Ocean5 is ECMWF’s current ocean and sea‐ice analysis system. It provides initial conditions for the ocean and sea‐ice component of ECMWF’s Earth system forecasting system. Ocean5 runs both a behind‐ real‐time stream that produces the Ocean Re‐Analysis System 5 (ORAS5) and a near‐real‐time (NRT) stream. ORAS5 is used for climate monitoring while Ocean5 NRT provides initial conditions for the Centre’s forecasting activities. Charts for each stream are now freely accessible to both internal and external users on the ECMWF website. The case of the North Pacific ‘Blob’ discussed below illustrates how the charts can be used.
ORAS5 page
The ORAS5 web page targets users interested in climate monitoring. The charts on that page show monthly averages of ORAS5 with around a month and a half delay with respect to the current month. Three‐month and yearly averages as well as 11‐year and 21‐year records are also shown. The full period (1979–now) of the reanalysis is covered for users interested in interannual to decadal climate variability. Ocean and sea‐ice charts range from maps to vertical sections and time series. The different charts aim to complement each other. For example, the source of an anomaly in sea‐surface temperature seen in the tropical Pacific can be tracked down by looking at the atmospheric fluxes received by the ocean, the vertical structure of the ocean and the magnitude of ocean transports. The anomaly can also be viewed in the context of the past months to decades and potentially linked to climate patterns, such as the El Niño Southern Oscillation (ENSO), a periodic warming and cooling of the equatorial Pacific Ocean.
Ocean5 NRT page
The Ocean5 NRT page is more limited. It provides daily maps of ocean and sea‐ice parameters to provide an overview of the response of the ocean to current weather events, such as tropical cyclones, or to modes of variability, such as ENSO and the North Atlantic Oscillation. One‐year long longitude–time diagrams at the equator are also available to monitor the evolution of the state of ENSO.
The North Pacific ‘Blob’
From the implementation of the NRT Ocean5 monitoring page in early November 2019 to the time of writing (mid‐December), the daily sea‐ surface temperature maps showed a persistent anomalously warm water mass off the west coast of North America and Alaska. This is illustrated in the figure for 26 November 2019. The climate community noticed these unusually warm sea‐surface temperatures as early as the end of the summer. The generation of this ‘marine heatwave’ was favoured by an atmospheric blocking event allowing stronger than usual solar radiation to warm the north‐east Pacific in August. The reason behind the scrutiny triggered by this event is its similarity with the long‐lasting marine heatwave in the same area from 2014 to 2016, which was baptised ‘the Blob’ by the community. The ‘Blob’ formed under similar atmospheric conditions as this year’s marine heatwave. It is thought that it may have been reinforced by a teleconnection with ENSO, in particular.
Details of the strength, duration and structure of the 2014–2016 ‘Blob’ can be studied in the ORAS5 charts. The ‘Blob’ is a unique event in the ORAS5 record and its impact on the marine ecosystem was disastrous. At the time of writing, it was unclear whether the current marine heatwave in the same area will be swept away during the winter storm season or whether it is here to stay.
The ORAS5 page is at: https://www.ecmwf.int/en/forecasts/charts/oras5/.
The ORAS5 NRT page is at: https://www.ecmwf.int/en/forecasts/charts/oras5_nrt/.