La Nina-like Sea Surface Temperature Research Articles

Long-lived cold blobs in the Northeast Pacific linked with the tropical La Niña

This study explores the counterpart of marine heatwave, known as the cold blob, in the Northeast Pacific (NEP) by selecting the long-lived events with the normalized sea surface temperature (SST) anomalies averaged over the blob area (160°–135° W, 40°–50° N) smaller than − 0.75 for at least 5 months. There are 10 summer-peak events with a stronger intensity and 7 winter-peak events. The summer-peak cold blob events are mostly attributed to the vertical entrainment when the mixed layer depth is shoaling. For the winter-peak group, the greater heat loss at surface is the principal factor to stimulate the cold blob, associated with the intense westerly anomalies around the blob area. However, the vertical entrainment generally hinders the amplification of winter-peak cold blobs. Moreover, the advection of colder-than-normal water to the blob area contributes much less for both two groups. We conjecture that the NEP cold blob and tropical La Nina-like state are potentially linked. The summer-peak cold blob event is often followed by a La Nina event in the ensuing winter. On the other hand, the equatorial La Nina-like SST cooling in late summer and autumn in turn enhances the winter-peak cold blobs. We find the subtropical region, particularly near the Baja California, may act as the oceanic bridge to connect the NEP cold blob and equatorial La Nina.

Dominant wintertime surface air temperature modes in the Northern Hemisphere extratropics

Recent observations showed that boreal winter exhibits significant cooling events in the Eurasian and North American continents despite the global warming trend. To better understand the boreal winter surface air temperature (SAT) variability and its physical characteristics, we investigated the three distinct Empirical Orthogonal Function (EOF) modes of the extratropical 2-m air temperature (T2m) variation and found that each mode can be related to the well-known physical processes: The first mode is characterized by the coherent change of wintertime SAT anomalies over northern Eurasia and central-eastern North America and it is found that snow cover variation in the Eurasian region can modulate Arctic Oscillation (AO) circulation through the troposphere–stratosphere interaction, which can affect wintertime SAT in the two continents. The second leading mode shows warm anomalies over the North Pacific especially around the Bering Strait and cold anomalies over most of Canada and the central part of North America. Associated with this mode, La Nina-like sea surface temperature (SST) anomaly can modulate the North American SAT variability through the generation of the tropospheric Pacific–North American (PNA) teleconnection pattern. The third mode represents warm anomalies over the Barents and Kara (B–K) seas and cold anomalies over East Asia. Sea ice loss over the B–K sea is analyzed to be closely related to the SAT variability over the East Asian region.

Decrease of tropical cyclone genesis frequency in the western North Pacific since 1960s

Tropical cyclone (TC) genesis frequency in the western North Pacific (WNP) during 1960–2014 shows a step-by-step decrease on interdecadal timescale, in accordance to the phase of the Interdecadal Pacific Oscillation (IPO). The environmental parameters responsible for the interdecadal change of TC genesis frequency were investigated. It was found that vertical wind shear especially the zonal wind shear plays a critical role, while other parameters such as sea surface temperature (SST), vertical velocity, divergence, humidity and maximum potential intensity cannot explain the step-by-step decrease of TC genesis frequency.A further diagnosis shows that the interdecadal change of vertical wind shear is caused by SST and associated rainfall pattern changes across the Indo-Pacific Ocean. A stronger warming in the Indian Ocean/western Pacific from 1960–1976 to 1977–1998 led to enhanced convection over the Maritime Continent and thus strengthened vertical shear over the key TC genesis region in the WNP. A La Nina-like SST pattern change from 1977–1998 to 1999–2014 led to a strengthened Walker circulation in the tropical Pacific, which further enhanced the vertical shear and decreased TC genesis frequency in the WNP.

