantic Meridional Overturning Circulation (AMOC). However, we also notice that the natural variations of WBCs might conceal the long-term effect of global warming in the available observational data sets, especially over the Northern Hemisphere. Therefore, long-term observations or proxy data are necessary to further evaluate the dynamics of the WBCs.

Quasi-Decadal Variability in the Atlantic Basin Involving Tropics–Midlatitudes and Ocean–Atmosphere Interactions

Observational data are used to emphasise a 5- to 7-yr lag between the main modes of variability in the midlatitudes and in the Tropics. Considering this finding a mechanism for quasidecadal variability based on Tropics‐midlatitudes and ocean‐atmosphere interaction is described. It appears that the signal associated with the SST anomalies in the northern region of the tropical Atlantic is transferred in midlatitudes through the atmosphere and it will modify the thermal conditions of the ocean upper layers. In 5‐7 years, thermal conditions will affect the SST anomalies in the northern Tropics reversing their sign. The results suggest that the Tropics get a negative feedback from midlatitudes so that the Tropics‐midlatitudes system is capable of generating an oscillatory mode.

Predicting the June 2013 European Flooding Based on Precipitation, Soil Moisture, and Sea Level Pressure

AbstractOver recent decades Europe has experienced heavy floods, with major consequences for thousands of people and billions of euros worth of damage. In particular, the summer of 2013 flood in central Europe showed how vulnerable modern society is to hydrological extremes and emphasized once more the need for improved forecast methods of such extreme climatic events. Based on a multiple linear regression model, it is shown here that 55% of the June 2013 Elbe River extreme discharge could have been predicted using May precipitation, soil moisture, and sea level pressure. Moreover, the model was able to predict more than 75% of the total Elbe River discharge for June 2013 (in terms of magnitude) by also incorporating the amount of precipitation recorded during the days prior to the flood, but the predicted discharge for the June 2013 event was still underestimated by 25%. Given that all predictors used in the model are available at the end of each month, the forecast scheme can be used to predict extreme ...

Fundamental and derived modes of climate variability: concept and application to interannual time-scales

The notion of mode interaction is proposed as a deterministic concept for understanding climatic modes at various timescales. This concept is based on the distinction between fundamental modes relying on their own physical mechanisms and derived modes that emerge from the interaction of two other modes. The notion is introduced and applied to interannual climate variability. Observational evidence is presented for the tropospheric biennial variability to be the result of the interaction between the annual cycle and a quasi-decadal mode originating in the Atlantic basin. Within the same framework, Pacific interannual variability at time-scales of about 4 and 6 yr is interpreted as the result of interactions between the biennial and quasi-decadal modes of climate variability. We show that the negative feedback of the interannual modes is linked to the annual cycle and the quasi-decadal mode, both originating outside the Pacific basin, whereas the strong amplitudes of interannual modes result from resonance and local positive feedback. It is argued that such a distinction between fundamental and derived modes of variability is important for understanding the underlying physics of climatic modes, with strong implications for climate predictability.

Interdecadal variability generated by interactions between Pacific and Atlantic oceans

Recent studies suggest that, interannual, decadal, and quasibidecadal periodicities dominate global sea surface temperature and sea level pressure variability over past hundred years. In this study, we present evidence that the quasibidecadal mode of variability can be considered in a first order approximation as resulting from a linear interaction between the dominant modes of variability in the Tropical Pacific (associated with interannual variability) and North Atlantic ocean (associated with decadal variability). A two coupled delayed action oscillators model is proposed to qualitatively explain this result.