Impact of the El Niño type and PDO on the Winter Sub-seasonal North American Zonal Temperature Dipole via the Variability of Positive PNA Events

In recent years, the winter (from December to February, DJF) North American surface air temperature (SAT) anomaly in midlatitudes shows a “warm west/cold east” (WWCE) dipole pattern. To some extent, the winter WWCE dipole can be considered as being a result of the winter mean of sub-seasonal WWCE events. In this paper, the Pacific SST condition linked to the WWCE SAT dipole is investigated. It is found that while the sub-seasonal WWCE dipole is related to the positive Pacific North American (PNA+) pattern, the impact of the PNA+ on the WWCE dipole depends on the El Niño SST type and the phase of Pacific decadal Oscillation (PDO). For a central-Pacific (CP) type El Niño, the positive (negative) height anomaly center of PNA+ is located in the western (eastern) North America to result in an intensified WWCE dipole, though the positive PDO favors the WWCE dipole. In contrast, the WWCE dipole is suppressed under an Eastern-Pacific (EP) type El Niño because the PNA+ anticyclonic anomaly dominates the whole North America. Moreover, the physical cause of why the El Niño type influences PNA+ is further examined. It is found that the type of El Niño can significantly influence the location of PNA+ through changing North Pacific midlatitude westerly winds associated with the Pacific Hadley cell change. For the CP-type El Niño, the eastward migration of PNA+ is suppressed to favor its anticyclonic (cyclonic) anomaly appearing in the west (east) region of North American owing to reduced midlatitude westerly winds. But for the EP-type El Niño, midlatitude westerly wind is intensified to cause the appearance of PNA+ anticyclonic anomaly over the whole North America due to enhanced Hadley cell.

Effects of prescribed CMIP6 ozone on simulating the Southern Hemisphere atmospheric circulation response to ozone depletion

The Antarctic ozone hole has led to substantial changes in the Southern Hemisphere atmospheric circulation, such as the strengthening and poleward shift of the midlatitude westerly jet. Ozone recovery during the twenty-first century is expected to continue to affect the jet's strength and position, leading to changes in the opposite direction compared to the twentieth century and competing with the effect of increasing greenhouse gases. Simulations of the Earth's past and future climate, such as those performed for the Coupled Model Intercomparison Project Phase 6 (CMIP6), require an accurate representation of these ozone effects. Climate models that use prescribed ozone fields lack the important feedbacks between ozone chemistry, radiative heating, dynamics, and transport. In addition, when the prescribed ozone field was not generated by the same model to which it is prescribed, the imposed ozone hole is inconsistent with the simulated dynamics. These limitations ultimately affect the climate response to ozone depletion. This study investigates the impact of prescribing the ozone field recommended for CMIP6 on the simulated effects of ozone depletion in the Southern Hemisphere. We employ a new state-of-the-art coupled climate model, Flexible Ocean Climate Infrastructure (FOCI), to compare simulations in which the CMIP6 ozone is prescribed with simulations in which the ozone chemistry is calculated interactively. At the same time, we compare the roles played by ozone depletion and by increasing concentrations of greenhouse gases in driving changes in the Southern Hemisphere atmospheric circulation using a series of historical sensitivity simulations. FOCI captures the known effects of ozone depletion, simulating an austral spring and summer intensification of the midlatitude westerly winds and of the Brewer–Dobson circulation in the Southern Hemisphere. Ozone depletion is the primary driver of these historical circulation changes in FOCI. The austral spring cooling of the polar cap in the lower stratosphere in response to ozone depletion is weaker in the simulations that prescribe the CMIP6 ozone field. We attribute this weaker response to a prescribed ozone hole that is different to the model dynamics and is not collocated with the simulated polar vortex, altering the strength and position of the planetary wavenumber one. As a result, the dynamical contribution to the ozone-induced austral spring lower-stratospheric cooling is suppressed, leading to a weaker cooling trend. Consequently, the intensification of the polar night jet is also weaker in the simulations with prescribed CMIP6 ozone. In contrast, the differences in the tropospheric westerly jet response to ozone depletion fall within the internal variability present in the model. The persistence of the Southern Annular Mode is shorter in the prescribed ozone chemistry simulations. The results obtained with the FOCI model suggest that climate models that prescribe the CMIP6 ozone field still simulate a weaker Southern Hemisphere stratospheric response to ozone depletion compared to models that calculate the ozone chemistry interactively.

North American Warm-west/Cold-East air temperature pattern and its linkage to different El Nino types

<p><strong> </strong></p><p>In recent years, the surface air temperature (SAT) anomalies in winter over North America show a “warm-West/cool-East” (WWCE) dipole pattern. The underlying mechanism of the North American WWCE dipole pattern has been an important research topic. This study examines the physical cause of the WWCE dipole generation.</p><p>It is found that the positive phase (PNA<sup>+</sup>) of the Pacific North American (PNA) pattern can lead to the generation of the WWCE SAT dipole. However, the impact of the PNA<sup>+ </sup>on the WWCE SAT dipole over North America depends on the type of the El Nino SST anomaly. When an Eastern-Pacific (EP) type El Nino occurs, the anticyclonic anomaly center of the PNA<sup>+ </sup>over the North American continent is displaced eastward near 100°W due to intensified midlatitude westerly winds over North Pacific so that its anticyclonic anomaly dominates the whole North America. In this case, the cyclonic anomaly of the PNA<sup>+</sup> almost disappears over the North America. Thus, the WWCE SAT dipole over the North America is weakened. In contrast, when a central-Pacific (CP) type El Nino appears, the anticyclonic anomaly center of the associated PNA<sup>+</sup> is located over the North America west coast due to reduced midlatitude westerly winds over North Pacific. As a result, the cyclonic anomaly of the PNA<sup>+</sup> can appear over the east United States to result in an intensified WWCE SAT dipole over the North America</p>

Response of Southern Ocean Ventilation to Changes in Midlatitude Westerly Winds

ABSTRACT Changes in ventilation of the Southern Hemisphere oceans in response to changes in midlatitude westerly winds are examined by analyzing the ideal age tracer from global eddy-permitting ocean–ice model simulations in which there is an abrupt increase and/or a meridional shift in the winds. The age response in mode and intermediate waters is found to be close to linear; the response of a combined increase and shift of peak winds is similar to the sum of the individual responses to an increase and a shift. Further, a barotropic response, following Sverdrup balance, can explain much of the age response to the changes in wind stress. There are similar peak decreases (of around 50 years) in the ideal age for a 40% increase or 2.5° poleward shift in the wind stress. However, while the age decreases throughout the thermocline for an increase in the winds, for a poleward shift in the winds the age increases in the north part of the thermocline and there are decreases in age only south of 35°S. As a consequence, the change in the volume of young water differs, with a 15% increase in the volume of water with ages younger than 50 years for a 40% increase in the winds but essentially no change in this volume for a 2.5° shift. As ventilation plays a critical role in the uptake of carbon and heat, these results suggest that the storage of anthropogenic carbon and heat in mode and intermediate waters will likely increase with a strengthening of the winds, but will be much less sensitive to a meridional shift in the peak wind stress.