Arnaud Czaja

Observed Impact of Atlantic SST Anomalies on the North Atlantic Oscillation

Abstract The large-scale patterns of covariability between monthly sea surface temperature (SST) and 500-mb height anomalies (Z500) in the Atlantic sector are investigated as a function of time lag in the NCEP–NCAR reanalysis (1958–97). In agreement with previous studies, the dominant signal is the atmospheric forcing of SST anomalies, but statistically significant covariances are also found when SST leads Z500 by several months. In winter, a Pan-Atlantic SST pattern precedes the North Atlantic oscillation (NAO) by up to 6 months. Such long lead time covariance is interpreted in the framework of the stochastic climate model, reflecting the forcing of the NAO by persistent Atlantic SST anomalies. A separate analysis of midlatitudes (20°–70°N) and tropical (20°S–20°N) SST anomalies reveals that the bulk of the NAO signal comes from the midlatitudes. A dipolar anomaly, with warm SST southeast of Newfoundland and cold SST to the northeast and southeast, precedes a positive phase of the NAO, and it should prov...

A Diagnostic Study of the Role of Remote Forcing in Tropical Atlantic Variability

This observational study focuses on remote forcing of the dominant pattern of north tropical Atlantic sea surface temperature (SST) anomalies by ENSO and NAO. Based on a spring SST index of the north tropical Atlantic (NTA) SST (58‐258N), it is shown that almost all NTA‐SST extreme events from 1950 to the present can be related to either ENSO or NAO. Since the SST NTA events lag NAO and ENSO events, NTA variability is interpreted as being largely a response to remote NAO or ENSO forcing. The local response of the tropical Atlantic to these external sources—whether it be ENSO or the NAO—is observed to be rather similar: changes in surface winds induce changes in latent heating that, in turn, generate SST anomalies. Once generated, the latter are damped through local air‐sea interaction, at a rate estimated to be 10 W m22 K21. Experiments with simple models, but driven by observations, strongly suggests that variability on interannual to interdecadal timescales—both time series and spectral signatures—can be largely explained as a result of direct atmospheric forcing, without the need to invoke a significant role for local unstable air‐sea interactions or ocean circulation.

Influence of the North Atlantic SST on the atmospheric circulation

Using the monthly COADS dataset and NMC-NCAR archives we show that significant anomalies of the atmospheric circulation are related to previous SST anomalies in the North Atlantic. A signal over the northwest Labrador Sea in late spring is associated with the dominant mode of SST variability during the preceeding winter. It is more clearly seen in the mid-troposhere than at sea level and appears to be related to the anomalous surface heat exchanges that slowly damp the SST anomalies. In addition, a NAO-like signal in early winter is associated with SST anomalies east of Newfoundland and in the eastern subtropical North Atlantic during the preceeding summer.

The Global Atmospheric Circulation on Moist Isentropes

The global atmospheric circulation transports energy from the equatorial regions to higher latitudes through a poleward flow of high-energy and -entropy parcels and an equatorward flow of air with lower energy and entropy content. Because of its turbulent nature, this circulation can only be described in some averaged sense. Here, we show that the total mass transport by the circulation is twice as large when averaged on moist isentropes than when averaged on dry isentropes. The additional mass transport on moist isentropes corresponds to a poleward flow of warm moist air near Earths surface that rises into the upper troposphere within mid-latitudes and accounts for up to half of the air in the upper troposphere in polar regions.

The Partitioning of Poleward Heat Transport between the Atmosphere and Ocean

Abstract Observations of the poleward heat transport of the earth (H) suggest that the atmosphere is the primary transporting agent poleward of 30°, that oceanic (HO) and atmospheric (HA) contributions are comparable in the tropical belt, and that ocean transport dominates in the deep Tropics. To study the partition we express the ratio HA/HO as where Ψ (with subscripts A and O denoting atmosphere and ocean, respectively) is the meridional mass transport within θ layers (moist potential temperature for the atmosphere, potential temperature for the ocean), and CΔθ (C being the specific heat) is the change in energy across the circulation defined by Ψ. It is argued here that the observed partitioning of heat transport between the atmosphere and ocean is a robust feature of the earths climate and reflects two limits: (i) dominance of atmospheric mass transport in mid-to-high latitudes (ΨA ≫ ΨO with CAΔθA ∼ COΔθO and hence HA/HO ≫ 1) and (ii) dominance of oceanic energy contrast in the Tropics (COΔθO ≫ CAΔθA...

