The double peaked El Niño and its physical processes

Recently, El Niño diversity has been paid much attention due to its different global impacts. However, most studies have focused on a single warm peak in sea surface temperature anomalies (SSTAs), either in the central Pacific or the eastern Pacific. Here, we demonstrate from observational analyses that several recent El Niño events show double warm peaks in SSTA – called “Double Peaked (DP) El Niño” – that have only been observed since 2000. The DP El Niño has two warm centers, which grow concurrently but separately, in both the central and eastern Pacific. In general, the atmospheric and oceanic patterns of the DP El Niño are similar to those of the Warm Pool (WP) El Niño from the development phase, such that the central Pacific peak is developed by the zonal advective feedback and reduced wind speed anomalies. However, a distinctive difference exists in the eastern Pacific where the DP El Niño has a second SSTA peak. In addition, the DP El Niño shows more distinctive anomalous precipitation along the Pacific intertropical convergence zone (ITCZ) compared with the WP El Niño. We demonstrate that the peculiar precipitation anomalies along the Pacific ITCZ play a critical role in enhancing the equatorial westerly wind stress anomalies, which help to develop the eastern SSTA peak by deepening the thermocline in the eastern Pacific.

Regional Impacts of the Westerly Winds on Southern Ocean Mode and Intermediate Water Subduction

Subduction processes in the Southern Ocean transfer oxygen, heat, and anthropogenic carbon into the ocean interior. The future response of upper-ocean subduction, in the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) classes, is dependent on the evolution of the combined surface buoyancy forcing and overlying westerly wind stress. Here, the recently observed pattern of a poleward intensification of the westerly winds is divided into its shift and increase components. SAMW and AAIW formation occurs in regional “hot spots” in deep mixed layer zones, primarily in the southeast Indian and Pacific. It is found that the mixed layer depth responds differently to wind stress perturbations across these regional formation zones. An increase only in the westerly winds in the Indian sector steepens isopycnals and increases the local circulation, driving deeper mixed layers and increased subduction. Conversely, in the same region, a poleward shift and poleward intensification of the westerly winds reduces heat loss and increases freshwater input, thus decreasing the mixed layer depth and consequently the associated SAMW and AAIW subduction. In the Pacific sector, all wind stress perturbations lead to increases in heat loss and decreases in freshwater input, resulting in a net increase in SAMW and AAIW subduction. Overall, the poleward shift in the westerly wind stress dominates the SAMW subduction changes, rather than the increase in wind stress. The net decrease in SAMW subduction across all basins would likely decrease anthropogenic carbon sequestration; however, the net AAIW subduction changes across the Southern Ocean are overall minor.

Effects of the Westerly Wind Stress over the Southern Ocean on the Meridional Overturning

Numerical experiments were conducted to clarify the processes through which the Southern Ocean wind affects the meridional overturning (NA cell) associated with North Atlantic Deep Water production. These were based on idealized single- and twin-basin (idealized Atlantic and Pacific Ocean) models with a periodically connected passage under various forcings at the surface. Relationships among the wind stresses, the NA cell, and the buoyancy fluxes were investigated. Increased westerly wind stresses increase the surface buoyancy gains in the Southern Ocean under the density-restoring boundary condition. The buoyancy anomalies excited in the Southern Ocean propagate as baroclinic waves into the northern North Atlantic, modify the density field, and enhance the NA cell, which increases buoyancy losses there until the global buoyancy flux budget balances. The results from experiments using a realistically configured global ocean model confirm that the Southern Ocean wind effects on the NA cell can be understood consistently through thermodynamics and that the wind stresses outside the channel latitudes, as well as those at the Cape Horn latitude, affect the global buoyancy fluxes and the NA cell.

A Mechanism for Abrupt Climate Change Associated with Tropical Pacific SSTs*

The tropical Pacific’s response to transiently increasing atmospheric CO2 is investigated using three ensemble members from a numerically efficient, coupled atmosphere–ocean GCM. The model is forced with a 1% yr−1 increase in CO2 for 110 yr, when the concentration reaches 3 times the modern concentration. The transient greenhouse forcing causes a regionally enhanced warming of the equatorial Pacific, particularly in the far west. This accentuated equatorial heating, which is slow to arise but emerges abruptly during the last half of the simulations, results from both atmospheric and oceanic processes. The key atmospheric mechanism is a rapid local increase in the super–greenhouse effect, whose emergence coincides with enhanced convection and greater high cloud amount once the SST exceeds an apparent threshold around 27°C. The primary oceanic feedback is greater Ekman heat convergence near the equator, due to an anomalous near-equatorial westerly wind stress created by increased rising (sinking) air to the east (west) of Indonesia. The potential dependence of these results on the specific model used is discussed. The suddenness and far-ranging impact of the enhanced, near-equatorial warming during these simulations suggests a mechanism by which abrupt climate changes may be triggered within the Tropics. The extratropical atmospheric response in the Pacific resembles anomalies during present-day El Niño events, while the timing and rapidity of the midlatitude changes are similar to those in the Tropics. In particular, a strengthening of the Pacific jet stream and a spinup of the wintertime Aleutian low seem to be forced by the changes in the tropical Pacific, much as they are in the modern climate.

Seasonal and Inter-annual variability in Chemical and Biological parameters in Storm Bay, Tasmania. I. Physics, Chemistry and the Biomass of components, of the food chain

As south-east Tasmania lies close to the subtropical convergence, the waters of the shelf regions are a complex mixture of subtropical and subantarctic water masses. Temperature, salinity and nutrient data from the waters off Maria Island have been recorded for 40 years and show considerable inter-annual variability. In a complementary study, physical, chemical and biological parameters were measured in Storm Bay over 3 years. The enlarged data base gives a detailed picture of the seasonality and inter- annual variability in south-east Tasmanian waters and the effects of such variability on the food chain. The temperature and salinity measurements show that the water in Storm Bay is strongly influenced at different times of the year by water of subtropical origin from the east and by water of subantarctic origin from the west. Satellite images and drift-card data confirm these findings. Chemical parameters (dissolved inorganic and organic nutrients) showed the expected temperate seasonal trends, but some features of the seasonal cycle differed greatly in magnitude and duration between years. Differences between the years are shown to be due to inter-annual differences in the westerly wind stress, which affects nitrate concentration. In years of strong westerlies, the phytoplankton biomass and productivity increases and the spring bloom lasts longer. In such years, zooplankton biomass increases by a factor of ten in late spring, and salps replace euphausiids as the dominant organisms.