• Speaker: Adèle Révelard, Laboratoire d’Océanographie et du Climat (LOCEAN, Paris, France)

Esporles, June 15, 2018. The aim of this work is to study the influence on the atmosphere of the variability of the oceanic fronts associated with the Kuroshio and Oyashio Extensions (KE and OE), and to differentiate it from the influence of the main SST mode of the North Pacific, in particular the Pacific Decadal Oscillation (PDO).

We use the atmospheric reanalysis ERA-Interim starting from 1979, and timeseries that describe the variability of the KE and OE that are already available.

In the first part of this work, we focus on the KE influence, using lag partial regression analysis with the ENSO signal removed. The KE index describes the dynamical oscillation of the current at decadal timescale. When the index is positive, the KE is strengthened, less turbulent, and its path is shifted north. The reverse holds when the index is negative. We show that the positive phase of the KE is associated with an anomalous warming of the oceanic surface in the region downstream of the KE, which generates enhanced heat flux to the atmosphere.

In October to January (ONDJ), this leads to a downstream barotropic high response in the central North Pacific and over western United States, and an enhanced polar vortex. We try to understand the mechanisms behind this atmospheric response analyzing the KE influence on the synoptic activity.

In the second part, we use a multivariate statistical method that allows us to analyze in parallel the atmospheric response to different oceanic forcings that may be correlated to each other.

In addition to the KE and OE indices, we include in the analysis the main modes of SST variability of the tropical and northern hemisphere oceans, such as the PDO in the North Pacific and ENSO in the tropical Pacific.

The atmospheric response being very dependent on the mean background flow, we distinguish between three seasons : early winter (OND), winter (DJF) and late winter (FMA). In OND, the multivariate analysis confirms the earlier results of the KE found by partial regression in ONDJ. In winter and late winter, however, the KE does not have any significant influence.

We found a significant influence of the OE in OND and FMA, which resembles the North Pacific Oscillation, indicating a poleward shift of the Jet Stream, in agreement with the analysis of the transient eddies.

The response to the PDO is similar in OND and FMA, indicating a strengthening of the Jet Stream. In DJF, however, the response is different and rather indicates a southward shift of the Jet Stream.

The disparity between our results and past studies are explained by the difference in the periods considered.

The non stationarity of the atmospheric response is therefore a key question for future researches.

Date and Time: Tuesday, June 19, 12:00-13:00h

Place: IMEDEA Seminar Room

Organiza: Baptiste Mourre i Joaquím Tintoré / SOCIB & IMEDEA TMOOC

Source: IMEDEA (CSIC-UIB) & SOCIB