The Global Atmospheric Circulation in Moist Isentropic Coordinates

Abstract Differential heating of the earth’s atmosphere drives a global circulation that transports energy from the tropical regions to higher latitudes. Because of the turbulent nature of the flow, any description of a “mean circulation” or “mean parcel trajectories” is tied to the specific averaging method and coordinate system. In this paper, the NCEP–NCAR reanalysis data spanning 1970–2004 are used to compare the mean circulation obtained by averaging the flow on surfaces of constant liquid water potential temperature, or dry isentropes, and on surfaces of constant equivalent potential temperature, or moist isentropes. While the two circulations are qualitatively similar, they differ in intensity. In the tropics, the total mass transport on dry isentropes is larger than the circulation on moist isentropes. In contrast, in midlatitudes, the total mass transport on moist isentropes is between 1.5 and 3 times larger than the mass transport on dry isentropes. It is shown here that the differences between ...

Sea Surface Temperature Variability along the Path of the Antarctic Circumpolar Current

The spatial and temporal distributions of sea surface temperature (SST) anomalies in the Antarctic Circumpolar Current (ACC) are investigated, using monthly data from the NCEP–NCAR reanalysis for the period 1980–2004. Patterns of atmospheric forcing are identified in observations of sea level pressure and air–sea heat fluxes. It is found that a significant fraction of SST variability in the ACC can be understood as a linear response to surface forcing by the Southern Annular Mode (SAM) and remote forcing by ENSO. The physical mechanisms rely on the interplay between atmospheric variability and mean advection by the ACC. SAM and ENSO drive a low-level anomalous circulation pattern localized over the South Pacific Ocean, inducing surface heat fluxes and Ekman heat advection anomalies. A simple model of SST propagating in the ACC, forced with heat fluxes estimated from the reanalysis, suggests that surface heat fluxes and Ekman heat advection are equally important in driving the observed SST variability. Further diagnostics indicate that SST anomalies, generated mainly upstream of Drake Passage, are subsequently advected by the ACC and damped after a couple of years. It is suggested that SST variability along the path of the ACC is largely a passive response of the oceanic mixed layer to atmospheric forcing.

Carbon dioxide and oxygen fluxes in the Southern Ocean: Mechanisms of interannual variability

(1) We analyze the variability of air-sea fluxes of carbon dioxide and oxygen in the Southern Ocean during the period 1993-2003 in a biogeochemical and physical simulation of the global ocean. Our results suggest that the nonseasonal variability is primarily driven by changes in entrainment of carbon-rich, oxygen-poor waters into the mixed layer during winter convection episodes. The Southern Annular Mode (SAM), known to impact the variability of air-sea fluxes of carbon dioxide, is also found to affect oxygen fluxes. We find that El Nino-Southern Oscillation (ENSO) also plays an important role in generating interannual variability in air-sea fluxes of carbon and oxygen. Anomalies driven by SAM and ENSO constitute a significant fraction of the simulated variability; the two climate indices are associated with surface heat fluxes, which control the modeled mixed layer depth variability. We adopt a Lagrangian view of tracers advected along the Antarctic Circumpolar Current (ACC) to highlight the importance of convective mixing in inducing anomalous air-sea fluxes of carbon dioxide and oxygen. The idealized Lagrangian model captures the principal features of the variability simulated by the more complex model, suggesting that knowledge of entrainment, temperature, and mean geostrophic flow in the mixed layer is sufficient to obtain a first-order description of the large-scale variability in air-sea transfer of soluble gases. Distinct spatial and temporal patterns arise from the different equilibration timescales of the two gases.

The atmospheric frontal response to SST perturbations in the Gulf Stream region

The link between sea surface temperature (SST) gradients and atmospheric fronts is explored in a general circulation model across the Gulf Stream (GS) region from December to February 1981–2000. Two model experiments are analyzed, one with a realistic control SST distribution and one with a spatially smoothed SST distribution. The analysis shows a noticeable change in regional atmospheric frontal frequency between the two experiments (up to 30%), with the distribution of change exhibiting a clear imprint of the GS SST front. Further analysis of the surface sensible heat flux gradient across cold fronts reveals the pattern of change to be mediated by a thermal interaction between the oceanic and atmospheric fronts (“thermal damping and strengthening”). These results not only emphasize the significance of the GS SST gradient for storm development in the North Atlantic but also highlight the importance of resolution in assessing the role of frontal air-sea interaction in midlatitude climate variability.

Why Is North Tropical Atlantic SST Variability Stronger in Boreal Spring

It is suggested that the seasonal dependence of interannual variability displayed by observed sea surface temperature (SST) over the north tropical Atlantic primarily reflects the seasonality of the remote forcing associated with the North Atlantic and Southern Oscillations and is controlled by the mean damping time scale of SST anomaly. A stochastic model including a seasonally dependent forcing is used to test this hypothesis against observations.