Changes in the autumn precipitation and tropical cyclone activity over Central Vietnam and its East Sea

Central Vietnam is particularly vulnerable to the impacts of climate change, with all of its incumbent socioeconomic and ecological consequences. This is due in part to the autumn timing of the peak rainfall season in Central Vietnam, at a time when tropical cyclones are most likely to make landfall. We conducted climate diagnostic analyses using meteorological and tropical cyclone data to understand the changing patterns of autumn rainfall and tropical cyclones, revealing an intensification of precipitation over Central Vietnam since the beginning of the 20 th century that is associated with increased tropical cyclones in the adjacent sea. A warming of the sea surface temperature (SST) and enhanced southerly low-level winds are coincident with the above noted increase in rainfall and tropical cyclones. The underlying regional SST and circulation patterns are part of a hemispheric-scale change in the general circulation, i.e. a La Nina-like SST anomaly and a strengthened Walker circulation with the ascending branch located near Vietnam and far-western Pacific. We make the case with this paper that increasing autumn rainfall and tropical cyclones should be taken into account any mitigation and adaptation plans anticipated for Central Vietnam.

Recent and future sea surface temperature trends in tropical pacific warm pool and cold tongue regions

Using coral data, sea surface temperature (SST) reanalysis data, and Climate Model Intercomparison Project III (CMIP3) data, we analyze 20th-century and future warm pool and cold tongue SST trends. For the last 100 years, a broad La Nina-like SST trend, in which the warming trend of the warm pool SST is greater than that of the cold tongue SST, has appeared in reanalysis SST data sets, 20C scenario experiments of the CMIP3 data and less significantly in coral records. However, most Coupled General Circulation Models subjected to scenarios of future high greenhouse gas concentrations produce larger SST warming trends in cold tongues than in warm pools, resembling El Nino-like SST patterns. In other words, warmer tropical climate conditions correspond to stronger El Nino-like response. Heat budget analyses further verify that warmer tropical climates diminish the role of the ocean’s dynamic thermostat, which currently regulates cold tongue temperatures. Therefore, the thermodynamic thermostat, whose efficiency depends on the mean temperature, becomes the main regulator (particularly via evaporative cooling) of both warm pool and cold tongue temperatures in future warm climate conditions. Thus, the warming tendency of the cold tongue SST may lead that of the warm pool SST in near future.

Evidence for strengthening of the tropical Pacific Ocean surface wind speed during 1979–2001

Using multiple surface wind speed (SWS) data sets and trend empirical orthogonal function analysis, we have explored the trend in SWS associated with the large-scale tropical Pacific atmospheric circulation for the period 1979–2001. The present research provides a robust evidence of strengthening of the tropical Pacific Ocean SWS during this period and the magnitude is generally in line with the finding of Wentz et al. The strengthening in SWS is closely associated with the so-called La Nina-like sea surface temperature (SST) trend pattern rather than the changes in the ENSO, ENSO Modoki, or PDO. The present results, together with those from some recent climate model simulations, suggest that global warming forcing may have caused an intensification of SWS in the tropical Pacific Ocean by inducing the La Nina-like SST trend pattern due to ocean dynamics. Meanwhile, the strengthening in the tropical Pacific Ocean surface trade winds may also feedback to enhance the La Nina-like SST trend pattern under the positive wind-upwelling dynamic feedback mechanism.

Glacial/interglacial changes in the East Australian current

The East Australian Current (EAC) is the western boundary current of the south Pacific gyre transporting warm tropical waters to higher southern latitudes. Recent modelling shows that the partial separation of the EAC (~32°S) and the coupled formation of the Tasman Front (~34°S) are caused by a steep gradient in the zonally integrated wind stress curl. Analysis of oxygen isotope ratios (δ18O) in the planktonic foraminifer, Globigerinoides ruber, from sediment cores from the Coral Sea and Tasman Sea indicates that the EAC separation shifted northward to between 23 and 26°S during the last glacial. We suggest these results indicate a significant change in the Pacific wind stress curl during the glacial. Given recent evidence for El Nino-like conditions in the Pacific during the last glacial, with a reduction in the east–west sea surface temperature (SST) gradient, we suggest that weaker trade winds combined with more northerly, stronger westerlies were associated with a change to the wind stress curl, which repositioned the EAC separation and Tasman Front. In contrast, by ~11 ka BP, the EAC separation was forced south of 26°S. This southward shift was synchronous with a rapid warming of tropical SSTs, and the onset of a La Nina-like SST configuration across the tropical Pacific. It appears that the south Pacific trade winds strengthened accordingly, causing the EAC to readjust its flow. This readjustment of the EAC marks the onset of modern surface-ocean circulation in the southwest Pacific, but the present EAC transport was only achieved in the late Holocene, after 5 ka BP.