The plots in this page display daily, pressure-height cross sections of geopotential height (GPH) over the polar cap, averaged from 65°N to 90°N and around all longitudes, from the ERA5 reanalysis dataset. Heights are expressed as normalized anomalies (in standard deviations, σ) relative to a climatological mean.

Season view: Summer vs Winter

• Summer (Jan-Dec): one calendar year along the x-axis, from 1 January to 31 December.
• Winter (Jul-Jun): a “seasonal” year from 1 July to 30 June, so each winter appears as a continuous season (e.g., 2017-2018) instead of being split across two calendar years.

What is being averaged?

• Data source: ERA5 reanalysis geopotential on pressure levels.
• First, the field is averaged zonally (all longitudes), then averaged over the 65–90°N latitude band.
• Geopotential is converted to geopotential height, and the resulting daily values are compared to a climatological mean for that calendar day.
• The anomalies are divided by the daily climatological standard deviation, giving a normalized anomaly in units of σ.

Mathematical details

Z = Φ / g₀    (g₀ = 9.80665 m s⁻²)

Z̄(φ, t, p) = (1 / Nλ) Σ Z(λᵢ, φ, t, p)

Zcap(t,p) = Σ[ Z̄(φⱼ, t, p) · cosφⱼ ] / Σ[ cosφⱼ ]

μ(d,p) = mean over base period
σ(d,p) = standard deviation over base period

Anom(t,p) = (Zcap(t,p) − μ(d,p)) / σ(d,p)

Sliding 30-year climatology

In sliding mode, each day is compared to a 30-year climatology that is centered near the period of interest, similar in spirit to how NOAA computes the Oceanic Niño Index (ONI) with multiple base periods:

• For each 5-year block in the record, a different 30-year base period is used (e.g., 1961-1990, 1966-1995, ...).
• This means earlier decades are evaluated against their contemporary climatology, reducing contamination from long-term trends.
• Once a 5-year block is assigned to a base period, its classification relative to that climatology is fixed in time (no future re-classification of past events).
• The reference climatology is indicated on the plot itself, in the subtitle.

The sliding approach is especially useful when comparing very different eras (e.g., 1950s vs 2010s), because a warming trend doesn’t artificially shift older winters toward “cold anomalies” or newer winters toward “warm anomalies” purely due to changing baseline.

Fixed 30-year climatology

In fixed mode, all years are compared to the same 30-year base period, such as 1981-2010 or 1991-2020. You can choose which base period to use in the “Base period” dropdown when “Fixed 30-year” is selected.

• Useful when you want a consistent reference period for all years (e.g., to match a particular operational climatology).
• However, a fixed base will implicitly include any long-term trend, so very early years and very recent years may not be strictly comparable in terms of “typical climate at the time.”

Vertical axis: pressure and approximate height

• Left axis: pressure (hPa) on a logarithmic scale, increasing downward (1000 hPa near the surface, 1 hPa in the upper stratosphere).
• Right axis: an approximate linear altitude scale (km), from the lower troposphere up into the stratosphere.

Interpreting the colors

• Blue shades: negative anomalies (heights lower than climatology). These are often associated with stronger, colder polar vortex conditions aloft.
• Red/orange shades: positive anomalies (heights higher than climatology). On the opposite case, these are often associated with a weakened or displaced polar vortex.
• The values are in σ (standard deviations), where in general sense:
• |σ| ≈ 0.5-1: modest anomaly
• |σ| ≈ 1-2: notable anomaly
• |σ| ≥ 2: strong event relative to the chosen climatology

Notes

• These plots are based on ERA5 reanalysis, not the NCEP/CPC height analyses used on some operational pages, so details will differ, but the overall structure should often be comparable.
• Sliding vs fixed climatology does not change the underlying ERA5 data, only the reference used to define “normal” and “anomalous” conditions.
• How to cite this work:
  If you use these plots, data transformations, or derived climatologies in publications, posts, or presentations, please cite:
  “ERA5-based polar-cap (65–90°N) geopotential height (GPH) anomaly composites, processed and visualized by Giuseppe Petricca. Data source: ECMWF ERA5 Reanalysis.”
  Including a link back to this page is appreciated.